Help needed. Zero crossing with RC snubber problem
Hi
I have a 12 v relay driving an large 220 volt AC relay . Across the contact of the driver relay i placed one RC snubber circut (27NF with 100 R resisitor in series) to help with some spikes that were influencing the low voltage driver circuits. The driver circuit is able to detect mains zero crossing and fire the driver relay at an angle i choose . From what i read the best point to switch off the power relay is at zero crossing . I did that and i show a large spike up to 1 KV at the relay contact followed by a decaying 500hz waveform to 0 volts . After some experimentation the best point came exactly when switching off at the peak of the mains voltage .At this point there is smooth decaying waveform to 0 volt after 5 periods of 500 HZ but no overshoot. The relay presents no arcing. If i remove the snubber and make the experiment the best place to switch is zero crossing but i also see large SHARP spikes up to 500 Volts Peak. My question is The switching with snubber must be made at zero crossing or at the peak of an ac voltage waveform ? What is the behaviour of the circuit ?. As i understand any large spikes can harm the X2 capacitor i'm using so what is the best operating practise ?. Any help will be appreciated Michael |
Help needed. Zero crossing with RC snubber problem
"michael nikolaou" schreef in bericht ... Hi I have a 12 v relay driving an large 220 volt AC relay . Across the contact of the driver relay i placed one RC snubber circut (27NF with 100 R resisitor in series) to help with some spikes that were influencing the low voltage driver circuits. The driver circuit is able to detect mains zero crossing and fire the driver relay at an angle i choose . From what i read the best point to switch off the power relay is at zero crossing . I did that and i show a large spike up to 1 KV at the relay contact followed by a decaying 500hz waveform to 0 volts . After some experimentation the best point came exactly when switching off at the peak of the mains voltage .At this point there is smooth decaying waveform to 0 volt after 5 periods of 500 HZ but no overshoot. The relay presents no arcing. If i remove the snubber and make the experiment the best place to switch is zero crossing but i also see large SHARP spikes up to 500 Volts Peak. My question is The switching with snubber must be made at zero crossing or at the peak of an ac voltage waveform ? What is the behaviour of the circuit ?. As i understand any large spikes can harm the X2 capacitor i'm using so what is the best operating practise ?. Any help will be appreciated Michael The best moment for switching off, highly depends on the load. As long as the load is resistive, the zero crossing point of the voltage is best as switching is done at minimum voltage and current. As soon as the load has a reactive component, zero crossing of the voltage differs from zero crossing of the current. It is the breaking of the current that causes the voltage spikes. petrus bitbyter |
Help needed. Zero crossing with RC snubber problem
On Feb 26, 3:00*am, "michael nikolaou"
wrote: Hi I have a 12 v relay driving an large 220 volt AC relay . Across the contact of the driver relay i placed one RC snubber circut (27NF with 100 R resisitor in series) to help with some spikes that were influencing the low voltage driver circuits. The driver circuit is able to detect *mains zero crossing and fire the driver relay at an angle i choose . From what i read the best point *to switch off the power relay is at zero crossing . I did that and i show a large spike up to 1 KV *at the relay contact followed by a decaying 500hz waveform to 0 volts . After some experimentation the best point came exactly when switching off at the peak of the mains voltage .At this point there is smooth decaying waveform *to 0 volt after 5 periods of * *500 HZ *but no overshoot. The relay presents no arcing. *If i remove the snubber and make the experiment the best place to switch is zero crossing but i also see large SHARP spikes up to 500 Volts Peak. My question is *The switching with snubber must be made at zero crossing or at the peak of an ac voltage waveform ? What is the behaviour of the circuit ?. As i understand any large *spikes can harm the X2 capacitor i'm using so what is the best operating practise ?. Any help will be appreciated *Michael Hi, Michael. First off, you should spec a 12V relay that's made to handle inductive loads (you can see a HP rating). This type of relay has contacts which open more quickly, and are farther apart when open. A small standard relay might not even open up far enough to stop an inductive arc at line voltage. Next, when using a relay to drive a relay, you have to be aware of the delay-on-make, which can be several milliseconds, especially for larger relays. That might help explain some of the curious results you're getting. Turn-on delay can be affected by wear and aging. It also varies from unit to unit, even in relays with the same part and lot number. Trying to get this kind of timing accuracy might be the wrong way to go. It might be better to take a look at suppressing the arc, which you've already started to do. Here's an intuitive way to start. First off, remember your basic goal: you want the voltage across the contacts to rise just slowly enough so the contacts can pull away without sustaining an arc. That's all. Remove the cover from the driving relay so you can see the contacts. Next, find the current rating of those contacts, and use Ohm's law to find a resistor which will result in about half that current. For example, if you have a 220VAC load, and your relay can handle 10A,: R = 220V / 5A = 44 ohms Choose a 47 ohm, 1 watt resistor here (carbon comp is best). Now get a selection of self-healing AC line-rated capacitors, and switch the inductive load with the 47 ohm and C snubber directly across the load, repeating and increasing the cap value until the contact arcing disappears, or at least is minimized. Without knowing anything about your 220VAC relay, it sounds like your 0.027uF cap might be on the small side. Note that ITW Paktron makes a series of Quencharc snubbers that you can just plug in, which makes selection a snap. They're one-piece, epoxy-encapsulated units, and are very easy to use. http://www.paktron.com/pdf/Quencharc_QRL.pdf If all else fails, remember that physical distance is also helpful. Do what you can to place the arcing contact as far away as possible from sensitive circuitry. Good luck Chris |
Help needed. Zero crossing with RC snubber problem
On Tue, 26 Feb 2008 11:00:51 +0200, "michael nikolaou"
wrote: Hi I have a 12 v relay driving an large 220 volt AC relay . Across the contact of the driver relay i placed one RC snubber circut (27NF with 100 R resisitor in series) to help with some spikes that were influencing the low voltage driver circuits. The driver circuit is able to detect mains zero crossing and fire the driver relay at an angle i choose . From what i read the best point to switch off the power relay is at zero crossing . I did that and i show a large spike up to 1 KV at the relay contact followed by a decaying 500hz waveform to 0 volts . After some experimentation the best point came exactly when switching off at the peak of the mains voltage .At this point there is smooth decaying waveform to 0 volt after 5 periods of 500 HZ but no overshoot. The relay presents no arcing. If i remove the snubber and make the experiment the best place to switch is zero crossing but i also see large SHARP spikes up to 500 Volts Peak. My question is The switching with snubber must be made at zero crossing or at the peak of an ac voltage waveform ? What is the behaviour of the circuit ?. As i understand any large spikes can harm the X2 capacitor i'm using so what is the best operating practise ?. Any help will be appreciated Michael If your load is inductive, the current lags the voltage by 90 degrees. Generally, to avoid switch stress, you would try to switch inductive loads at zero current, capacitive loads at zero voltage. RL |
Help needed. Zero crossing with RC snubber problem
"michael nikolaou" wrote in message ... Hi I have a 12 v relay driving an large 220 volt AC relay . Across the contact of the driver relay i placed one RC snubber circut (27NF with 100 R resisitor in series) to help with some spikes that were influencing the low voltage driver circuits. The driver circuit is able to detect mains zero crossing and fire the driver relay at an angle i choose . From what i read the best point to switch off the power relay is at zero crossing . I did that and i show a large spike up to 1 KV at the relay contact followed by a decaying 500hz waveform to 0 volts . After some experimentation the best point came exactly when switching off at the peak of the mains voltage .At this point there is smooth decaying waveform to 0 volt after 5 periods of 500 HZ but no overshoot. The relay presents no arcing. If i remove the snubber and make the experiment the best place to switch is zero crossing but i also see large SHARP spikes up to 500 Volts Peak. My question is The switching with snubber must be made at zero crossing or at the peak of an ac voltage waveform ? What is the behaviour of the circuit ?. As i understand any large spikes can harm the X2 capacitor i'm using so what is the best operating practise ?. Any help will be appreciated Michael Use a solid state releay and dont worry about it. Bob |
Help needed. Zero crossing with RC snubber problem
What chris writes really makes sense. I am adding my extra's here too.
