Home |
Search |
Today's Posts |
|
Home Repair (alt.home.repair) For all homeowners and DIYers with many experienced tradesmen. Solve your toughest home fix-it problems. |
Reply |
|
LinkBack | Thread Tools | Display Modes |
#1
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
Why do Microwave ovens that have a rotating platter, change direction?
The platter seems to usually spin counter-clockwise. However, if I turn it off to stir or check the food, the platter begins to turn clockwise. It dont change direction once it's running, only when I stop it and restart. This is the second microwave I've had that does this, and they were different brand names. Not only do I wonder *why* they change rotation, but *how*? I'm guessing they use a shaded-pole motor (I'm not sure), and as far as I know, those motors only turn one direction. Either CW or CCW. Unless there's a belt driving them, there is not much room for any other type of motor in the base of the oven. Anyone know anything about this? |
#2
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
wrote:
Why do Microwave ovens that have a rotating platter, change direction? The platter seems to usually spin counter-clockwise. However, if I turn it off to stir or check the food, the platter begins to turn clockwise. It dont change direction once it's running, only when I stop it and restart. This is the second microwave I've had that does this, and they were different brand names. Not only do I wonder *why* they change rotation, but *how*? I'm guessing they use a shaded-pole motor (I'm not sure), and as far as I know, those motors only turn one direction. Either CW or CCW. Unless there's a belt driving them, there is not much room for any other type of motor in the base of the oven. Anyone know anything about this? They Google randomly start Google in either Google direction because Google of the inexpensive Google AC motors that are Google used. |
#3
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
On Jan 21, 9:22 pm, wrote:
Why do Microwave ovens that have a rotating platter, change direction? The platter seems to usually spin counter-clockwise. However, if I turn it off to stir or check the food, the platter begins to turn clockwise. It dont change direction once it's running, only when I stop it and restart. This is the second microwave I've had that does this, and they were different brand names. Not only do I wonder *why* they change rotation, but *how*? I'm guessing they use a shaded-pole motor (I'm not sure), and as far as I know, those motors only turn one direction. Either CW or CCW. Unless there's a belt driving them, there is not much room for any other type of motor in the base of the oven. Anyone know anything about this? It has to "unwind" so the turntable won't kink ..... |
#4
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
|
#6
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
On Jan 22, 12:26*pm, wrote:
On Tue, 22 Jan 2013 07:36:49 -0800, Jon Danniken wrote: On 01/21/2013 07:22 PM, wrote: Why do Microwave ovens that have a rotating platter, change direction? The platter seems to usually spin counter-clockwise. *However, if I turn it off to stir or check the food, the platter begins to turn clockwise.. It dont change direction once it's running, only when I stop it and restart. *This is the second microwave I've had that does this, and they were different brand names. Not only do I wonder *why* they change rotation, but *how*? *I'm guessing they use a shaded-pole motor (I'm not sure), and as far as I know, those motors only turn one direction. *Either CW or CCW. Unless there's a belt driving them, there is not much room for any other type of motor in the base of the oven. They're just little sync motors, like you'd find in a clock. *To save money, they don't have any provision to determine which way they spin, so direction of travel is determined by pole proximity. Jon Thanks for the info. *That makes sense. *The direction should not matter as far as how it cooks, since the cooking is done by the magnetron. *The rotation is just so it cooks evenly. *That much I knew. *Now I know why it changes direction.- Hide quoted text - - Show quoted text - And in the similarly-designed clock motors, there is a little mechanical gizmo that won't allow the motor to run the clock backwards. The motor will try to run backwards, the try forward, and then the gizmo lets it proceed. |
#7
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
On 1/22/2013 1:01 PM, hr(bob) wrote:
On Jan 22, 12:26 pm, wrote: On Tue, 22 Jan 2013 07:36:49 -0800, Jon Danniken wrote: On 01/21/2013 07:22 PM, wrote: Why do Microwave ovens that have a rotating platter, change direction? The platter seems to usually spin counter-clockwise. However, if I turn it off to stir or check the food, the platter begins to turn clockwise. It dont change direction once it's running, only when I stop it and restart. This is the second microwave I've had that does this, and they were different brand names. Not only do I wonder *why* they change rotation, but *how*? I'm guessing they use a shaded-pole motor (I'm not sure), and as far as I know, those motors only turn one direction. Either CW or CCW. Unless there's a belt driving them, there is not much room for any other type of motor in the base of the oven. They're just little sync motors, like you'd find in a clock. To save money, they don't have any provision to determine which way they spin, so direction of travel is determined by pole proximity. Jon Thanks for the info. That makes sense. The direction should not matter as far as how it cooks, since the cooking is done by the magnetron. The rotation is just so it cooks evenly. That much I knew. Now I know why it changes direction.- Hide quoted text - - Show quoted text - And in the similarly-designed clock motors, there is a little mechanical gizmo that won't allow the motor to run the clock backwards. The motor will try to run backwards, the try forward, and then the gizmo lets it proceed. A sprag clutch. ^_^ http://en.wikipedia.org/wiki/Sprag_clutch TDD |
