PSC motor speed affected by capacitor?

Question

I replaced the induction motor in my furnace (PSC type), and it was suggested that I replace the run capacitor at the same time. (They are both 20 years old). The new motor is a Fasco A136, identical in model to the old one.

My original capacitor was 4 MFD 250 VAC, the new one is 4 MFD 370 VAC. I am trying to diagnose why my induction motor is not moving enough air, and I wondered if the capacitor could be the cause.

1. Would the different capacitor voltage rating affect motor speed?

2. Could a defective (new) capacitor affect motor speed?

Answer 1

The new motor could be a suitable replacement but be designed for a different capacitor value. The capacitor value should be marked on the motor. However, the wrong capacitor value would probably not cause a big difference in operating speed. It seems more likely that the motor is wrong or that it is a two-speed motor that is connected for the slow speed.

A defective new capacitor or a capacitor that has a much lower capacitance like 1 or 2 MFD could cause the motor speed to be significantly low.

Answer 2

Fasco A136 3.3" Frame Permanent Split Capacitor OEM Replacement Specific Purpose Blower with Sleeve Bearing, 1/20HP, 3,450 rpm, 115V, 60 Hz, 0.75 amps

The motor is 115V, so the max voltage rating of the capacitor is irrelevant in that it is sufficiently higher than the operating peak voltage, which is all that is about. It's the capacitance rating that is important, and yours is the same (ostensibly). Just make sure it is not .4mFD instead of 4mFD!

So if that's not it, then it is either a defective new capacitor, or there is some other reason why your fan is not moving enough air, i.e. a blockage somewhere.

Answer 3

PSC motor troubleshooting:

• Motor is overloaded. Disconnect fan from motor. Turn motor shaft to ensure free rotation. Ensure fan not rubbing on housing or some other mechanical or airflow obstruction.
• Insufficient voltage supply. Check power from primary control wiring. Voltage to motor while running should match motor nameplate volts and frequency, such as 120vAC, 60Hz.
• Capacitor on wrong winding. Ensure motor is spinning in the correct direction. If not, investigate.
• Capacitor or winding(s) faulty.

To test the windings:

• Motor should have three wires. In your case, five: two from motor, two from cap, and a ground. Disconnect motor.
• Discharge the cap (with a screwdriver across it's terminals) and disconnect it.
• Using a multimeter set to Resistance (Ohms) measure between the four motor wires. Should read something between 10 Ohms and probably 50 Ohms to a "common" wire, and measuring across the two "legs" gives the sum of these values. One of the capacitor leads is likely connected to one of the motor leads (0Ω, shorted.)
• If this is not the case (one or more wires read infinite Ohms or <10 Ohms) then a winding is likely bad.
• There should be infinite resistance from all motor leads (except ground wire) to the motor chassis.

To test the capacitor:

• Discharge the capacitor and disconnect it.
• Measure the resistance across the cap. When touched, the meter should start to display something, then very quickly go to infinite (meaning open-circuit.)
• Reverse the connections and repeat. The results should be the same (initial blip, then nothing.)
• If this does not happen (no initial Ohms) then the capacitor is open and is bad. If instead the cap does not go to infinite (reads any constant value of Ohms) then it is shorted. In either case, the cap is defective.
• Measure from each terminal to the metal case of the capacitor. There should be no resistance (infinite Ohms.) If there is any reading, it is bad.
• If your meter has a "capacitance" test function, use that. The capacitor should be within about 10% of the rated 4mF. If not, it is bad.
• If lucky enough to have a capacitance ESR test function, the equivalent-series-resistance should be quite low - well under 1Ω. If it is higher than this, it is bad.

Now, increasing the capacitance value in a PSC motor will have the following effects:

• Increased starting torque
• Increased power consumption
• Increased heat generation
• Decreased max speed

If the capacitor seems good, and there is very little starting torque, you could try a slightly larger value capacitor, such as 5mF or 6mF. I wouldn't recommend going much higher, as it will make the motor run hotter and fail sooner.

If the capacitor seems good, but there is too much starting torque (spins up almost instantly) and gets warm quickly, then perhaps the capacitor is already too large in value and it needs reduced to 3mF or even 2mF to allow the max speed to be attained.

In either case, this would be a problem with the item (cap not matching motor.)

P.S. Motor datasheet seems to be available here. It says that "4M" motors come with a cap, but does not specify the capacitance.