On Tue, 26 Feb 2008 04:43:44 -0800 (PST), Chris wrote: On Feb 26, 3:00*am, "michael nikolaou" wrote: Hi I have a 12 v relay driving an large 220 volt AC relay . Across the contact of the driver relay i placed one RC snubber circut (27NF with 100 R resisitor in series) to help with some spikes that were influencing the low voltage driver circuits. The driver circuit is able to detect *mains zero crossing and fire the driver relay at an angle i choose . From what i read the best point *to switch off the power relay is at zero crossing . I did that and i show a large spike up to 1 KV *at the relay contact followed by a decaying 500hz waveform to 0 volts . After some experimentation the best point came exactly when switching off at the peak of the mains voltage .At this point there is smooth decaying waveform *to 0 volt after 5 periods of * *500 HZ *but no overshoot. The relay presents no arcing. *If i remove the snubber and make the experiment the best place to switch is zero crossing but i also see large SHARP spikes up to 500 Volts Peak. My question is *The switching with snubber must be made at zero crossing or at the peak of an ac voltage waveform ? What is the behaviour of the circuit ?. As i understand any large *spikes can harm the X2 capacitor i'm using so what is the best operating practise ?. Any help will be appreciated *Michael Hi, Michael. First off, you should spec a 12V relay that's made to handle inductive loads (you can see a HP rating). This type of relay has contacts which open more quickly, and are farther apart when open. A small standard relay might not even open up far enough to stop an inductive arc at line voltage. Very true! Next, when using a relay to drive a relay, you have to be aware of the delay-on-make, which can be several milliseconds, especially for larger relays. That might help explain some of the curious results you're getting. Turn-on delay can be affected by wear and aging. It also varies from unit to unit, even in relays with the same part and lot number. Trying to get this kind of timing accuracy might be the wrong way to go. There also is a relay-off delay. And the RC snubber makes the turn-off delay even worse as the current will run a little longer. Expect something here between 5 to 20 milliseconds. It might be better to take a look at suppressing the arc, which you've already started to do. Here's an intuitive way to start. First off, remember your basic goal: you want the voltage across the contacts to rise just slowly enough so the contacts can pull away without sustaining an arc. That's all. What you need to know is the current running through the contacts, and the voltage spike you want to allow. What a snubber does is store the energy of the coil in the capacitor. The resistors "eats" this up as the capacitor discharges over the load. The voltagespike at turnoff (aasuming an empty capacitor at that moment) is the load current * resistor. So if you have R= 100, C= 47n, I=2A, you get a voltage spike of 100*2 = 200 volt even before your capacitor charges. The load inductance with the current give the stored energy: Q=L*I=C*U. So the capacitor size should match the load inductance, otherwise you get a high voltage there too. The capacitor will for example be loaded to 100 volts with a Q = 27n*100V = 2,7 uCoulomb. An inductance that contains this would be L=Q/I= 2,7uC/2A = 1,35 uH. But the resistor already "eats" up a lot, so the voltage will be lower. When you turn off larger motors, transformers, inductors (the coil of a large relay!), the capacitor must match the load to keep voltages limited. The prevent oscillations, resistance values must not be too low, ususally between 30 to 100 Ohm is ok. For large inductive loads I would not take 100 Ohm but 47 Ohm as normal value (see below what Chris wrote), you contact must ve able to handle that. The capacitance could be anything you want, for larger devices 220n, 470n etc. Remove the cover from the driving relay so you can see the contacts. Next, find the current rating of those contacts, and use Ohm's law to find a resistor which will result in about half that current. For example, if you have a 220VAC load, and your relay can handle 10A,: R = 220V / 5A = 44 ohms Choose a 47 ohm, 1 watt resistor here (carbon comp is best). Now get a selection of self-healing AC line-rated capacitors, and switch the inductive load with the 47 ohm and C snubber directly across the load, repeating and increasing the cap value until the contact arcing disappears, or at least is minimized. Without knowing anything about your 220VAC relay, it sounds like your 0.027uF cap might be on the small side. Also very true: the RC snubber also gives a current peak when you turn ON the relay. Nice thing about inductive loads is that the current does not rise fast, so the load and RC current do not occure exactly at the same time (also depending on RC, where a smaller R gives a higher rush-in current but also a better timing). The peak where the snubber works is at turning the relay OFF again. What I often use is 47 Ohm to 100 Ohm with 100 nF. Note that ITW Paktron makes a series of Quencharc snubbers that you can just plug in, which makes selection a snap. They're one-piece, epoxy-encapsulated units, and are very easy to use. http://www.paktron.com/pdf/Quencharc_QRL.pdf If all else fails, remember that physical distance is also helpful. Do what you can to place the arcing contact as far away as possible from sensitive circuitry. Good luck Chris You need TWO RC networks: one across the driving relay, one across the large relay, and do not forget the diode (or also RC) across the coil of the driving relay. So you have 3 things that need attention. Do not forget that long cables (also with resistive loads like lamps) act as inductance. 10 meters or more may also need a RC! Regards, Pieter |
Help needed. Zero crossing with RC snubber problem
I have a 12 v relay driving an large 220 volt AC relay . Across the contact of the driver relay i placed one RC snubber circut (27NF with 100 R resisitor in series) to help with some spikes that were influencing the low voltage driver circuits. The driver circuit is able to detect mains zero crossing and fire the driver relay at an angle i choose . From what i read the best point to switch off the power relay is at zero crossing . I did that and i show a large spike up to 1 KV at the relay contact followed by a decaying 500hz waveform to 0 volts . After some experimentation the best point came exactly when switching off at the peak of the mains voltage .At this point there is smooth decaying waveform to 0 volt after 5 periods of 500 HZ but no overshoot. The relay presents no arcing. If i remove the snubber and make the experiment the best place to switch is zero crossing but i also see large SHARP spikes up to 500 Volts Peak. WTF are you talking about, the arc across the driver relay contacts or the arc across the "large 220 volt AC relay?" It gets reall aggrvating when you don't make yourself clear... My question is The switching with snubber must be made at zero crossing or at the peak of an ac voltage waveform ? What is the behaviour of the circuit ?. As i understand any large spikes can harm the X2 capacitor i'm using so what is the best operating practise ?. Your whole idea is dumb anyway, you're working with relays and pull-in times that are a substantial fraction of line voltage period and vary with age and operating conditions. Snubbers are spedc'd around worst case and not timing. They are only an approximation, the inductance of the "large 220 volt AC relay" is non-linear and may differ by 10:1 between pulled in and out. And some relays require an arc for contact longevity. You let the relay manufacturer take care of the switching and just work out a better 12V drive circuit immune to the dV/dt current from that HV ringing, usually means lower impedance drive. |
