3 HP motor question (wood saw bench)

[ProDave]

Called to a customer today. amongst other things to look at a problem with a saw bench.

The saw bench was bought second hand a few months back and initially worked well, but now it's "lacking in power"

Here's what I found:

The motor is rated at 230V / 15A / 3HP (from it's rating plate)

There are 4 connections to the motor (5 core cable including earth)

Measurements sugest two independant motor windings. One connecting between the brown and blue wires, and one between the two black wires (labeled 1 and 2) Both motor windings measure 2.7 ohms and IR is good.

Tracing out part of the control wiring shows that the 230V is applied (via stop start contactor etc) to the blue / brown motor winding, and via a 50uF 400V ac capacitor to the black winding.

The machine runs, but under moderate load the motor slows considerably, and under full load (motor almost stalled), the motor is only drawing 5A maximum (according to my clamp meter) so less than half power.

Supply voltage is 230V and no signifcant drop under load, so it's not a supply problem. No volt drop within switchgear as the terminal voltage of the direct winding measures 230V.

The only "anomoly" is when trying to measure the applied voltage to the phase shifted winding, my digital meter shows 900V. Next time I go back I'll take an old analogue volt meter and repeat that test as I suspect my digital meter is telling porkies.

There are actually two 50uF 400V capacitors. I haven't fully traced out where the other one goes, but as a test I swapped them over and it made no difference.

I tried swapping the motor windings over so the two blacks were directly powered and the brown / blue came from the phase shift capacitor. It made absolutely no difference.

So what's at fault here?

The customer doesn't mind paying for a new motor, but only if the motor is at fault.

My inclination is first to try replacing both capacitors.

Incidently, there's no wiring details in the moter terminal box, so I am making the assumption that it is wired correctly since it initially worked properly.

 

[bil]

Did you test the capacitors with your meter?

 

[bengie]

first thing i'd do is search the interwebs for a wiring diagram/comon faults on the specific make/model saw. it might just be connected up wrong.

 

[bil]

first thing i'd do is search the interwebs for a wiring diagram/comon faults on the specific make/model saw. it might just be connected up wrong.

Definitely get a drawing if possible but OP stated that it had been working fine, which would rule out wrong connections??

 

[ProDave]

Did you test the capacitors with your meter?

No. didn't test the capacitors. I don't have a capacitance meter, and have never had much luck trying to "test" a capacitor with a digital ohm meter.

When I return with my trusty old Avo 8 I'll test them on ohms, you can usually guage by the "kick" how ggod they are, and certainly to compare one with the other.

first thing i'd do is search the interwebs for a wiring diagram/comon faults on the specific make/model saw. it might just be connected up wrong.

Good point. I didn't record the make or model, but I'll email the customer and see if he can find it written anywhere on the machine.

Is there any way to test the motor? One assumes if it's still low power with new capacitors, it has the be the motor. But most failed motors I have seen draw too much power and overheat due to shorted windings. What can cause a lack of power? I'm assuming it's a squiral cage motor, so unlike a DC motor, it can't be demagnetised magnets can it?

 

[kme]

did you take an ohmic reading of the two coils, by any chance??

 

[ProDave]

did you take an ohmic reading of the two coils, by any chance??

Yes as per the original post, both motor windings have an identical DC resistance of 2.7 ohms

 

[benji]

you need motor connection data from manufacturer ,it might need both caps. to start or could be cap.start & cap. run

yours benji

 

[ProDave]

Thanks for all your help so far.

I don't think I can do much more until I get some more data. I'm awaiting a machine or motor part number from the customer.

I have to assume at the moment it is wired correctly, as I know the customer and he doesn't touch electrics at all (he's a carpenter / joiner) and it was working correctly to start with.

Just another thought. Could the cold temperature affect the capacitor function? It's in an unheated open barn, and today it was -7 up there. Could it be the performance has steadilly diminished as it's got colder?

I see I can get two replacement 50uF 450V motor capacitors for

 

[kme]

My bad - I didn`t spot that in your first post when I read it (oops)Blushing

OR......did you sneakily "edit it in" when I wasn`t looking??? :^O :^O:^O

ermmm. Off to check my bits & pieces, see what I have in the way of info.......

 

[revjames]

I test capacitors with a megger (an old one just in case) just charge em up but pull the leads off before releasing the button. Then short out and you should get a healthy 'crack' off the terminals.

 

[bil]

Is it belt driven? Could we be missing the obvious - belts slipping??

 

[ProDave]

Yes, it's belt driven, two belts in fact.

One was just a little slack, but I doubt enough to slip. They are both vee belts, which unless incredibly slack usually self tighten and grip on the tension side. And if the belt slips you usually hear and smell it.

I'll certainly ask him to tighten that belt and try it again before I go back just to be sure. But yes I agree a slipping belt would cause exactly what we are seeing.

 

[bil]

I'll keep thinking...

 

[ProDave]

The customer is going to look for the machine and motor part numbers, and tighten both belts fully, and will report back to me tomorrow.

 

[Riggy]

I suspect this is an old motor with a centrifugal switch in series with the start winding, and that the switch is welded closed. Wire a switch in series, when the motor is running, open the switch, I suspect you will then have full power.

 

[ProDave]

Hi all

No it's not an old centrifugal motor. I know about them, I have one on my Lathe, but almost always with those the starting capacitor is built into the motor. This one is quite new and certainly not like that.

Anyway, word back from the customer. Having tightened the belt, all is working fine now.

So after all that it was just the slightly slack belt slipping. Strange you couldn't hear the belt squealing.

I guess that confirms why I'm an electrician, because I would make a lousy mechanic.