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RPC run capacitors
"Ignoramus19023" wrote in message .. . I am considering adding some run capacitors to my self starting RPC. I am reading Jim Hanrahan's article at http://www.metalwebnews.com/howto/ph-conv/ph-conv.html and I am confused by something. I understand how self starting RPCwould start with one cap between one leg 1 and leg 3 (the generated one). That's how mine is wired. Jim makes a point that it works, but makes unbalanced voltage. But why would it start is capacitors are connected between 1-3 AND 2-3, like in this pictu http://www.metalwebnews.com/howto/ph-conv/fig1.html I cannot see how it would create assymmetric fields needed to spin up the motor. Is that because capacitances across legs 1-3 are greater than capacitance between leg 2-3? I could try to use run caps at run time and start caps at start time. In fact, I won a time delay relay for $9 on ebay yesterday, so I could set the RPC to start on start cap (both caps between leg 1-3) and then reconnect the same caps to become run caps, one between 1-3 and another between 2-3. Same TDR could, then, turn out output current aftet time delay, allowing the RPC to spin up and switch to the run mode. As you can see, I am quite confused, but am willing to experiment. I have 4 unused Furnas 75 A contactors that I can wire, with the time delay relay, to do just about anything. Idler: 10 HP Capacitors: 92 mF each, 535 VAC rated, oil filled. I have 5 total, and use 2 for the starting leg, so three are unused. i You are correct saying that it's because the capacitance between 1 + 3 is higher than between 2 + 3. My RPC, 10HP 440V 50Hz motor, has 40uF between 2 + 3, 60uF between 1 + 3 and 500uF switched by a start circuit between 1 + 3. The 500uF is a bit too much, it starts very quickly (around 1/3 second) so I could probably drop that to around 200uF. Martin -- martindot herewhybrowat herentlworlddot herecom |
"Ignoramus19023" wrote in message .. . I am considering adding some run capacitors to my self starting RPC. I am reading Jim Hanrahan's article at http://www.metalwebnews.com/howto/ph-conv/ph-conv.html and I am confused by something. I understand how self starting RPCwould start with one cap between one leg 1 and leg 3 (the generated one). That's how mine is wired. Jim makes a point that it works, but makes unbalanced voltage. But why would it start is capacitors are connected between 1-3 AND 2-3, like in this pictu http://www.metalwebnews.com/howto/ph-conv/fig1.html I cannot see how it would create assymmetric fields needed to spin up the motor. Is that because capacitances across legs 1-3 are greater than capacitance between leg 2-3? A 3-phase motor will start on single-phase current because the (3rd leg, middle leg, whatever) has phase-shifted current applied to it. That leg in a 3-phase motor operated on single-phase, has phase-shifted current applied via the start cap. That current is "sufficiently out of phase" with the current in the main winding to provide a starting field, if you will, for the main field to operate against, thus causing start-up. Remember, we are talking about a single-phase motor. After the starting interval, the motor will continue to operate as a single phase machine. The start cap, if left permanently in place, forms a "self starting" RPC. A rather large amount of capacitance is required for starting any motor, be it single-phase or 3-phase, when starting on single-phase current. Therefore, a 3-phase motor running with an over-large starting capacitance, permanently in place, is likely to exhibit severly unbalanced running voltages. This is why serious RPC builders always seperate the starting and running functions. Assymetric fields are necessary for starting, as above. The "running current" flow paths in a RPC are quite complex and are also assynmetric even in a tuned, voltage balanced RPC because of direction of rotation, among other things. Suffice it to say, the current flow in a RPC and its load requires "steerage" (think series resonance) in order for the fundamental single-phase running current to be guided into paths that emulate 3-phase conditions. Remember, you are still dealing with single-phase power. A RPC does not "generate" 3-phase power - it merely performs adjustments in a fundamentally single-phase scenario which emulates 3-phase. I could try to use run caps at run time and start caps at start time. In fact, I won a time delay relay for $9 on ebay yesterday, so I could set the RPC to start on start cap (both caps between leg 1-3) and then reconnect the same caps to become run caps, one between 1-3 and another between 2-3. Same TDR could, then, turn out output current aftet time delay, allowing the RPC to spin up and switch to the run mode. By all means do "use run caps at run time and start caps at start time". Forget about reconnecting start caps and using them as run caps. Either use a