Help needed. Zero crossing with RC snubber problem
Thank guys for your replies .Some of them i have to study first
Let me make some things clear about the circuit and values chosen 1. I've measured turn-on , turn-off delay at 3.3 ms for the driver relay.All results are after calculating this delay .So what is see on the scope is at the moment i'm explaining 2. the arc is across the driver relay .The power board is inside a control unit box so i have to leave with small distances and cpu disturbances.Its actually a microcontroller having the problem .Driver relay contact current rating is 5A at 220V ..Power relay coil is rated is 6 watts consumption at 220V. 3. Using large value capacitors over 33 nf was causing sometimes latch of the power relay so i have value limitation here 4. The capacitors i've chosen are X2 self healing 275VAC. With no ZC control they are blown to 0 nf value (some of them) after 10-12 months of operation. 5. I don't have the space or budget to use large sized capacitors rated at higher voltages or SSR . The idea was to use ZC to avoid using expensive and large size protection snubber So the question is . Does the relay On/OFF time differs with time .If it's 10% it's not a problem since again the arc will not be so high .Since its the current break that causes the arc i must switch off at Peak of the ac voltage .This is what my reading confirmed .In this case switching a resistive load must i change the driver algorithm ??? Any helpful comments will be apreciated "Fred Bloggs" wrote in message ... I have a 12 v relay driving an large 220 volt AC relay . Across the contact of the driver relay i placed one RC snubber circut (27NF with 100 R resisitor in series) to help with some spikes that were influencing the low voltage driver circuits. The driver circuit is able to detect mains zero crossing and fire the driver relay at an angle i choose . From what i read the best point to switch off the power relay is at zero crossing . I did that and i show a large spike up to 1 KV at the relay contact followed by a decaying 500hz waveform to 0 volts . After some experimentation the best point came exactly when switching off at the peak of the mains voltage .At this point there is smooth decaying waveform to 0 volt after 5 periods of 500 HZ but no overshoot. The relay presents no arcing. If i remove the snubber and make the experiment the best place to switch is zero crossing but i also see large SHARP spikes up to 500 Volts Peak. WTF are you talking about, the arc across the driver relay contacts or the arc across the "large 220 volt AC relay?" It gets reall aggrvating when you don't make yourself clear... My question is The switching with snubber must be made at zero crossing or at the peak of an ac voltage waveform ? What is the behaviour of the circuit ?. As i understand any large spikes can harm the X2 capacitor i'm using so what is the best operating practise ?. Your whole idea is dumb anyway, you're working with relays and pull-in times that are a substantial fraction of line voltage period and vary with age and operating conditions. Snubbers are spedc'd around worst case and not timing. They are only an approximation, the inductance of the "large 220 volt AC relay" is non-linear and may differ by 10:1 between pulled in and out. And some relays require an arc for contact longevity. You let the relay manufacturer take care of the switching and just work out a better 12V drive circuit immune to the dV/dt current from that HV ringing, usually means lower impedance drive. |
Help needed. Zero crossing with RC snubber problem
On Tue, 26 Feb 2008 19:39:06 +0200, "michael nikolaou"
wrote: Thank guys for your replies .Some of them i have to study first Let me make some things clear about the circuit and values chosen 1. I've measured turn-on , turn-off delay at 3.3 ms for the driver relay.All results are after calculating this delay .So what is see on the scope is at the moment i'm explaining 2. the arc is across the driver relay .The power board is inside a control unit box so i have to leave with small distances and cpu disturbances.Its actually a microcontroller having the problem .Driver relay contact current rating is 5A at 220V .Power relay coil is rated is 6 watts consumption at 220V. 3. Using large value capacitors over 33 nf was causing sometimes latch of the power relay so i have value limitation here 4. The capacitors i've chosen are X2 self healing 275VAC. With no ZC control they are blown to 0 nf value (some of them) after 10-12 months of operation. 5. I don't have the space or budget to use large sized capacitors rated at higher voltages or SSR . The idea was to use ZC to avoid using expensive and large size protection snubber So the question is . Does the relay On/OFF time differs with time .If it's 10% it's not a problem since again the arc will not be so high .Since its the current break that causes the arc i must switch off at Peak of the ac voltage .This is what my reading confirmed .In this case switching a resistive load must i change the driver algorithm ??? However the timing is controlled, yes, that's probably what you need to do. The larger relay coil will be rated for power consumption in the continuous active condition - with the armature closed and inductance fairly high. The inductance limits the current flow that generates power loss in the coil, if your phase angle observations are correct. It's therefor possible that current in the coil is higher than a guestimate (using the 6W label) might produce. What's the DC resistance of the bigger coil? What's the actual coil current with the voltage applied? Larger currents could account for the control contact arcing and large voltages that you see with 100R snubber....unless the resistor is open circuit or intermittent. RL |
Help needed. Zero crossing with RC snubber problem
I have used a triac assisted relay before. The triac shorts out the relay contacts just before the relay is switched on and off. Because the relay is no longer swicthing current they pretty much last forever. You can get away with a small triac as its not on for long. |
Help needed. Zero crossing with RC snubber problem
