AC repair question

Well, not quite.

"Engine" and "Motor" are often used interchangeably, and in some cases either term is considered correct. Generally "motor" refers to a device to convert electrical energy into mechanical energy, while "engine" refers to a device to convert chemical energy (gasoline for example) into mechanical energy.

A device to convert mechanical energy into electrical energy is a generator, of which there are many sub types with more specific names.

Any electric motor can operate as a generator under the right circumstances, and any electrical generator can operate as a motor.

Any guesses as to what is wrong with the winding? Someone suggested looking at the bearings. What do I look for, excessive wear? My electrician friend recommended just replacing the motor rather than trying to diagnose it farther than that. However, my time is worth about $5/hr, so if I can repair it in less than I day, I have come out ahead.

why would the speed change?

Hmmm, You and your frieed lectrician better learn what is the function of capacitor for the motor first. If same value is not available always replace it with next higher value. Same with W.V. You do the math to see why it is so.

The only time I ever had a blower motor fail, the bearings seized. I took it apart and found some mild scoring, scrubbed them out with a scotchbrite pad, put some 10w30 motor oil on them and reassembled, surprisingly it ran fine still 2 years later when I upgraded to a more efficient furnace.

If it has open windings, the thermal fuse may have opened.

I found one for $65 shipped at:

Does anyone know where I can buy one for $30?

ps1. I am interested in the start capacitance, (the motor makes the humming sound trying to start up). ps2. my friend the electrician is not the same as the AC repairman and not the same as the earlier guy who put in the 7.5 MFD cap (which seems to be the one that the sites sell with it [A.O. Smith recommends

10 MFD]). ps3. The reason I was looking for the calculations is that I can see how sensitive they are to a change in capacitors.

I can walk in to several places here in town and buy one for less. $60 plus $15 shipping? Kinda runs the price up.

TDD

Speed changes in PSC motors when the value of the capacitor is changed. Not a major speed change for the previously discussed values of 7.5uF to 10uF.

For the theoretical explanations, try a book with a detailed section on Permanent Split Capacitor (PSC) type AC motors. These motors have been used for deades and are still presently utilized in many applications from fans and blowers to fractional HP gearhead motors.

PSC type motors differ greatly from split-phase capacitor start motors.

-- Cheers, WB .............

If the capacitor was a starting capacitor it won't make any difference in speed, as it is only use to start the motor rotating, and is disconnected once it is spinning near operating speed. The motor windings will determine the speed and torque while running.

Most AC blower motors use a running capacitor that stays active while the motor runs. A running capacitor shifts the Alternating Current sine-wave between the windings to give the motor torque. A larger value capacitor will give more torque, and perhaps a little more speed. Induction motors are greatly affected by the load upon them. If the load is less than the maximum, a smaller capacitor value is used to keep the rotation speed within reasonable parameters. An induction motor run without any load will run too fast and the windings will overheat and break down the lacquer insulation causing an internal short.

If the motor just hums and does not rotate - there is something else wrong. A failed capacitor results in this same symptom and is the easiest solution to try, hence the reason it was replaced. Even a 4µF or 5µF capacitor should turn the motor. Check the bearings by rotating the unloaded motor shaft - it should rotate fairly easily. Usually sealed ball bearing sets are used in these motors, pressed onto the shaft - replacement will require a bearing puller.

If the blower is directly attached to the shaft, the only bearings to check are in the motor. If it is a belt driven set-up (older), the bearings on the squirrel cage will be suspect.

If the motor spins freely, but still only hums on start up, either a bad connection to the capacitor (or other component) or a short or break in the motor windings, not to mention another capacitor failure. These motors tend to have very fine and compact winding wraps and then take a lacquer bath, making service difficult if not impossible. If there is a short in the motor, the first place to check is the bundle where the insulated wires are soldered to the winding wires, if that is the connection method.

Scott

Surplus City usually has quite a few, but it's a crapshoot if they'll have the one you need.

Not even close. Induction motor speeds vary little from no-load to full-load (slip from no-load to full-load of typical induction motor [not some ceiling fan junk] is

Well it's possible that this is still the original problem and the capacitor was not bad to begin with.

The symptoms point to something in one of the two winding circuits. There are two separate windings in a single phase induction motor. In many motors one of the windings is only energized during starting and then turned off by a centrifugal switch. But this thread has been talking about capacitive run type (and that's common for blower motors) so we can ignore problems with the switch.

If the two windings are energized from the same power source without any capacitor at all, then no torque is developed and the motor sits and hums. So one thing is it may be wired up wrong. Recheck your work against the circuit diagram (often inside the panel or in a manual).

If the bearings or blower are seized up, the motor can't develop enough torque to start spinning. Or you may get it started but the load is so great that it overheats and shuts down. I think you mentioned it ran for a while one time? When everything is turned off, you should be able to easily turn the thing by hand, blower and all.

If one of the two windings has developed an open, then the other winding alone can't develop torque to start and it will just hum. This *may* be just an open in a connecting wire and would be easy to fix. But if it's down inside the winding, rewinding a motor is quite a task and often not worth the effort.

If your handy with an ohm-meter you can check for opens easy enough. Just lift the leads and read between them. Be sure to turn off the power though, ohm-meters tend to smoke when connected to live AC power.

daestrom

Ah, I should have qualified that with "low-torque/low speed" induction motors (less than 300 RPM) often found in smaller squirrel cage blower fans and in some ceiling fans. I have not worked on many high torque, high speed induction motors to consider them typical of induction motors. Low torque motors are designed to carry a specified load and can run up to 25% faster without that load. But I concede, a blower motor requiring a 10µF running capacitor probably is a high torque induction motor that will not suffer from heat-rise unless it is from the bearings.