I have always considered it acceptable when replacing a PSC capacitor to exceed the voltage rating of the original, but not the mfd rating. For microfarads I've always matched what the motor called for. A fellow HVAC tech and I were debating this the other day, he thinks going with a higher microfarad capacitor is ok. I'm sticking with how I've always done it, but I'm interested if anyone has any documentation on this. Yes I can Google the forums and hear everyone's opinion, but have you seen anything official?
Oodly
You can probably get away with a slightly higher mfd rating on a start capacitor, since it's only in the circuit for a fraction of a second, but I wouldn't recommend risking it for a run capacitor.
Maybe if its temporary just to get yourself out of a jam until you can return with the right one, otherwise just stick with what the motor manufacturer recommends.
Maybe if its temporary just to get yourself out of a jam until you can return with the right one, otherwise just stick with what the motor manufacturer recommends.
I remember someone saying that if you put a higher or lower mfd capacitor than rated for the motor your amperage will go up. I never try it, but it makes sense.
Higher voltage rating is okay. It should be a higher reliability capacitor with higher breakdown voltage rating spec margin.
The proper Run capacitor mfd. value provides 90 degrees of current phase lead to start (auxiliary) winding of motor compared to run winding current phase. This provides the best motor torque for given motor load current. Matching proper Run capacitor mfd. is important for peak motor performance.
Proper mfd value Run capacitor provides:
reduces rotor slip.
(rotor spins at slightly lower rpm's than motor synchronous electrical field to induce current in rotor. Slower rotor speed is called 'slip' and is in the range of 2-7% slower than synchronous speed depending on mechanical load, which for 60Hz AC supply is 3600 rpm for two pole, and 1800 rpm for four pole motor.) Slip is required to induce current in rotor and the % slip is greater with increasing mechanical load on motor.
improves run Power Factor. (less Run current through motor for given mechanical load)
improves breakdown torque (maximum mechanical load which causes motor to slow down and stop).
improves motor efficiency.
If you measure current on compressor's Run winding wire, Start winding wire, and Common wire, the correct mfd Run capacitor will result in:
Compressor common wire current = square root of the sum of run winding current squared plus start winding current squared.
Example: 12.2 amps common wire current = square root (10 amps run winding current squared + 7 amps start winding current squared) = 12.2A common terminal current = sqrt (100 + 49) = sgrt (149) = 12.2A common wire current.
A PSC motor is actually a two-phase motor with the second phase source synthesized by the series Run capacitor, with two source's phases being 90 degrees difference. This is in contrast to a three-phase motor which has three phases at 120 degrees difference between sources.
The proper Run capacitor mfd. value provides 90 degrees of current phase lead to start (auxiliary) winding of motor compared to run winding current phase. This provides the best motor torque for given motor load current. Matching proper Run capacitor mfd. is important for peak motor performance.
Proper mfd value Run capacitor provides:
reduces rotor slip.
(rotor spins at slightly lower rpm's than motor synchronous electrical field to induce current in rotor. Slower rotor speed is called 'slip' and is in the range of 2-7% slower than synchronous speed depending on mechanical load, which for 60Hz AC supply is 3600 rpm for two pole, and 1800 rpm for four pole motor.) Slip is required to induce current in rotor and the % slip is greater with increasing mechanical load on motor.
improves run Power Factor. (less Run current through motor for given mechanical load)
improves breakdown torque (maximum mechanical load which causes motor to slow down and stop).
improves motor efficiency.
If you measure current on compressor's Run winding wire, Start winding wire, and Common wire, the correct mfd Run capacitor will result in:
Compressor common wire current = square root of the sum of run winding current squared plus start winding current squared.
Example: 12.2 amps common wire current = square root (10 amps run winding current squared + 7 amps start winding current squared) = 12.2A common terminal current = sqrt (100 + 49) = sgrt (149) = 12.2A common wire current.
A PSC motor is actually a two-phase motor with the second phase source synthesized by the series Run capacitor, with two source's phases being 90 degrees difference. This is in contrast to a three-phase motor which has three phases at 120 degrees difference between sources.