"michael nikolaou" wrote in message ... Thank guys for your replies .Some of them i have to study first Let me make some things clear about the circuit and values chosen 1. I've measured turn-on , turn-off delay at 3.3 ms for the driver relay.All results are after calculating this delay .So what is see on the scope is at the moment i'm explaining 2. the arc is across the driver relay .The power board is inside a control unit box so i have to leave with small distances and cpu disturbances.Its actually a microcontroller having the problem .Driver relay contact current rating is 5A at 220V .Power relay coil is rated is 6 watts consumption at 220V. [ ... ] From your info', my guesstimate says the big relay has a 8kohm, 4H coil (I'm working from a UK 50Hz). This means the current running through it will be nearly in phase with the voltage across it. Let's say the current is running about 1mS late, then add on the 3.3mS delay for the relay to pull in and it seems not unreasonable that you need to be switching ON about 4-5 mS from the sensed 0V Xover to give minimum transients. This -coincidentally- just happens to be at the peak AC voltage point. A different relay may well have bigger inductance and lower resistance, these would need measuring and the Arrctan thing doing to get a new phase angle/delay for the current. If you put say a 10k ohm 2Watt resistor in // with the power relay coil this will be effective in quickly quenching the relay coil energy. Or better still, fitting a Varistor will simply clip the transient to whatever it's rated voltage is. |
Help needed. Zero crossing with RC snubber problem
michael nikolaou wrote:
Hi I have a 12 v relay driving an large 220 volt AC relay . Across the contact of the driver relay i placed one RC snubber circut (27NF with 100 R resisitor in series) to help with some spikes that were influencing the low voltage driver circuits. The driver circuit is able to detect mains zero crossing and fire the driver relay at an angle i choose . From what i read the best point to switch off the power relay is at zero crossing . I did that and i show a large spike up to 1 KV at the relay contact followed by a decaying 500hz waveform to 0 volts . After some experimentation the best point came exactly when switching off at the peak of the mains voltage .At this point there is smooth decaying waveform to 0 volt after 5 periods of 500 HZ but no overshoot. The relay presents no arcing. If i remove the snubber and make the experiment the best place to switch is zero crossing but i also see large SHARP spikes up to 500 Volts Peak. My question is The switching with snubber must be made at zero crossing or at the peak of an ac voltage waveform ? What is the behaviour of the circuit ?. As i understand any large spikes can harm the X2 capacitor i'm using so what is the best operating practise ?. Any help will be appreciated Michael WHen you say relay, I assume you mean a mechanical contact? if so, It takes time for the contacts the release. If you turn it off at what you detect as the zero crossing point, the contacts most likely will not actually release until some where in some mid point . Many contactors are fast but not fast enough to open before current can get a charge going. That's just my evaluation of what you're doing. By you signaling to turn off the relay at a peak, the contactor will most likely not open until it gets near the zero crossing point. -- |
Help needed. Zero crossing with RC snubber problem
On Feb 26, 7:14*am, "sycochkn" wrote:
"michael nikolaou" wrote in message ... Hi I have a 12 v relay driving an large 220 volt AC relay . Across the contact of the driver relay i placed one RC snubber circut (27NF with 100 R resisitor in series) to help with some spikes that were influencing the low voltage driver circuits. The driver circuit is able to detect *mains zero crossing and fire the driver relay at an angle i choose . From what i read the best point *to switch off the power relay is at zero crossing . I did that and i show a large spike up to 1 KV *at the relay contact followed by a decaying 500hz waveform to 0 volts . After some experimentation the best point came exactly when switching off at the peak of the mains voltage .At this point there is smooth decaying waveform *to 0 volt after 5 periods of * *500 HZ *but no overshoot. The relay presents no arcing. *If i remove the snubber and make the experiment the best place to switch is zero crossing but i also see large SHARP spikes up to 500 Volts Peak. My question is The switching with snubber must be made at zero crossing or at the peak of an ac voltage waveform ? What is the behaviour of the circuit ?. As i understand any large *spikes can harm the X2 capacitor i'm using so what is the best operating practise ?. Any help will be appreciated Michael Use a solid state releay and dont worry about it. Bob- Hide quoted text - - Show quoted text - Right. If you have a resistive load for the final 220VAC load, a plain old solid state relay will work fine (be sure to heat-sink the SSR at about 1.5 watts per amp load). If you have an inductive load, you may want to spec a SSR made to switch these loads, which have back- to-back SCRs to eliminate the possibility of not being able to turn off the SSR). If this is a class project, you won't lose any points by going for the simple solution, as long as it also happens to be the best one. Good luck Chris |
Help needed. Zero crossing with RC snubber problem
On 27 Φε, 03:24, Jamie
t wrote: michael nikolaou wrote: Hi I have a 12 v relay driving an large 220 volt AC relay . Across the contact of the driver relay i placed one RC snubber circut (27NF with 100 R resisitor in series) to help with some spikes that were influencing the low voltage driver circuits. The driver circuit is able to detect *mains zero crossing and fire the driver relay at an angle i choose . From what i read the best point *to switch off the power relay is at zero crossing . I did that and i show a large spike up to 1 KV *at the relay contact followed by a decaying 500hz waveform to 0 volts . After some experimentation the best point came exactly when switching off at the peak of the mains voltage .At this point there is smooth decaying waveform *to 0 volt after 5 periods of * *500 HZ *but no overshoot. The relay presents no arcing. *If i remove the snubber and make the experiment the best place to switch is zero crossing but i also see large SHARP spikes up to 500 Volts Peak. My question is *The switching with snubber must be made at zero crossing or at the peak of an ac voltage waveform ? What is the behaviour of the circuit ?. As i understand any large *spikes can harm the X2 capacitor i'm using so what is the best operating practise ?. Any help will be appreciated *Michael WHen you say relay, I assume you mean a mechanical contact? * if so, It takes time for the contacts the release. If you turn it * off at what you detect as the zero crossing point, the contacts most likely will not actually release until some where in some mid point . * *Many contactors are fast but not fast enough to open before current can get a charge going. * * That's just my evaluation of what you're doing. * *By you signaling to turn off the relay at a peak, the contactor will most likely not open until it gets near the zero crossing point. --- Απόκρυψη κειμένου σε παράθεση - - Εμφάνιση κειμένου σε παράθεση - I observe all signals with a scope . I see actually the "driver relay" signal .So all my observations about timing are correct . My notion was different though .I expected to turn off at zero crossing voltage and have no arc .Actualy when the transition happents at the peak of voltage then i see only a decaying waveform ,no overshoot, for which i suspect since i know the capacitor and measure the frequency one could calculate the total inductance value (cables+relay coil). Since its clear that V(emf)=-L*di/Dt as pointed out by your emails also then the correct point to switch off the driver relay is at peak voltage since current lags voltage by 90 deg in any inductor. Now i only have to find out how much can a relay type deviate from the measurements i have made. Thank's for all you help guys |