simple push-button switch to temporarily connect the start caps or a NC potential relay that senses 3rd leg voltage to open up the start circuit. Then leave the start circuit alone. Period. As you can see, I am quite confused, but am willing to experiment. I have 4 unused Furnas 75 A contactors that I can wire, with the time delay relay, to do just about anything. Idler: 10 HP Capacitors: 92 mF each, 535 VAC rated, oil filled. I have 5 total, and use 2 for the starting leg, so three are unused. It occurs, you are attempting to use components that may not be appropriate, or the best way, just becasue you got them on the cheap. No amount of aimless, and possibly dangerous experimentation, with the wrong things can necessarily force success. Bob Swinney i |
On Fri, 05 Aug 2005 17:29:48 GMT, Ignoramus19023
wrote: On Fri, 05 Aug 2005 17:16:59 GMT, Martin Whybrow wrote: "Ignoramus19023" wrote in message .. . I am considering adding some run capacitors to my self starting RPC. I am reading Jim Hanrahan's article at http://www.metalwebnews.com/howto/ph-conv/ph-conv.html and I am confused by something. I understand how self starting RPCwould start with one cap between one leg 1 and leg 3 (the generated one). That's how mine is wired. Jim makes a point that it works, but makes unbalanced voltage. But why would it start is capacitors are connected between 1-3 AND 2-3, like in this pictu http://www.metalwebnews.com/howto/ph-conv/fig1.html I cannot see how it would create assymmetric fields needed to spin up the motor. Is that because capacitances across legs 1-3 are greater than capacitance between leg 2-3? I could try to use run caps at run time and start caps at start time. In fact, I won a time delay relay for $9 on ebay yesterday, so I could set the RPC to start on start cap (both caps between leg 1-3) and then reconnect the same caps to become run caps, one between 1-3 and another between 2-3. Same TDR could, then, turn out output current aftet time delay, allowing the RPC to spin up and switch to the run mode. As you can see, I am quite confused, but am willing to experiment. I have 4 unused Furnas 75 A contactors that I can wire, with the time delay relay, to do just about anything. Idler: 10 HP Capacitors: 92 mF each, 535 VAC rated, oil filled. I have 5 total, and use 2 for the starting leg, so three are unused. i You are correct saying that it's because the capacitance between 1 + 3 is higher than between 2 + 3. My RPC, 10HP 440V 50Hz motor, has 40uF between 2 + 3, 60uF between 1 + 3 and 500uF switched by a start circuit between 1 + 3. The 500uF is a bit too much, it starts very quickly (around 1/3 second) so I could probably drop that to around 200uF. Thank you Martin. I have five 92 mF caps total. What I have now is 2 capacitors connected between leg 1 and 3. It seems that about the only thing that I need, then, wire it as follows: 1. 1 cap (92 mF) between legs 1 and 2, permanently connected. 2. 1 cap (92 mF) between 1 and 3, permanently connected. 3. 2-3 more caps (180-270 mF) between legs 1 and 3, connected at start time and switched off by the time delay relay (TDR), say 4 seconds after startup. 4. (optional) add another contactor on output side that would only switch output on when the TDR actuates and switches off the start caps. Theat would get me an RPC that is: - properly balanced across the range of output HP - starts quickly - only turns on when balanced power is output and full rotational speed is achieved, that is, when good quality 3 phase power is available. Does it make sense? i With five equal value oil filled capacitors your options are fairly limited but you can still finish up wth a perfectly good converter. One thing to remember is that capacitors of this type have extremely low internal series resistance (milliohms)- that's why you get a sizable bang if you short circuit a charged one! The same thing happens when you connect a charged capacitor to an uncharged one - VERY high peak currents flow as the charge voltage equalises on the two capacitors. The capacitors are not greatly bothered by this treatment but it's very unkind to the switch contacts or relay contacts used to parallel connect two capacitors if there is a substantial voltage difference at the instant of connection. Brute force oversizing of the switching contacts can give reasonable contact life but it's much neater to avoid the problem by using separate start and run capacitors that are never parallel connected. This is easily done with your capacitor collection. Use three parallel connected as your start capacitor. Only in circuit for the few seconds needed for the idler to run up to speed. You now have a bit unbalanced but perfectly usable converter system. Ideally the load motor should not be switched in until the idler is up to speed. However, if the load motor is initially running light, this start capacitance is probably enough for a simultaneous idler and load start. Your START switching should be a changeover contact which EITHER connects the start capacitor OR the run capacitor across L1 and L2. For the run capacitor, try one capacitor or two capacitors series connected to halve the effective value - whichever gives the best voltage balance with full load on the load motor. Do not place a capacitor across L2 L3. With the values you have available this would do more harm than good. Jim |
