Author Topic: AC Motor Diagnosis  (Read 1456 times)

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Offline shanegrilah

  • Posts: 31
AC Motor Diagnosis
« on: August 18, 2018, 09:58 AM »

My band saw motor is having trouble starting. If I have tension on the blade it might start but motor often it hums and kicks out the breaker. Sometimes I can get it going by jamming a scrap piece of wood against the blade and pushing down to help it along. With no tension it starts OK. I've replaced the capacitor but that made no difference. Anything else I can check or do I just need a new motor?

Specs on the motor plate: IEC 34-1 Single Phase 220V 3390RPM 60HZ HFM 90LA B5

I tried searching for a new motor but in N America the above specs seem to be rare. Anybody had to replace a similar motor? Are there IEC to NEMA adapters available?

Band saw manufacturer has said they will snail mail me a quote for a new motor but after two requests I have not received it yet. I'll keep trying them but they sure are making it harder than it should be.


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Offline aloysius

  • Posts: 340
Re: AC Motor Diagnosis
« Reply #1 on: August 18, 2018, 10:24 AM »
My immediate thought was (as is usual in similar circumstances) the start capacitor.  You've stated that it's been replaced, to no avail.  Are you sure that you've substituted like-for-like?  That the capacity (measured in microfarads) & charge/discharge characteristics are similar?

Generally, it's the hard-working start capacitor that's at the root of 99% of all induction motor problems, especially those with similar symptoms to those described.
FOG-wit since '95:  Some say since birth...

Offline shanegrilah

  • Posts: 31
Re: AC Motor Diagnosis
« Reply #2 on: August 18, 2018, 01:35 PM »

The replacement is a 40 micro farad/440 VAC capacitor. Which is what I could find quickly at the time and was told the higher voltage shouldn't matter. Maybe that was bad advice and the voltage is important?

I did keep the original cap but at the moment it is in such a secret location not even I can find it. I know it was 40 mf but thought it was also rated more than the 220 VAC.


Offline RKA

  • Posts: 1668
Re: AC Motor Diagnosis
« Reply #3 on: August 18, 2018, 09:21 PM »
As long as the new capacitor is equal to or higher than the 220 VAC on the original, you’re fine.  The uF rating is what you have to get right.  It’s worth finding someone with a meter to check it as well.  It’s unlikely the part is bad, but it happens and if it is, the fix is easy.  Have you tracked down the run capacitor as well?  Check it while you’re testing the other one.

I would examine the wiring going in and out of the start capacitor for signs of damage, burned insulation, charred connectors, loose connectors, anything simple that might need to be addressed.  (I just spent the afternoon fixing issues one at a time on my AC compressor and have a pile of blown fuses to show for it, so I feel your pain)

Offline Gregor

  • Posts: 1271
Re: AC Motor Diagnosis
« Reply #4 on: August 19, 2018, 01:57 AM »
A 1-phase Motor is usually a 3-phase one wired with two of the windings in series and the 3rd driven by the capacitor.

One broken winding (usually the one with the capacitor) or a defective capacitor and it might continue to run after being started (kept in motion by inertia) with reduced torque, but no longer start reliebly on it's own. It's a good idea to unplug and measure the three motor windings for resistance (they should read roughly the same value) when replacing the capacitor, at times the capacitor isn't the problem but the one winding that permanently sees the full line power (as the other two are connected in series and see only half line power, see eg. here for some ways in which your motor might be wired).
« Last Edit: August 19, 2018, 02:00 AM by Gregor »