Help needed. Zero crossing with RC snubber problem
I have had some occasions using a solid state relay to switch a large
contactor. We found that a random turn-on relay was as good as a zero crossing turn-on (and cost a bit less too). Turn-off with a solid state relay will be at the next zero *current* crossing regardless of the type of SSR used. As to snubbing, I found that a snubber across the contactor's coil was more significant than one across the SSR. A secondary snubber can be applied across the SSR if still needed. The main purpose of snubbing with a SSR is to control the slew rate and peak voltage across the SSR's switching element (traic or scr) to keep it below the point of causing a spurrious trigger/conduction cycle. You should also be mindful that triacs are more sensitive to triggering in certain quadrants and may lead to an asymetrical output conduction (aka a net DC component) which can cause inductive loads to saturate. Oppie "michael nikolaou" wrote in message ... Hi I have a 12 v relay driving an large 220 volt AC relay . Across the contact of the driver relay i placed one RC snubber circut (27NF with 100 R resisitor in series) to help with some spikes that were influencing the low voltage driver circuits. The driver circuit is able to detect mains zero crossing and fire the driver relay at an angle i choose . From what i read the best point to switch off the power relay is at zero crossing . I did that and i show a large spike up to 1 KV at the relay contact followed by a decaying 500hz waveform to 0 volts . After some experimentation the best point came exactly when switching off at the peak of the mains voltage .At this point there is smooth decaying waveform to 0 volt after 5 periods of 500 HZ but no overshoot. The relay presents no arcing. If i remove the snubber and make the experiment the best place to switch is zero crossing but i also see large SHARP spikes up to 500 Volts Peak. My question is The switching with snubber must be made at zero crossing or at the peak of an ac voltage waveform ? What is the behaviour of the circuit ?. As i understand any large spikes can harm the X2 capacitor i'm using so what is the best operating practise ?. Any help will be appreciated Michael |
Help needed. Zero crossing with RC snubber problem
Thank guys for your replies .Some of them i have to study first Let me make some things clear about the circuit and values chosen 1. I've measured turn-on , turn-off delay at 3.3 ms for the driver relay.All results are after calculating this delay .So what is see on the scope is at the moment i'm explaining 2. the arc is across the driver relay .The power board is inside a control unit box so i have to leave with small distances and cpu disturbances.Its actually a microcontroller having the problem .Driver relay contact current rating is 5A at 220V .Power relay coil is rated is 6 watts consumption at 220V. 3. Using large value capacitors over 33 nf was causing sometimes latch of the power relay so i have value limitation here 4. The capacitors i've chosen are X2 self healing 275VAC. With no ZC control they are blown to 0 nf value (some of them) after 10-12 months of operation. 5. I don't have the space or budget to use large sized capacitors rated at higher voltages or SSR . The idea was to use ZC to avoid using expensive and large size protection snubber So the question is . Does the relay On/OFF time differs with time .If it's 10% it's not a problem since again the arc will not be so high .Since its the current break that causes the arc i must switch off at Peak of the ac voltage .This is what my reading confirmed .In this case switching a resistive load must i change the driver algorithm ??? Any helpful comments will be apreciated See section 15.2.12.5 and in particular Circuit B of figure 15.6 in the link below. You will want 450WVDC electrolytics and 600PIV rated diodes along with possibly a small inrush limiting resistor in series with the whole lot. The idea is to dynamically form a peak clamp circuit and this has been used in many high end line surge protection devices for many years with good success. The zero crossing helps a bit but is largely a waste of time. http://www.leachintl2.com/english/en...ies/00053.html |
Help needed. Zero crossing with RC snubber problem
|
Help needed. Zero crossing with RC snubber problem
Thank guys for your replies .Some of them i have to study first Let me make some things clear about the circuit and values chosen 1. I've measured turn-on , turn-off delay at 3.3 ms for the driver relay.All results are after calculating this delay .So what is see on the scope is at the moment i'm explaining 2. the arc is across the driver relay .The power board is inside a control unit box so i have to leave with small distances and cpu disturbances.Its actually a microcontroller having the problem .Driver relay contact current rating is 5A at 220V .Power relay coil is rated is 6 watts consumption at 220V. 3. Using large value capacitors over 33 nf was causing sometimes latch of the power relay so i have value limitation here 4. The capacitors i've chosen are X2 self healing 275VAC. With no ZC control they are blown to 0 nf value (some of them) after 10-12 months of operation. 5. I don't have the space or budget to use large sized capacitors rated at higher voltages or SSR . The idea was to use ZC to avoid using expensive and large size protection snubber So the question is . Does the relay On/OFF time differs with time .If it's 10% it's not a problem since again the arc will not be so high .Since its the current break that causes the arc i must switch off at Peak of the ac voltage .This is what my reading confirmed .In this case switching a resistive load must i change the driver algorithm ??? Any helpful comments will be apreciated The circuit below simulates fairly well. You don't want to use a conventional snubber across the contacts because on opening the relay coil voltage reverses and adds to the 220VAC source. Placing a snubber in shunt with the coil with peak current limiting resistor as shown increases operating power by about 10% but tends to maintain the contact voltage and results in a very slew rate limited 0.5V/us contact voltage peaking in the 450V range. There should be no arc at all with this circuit, with or without zero crossing logic. K1 are the 12V relay contacts and K2 is the 220VAC coil. I did not consider contact bounce on closure, will leave that to you. View in a fixed-width font such as Courier. |
