Motor-Start Capacitor Questions

[Rod Ango]

I picked up a 7.5W 110V TEFC single-phase induction motor for super cheap. I may use it on a home made lathe or stationary-sander for my woodshop.

It's not running. I read that the run windings would probably have less impedance than the start windings. I was only able to get and measure conductivity, ~4.5 ohms, between one pair made from the 3 wires extant (checked them at the motor terminals.) I opened the run-capacitor cover and shorted it across its pair of terminals - no spark. Should it have sparked if it was a good cap?

The capacitor also appeared to have a bit of a leak at the top, (picture).

Since a new one that offers the same spec's was US$6.50 shipped, I ordered the replacement. Does it look like this cap was probably bad based on that appearance? Since it's alternating current, polarity won't matter on the re-installation, right?

The cap's side view and spec's. The new one has same mfd (161-193) value range and 120v specified, but close enough, right?

Last, in wiring it up, I'll have 120V with hot + neutral + ground. Which wire goes where? Should the be another part such as a 'potential relay' to operate this motor?

 

[Lucien Nunes]

Old cap looks suspect, yes. It seems to have burst at the terminal. You will only be guaranteed to get a spark off a motor cap if you disconnect it from the winding and charge it with DC. In-circuit the winding will immediately discharge it. Voltage rating is a maximum limit, so you are fine to use a higher voltage cap than original.

Re. Relay, does the motor have a centrifugal switch?

Hey, wait, this is a 2-speed motor right? It has 4-pole and 6-pole windings. I think the teminals are stamped H (blue) L (orange) and C for common (white). So you would feed either H or L with the hot. I think. Check inside the terminal box cover for a diagram.

 

[Rod Ango]

Old cap looks suspect, yes. It seems to have burst at the terminal. You will only be guaranteed to get a spark off a motor cap if you disconnect it from the winding and charge it with DC. In-circuit the winding will immediately discharge it. Voltage rating is a maximum limit, so you are fine to use a higher voltage cap than original.

Re. Relay, does the motor have a centrifugal switch?

Hey, wait, this is a 2-speed motor right? It has 4-pole and 6-pole windings. I think the teminals are stamped H (blue) L (orange) and C for common (white). So you would feed either H or L with the hot. I think. Check inside the terminal box cover for a diagram.

High|Low|Common, and the other answers, thank you so much! I will look specifically to confirm this, and I know what a centrifugal clutch is, in concept how it works, but to find it do I pull off the 'end bell' and just look for a likely mechanical suspect?

 

[DPG]

High|Low|Common, and the other answers, thank you so much! I will look specifically to confirm this, and I know what a centrifugal clutch is, in concept how it works, but to find it do I pull off the 'end bell' and just look for a likely mechanical suspect?

I am going to resist commenting on this, even though my juvenile mind really wants to.

 

[Lucien Nunes]

If the three visible wires are the only outgoing connections, and nothing is missing from inside the capacitor box, the motor does not need a start relay. An externally-connected start relay would require the start and run winding leads to exit the motor separately. The start winding might be internally connected directly to the switch.

I would measure resistance from C to H (or L) first with the capacitor leads disconnected (or indeed connected to the new good capacitor and then short-circuit across the capacitor. If this causes the resistance to drop, but not to zero, that suggests your two windings are present and correct and you are good to go.

 

[Rod Ango]

Capacitor is scheduled to arrive Monday, and I will test it and perform the tests you recommended. I hadn't spotted that it is a 2-speed motor. Makes sense and adds value, even for repurposing a 1/3 - 1/2 HP motor.

Digging through boxes, I found a DC motor a friend gave me years ago. Its label shows it's meant to run at 7100RPM, @12VDC/18AMPS. It looks like it was maybe part of a treadmill. Now, if it works, I'll have to consider a good new use for it. I might be able to fit it with a pulley to reduce speed and power a wood-shaper spindle, or maybe as a basis for a sander.

This motor probably has a feedback circuit to help it maintain speed precisely, so I guess the black wire here is more likely part of a sensor rather than for a carbon brush (on this permanent magnet motor?)

 

[Rod Ango]

Capacitor is scheduled to arrive Monday, and I will test it and perform the tests you recommended. I hadn't spotted that it is a 2-speed motor. Makes sense and adds value, even for repurposing a 1/3 - 1/2 HP motor.

Digging through boxes, I found a DC motor a friend gave me years ago. Its label shows it's meant to run at 7100RPM, @12VDC/18AMPS. It looks like it was maybe part of a treadmill. Now, if it works, I'll have to consider a good new use for it. I might be able to fit it with a pulley to reduce speed and power a wood-shaper spindle, or maybe as a basis for a sander.

This motor probably has a feedback circuit to help it maintain speed precisely, so I guess the black wire here is more likely part of a sensor rather than for a carbon brush (on this permanent magnet motor?)

120VDC

 

[Rod Ango]

The capacitor got the motor working on the high speed as expected. Low speed not working. No continuity at all on the orange wire. Any suggestions on things to check?