#8
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
|
#9
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
|
#10
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
|
#11
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
Ashton Crusher wrote:
On Mon, 21 Jan 2013 21:22:25 -0600, wrote: Why do Microwave ovens that have a rotating platter, change direction? The platter seems to usually spin counter-clockwise. However, if I turn it off to stir or check the food, the platter begins to turn clockwise. It dont change direction once it's running, only when I stop it and restart. This is the second microwave I've had that does this, and they were different brand names. Not only do I wonder *why* they change rotation, but *how*? I'm guessing they use a shaded-pole motor (I'm not sure), and as far as I know, those motors only turn one direction. Either CW or CCW. Unless there's a belt driving them, there is not much room for any other type of motor in the base of the oven. Anyone know anything about this? I see some suggest it's just random but I think it's random "on purpose". In the old days they always went the same direction. What I read was that they purposely make, or at least allow, them to revere direction because they found that was a good way to dislodge crumbs that would otherwise get stuck on/under the little support wheels and make the turntable jerk/squeak/squeal. Looks like a winner. From what I see, commercial units don't have rotating dishes. My first microwave used a circulator what looks like flat fan blades in the waveguide. I suspect commercial units still do this. And no, the dish does not change direction because of left hand, right hand circular polarization. Greg |
#12
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
On Tue, 22 Jan 2013 16:13:30 -0700, Ashton Crusher
wrote: Not only do I wonder *why* they change rotation, but *how*? I'm guessing they use a shaded-pole motor (I'm not sure), and as far as I know, those motors only turn one direction. Either CW or CCW. Unless there's a belt driving them, there is not much room for any other type of motor in the base of the oven. Anyone know anything about this? I see some suggest it's just random but I think it's random "on purpose". In the old days they always went the same direction. What I read was that they purposely make, or at least allow, them to revere direction because they found that was a good way to dislodge crumbs that would otherwise get stuck on/under the little support wheels and make the turntable jerk/squeak/squeal. Someone had their smarts working to figure that out.... |
#13
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
On Tue, 22 Jan 2013 15:48:13 -0600, Gordon Shumway
wrote: On Mon, 21 Jan 2013 21:22:25 -0600, wrote: Why do Microwave ovens that have a rotating platter, change direction? The platter seems to usually spin counter-clockwise. However, if I turn it off to stir or check the food, the platter begins to turn clockwise. It dont change direction once it's running, only when I stop it and restart. This is the second microwave I've had that does this, and they were different brand names. Not only do I wonder *why* they change rotation, but *how*? I'm guessing they use a shaded-pole motor (I'm not sure), and as far as I know, those motors only turn one direction. Either CW or CCW. Unless there's a belt driving them, there is not much room for any other type of motor in the base of the oven. Anyone know anything about this? The answer should be intuitively obvious to even the most casual observer. You do have it plugged into ALTERNATING CURRENT don't you? Actually, I just plug it into a CURRENT BUSH! |
#14
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
You don't want the food to get dizzy. After all, you are what you eat. -- Wes Groleau Alive and Well http://freepages.religions.rootsweb.com/~wgroleau/ |
#15
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
On 01-22-2013 10:36, Jon Danniken wrote:
They're just little sync motors, like you'd find in a clock. To save money, they don't have any provision to determine which way they spin, so direction of travel is determined by pole proximity. That may be true of some, but most I've seen NEVER go in the same direction twice. -- Wes Groleau In any formula, constants (especially those obtained from handbooks) are to be treated as variables. |
#16
|
|||
|
|||
Quote:
This post is going to be off topic because I believe your question has been answered. All electric motors operate on the principle that a magnet will spin if you put it in a rotating magnetic field. It's producing a rotating magnetic field that can be a bit of a trick sometimes. With three phase electric power, producing a rotating magnetic field is easy. If each phase of your power is 120 degrees apart, you just arrange the three windings 120 degrees apart around the stator and you have a near perfect rotating magnetic field. With two phase power, you can do a similar thing and get good results. It's when you get to single phase 120 VAC power that you have to get creative in using that single phase power to produce what appears to be a rotating magnetic field instead of just an oscillating magnetic field. ALL of the different kinds of 120 volt electric motors that you hear about (like split phase motors, shaded pole motors, capacitor start motors, etc.) are different only because they use a different way of making the rotor see what appears to be a rotating magnetic field. The easiest of these to explain is the capacitor start motor. If you imagine two metal plates in close proximity, if you apply a voltage to one plate, that applied voltage will repel the electrons in the second plate, and you'll get a small current flowing out of that second plate. Now, if the voltage you apply to the first plate were in the form of a sinusoidal wave, just like the voltage in your wall outlets, the current coming out of the second plate would be at a maximum when the RATE OF CHANGE IN VOLTAGE in the first plate was at a maximum, and that actually occurs when the applied voltage is going from positive to negative or negative to positive, or when the applied voltage is actually ZERO for a very short period of time. That is, by putting a capacitor in a circuit, you completely change the relationship between the applied voltage and the resulting current through the circuit. In a simple circuit with only a single resistor in the circuit, current through the resistor is maximum when the applied voltage is maximum. Similarily, current is theoretically zero when the voltage goes from positive to negative, or negative to positive, or when the applied voltage is temporarily zero. If you replace that resistor with a capacitor, the current through the circuit is a maximum when the applied sinusoidal voltage is changing the fastest, and that occurs when the voltage goes from negative to positive, or positive to negative, or when it's actually temporarily zero volts. So, one way to use single phase power to create an apparant rotating magnetic field is to build an electric motor for two phase power (with the windings 90 degrees apart) and apply the same 120 VAC to both windings. BUT, if you put a capacitor in series with one of those windings, the current through that winding will be 90 degrees out of phase with the current through the other winding. Since a coil of wire develops it's magnetism as a result of the CURRENT flowing through the coil and not the voltage applied to it, the magnetic field of one winding will develop 1/4 of a AC voltage cycle before or after the other winding, thereby creating much the same thing as the rotor would see if it were in a two phase motor while the motor is starting. In actuality, in a capacitor start motor the winding with the capacitor in series is cut out of the circuit by a centrifugal switch once the motor comes up to speed. After the "start" winding is shut off, the capacitor start motor continues to run on the other "run" winding only. It's been found that the motor will run smoother and more efficiently that way, and the explanation of "why" is something I just don't know. In a "split phase" electric motor, you have very much the same thing happening as in a capacitor start motor, except that you don't have a capacitor in series with one of the windings. Instead, in a split phase motor, one of the windings consists of a lot of turns of thin wire whereas the other winding consists of only a few turns of thick wire. This difference causes the two windings to have different "impedance", and that results in the winding with the thin wire developing it's magnetic field earlier in the AC voltage cycle than the winding with the thick wire. And, that difference in the timing of the magnetic field from each winding creates the impression of a rotating magnetic field for the rotor to follow. And, just like in a capacitor start motor, once a split phase motor gets up to speed, a centrifugal switch cuts out the start winding and the motor will continue to turn on it's run winding only. (I can't remember now whether the start winding is the one with thin or thick wires.) Now, both a capacitor start motor and a split phase motor will be happy to turn in the opposite direction if you want them to. All you have to do is reverse the polarity of ONE of the windings. It can be the start winding or the run winding; doesn't matter which. If you reverse the polarity of one of the windings, the motor will turn happily in the opposite direction. Washing machines use that feature to advantage by having the timer (or something called a "motor reversing relay") reverse the polarity of one of the windings on the motor so that the motor turns in one direction while the washer is in the agitate cycle, and in the opposite direction when the washer is in the spin cycle. This is important because during the spin cycle, the motor turns the washer's pump one way to pump water OUT of the washer, but during the agitate cycle, the pump turns so as to pump any water that leaks into it back into the wash basket. Similarily, Maytag top loading washing machines have a pulley which turns on a threaded shaft. The pulley screws UP the shaft when the motor turns one way, and down the shaft when the motor reverses direction, and it's which direction that pulley is pushing that determines what the transmission does; agitate or spin. I've been told not to reverse the polarity of BOTH windings on a capacitor start or split