Help needed. Zero crossing with RC snubber problem
On Thu, 28 Feb 2008 11:02:31 -0500, Fred Bloggs
wrote: The circuit below simulates fairly well. You don't want to use a conventional snubber across the contacts because on opening the relay coil voltage reverses and adds to the 220VAC source. Placing a snubber in shunt with the coil with peak current limiting resistor as shown increases operating power by about 10% but tends to maintain the contact voltage and results in a very slew rate limited 0.5V/us contact voltage peaking in the 450V range. There should be no arc at all with this circuit, with or without zero crossing logic. K1 are the 12V relay contacts and K2 is the 220VAC coil. I did not consider contact bounce on closure, will leave that to you. View in a fixed-width font such as Courier. . . . . . 220VAC . o o . | | . | | . | - K1 . | - . | | . | | . | R 100R, 1.5W . | | . | | . | +-----. . | | | . | - | . | |\| | . | K2|\| === . | |\| | 47nF . | |\| | . | - | . | | | . | | | . '------+-----' . . Placing impedance in series with the working solenoid could produce a reduction in speed/dropout performance in the armature of the relay switching the main working load. (not shown in the above drawing) I'm not sure how you modelled the relay coil, but if it used a linear inductor, it will not likely reflect actual performance. A relay drive coil is coupled to a mechanically changing magnetic circuit. As the OP already has a cost-free solution involving programmed timing adjustments, perhaps it's best to let the issue drop? RL |
Help needed. Zero crossing with RC snubber problem
legg wrote: On Thu, 28 Feb 2008 11:02:31 -0500, Fred Bloggs wrote: The circuit below simulates fairly well. You don't want to use a conventional snubber across the contacts because on opening the relay coil voltage reverses and adds to the 220VAC source. Placing a snubber in shunt with the coil with peak current limiting resistor as shown increases operating power by about 10% but tends to maintain the contact voltage and results in a very slew rate limited 0.5V/us contact voltage peaking in the 450V range. There should be no arc at all with this circuit, with or without zero crossing logic. K1 are the 12V relay contacts and K2 is the 220VAC coil. I did not consider contact bounce on closure, will leave that to you. View in a fixed-width font such as Courier. . . . . . 220VAC . o o . | | . | | . | - K1 . | - . | | . | | . | R 100R, 1.5W . | | . | | . | +-----. . | | | . | - | . | |\| | . | K2|\| === . | |\| | 47nF . | |\| | . | - | . | | | . | | | . '------+-----' . . Placing impedance in series with the working solenoid could produce a reduction in speed/dropout performance in the armature of the relay switching the main working load. (not shown in the above drawing) I'm not sure how you modelled the relay coil, but if it used a linear inductor, it will not likely reflect actual performance. A relay drive coil is coupled to a mechanically changing magnetic circuit. As the OP already has a cost-free solution involving programmed timing adjustments, perhaps it's best to let the issue drop? RL He was working with a largely resistive 8KR coil, the 100R has no effect on pull-in or hold-in. The programmed timing can be dropped, what happens at line loss, does his controller lock up because things didn't go exactly as planned, you tell me. |
Help needed. Zero crossing with RC snubber problem
On Thu, 28 Feb 2008 12:51:29 -0500, Fred Bloggs
wrote: Placing impedance in series with the working solenoid could produce a reduction in speed/dropout performance in the armature of the relay switching the main working load. (not shown in the above drawing) I'm not sure how you modelled the relay coil, but if it used a linear inductor, it will not likely reflect actual performance. A relay drive coil is coupled to a mechanically changing magnetic circuit. As the OP already has a cost-free solution involving programmed timing adjustments, perhaps it's best to let the issue drop? RL He was working with a largely resistive 8KR coil, the 100R has no effect on pull-in or hold-in. The programmed timing can be dropped, what happens at line loss, does his controller lock up because things didn't go exactly as planned, you tell me. I've asked for this information, but still see only the reference to a 6W AC coil in the relay doing the work. Larger AC working relay coil current is seldom determined by the DC impedance of the coil. You can refer to the Leach tutorial on this issue, and their catalog, though they don't seem to supply relays or contactors with low frequency AC coils, at present. For example, a 115V 400Hz coil in a 4W series, is listed as consuming 90mA. This indicates that the DC resistance of the coil must be less than half an RDC value that would produce the specified current, in order to keep combined power loss from coil and armature poles to the value expected. The losses in AC activated coils is typically 5 times that for DC rated ones, in simpler commodity forms like OMRON MGN. http://components.omron.com/components/web/webfiles.nsf$FILES/family.html?ID=CNEN-6TJQPU But that wasn't always the case. Older SquareD parts anticipate coil current ratios of more tha 4:1 between closed and open armature, with different values expected for 50 and 60Hz dedicated parts.. http://ecatalog.squared.com/catalog/...6.html#1013844 The same relationship shows up in lower-powered parts: http://catalog.tycoelectronics.com/T...354,16453&LG=1 RL |
Help needed. Zero crossing with RC snubber problem
legg wrote: On Thu, 28 Feb 2008 12:51:29 -0500, Fred Bloggs wrote: Placing impedance in series with the working solenoid could produce a reduction in speed/dropout performance in the armature of the relay switching the main working load. (not shown in the above drawing) I'm not sure how you modelled the relay coil, but if it used a linear inductor, it will not likely reflect actual performance. A relay drive coil is coupled to a mechanically changing magnetic circuit. As the OP already has a cost-free solution involving programmed timing adjustments, perhaps it's best to let the issue drop? RL He was working with a largely resistive 8KR coil, the 100R has no effect on pull-in or hold-in. The programmed timing can be dropped, what happens at line loss, does his controller lock up because things didn't go exactly as planned, you tell me. I've asked for this information, but still see only the reference to a 6W AC coil in the relay doing the work. Larger AC working relay coil current is seldom determined by the DC impedance of the coil. You can refer to the Leach tutorial on this issue, and their catalog, though they don't