FP1 spindle motor problem

RobCee · Mar 8, 2020

Hi guys,

I have a 1985 FP1 Activ which has a slight challenge with it's spindle motor.
When the spindle motor is engaged, it causes a short drop in power to the controller which resets the DRO/controller.
This was just happening if I engaged high speed, but now it is behaving the same way when starting at low speed too.

The machine is powered from a rotary phase converter, but I have no issues running other devices from the same RPC, so I don't believe it is to do with that.

My thinking is that there may be something wrong with the motor, so that it is drawing more current than expected when starting, but I am unsure where to start with my investigations.

I have rooted through the posts on the forum, without finding anything similar to this, so far.

Any suggestions would be most appreciated.

Cheers,

Rob

ballen · Mar 8, 2020

RobCee said:
I have a 1985 FP1 Activ which has a slight challenge with it's spindle motor. When the spindle motor is engaged, it causes a short drop in power to the controller which resets the DRO/controller.

Rob, a simple and inexpensive solution to this is to add a NTC (Negative Temperature Coefficient) Thermistor as a "soft start" element to the spindle motor or to the supply that feeds it. This is a two-pin electronic component which has relatively high resistance when cold at room temperature and whose resistance drops dramatically when it gets warm. If you put this in series with the power supply to the spindle, it reduces the start up "inrush" current which lasts a fraction of a second.

NTC thermistors cost one or two dollars or euros or pounds. You can buy them online or from any electronics supply outfit. Details of how to size them (cold and hot resistance, max current) can be found here: https://www.mouser.com/datasheet/2/136/NTC_ICL_S364-226349.pdf

I use these on some of my machines. For example I have an NTC thermistor wired in series with the power supply transformer on my Studer RHU-450 grinder. Before I did that, the inrush current in the transformer would occasionally flip the circuit breaker, which was annoying and also made the DRO lose its settings. That no longer happens.

Peter from Holland · Mar 8, 2020

Check the amps on each phase On a bad motor you generaly see a signifacant difference

Peter

RobCee · Mar 9, 2020

Peter, Ballen,

Thank you for your suggestions. I will look into the current values and see what I can find.

Ballen, - I don't want to go straight to a NTC Thermistor solution, as I think that there must be an underlying problem that this would mask. If I can't find anything else wrong, I will look at adding a thermistor, as it is a great and simple solution.

Rob

AlfaGTA · Mar 9, 2020

Might want to check the motor brake for proper adjustment, and to be sure its clean and free running when powered.
Cheers Ross

RobCee · Mar 10, 2020

Ross, thaks for that suggestion. It was on my list of things I found that may need checking.
I do believe that it is ok, but definitely worth verifying.

RobCee · Mar 16, 2020

I have had a poke around insude the control cabinet and measured the current on the spindle motor windings.

Low Speed 2.2 A, 3.1 A, 3.0 A
High Speed 1.3 A, 1.2 A, 1.5 A

Peter, would this classify as significantly different?

I had a look at the output voltages from the various transformers and all of them seemed to be as expected.

The spindle is easily turned by hand with the brake disengaged.

My next thought was to start disconnecting things and see if the behaviour persists, as maybe something in the drive train is causing the problem.

Peter from Holland · Mar 16, 2020

That is too much IMHO As you have a RPC in line switch phases to see if the amps stay on the same windings or move with the phases
If it stays on the same windings your motor is bad You have a small chance if you bake your motor
I think about 200°C it was for 8 hours But someone more knowledgable might correct me

Peter

RobCee · Mar 16, 2020

Super! Thanks for the info Peter.
I will play around with the connections and see what is happening and report back.

Rob

RobCee · Mar 17, 2020

Right then, I have measured some more current values, but am not sure it helps much.

All on the Low speed

2.2 A 3.1 A 3.0 A
3.2 A 2.6 A 2.4 A
3.0 A 1.9 A 3.5 A

Does this make any sense?

It looks to me as if the lower current value is switching between the three phase legs as I shift the three phases coming into the mill.

Rob

ballen · Mar 18, 2020

RobCee said:
Does this make any sense? It looks to me as if the lower current value is switching between the three phase legs as I shift the three phases coming into the mill.

If I have understood correctly, then one of the three RPC outputs is providing less current than the other two outputs. Is that right? There might be a simple explanation (thinner wiring or a bad connection in that leg) or it might be more complicated.

Peter from Holland · Mar 18, 2020

I would check the voltage of each leg when the motor is running
And if you have someone nearby with real 3phase I would hook up the motor to real 3phase and check the Amps
Or perhaps a VFD Just to test if the motor is good

Peter

RobCee · Mar 23, 2020

Thank you for your suggestions.
I have replaced the cable from the RPC to the Control cabinet, as I noticed it was looking a bit old. Nothing different.
I will check out the voltages on the legs of the supply next time I get down there.
Unfortunately, checking with a 3ph supply is not possible as the machine was an absolute fight to get into position and getting it back out again is a significant challenge.
Maybe, once all this quarantine stuff is over, I can remove the motor and take it somewhere that has 3ph and check it out. Until then I am somewhat limited in terms of testing.

Rob

RobCee · Apr 3, 2020

Ok, I have tested the different legs of the RPC and found the following:

335 V L1 to Ground
115 V L2 to Ground
347 V L3 to Ground
150 V N to Ground

I also checked the phases to Neutral
250 V L1 to N
265 V L2 to N
251 V L3 to N

I understand that there will always be a difference with the generated power leg. Is mine too different?

Peter from Holland · Apr 3, 2020

I would have checked(with motor running at best)
L1 to L2
L2 to L3
L1 to L3
That is what the motor feels
I suspect it is off too

Peter

RobCee · Apr 3, 2020

With the motor running In low speed I get:
417 across L1 & L2
422 across L2 & L3
394 across L3 & L1
In high speed it is a bit closer:
418 across L1 & L2
414 across L2 & L3
402 across L3 & L1

It doesn’t seem to be too far off under load.

Peter from Holland · Apr 3, 2020

It looks to me like it is your motor
But not 100% sure
It could be its a bit of both
Peter

65AMC · Apr 3, 2020

Where was the voltage measured at? Ideally it would be right at the motor terminals. That would help to expose any voltage drops in the system from bad connections anywhere.

The fact that the voltage is in spec (10% from nominal is acceptable, 5% is ideal) and the current is off means that the excess current is going somewhere. Also it seems the issue is primarily isolated to the low speed windings.

I am going to follow a couple of different assumptions for some direction, first that the voltage was measured at the motor. In this case there is a problem with the motor and it should be megged to verify.

The second version is the voltage was measured either at the RPC or before entrance to the mill electric cabinet. In this case the switch or contactor that turns on the low speed of the spindle is at fault. I find this to be highly likely, I have seen a lot of older equipment have problems with the contacts in contractors even though it still pulls in OK. Check for voltage drop across the component (contactor or switch) while the spindle is on. Current has to be flowing to check for voltage drop.

Shawn

RobCee · Apr 5, 2020

Thanks Shawn, you have some helpful ideas in there.
Measurement was at the supply side into the control panel - as it was easily accessed at the terminals.
You make a good point on the excess current going somewhere.
I will check the voltage at the motor and see if there is a significant difference. Also checking across the contactors sounds sensible too. They are all a bit elderly now!

Rob

doug8cat · Apr 5, 2020

You could meg the motor not sure if it would help or just provide more data.
I know meggers are expensive but sure beats pulling the motor to have it done. There are cheap one on all the usual scum bags for cheap; don't bother with them you want one with at least 2000 2500 voltage output.

FP1 spindle motor problem

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