phase motor. Apparantly, doing that will create a dislocation in the space-time continuum causing you to complete the reversal procedure before you began, with the necessary result that the motor will turn in it's original direction. (smirk) A shaded pole motor is a different kettle of fish. A shaded pole motor has only two poles in it's stator. But, there will be a thick loop of copper wire going through the middle of both of those two poles, thereby separating each pole in half. A current flows in that thick wire as a result of the magnetism produced at the two poles, and the magnetic field of the thick wire opposes the magnetic field on one side of the pole and adds to the magnetic field on the other side of the pole. So, what the rotor sees is a weak magnetic field on one side of the pole becoming a strong magnetic field on the other side of the pole, and that gives the rotor the illusion of the magnetism "sweeping across" the pole, or something similar to a rotating magnetic field. The thing to remember here is that there is only ONE kind of three phase motor and only ONE kind of two phase motor, but several different kinds of single phase electric motors. That's because each different kind of 120 VAC electric motor uses a different method of getting the stator to create the appearance of a rotating magnetic field using only single phase electric power. Two and three phase electric motors don't need to do that because their stators actually produce a rotating magnetic field for the rotor to follow. There, now you know more about electric motors than 99 percent of people named "Homeowner". Last edited by nestork : January 23rd 13 at 06:47 AM |
#17
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
In article ,
Wes Groleau wrote: On 01-22-2013 10:36, Jon Danniken wrote: They're just little sync motors, like you'd find in a clock. To save money, they don't have any provision to determine which way they spin, so direction of travel is determined by pole proximity. That may be true of some, but most I've seen NEVER go in the same direction twice. Maybe so... from what your saying, I'm guessing there are both types out there. For the record my 4 year old Sharp usually rotates clockwise (as viewed from above), but now and again (maybe 10% of the time) it'll start up counter clockwise. On one page of the manual it says "It is normal for the turntable to turn in either direction" and on another states "The turntable will turn both clockwise and counterclockwise." I've always had a hunch it has a shaded pole motor sans the 'shade'... thus saving a penny or two in copper per unit. Will such an unshaded motor start itself? It's on my list of things to look into some day. Erik |
#18
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
On Wednesday, January 23, 2013 5:27:11 AM UTC, Wes Groleau wrote:
On 01-22-2013 10:36, Jon Danniken wrote: They're just little sync motors, like you'd find in a clock. To save money, they don't have any provision to determine which way they spin, so direction of travel is determined by pole proximity. That may be true of some, but most I've seen NEVER go in the same direction twice. They must do as there's only two directions clockwise and anti-clockwise ;-) -- Wes Groleau In any formula, constants (especially those obtained from handbooks) are to be treated as variables. |
#19
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
On Jan 22, 11:50 pm, nestork wrote:
;3001034 Wrote: Not only do I wonder *why* they change rotation, but *how*? I'm guessing they use a shaded-pole motor (I'm not sure), and as far as I know, those motors only turn one direction. Homeowner: This post is going to be off topic because I believe your question has been answered. All electric motors operate on the principle that a magnet will spin if you put it in a rotating magnetic field. It's producing a rotating magnetic field that can be a bit of a trick sometimes. With three phase electric power, producing a rotating magnetic field is easy. If each phase of your power is 120 degrees apart, you just arrange the three windings 120 degrees apart around the stator and you have a near perfect rotating magnetic field. With two phase power, you can do a similar thing and get good results. It's when you get to single phase 120 VAC power that you have to get creative in using that single phase power to produce what appears to be a rotating magnetic field instead of just an oscillating magnetic field. ALL of the different kinds of 120 volt electric motors that you hear about (like split phase motors, shaded pole motors, capacitor start motors, etc.) are different only because they use a different way of making the rotor see what appears to be a rotating magnetic field. The easiest of these to explain is the capacitor start motor. If you imagine two metal plates in close proximity, if you apply a voltage to one plate, that applied voltage will repel the electrons in the second plate, and you'll get a small current flowing out of that second plate. Now, if the voltage you apply to the first plate were in the form of a sinusoidal wave, just like the voltage in your wall outlets, the current coming out of the second plate would be at a maximum when the RATE OF CHANGE IN VOLTAGE in the first plate was at a maximum, and that actually occurs when the applied voltage is going from positive to negative or negative to positive, or when the applied voltage is actually ZERO for a very short period of time. That is, by putting a capacitor in a circuit, you