seem to supply relays or contactors with low frequency AC coils, at present. For example, a 115V 400Hz coil in a 4W series, is listed as consuming 90mA. This indicates that the DC resistance of the coil must be less than half an RDC value that would produce the specified current, in order to keep combined power loss from coil and armature poles to the value expected. The losses in AC activated coils is typically 5 times that for DC rated ones, in simpler commodity forms like OMRON MGN. http://components.omron.com/components/web/webfiles.nsf$FILES/family.html?ID=CNEN-6TJQPU But that wasn't always the case. Older SquareD parts anticipate coil current ratios of more tha 4:1 between closed and open armature, with different values expected for 50 and 60Hz dedicated parts.. http://ecatalog.squared.com/catalog/...6.html#1013844 The same relationship shows up in lower-powered parts: http://catalog.tycoelectronics.com/T...354,16453&LG=1 RL I'm aware of that. Generally AC coils with predominantly reactive impedance are rated in VA and the so-called impedance limited coils with impedance dominated by coil wire resistance are rated in Watts. The OP is working with a 6W impedance limited coil. |
Help needed. Zero crossing with RC snubber problem
On Thu, 28 Feb 2008 16:09:22 -0500, Fred Bloggs
wrote: legg wrote: On Thu, 28 Feb 2008 12:51:29 -0500, Fred Bloggs wrote: Placing impedance in series with the working solenoid could produce a reduction in speed/dropout performance in the armature of the relay switching the main working load. (not shown in the above drawing) I'm not sure how you modelled the relay coil, but if it used a linear inductor, it will not likely reflect actual performance. A relay drive coil is coupled to a mechanically changing magnetic circuit. As the OP already has a cost-free solution involving programmed timing adjustments, perhaps it's best to let the issue drop? RL He was working with a largely resistive 8KR coil, the 100R has no effect on pull-in or hold-in. The programmed timing can be dropped, what happens at line loss, does his controller lock up because things didn't go exactly as planned, you tell me. I've asked for this information, but still see only the reference to a 6W AC coil in the relay doing the work. Larger AC working relay coil current is seldom determined by the DC impedance of the coil. You can refer to the Leach tutorial on this issue, and their catalog, though they don't seem to supply relays or contactors with low frequency AC coils, at present. For example, a 115V 400Hz coil in a 4W series, is listed as consuming 90mA. This indicates that the DC resistance of the coil must be less than half an RDC value that would produce the specified current, in order to keep combined power loss from coil and armature poles to the value expected. The losses in AC activated coils is typically 5 times that for DC rated ones, in simpler commodity forms like OMRON MGN. http://components.omron.com/components/web/webfiles.nsf$FILES/family.html?ID=CNEN-6TJQPU But that wasn't always the case. Older SquareD parts anticipate coil current ratios of more tha 4:1 between closed and open armature, with different values expected for 50 and 60Hz dedicated parts.. http://ecatalog.squared.com/catalog/...6.html#1013844 The same relationship shows up in lower-powered parts: http://catalog.tycoelectronics.com/T...354,16453&LG=1 RL I'm aware of that. Generally AC coils with predominantly reactive impedance are rated in VA and the so-called impedance limited coils with impedance dominated by coil wire resistance are rated in Watts. The OP is working with a 6W impedance limited coil. Sorry, but I don't see a resistance limited coil described in any correspondence from the OP. The most common ( old P&B now Tyco ) contactor for high current that still offers AC coils and a fair description of their impedance and wattage expectations: http://tinyurl.com/3845jx RL |
Help needed. Zero crossing with RC snubber problem
legg wrote: On Thu, 28 Feb 2008 16:09:22 -0500, Fred Bloggs wrote: legg wrote: On Thu, 28 Feb 2008 12:51:29 -0500, Fred Bloggs wrote: Placing impedance in series with the working solenoid could produce a reduction in speed/dropout performance in the armature of the relay switching the main working load. (not shown in the above drawing) I'm not sure how you modelled the relay coil, but if it used a linear inductor, it will not likely reflect actual performance. A relay drive coil is coupled to a mechanically changing magnetic circuit. As the OP already has a cost-free solution involving programmed timing adjustments, perhaps it's best to let the issue drop? RL He was working with a largely resistive 8KR coil, the 100R has no effect on pull-in or hold-in. The programmed timing can be dropped, what happens at line loss, does his controller lock up because things didn't go exactly as planned, you tell me. I've asked for this information, but still see only the reference to a 6W AC coil in the relay doing the work. Larger AC working relay coil current is seldom determined by the DC impedance of the coil. You can refer to the Leach tutorial on this issue, and their catalog, though they don't seem to supply relays or contactors with low frequency AC coils, at present. For example, a 115V 400Hz coil in a 4W series, is listed as consuming 90mA. This indicates that the DC resistance of the coil must be less than half an RDC value that would produce the specified current, in order to keep combined power loss from coil and armature poles to the value expected. The losses in AC activated coils is typically 5 times that for DC rated ones, in simpler commodity forms like OMRON MGN. http://components.omron.com/components/web/webfiles.nsf$FILES/family.html?ID=CNEN-6TJQPU But that wasn't always the case. Older SquareD parts anticipate coil current ratios of more tha 4:1 between closed and open armature, with different values expected for 50 and 60Hz dedicated parts.. http://ecatalog.squared.com/catalog/...6.html#1013844 The same relationship shows up in lower-powered parts: http://catalog.tycoelectronics.com/T...354,16453&LG=1 RL I'm aware of that. Generally AC coils with predominantly reactive impedance are rated in VA and the so-called impedance limited coils with impedance dominated by coil wire resistance are rated in Watts. The OP is working with a 6W impedance limited coil. Sorry, but I don't see a resistance limited coil described in any correspondence from the OP. The most common ( old P&B now Tyco ) contactor for high current that still offers AC coils and a fair description of their impedance and wattage expectations: http://tinyurl.com/3845jx RL That link proves my point, the AC coils are specified in Volt-Amps and the listed DC resistance of those coils is ~20% of the reactance. The fact that the OP describes his coil as AC and 6W means it's impedance limited. |
Help needed. Zero crossing with RC snubber problem
On Fri, 29 Feb 2008 03:43:59 -0500, Fred Bloggs
