Spindle motor low tacho voltage

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donm0098
Posts: 6
Joined: Thu 28 Apr , 2011 14:26 pm
Location: Leeds

Spindle motor low tacho voltage

Post by donm0098 » Wed 14 Mar , 2018 15:16 pm

I have a 1998 Triac PC. It has problems with RCD trip out at high revs. The spindle motor is an SMC MT30V4-36. It is rated at 140 volts, 4000 rpm and tacho output of 9.5 volts per 1000 revs. The insulation of the armature measured at about 4 meg, so I had a refurbishment of the brushes and commutator. That cured the low insulation, but the original problem remains.

I disconnected the speed controller and tested the motor with a variac and a rectifier bridge. If I wind up the variac to 140 volts and measure the voltage from the tacho, it is only around 12 volts. If I measure the speed with a strobe, it seems to be doing 2400 rpm. Increasing the voltage to over 140 to try and get the revs up tripped the RCD and the fuse blew in the variac. I think that the motor flashed over, but it is sealed and you can't see the armature.

So it looks like the speed controller is overloading the motor trying to get the feedback voltage up.

Could the problem be that the permanent magnets are demagnetised?

Any help with this problem will be greatly appreciated.

Martin
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Re: Spindle motor low tacho voltage

Post by Martin » Wed 14 Mar , 2018 21:33 pm

Yes it sounds like the permanent magnets have failed.

donm0098
Posts: 6
Joined: Thu 28 Apr , 2011 14:26 pm
Location: Leeds

Re: Spindle motor low tacho voltage

Post by donm0098 » Sun 18 Mar , 2018 4:00 am

Thanks Martin.

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