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Single-phase induction motor draws above nameplant amps at no load.

Marco Polo

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Nov 2, 2012
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I removed a single-phase induction motor from the cistern pump that is in my basement. The cistern has been removed and the pump cylinder is stuck. AFAIK the unit was seldom used. At any rate, we almost never used the thing as long as we have lived here.

The motor is a Packard Electric model S-7758. It's 1/3 hp. Rated at 115 volts, and 6.5 amps. It is a 4-pole motor, 1725 RPM. I believe it's capacitor start, but the cap must be inside the end bell, because it's not on the outside. I had the motor apart and neglected to look for it because I was only removing dust and whatnot from the windings.

The motor seems to work fine, but according to my DMM it is pulling 6.9 amps with no load. Bearings seem fine, the motor turns silky-smooth with no noise at all. There is a bid of end-play, but no side-play or rattle whatsoever. The only friction sound is due to the start-winding cutout switch. The cutout switch does function as intended, and once it cuts out there is no friction or bearing noise at all.

The motor starts up just fine. Really strong starting torque, turn on the power and you're at full speed in a fraction of a second. Start-winding cutout sometimes makes a flash and usually some ozone. I note that the flyball mechanism that actuates the switch is supposed to have two springs on it, but one broke and now it only has one. This does not seem to make much difference. It probably means that the start-winding is cut out sooner than usual, but again, motor comes up to speed just fine. I thought about putting a fan on the shaft, but the starting torque would probably rip the blades off of it.

The motor does become somewhat hot, it is a little uncomfortable to touch or grab tightly, but at that point the temperature seems to stabilize and it doesn't seem to get any hotter. It doesn't smell like the motor is burning; hot/burned motor smell is pretty unmistakeable and all I detect is old-basement smell. From a cold start, it takes ~10 minutes or so to warm up to the point at which temperature stabilizes. It has nothing that I would call a cooling fan, just small bar-like protrusion at the end of the rotor. It does have vent slots, and does pull air in through a couple, and moves warm air out through the others, but not a lot. AFAIK most/all motors of this sort normally run warm/hot to the touch and it's no problem. This motor has a thermal-protection "push to reset" button, and the protection has not tripped.

I'm curious as to why it is pulling 6.9 amps at idle though. Is it supposed to do that? I thought most single-phase motors pulled half nameplate amps or less at no load. Some thoughts I had:

* Maybe the motor is highly fluxed by the design, which I've read can cause higher current draw than if the flux density were lower.

* It's rated at 115 volts, but actual mains voltage is 124 volts. If the motor were highly fluxed to begin with, this could push it into saturation, which would result in excess current draw. My variac could drop the voltage to 115, but it's only good for 5 amps so I'm not sure I want to hook the motor to it.

* Maybe the motor just has really poor power factor at no load?

* Start winding could be engaged? I really doubt it, because the cutout switch apparently functions as intended. Plus, the start windings tend to burn up in a hurry if left connected to line, don't they? They generally don't last an hour, which is how long the motor has been running at this time.

* DMM is wrong. Can reactive power confuse the meter?

* I'm reading the nameplate wrong, is that possible? I thought nameplate amps indicates the full-load amps.

Does anyone here have any ideas/thoughts on this?
 
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Cyparagon

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Sep 12, 2007
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My guess is power factor. You can check that with one of these nifty $15 plug-in power meters. Oil filled caps are typically used to correct a poor power factor (for inductive loads, of course).

Two other points: your mains voltage will drop several volts with a 7A load. Variacs are conservatively rated. They can handle well over twice the rated current as long as they're not run continuously. A 5A variac will have no trouble operating at 7A continuously.
 
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