completely change the relationship between the applied voltage and the resulting current through the circuit. In a simple circuit with only a single resistor in the circuit, current through the resistor is maximum when the applied voltage is maximum. Similarily, current is theoretically zero when the voltage goes from positive to negative, or negative to positive, or when the applied voltage is temporarily zero. If you replace that resistor with a capacitor, the current through the circuit is a maximum when the applied sinusoidal voltage is changing the fastest, and that occurs when the voltage goes from negative to positive, or positive to negative, or when it's actually temporarily zero volts. So, one way to use single phase power to create an apparant rotating magnetic field is to build an electric motor for two phase power (with the windings 90 degrees apart) and apply the same 120 VAC to both windings. BUT, if you put a capacitor in series with one of those windings, the current through that winding will be 90 degrees out of phase with the current through the other winding. Since a coil of wire develops it's magnetism as a result of the CURRENT flowing through the coil and not the voltage applied to it, the magnetic field of one winding will develop 1/4 of a AC voltage cycle before or after the other winding, thereby creating much the same thing as the rotor would see if it were in a two phase motor while the motor is starting. In actuality, in a capacitor start motor the winding with the capacitor in series is cut out of the circuit by a centrifugal switch once the motor comes up to speed. After the "start" winding is shut off, the capacitor start motor continues to run on the other "run" winding only. It's been found that the motor will run smoother and more efficiently that way, and the explanation of "why" is something I just don't know. In a "split phase" electric motor, you have very much the same thing happening as in a capacitor start motor, except that you don't have a capacitor in series with one of the windings. Instead, in a split phase motor, one of the windings consists of a lot of turns of thin wire whereas the other winding consists of only a few turns of thick wire. This difference causes the two windings to have different "impedance", and that results in the winding with the thin wire developing it's magnetic field earlier in the AC voltage cycle than the winding with the thick wire. And, that difference in the timing of the magnetic field from each winding creates the impression of a rotating magnetic field for the rotor to follow. And, just like in a capacitor start motor, once a split phase motor gets up to speed, a centrifugal switch cuts out the start winding and the motor will continue to turn on it's run winding only. (I can't remember now whether the start winding is the one with thin or thick wires.) Now, both a capacitor start motor and a split phase motor will be happy to turn in the opposite direction if you want them to. All you have to do is reverse the polarity of ONE of the windings. It can be the start winding or the run winding; doesn't matter which. If you reverse the polarity of one of the windings, the motor will turn happily in the opposite direction. Washing machines use that feature to advantage by having the timer (or something called a "motor reversing relay") reverse the polarity of one of the windings on the motor so that the motor turns in one direction while the washer is in the agitate cycle, and in the opposite direction when the washer is in the spin cycle. This is important because during the spin cycle, the motor turns the washer's pump one way to pump water OUT of the washer, but during the agitate cycle, the pump turns so as to pump any water that leaks into it back into the wash basket. Similarily, Maytag top loading washing machines have a pulley which turns on a threaded shaft. The pulley screws UP the shaft when the motor turns one way, and down the shaft when the motor reverses direction, and it's which direction that pulley is pushing that determines what the transmission does; agitate or spin. I've been told not to reverse the polarity of BOTH windings on a capacitor start or split phase motor. Apparantly, doing that will create a dislocation in the space-time continuum causing you to complete the reversal procedure before you began, with the necessary result that the motor will turn in it's original direction. (smirk) A shaded pole motor is a different kettle of fish. A shaded pole motor has only two poles in it's stator. But, there will be a thick loop of copper wire going through the middle of both of those two poles, thereby separating each pole in half. A current flows in that thick wire as a result of the magnetism produced at the two poles, and the magnetic field of the thick wire opposes the magnetic field on one side of the pole and adds to the magnetic field on the other side of the pole. So, what the rotor sees is a weak magnetic field on one side of the pole becoming a strong magnetic field on the other side of the pole, and that gives the rotor the illusion of the magnetism "sweeping across" the pole, or something similar to a rotating magnetic field. The thing to remember here is that there is only ONE kind of three phase motor and only ONE kind of two phase motor, but several different kinds of single phase electric motors. That's because each different kind of 120 VAC electric motor uses a different method of getting the stator to create the appearance of a rotating magnetic field using only single phase electric power. Two and three phase electric motors don't need to do that because their stators actually produce a rotating magnetic field for the rotor to follow. There, now you know more about electric motors than 99 percent of people named "Homeowner". -- nestork Interesting..... Now, why does the turntable alternate in direction each time it is stopped and started...????? This is a reliable thing, and can easily be verified..... it isn't random.... |