wrote: I'm aware of that. Generally AC coils with predominantly reactive impedance are rated in VA and the so-called impedance limited coils with impedance dominated by coil wire resistance are rated in Watts. The OP is working with a 6W impedance limited coil. Sorry, but I don't see a resistance limited coil described in any correspondence from the OP. The most common ( old P&B now Tyco ) contactor for high current that still offers AC coils and a fair description of their impedance and wattage expectations: http://tinyurl.com/3845jx RL That link proves my point, the AC coils are specified in Volt-Amps and the listed DC resistance of those coils is ~20% of the reactance. The fact that the OP describes his coil as AC and 6W means it's impedance limited. Could you post a link to a data sheet for any device meeting this description? I am unable to find a relay in this coil power range that even has an AC-operated rating specified in W, never mind one operating at this power level resistively. I see some smaller ones that come close to 45 degrees, but are still on the inductive side. I guess it's hard to avoid, being a magnetic component. I appreciate that there's some confusion here, but I have a suspicion that it is most likely to originate with poor characterization by the OP. I see no reason to carve the misunderstanding into electronic stone on the news server. RL |
Help needed. Zero crossing with RC snubber problem
legg wrote: On Fri, 29 Feb 2008 03:43:59 -0500, Fred Bloggs wrote: I'm aware of that. Generally AC coils with predominantly reactive impedance are rated in VA and the so-called impedance limited coils with impedance dominated by coil wire resistance are rated in Watts. The OP is working with a 6W impedance limited coil. Sorry, but I don't see a resistance limited coil described in any correspondence from the OP. The most common ( old P&B now Tyco ) contactor for high current that still offers AC coils and a fair description of their impedance and wattage expectations: http://tinyurl.com/3845jx RL That link proves my point, the AC coils are specified in Volt-Amps and the listed DC resistance of those coils is ~20% of the reactance. The fact that the OP describes his coil as AC and 6W means it's impedance limited. Could you post a link to a data sheet for any device meeting this description? I am unable to find a relay in this coil power range that even has an AC-operated rating specified in W, never mind one operating at this power level resistively. I see some smaller ones that come close to 45 degrees, but are still on the inductive side. I guess it's hard to avoid, being a magnetic component. I appreciate that there's some confusion here, but I have a suspicion that it is most likely to originate with poor characterization by the OP. I see no reason to carve the misunderstanding into electronic stone on the news server. RL Here is one that does MUCH MUCH better in my original snubber circuit that you, in your infinite wisdumb, cut. http://www.components.omron.com/components/web/PDFLIB.nsf/0/109B19860C4214F385257201007DD570/$file/G2R_0607.pdf |
Help needed. Zero crossing with RC snubber problem
On Fri, 29 Feb 2008 11:36:51 -0500, Fred Bloggs
wrote: legg wrote: On Fri, 29 Feb 2008 03:43:59 -0500, Fred Bloggs wrote: I'm aware of that. Generally AC coils with predominantly reactive impedance are rated in VA and the so-called impedance limited coils with impedance dominated by coil wire resistance are rated in Watts. The OP is working with a 6W impedance limited coil. Sorry, but I don't see a resistance limited coil described in any correspondence from the OP. The most common ( old P&B now Tyco ) contactor for high current that still offers AC coils and a fair description of their impedance and wattage expectations: http://tinyurl.com/3845jx RL That link proves my point, the AC coils are specified in Volt-Amps and the listed DC resistance of those coils is ~20% of the reactance. The fact that the OP describes his coil as AC and 6W means it's impedance limited. Could you post a link to a data sheet for any device meeting this description? I am unable to find a relay in this coil power range that even has an AC-operated rating specified in W, never mind one operating at this power level resistively. I see some smaller ones that come close to 45 degrees, but are still on the inductive side. I guess it's hard to avoid, being a magnetic component. I appreciate that there's some confusion here, but I have a suspicion that it is most likely to originate with poor characterization by the OP. I see no reason to carve the misunderstanding into electronic stone on the news server. RL Here is one that does MUCH MUCH better in my original snubber circuit that you, in your infinite wisdumb, cut. http://www.components.omron.com/components/web/PDFLIB.nsf/0/109B19860C4214F385257201007DD570/$file/G2R_0607.pdf This is a 400mw/0.9VA relay coil. RL |
Help needed. Zero crossing with RC snubber problem
Fred Bloggs wrote in :
wisdumb Cute. *Saves for future use. |
Help needed. Zero crossing with RC snubber problem
michael nikolaou wrote:
Hi I have a 12 v relay driving an large 220 volt AC relay . Across the contact of the driver relay i placed one RC snubber circut (27NF with 100 R resisitor in series) to help with some spikes that were influencing the low voltage driver circuits. The driver circuit is able to detect mains zero crossing and fire the driver relay at an angle i choose . From what i read the best point to switch off the power relay is at zero crossing . I did that and i show a large spike up to 1 KV at the relay contact followed by a decaying 500hz waveform to 0 volts . After some experimentation the best point came exactly when switching off at the peak of the mains voltage .At this point there is smooth decaying waveform to 0 volt after 5 periods of 500 HZ but no overshoot. The relay presents no arcing. If i remove the snubber and make the experiment the best place to switch is zero crossing but i also see large SHARP spikes up to 500 Volts Peak. My question is The switching with snubber must be made at zero crossing or at the peak of an ac voltage waveform ? What is the behaviour of the circuit ?. As i understand any large spikes can harm the X2 capacitor i'm using so what is the best operating practise ?. Any help will be appreciated Michael Hello, Two comments: If the load is not purely resistive, there will be a voltage current shift, aka Eli ICE Man... therefore in this case zero voltage crossing is not at all zero current crossing and you may be creating more problem by switching at the worst (or just a bad) time) . BY looking at the signal that the rc snubber is trying to "tame" , you adjust RC unitl you get a "critically damped response. you can dampen more, but at the cost of higher stand by leakage via the RC, as it becomes a part of the load too! Good luck, this is an often asked problem, and the ability to visualizes on scope and make changes and observations, will help you forever. This is a re-occcuring problem and it is the variations in the load that cause engineers to have to re-visit the solutions Best regards Marco |
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