You can think of two-phase PSC or three phase motor as an engine crankshaft with piston rods arranged 90 degrees offset on PSC motor or 120 degrees offset on three-phase motor. When a motor has two poles, (not to be confused with phases), it is as stated above. A four pole motor runs at half the rpm's in order to keep to the 90 or 120 degree electrical drive phase peaks for PSC or three-phase motor respectively. Since horsepower is equal to torque x RPM's, the four-pole motor will have twice the torque at half the rpm's for equivalent horsepower motors.
A three-phase motor with only two phases connected in like a dead cylinder in three-cylinder piston internal combustion engine. Still runs but produces less maximum power. For given mechanical load, the two connected phases of three-phase motor will draw more current to make up for missing phase.
One other small detail on PSC motor, you may have noticed, the start winding has a higher AC voltage during run period. This is because of the series capacitor, synthesized second AC phase source with 90 degrees phase shift, also has an impedance transformation that bumps up the start (auxiliary) winding voltage as a side effect.
Start (auxilary) windings in a PSC motor is wound with more turns of wire with smaller gauge wire to fit in same area to work with the slightly higher AC voltage, typically about 300 vac for a 240vac motor. This is another secondary negative effect if Run capacitor mfd. value is incorrect, so it is not providing the proper matching start winding design voltage level.
The synthesized second phase source on a PSC motor is not perfect as the optimum Run capacitor value to provide 90 degree current phase shift is only optimum at a given motor mechanical load. As motor mechanical load varies, the fixed mfd value Run capacitor will be a bit off of optimum value, with some deviation for the desired 90 degree current offset. Usually, the mfd capacitor value is selected at near maximum mechanical load on motor.
A three-phase motor with only two phases connected in like a dead cylinder in three-cylinder piston internal combustion engine. Still runs but produces less maximum power. For given mechanical load, the two connected phases of three-phase motor will draw more current to make up for missing phase.
One other small detail on PSC motor, you may have noticed, the start winding has a higher AC voltage during run period. This is because of the series capacitor, synthesized second AC phase source with 90 degrees phase shift, also has an impedance transformation that bumps up the start (auxiliary) winding voltage as a side effect.
Start (auxilary) windings in a PSC motor is wound with more turns of wire with smaller gauge wire to fit in same area to work with the slightly higher AC voltage, typically about 300 vac for a 240vac motor. This is another secondary negative effect if Run capacitor mfd. value is incorrect, so it is not providing the proper matching start winding design voltage level.
The synthesized second phase source on a PSC motor is not perfect as the optimum Run capacitor value to provide 90 degree current phase shift is only optimum at a given motor mechanical load. As motor mechanical load varies, the fixed mfd value Run capacitor will be a bit off of optimum value, with some deviation for the desired 90 degree current offset. Usually, the mfd capacitor value is selected at near maximum mechanical load on motor.
Huh? Does that mean that you don't know the answer to my question?
I was wondering if a single phase PSC motor can tell the difference between being hooked up to real single phase power vs. 2 legs of a 3 phase power source.
How do these motors seem not to care when the sine wave is all out of balance between L1 and L2? It almost seems like we should be using a different run capacitor when the motor gets it's power from a 3 phase source.
Does anyone know if efficiency or horsepower is reduced when the 2 hots are out of balance (like it would be on any single phase motor that's inside of a 3 phase rtu)? Would it be like 1%, or 5%, or no reduction at all because they truly don't care?
I was wondering if a single phase PSC motor can tell the difference between being hooked up to real single phase power vs. 2 legs of a 3 phase power source.
How do these motors seem not to care when the sine wave is all out of balance between L1 and L2? It almost seems like we should be using a different run capacitor when the motor gets it's power from a 3 phase source.
Does anyone know if efficiency or horsepower is reduced when the 2 hots are out of balance (like it would be on any single phase motor that's inside of a 3 phase rtu)? Would it be like 1%, or 5%, or no reduction at all because they truly don't care?
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