#20
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
On Wed, 23 Jan 2013 05:50:07 +0000, nestork
wrote: The thing to remember here is that there is only ONE kind of three phase motor and only ONE kind of two phase motor, but several different kinds of single phase electric motors. That's because each different kind of 120 VAC electric motor uses a different method of getting the stator to create the appearance of a rotating magnetic field using only single phase electric power. Two and three phase electric motors don't need to do that because their stators actually produce a rotating magnetic field for the rotor to follow. There, now you know more about electric motors than 99 percent of people named "Homeowner". -- nestork Interesting. I learned a lot about motors.... I've installed many of them, repaired some, but never knew exactly how they operated, especially the capacitor types. That does bring up a question. There are some that use both a start ans a run capacitor. Why do they need both? Thanks for the detailed reply. |
#21
|
|||
|
|||
Quote:
But, a shaded pole motor doesn't have two windings, only one. So the magnetic rotor will be attracted to the closest winding of the opposite magnetism. That is, if the north magnetic pole of the rotor is clockwise of the south magnetic pole when power is applied to the stator, the motor will start (and continue) turning counter clockwise. If the north magnetic pole of the rotor is counter clockwise of the south magnetic pole when power is applied to the stator, the motor will start (and continue) to turn clockwise. Now, if the turn direction is random, then it's as explained in this and a previous post, that the direction of rotation depends entirely on where the rotor is when power is applied to the stator. But, if the direction of rotation alternates predictibly, then I would expect that there's some mechanism at work in the microwave that senses the direction or rotation of the motor and reverses it each time the microwave is restarted. Quote:
By fine turning the capacitor strength, you can tweak the timing of the development of the magnetic field of a winding, and that allows you to make an electric motor that will run smoother, more quietly and with better efficiency (so that they use less electricity). The problem is that in the past, the capacitor was only on the start winding, and so as soon as the motor came up to speed, that winding would be kicked out of the circuit, and the motor would continue to turn on it's run winding alone. So, tweaking the strength of the start capacitor wouldn't do any good when the motor was running. By putting the capacitor on the run winding instead, then tweaking the strength of that run capacitor would allow the motor to run smoother, more quietly and with better efficiently during the 99.999 percent of the time when it was actually running, and not just starting. And, by first determining the ideal run capacitor size for optimum motor performance, they can then add, and tweak, a start capacitor to obtain maximum starting torque. Thus, the most optomized single phase electric motors you can get now are capacitor start/capacitor run motors. In the past, the small savings in electrical costs that could have been had were ignored in favour of just using a bigger motor. But with the push for conservation in the past 20 years, lots of manufacturers are going the extra mile to fine tune the operation of their motors for optimum efficiency. Really, a capacitor start/capacitor run motor isn't going to save you much money on your electric bill because electric motors always were quite efficient, but it will run smoother and more quietly, and that's enough of a reason to spend the few extre dollars they cost. Still, if it were my money, I would prefer to just have a larger more powerful motor rather than an optimized smaller motor. The larger motor simply has more power available so that it can overcome inefficiencies that come about with age and use, such as door gaskets leaking so that the motor has to work harder to keep the fridge cold, or dust accumulating on the condenser coils so that the motor has to work harder to keep the fridge cool, and stuff like that. But, the politically correct solution is to use smaller, but more optomized electric motors, so my vote doesn't matter. Hope this helps. Last edited by nestork : January 23rd 13 at 06:52 PM |
#22
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
On 1/23/2013 12:21 PM, nestork wrote:
