1/2 Horsepower 120V AC Motor Debug

[PB2]

OK, I know many will say the right way to repair this is to buy a new motor, and I might have said it myself but ...

Our forced hot air blower motor is a 1/2 HP 120Vac unit about 20 years old with a starter winding that switches off by a centrifugal switch. It is sticking off, so the motor hums and doesn't start. Give it a kick and it starts. I cleaned and lubed it, the contacts are big and easy to get to, worked for about a week ... now the same thing.
Anyone know if these circuits have gone fully electronic? I could use a TRIAC and time it to turn off after 5 seconds - would be very simple and low cost. I considered sensing current but why bother, it just takes a few seconds to get going. Something like Eva's slow start up in reverse, full on then turns off, actually I'd like soft start - 5 sec on - then off, the start up now is abrupt:
Soft-start circuit with no relays and no aux. transformers

If the motor needed bearings or showed signs of wear I would simply replace it, but the bearings are fine, it looks brand new and I kind of like this TRIAC idea. Am I missing something that would complicate this?

I will probably just get a new motor, anything neat in high efficiency? It is belt drive so just about anything reasonable should mount fine.

Any motor experts ?

[Conrad Hoffman]

It sound like a simple mechanical problem. The operation of the switch should be obvious, so there's either a broken spring or something is gummed up so it doesn't return to the home (on) position. Can a weight come off? The more usual problem I have is bad start capacitors or bad start windings, but if your switch isn't closing you just need to figure out what's binding it up.

CH

[PB2]

Originally Posted by Conrad Hoffman

It sound like a simple mechanical problem. The operation of the switch should be obvious, so there's either a broken spring or something is gummed up so it doesn't return to the home (on) position. Can a weight come off? The more usual problem I have is bad start capacitors or bad start windings, but if your switch isn't closing you just need to figure out what's binding it up.

CH

I'm fairly mechanically inclined and there was nothing obviously wrong. It was a bit dirty at first but I cleaned it and the contacts. I'm guessing that the moving parts are worn and binding or that the contacts are just shot. There is no cap. Perhaps it was not the obvious and there is a loose or corroded contact somewhere, so I'll look for that if I have another look at it.

No moving parts and no contacts is kind of nice with the TRIAC.

[star882]

I once designed a simple compressor starter that uses a triac. There is a low value resistor in series with the main winding, and the triac is connected across it through an RC network. It then switches the starting capacitor in and out of the circuit, with an inductor to prevent excessive peak currents from the run capacitor.

[PB2]

Was this a repair or were you replacing a part?

Has it worked reliably?

This motor doesn't use a capacitor BTW.

[star882]

I built the circuit to replace a worn out starting relay.

[PB2]

Ah interesting, what's that saying about thinking alike?
I wonder if TRIACS are being used in newer motors.

I need to find a consumer reports for motors.

[PB2]

This motor starts most of the time. I marked the position where it will not start and it seems to be position dependent. Is there anything in the magnetics that could cause this? Are there multiple starter windings where one might be dead making it dependent on position?

[Conrad Hoffman]

Any chance of a picture of the mechanism? I'm surprised there's no cap buried in there somewhere. That's how they get the phase shift for the start winding, though no doubt there are other ways to do it. The usual cap is in a bulge on the outside, but I've seen 'em buried inside too. Regardless, the switch has to work right. IMO, the switches are usually reliable for many decades and I can't imagine wanting a solid state solution over a properly functioning switch.

CH

[zigzagflux]

Sounds like you have a split phase motor, where the start winding is wound 90 degrees from the run winding, effectively eliminating the need for the start capacitor. These are typically 1/3 hp or less. Larger hp sizes require the capacitor start for increased torque.

I see one of two problems:

1. The switch contacts are poor, which impedes the flow of current when called upon to start. Too low of start current into the start winding means too low of starting torque, and the motor won't move. Giving the assembly a kick helps the motor get a little bit of movement going, which allows the start process to commence. The kick may also dislodge the switch enough to aid in better contact.

2. A problem with the start winding, whereby you get too little starting torque. This could be a shorted turn (which would increase current, potentially damaging the switch) or other problem such that you need to start the motor at the sweet spot of the sine wave for an initial bump.

You could temporarily short out the switch and perform a number of start cycles (turn the motor off shortly after startup) to verify if you can consistently start with a shorted switch. If not, you have a problem with the start winding, and it's time for replacement. At 1/2 hp, I would consider a capacitor start motor.