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rustydog
10-21-2012, 11:15 PM
I have a Dayton 3/4 hp thermally protected motor. It is an internal automatic Klixon protector.

The problem is that the motor turns over at about 40 RPM unloaded and will not turn over at all on the horizontal band saw it has been successfully running for 3 years.

There is no humming. There are no roasted coils and the bushings are in good shape and run freely. The slow speed is not fast enough to trip the start circuit switch.

Is this characteristic of a specific problem? .

rock_breaker
10-22-2012, 01:08 AM
Hopefully some electricians will come to the rescue !!! My limited experiences say the starting windings are not getting power. This is from an armchair electrician.

Does Dayton provide online trouble shooting guides and wiring schematics?

My skills would make me take it to a motor shop.

Good luck

Ray

Jim Hubbell
10-22-2012, 01:40 AM
The fact that it turning at all seems to indicate the windings are not bad. I would check the power at the motor as it sounds like very low voltage is your problem. If you have another tool to plug into this tools outlet and it runs fine, the circuit should be ok. Check voltage and let us know.

The Artful Bodger
10-22-2012, 04:20 AM
I suspect the start capacitors have lost almost all their capacitance and there is only miniscule current through the start windings.

Bluechips
10-22-2012, 04:31 AM
I suspect the start capacitors have lost almost all their capacitance and there is only miniscule current through the start windings.

+1 on a capacitor problem, they go out from time to time. The centrifugal switch that kicks the capacitor in and out may be stuck in the run position and not allowing the start mode.

Mike

The Artful Bodger
10-22-2012, 05:23 AM
Hmmm... if the centrifugal switch is stuck in the run position (that is 'open') there would be no current through the start capacitor at all which may be the explanation.

J Tiers
10-22-2012, 08:05 AM
The description of the symptom says "virtually no starting torque".

The most likely cause is a bad capacitor or problem with the start switch (we assume it is single phase, because this is the HSM board, and not PM).

The odds are on the capacitor, because the start switch if open would generally put the start torque to "zero", not "very little".

A possible cause is that the bearings are so tight for some reason that it can't move freely... you can put that to rest by spinning it by hand. I doubt that is the issue because you'd have tried that first.

At the risk of repeating what has been said so many times before... the purpose of the start circuit is to provide a phase shift so that there is some starting torque. The motor is temporarily made into a "two phase" motor, and the start winding provides the second phase.

Once the motor is turning, the start circuit is switched out, and the running torque is provided by the rotor being given a kick" every half cycle. if the motor speed falls too far, the torque drops off.

A 3 phase motor has torque at zero RPM, and needs no start circuit.

A motor which has no start torque generally has either a bad start circuit, or if 3 phase, has a missing phase.

MaxHeadRoom
10-22-2012, 12:40 PM
Normally all you have to do to get a 1phase (split phase) motor turning if the cap or switch is open, you normally should be able to give the shaft a spin by hand and it will run up to speed, if the run winding is OK.
I am very surprised therefore that 40rpm is not enough to kick start it to get it up to speed?
I suspect something else is at foot here as a weak cap turning it at 40 rpm should not deter it ramping up to speed?
Also a current check will tell you as normally the current is going to be quite a bit higher at zero rpm when stalled by the saw if the start circuit is open or at fault and the run winding is intact.
Measure the resistance through both windings.
The classic symptom of what you are seeing is on older motors where the rotor bars were soldered on their ends, often the rotor would heat up and fling the solder off or loosened rivets, the result next to zero current/torque.
In modern motors the rotor bars are cast aluminum into the rotor.
In effect, an induction motor that is not rotating when power is applied is a transformer with a shorted secondary winding.
Max.
.

OKChipmaker
10-22-2012, 12:46 PM
Check the points on the start switch ,I have had the points to burn and only let some of the current through.Enough to turn slowley but not come up to speed.

rustydog
10-23-2012, 12:21 AM
Wow - what a great response. Wait a minute and I'll go thru all the suggestions and check things off. I' expect I'll be back for more ideas. THANKS AGAIN

Circlip
10-23-2012, 04:43 AM
When you turn the power off, you should hear a click as the centrifugal switch drops out.

Regards Ian.

Jim Hubbell
10-23-2012, 09:27 PM
I repaired a motor used on a table saw. Complaint was low start torque. The motor maker had made a "rubber" dust barrier around the start switch pionts which suceeded in capturing fine wood dust. The arc of the opening points burned the dust into a nice resister. Clean up points and omit dust barrier. Problem solved.