Unisaw 3 phase motor hummus won't start.

[Joe Tuminello]

Our 3 phase unisaw intermittently hums and will only start if the blade is turned by hand a bit (saw unplugged) and switch activated a second time . All the wire connections have been checked and are tight. I assume it is the centrifugal switch. There does not seem to be any easy way to access it. Any suggestions appreciated.

[Darcy Warner]

There is no centrifugal switch on a 3phase motor.
You have a leg that is dropped somewhere.
Typical single phasing syndrome of a 3phase motor.

[Peter Aeschliman]

Hummus won't fix your issue, I'm afraid.

[Dan Friedrichs]

Darcy is right. 3-phase motors don't have centrifugal switches. One of your 3 wires is disconnected, somehow. Check the supply and also the switch for something loose/broke/burned.

[Bradley Gray]

If you have an old fused starter switch, check the fuses.

Peter- hummus is good for everything.

[Matt Mattingley]

I am guessing this is on single phase using a static converter? If so, your static converter has blown capacitors in it.
or
As Dan said. You have lost a hot leg somewhere.

[Joe Tuminello]

Yeah, I know, I tried it.

[Joe Tuminello]

it's the original 3 phase motor that came new with the saw hooked to 3 phase power. All connections from switch to starter are tight. Intermittent problem, and can not figure out why manually moving the blade seems to reset something to allow it to start.

[Adam Herman]

as above.

turning the blade lines up the stater to one of the fields that is powered and causes enough inductive current to get it turning. when it is stationary, you have one field not powered which is making it "stick" its going to be a connection somewhere on one phase. you could have a break or short in one of the windings as well.

check the windings per the last part of this: http://www.pcbheaven.com/userpages/c...hase_ac_motor/

[Dan Friedrichs]

It's not that moving the blade "resets" anything, it just gives the motor a little inertia in the "right" direction so it can start spinning. If you spun the blade "backwards" then turned the power on, it would spin backwards. That's how all single-phase power works (which is why single-phase motors have a start capacitor and centrifugal switch). In a 3-phase system, the 3 different phased electrical signals naturally provide a "direction" in time, which is why the centrifugal switch and start capacitor aren't needed, and why you can change the rotation direction of a 3-phase motor by simply swapping any 2 of the wires.

However, if you disconnect 1 of the 3 wires, as far as the motor knows, it's connected to a single-phase supply (ie - 2 wires), so now it doesn't know what direction to turn. Also, the amount of power available at the motor will be reduced.

You have a bad connection, somewhere. Get a voltmeter and measure between each pair of wires.

[Matt Mattingley]

it's the original 3 phase motor that came new with the saw hooked to 3 phase power. All connections from switch to starter are tight. Intermittent problem, and can not figure out why manually moving the blade seems to reset something to allow it to start.

this is all fine. Is it hooked up to single phase in your home, or is it hooked up to a three phase??? If it is hooked up to single phase, what are you using as phase conversion?

[Chris Hachet]

If it hums...teach it the words to the song...(yes I know, bad joke!)

I think there is a phase loss issue somewhere. Check to ensure that you ahve three phases of power at the motor. Four bolts and the top should come off of the saw once the fence is removed, motor should be easily accessible.

Could be a loose connection at the motor.

[Darcy Warner]

Bad heater, bad overload, bad connection.

Time to get out a meter and a clamp on amp meter.

Check the pecker head on the motor too.

[Joe Tuminello]

I had an electrician check tha all connections are tight and measure line voltages on all three legs. One leg had a very slight voltage reading. He found a couple of places that needed tightening , also in the plug itself. We will see how it goes for now. There may be a problem in one of the windings if it redevelops is what his guess is as suggested above. I'll keep the thread updated as to the result. Thank you all for the very good suggestions.

[Joe Tuminello]

I had an electrician check that all connections are tight and measure line voltages on all three legs. One leg had a very slight voltage reading. He found a couple of places that needed tightening , also in the plug itself. We will see how it goes for now. Ran ok today. There may be a problem in one of the windings if it redevelops is what his guess is as suggested above. I'll keep the thread updated as to the result. Thank you all for the very good suggestions.