'Robert[_18_ Wrote: ;3001703'] Interesting..... Now, why does the turntable alternate in direction each time it is stopped and started...????? Capacitor start and split phase motors will always turn in the same direction because the rotor will follow the direction of the apparant rotating magnetic field, and that's always going to be the same direction unless and until something is changed in the wiring of the motor. That's because the strongest point of the apparant magnetic field will go from the pole of one winding (start or run) to the pole of the next winding (start or run, I forget which). But, a shaded pole motor doesn't have two windings, only one. So the magnetic rotor will be attracted to the closest winding of the opposite magnetism. That is, if the north magnetic pole of the rotor is clockwise of the south magnetic pole when power is applied to the stator, the motor will start (and continue) turning counter clockwise. If the north magnetic pole of the rotor is counter clockwise of the south magnetic pole when power is applied to the stator, the motor will start (and continue) to turn clockwise. In a shaded pole motor a small part of each pole has a shorting ring on it. That delays the magnetic field on the shaded part of the pole and produces a 'rotating magnetic field' just like motors with start windings. Shaded pole motors will always start in the same direction. They are commonly used in fans and dial type clocks, both of which always rotate in the same direction. (Clocks are synchronous motors, fans are induction motors. Some clock motors can be reversed by taking the motor apart and reversing the side of the pole that has the shading.) Now, if the turn direction is random, then it's as explained in this and a previous post, that the direction of rotation depends entirely on where the rotor is when power is applied to the stator. But, if the direction of rotation alternates predictibly, then I would expect that there's some mechanism at work in the microwave that senses the direction or rotation of the motor and reverses it each time the microwave is restarted. I don't know how you get a random direction motor in microwaves. A shaded pole type motor can be made with shading on each side of the poles. The shading is wound (not a shorting ring) and the direction is determined by which pair of shading windings are shorted and which are left open. Could be a mechanical switch operated by the rotation of the motor to reverse direction at each start. If the poles are not shaded at all I don't think the motor would reliably start. There are some that use both a start and a run capacitor. Why do they need both? What you're saying is absolutely correct. Capacitor start motors have been around for a long long time, but in the past 15 to 20 years we've seen capacitor run motors (where the capacitor is on the run winding instead) and capacitor start/capacitor run motors where there's a different strength capacitor on the start winding and on the run winding. Capacitor-run motors have 2 windings with the "run" capacitor permanently in series with what would be the start winding. The main winding connects across the line, as with other induction motors. It is, in effect, a 2-phase motor. Starting torque is relatively low. Capacitor-start capacitor-run motors also have a "run" capacitor permanently in series with the start winding. A second "start" capacitor is temporarily connected across the run capacitor with the usual starting switch to start the motor. Far as I know the main advantage of these motors is higher power factor. |
#23
Posted to alt.home.repair
|
|||
|
|||
Why do Microwave ovens change direction?
I poked around the internet with this subject.
Microwave oven electrical schematics "tell all". I found many schematics which just had an "on/off" for the turn table motor. No circuitry for CW or CCW rotation! Then I also saw "microwave oven reversible synchronous motors" for sale. That implies some microwave ovens might intentionally reverse direction after the door has been opened. And I read elsewhere they would do that to minimize "hot spots". Anyway to settle this, test your microwave oven by opening the door several times and seeing if it does in fact reverse direction when door closed and restarted... Then post the brand and model number. Then let's see if we can find the schematics / service manual for that microwave. See if there is circuitry to reverse the motor direction. |
#24
|
|||
|
|||
For anyone interested in learning more about electric motors, the information included in my post is largely from your own US Navy's training manuals.
There's a company called "Integrated Publishing", and they have a web site he Repair and Maintenance Manuals - Integrated Publishing Integated Publishing is a bit of a scam. What they do is sell people the information contained in your US military's training courses. Now, since all of the training manuals the US military uses to train their recruits was compiled at tax payer's expense, it's publicly owned material, and Integrated Publishing cannot sell the stuff to you because you already own it. So, what Integrated Publishing charges you for is the service of photocopying those training manuals and sending you paper copies, or putting the information on a CD-ROM and sending you that disk. But, the information is great. The stuff on electric motors comes from a section called "NEETS" which stands for Navy Electrical Engineering Training Series. The NEETS series of training manuals starts on this page: http://electriciantraining.tpub.com/ and the section that discusses AC electric motors (which is Chapter 4) starts on this page: http://electriciantraining.tpub.com/...s/14177_85.htm Last edited by nestork : January 26th 13 at 03:25 AM |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Microwave Ovens | UK diy | |||
Microwave Ovens | Electronics Repair | |||
Of pyramids and microwave ovens | UK diy | |||
Microwave ovens and cellphones | Electronics | |||
Microwave ovens | Home Repair |