On Sat, 06 Aug 2005 00:46:19 GMT, Ignoramus19023 wrote: SNIP For the run capacitor, try one capacitor or two capacitors series connected to halve the effective value - whichever gives the best voltage balance with full load on the load motor. Looks like one run cap per side should be good, based on Jim Hanrahan's writings. I'll see. Do not place a capacitor across L2 L3. With the values you have available this would do more harm than good. Hm, why is that so? Anyway, my main question is: if I do all this (as I outlined in steps 1-6), will I get a Mercedes Benz of phase converters? Or am I wasting my time for load up to and under 5 HP? i This is an extract from an earlier post which goes some way to explaining the peculiarities of "balancing" capacitors. Snip A converter of this type is basically a capacitor/inductor phase shift system which produces an open vee 3 phase system. This phase shifter is a series resonant circuit and when it is set up to give the 60 deg phase shift it is working a long way below its natural resonant frequency. 60 deg is of course the correct phase angle between the two legs of an open vee system. The motor(s) is the inductor in the system and unfortunately the apparent inductance of the motor changes with rotor speed. For any particular rotor speed greater than about 90% of synchronous speed (the lower limit varies a bit with motor type) it is possible to choose a capacitor combination which produces a pretty close approximation to balanced 3 phase at the motor terminals. For near the full load rated speed of the motor, large run capacitance is needed with most or all of it as a single capacitor feeding the phantom phase from supply live. At light load the speed of the rotor rises and if the capacitor value is chosen to achieve the right phase angle the phantom phase voltage will be excessive. This could be corrected by feeding the capacitor from a lower voltage single phase source but this would mean feeding it from an auto transformer across the supply. It is much simpler (and of course everybody does this) to use two capacitors arranged as a voltage divider to simultaneously achieve the correct phase angle and phase voltage. The effective capacitanceof the two capacitors connected in series across the supply is the sum of the capacitances because the source impedance of the supply is zero and this effectively parallels the two capacitors. Because the they also act as a voltage divider, this sum capacitance is effectively fed from a voltage of supply voltage times C1/(C1+C2) where C1 is the top capacitor and C2 is connected phantom phase to neutral. Because it looks nicely symmetrical there seems to be a tendency to believe that C1 and C2 should be equal and any inequality in their optimum value must result from some strange second order effect. This is NOT true. There is nothing magic about equal C1 and C2. It simply results in a capacitor of value C1+C2 fed from half the supply voltage. At this low effective supply voltage it is only possible to get close to balanced operation at no load or light loads which enable the rotor to operate close to synchronous speed. As the load increases with consequent slowing of the rotor speed the total capacitance needs to increase with both more in C1 and less in C2. By the time full load is reached the optimum value for C2 is usually zero. These effects are very noticeable if you're using a single motor on a variable load up to near rated full load power and some compromise necessary. The saving grace is that industrial motors are surprisingly tolerant of reasonable overvoltage when operating at light loads so the trick is to size the capacitors for at or near full loads and to accept some overvoltaqe at light loads. This increases the motor losses at light load but the total motor losses still remain below the losses at rated full load so temperature rise is acceptable. SNIP In your case, while a small additional capacitor across L2 L3 might yield better phase balance for some load conditions, the 92uF you have available is far too large and would do more harm than good. I fully support Jerry Martes comment to the effect that fine tuning rarely achieves significant practical benefit. If you've a nice big idler and have solved the starting problem, careful choice of balancing capacitors may give you a nice warm feeling but you're unlikely to notice much practical difference in the overall performance! Jim |
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