What’s wrong with hybrid stepper motor setup?

[blowlampParticipant @blowlamp]

[blowlampParticipant @blowlamp]

I'm replacing the conventional stepper motors on my cnc lathe with a pair of these.

The original Z axis driver has been replaced and this drives the motor beautifully when the motor body isn't touching the lathe. However, once I bolt the motor in position the driver throws a fault and starts making a growling sound.

If the motor is laid on any painted metal part of the lathe it can be jogged, but if there is any metal to metal contact whilst jogging, sparks are produced and it runs roughly or stops altogether if there's a good enough connection.

Interestingly, my multi-meter shows NO continuity between any wires and ground/body of the motor so could this be an induced voltage? If so, how do I mount the motors and why wasn't this a problem with the other motors/drives?

 

Thanks for any help with this.

 

Martin.

Edited By blowlamp on 31/03/2018 22:07:55

[Grindstone CowboyParticipant @grindstonecowboy]

I'm no expert, but I would have thought an induced voltage would also affect it when on a painted surface as well? As regards a solution… maybe paint the motor, or some sort of insulation?

Regards,

Rob

[Martin DowingParticipant @martindowing58466]

Maybe there is a small current leakage from motor to the earth through your lathe and this is detected on controller.

Some sort of residual current detector.

Martin

[MuzzerParticipant @muzzer]

You shouldn't be seeing sparking and malfunctions when you ground the motor frame. Sounds as if you have a short between the windings and the housing. I'd tell the vendor and get them to send a replacement.

If you connect all the winding connections together and put a bench power supply between them and the motor housing and apply a similar voltage (36V or so?), I expect you'll find there's a short circuit. An insulation tester (typically 500V or so) would be a better test instrument but few of us have them in the workshop.

Murray

[I.M. OUTAHEREParticipant @i-m-outahere]

Could checking continuity between the wires and housing give you false readings because the voltage a multimeter applies to do this is only small – less than 9v and the driver could be supplying a much higher voltage – say 36v ? , i know thats why when a transformer or mains motor is tested they use voltages much highr than the supply voltage .

What if you set the stepper up on an insulated surface and check if there is any voltage between the motor housing and driver ground or negative terminal with the stepper running then do the same between the motor and a clean metal surface of the lathe .

What if you disconnect the power lead to the lathe ( unplug from wall socket ) and power the stepper drive up and see if it works with the lathe disconnected , maybe something to do with the earth circuit between the lathe and stepper controller power supply or driver ?

Muzzer beat me to it ! 

 

Edited By XD 351 on 31/03/2018 22:55:40

[SillyOldDufferModerator @sillyoldduffer]

Odd.

Can you provide details of what the power supply is and how it's connected to both the mains and to the servo drive? Some photos would be good.

I don't understand what the RS232 connection is for. Is it connected to or touching anything? (The motor frame may be connected to an RS232 ground wire, which might explain sparking on an OK motor.)

Something is making the motor hot OR the lathe is already hot and the motor is grounding it . Worth triple checking there isn't a wiring error.

Can you share the manual (if any!)

Dave

 

 

Edited By SillyOldDuffer on 31/03/2018 22:57:10

[Mike PooleParticipant @mikepoole82104]

Dave, I think you will find the D plug is for the encoder fitted on these hybrid motors.

Mike

[blowlampParticipant @blowlamp]

The RS232 is there to allow tuning of the driver via a software app.

The power supply is a transformer, bridge rectifier and capacitor, all running at ~ 35VDC.

The motor is running very cool in its standby mode.

Since asking the question I've been back in the workshop and tried the other new motor and driver with the same result.

I've also checked again for a short circuit to the body on both motors using the 30MΩ setting on my meter and it's not showing so much as a flicker. I don't have anything better to test things with I'm afraid.

Thanks for the answers so far. I'll try to have another look tomorrow and report back with my findings.

Martin.

[I.M. OUTAHEREParticipant @i-m-outahere]

Did you also check the encoder for grounding ?

[blowlampParticipant @blowlamp]

Posted by XD 351 on 01/04/2018 00:55:41:

Did you also check the encoder for grounding ?

Yes, I checked every wire on both motors for a possible short to the motor frame. I couldn't find anything amiss.

Martin.

[Enough!Participant @enough]

Posted by blowlamp on 01/04/2018 01:25:44:

I checked every wire on both motors for a possible short to the motor frame. I couldn't find anything amiss.

Was it connected to the encoder at the time? My guess is that it needs to be …. and it may also be that the problem only occurs while the encoder is switching which will make it hard to fathom from simple static tests.

You may end up having to mount the motor using a rigid, isolated mounting. Which might not actually be that difficult.

[John OlsenParticipant @johnolsen79199]

I would measure the voltage between the motor and the lathe with everything connected up but the motor not touching the lathe. The voltage you measure will give some clue as to where it is coming from. It doesn't sound right to me since it would not be usual for this sort of motor to have a connection between the winding and the frame. (It is quite common on automotive stuff for one side to be grounded through the case, but not for this sort of stepper.)

Is the frame of this motor earthed via its controller. If it is, is this the same earth as the lathe is using? Sparking sounds like something with a bit of grunt, not just some leakage through a high resistance.

John

[I.M. OUTAHEREParticipant @i-m-outahere]

Have you tried with the encoder disabled / disconnected ? You may have to disable it in your software set up before disconnecting it , is the encoder cable shielded ? Did you measure the continuity from the motor housing to the drivers encoder terminals with it wired up and the encoder leads connected ? There may be something wrong with the cable or plug and socket that will only show up when everything is connected ..

As,John has stated the voltage has to be coming from somewhere either stepper driver or the lathe it can't create itself ! 

Edited By XD 351 on 01/04/2018 03:35:53

[John HaineParticipant @johnhaine32865]

Did you literally mean sparks? That would imply a significant voltage.

But, given that everything seems to work when the motor is isolated, maybe you have a ground loop problem that is generating spurious drive pulses, hence the growling. I assume that the lathe is earthed via the mains connection, perhaps you may need to disconnect that and earth it only via the servo cabling.

[Raglan LittlejohnParticipant @raglanlittlejohn]

Some DC equipment is +ve earth. If the original psu is -ve earth, and the new equipment is +ve earth, this is the sort of problem you would get. I'd put the motor on an insulated surface, and check with a voltmeter.

[Neil WyattModerator @neilwyatt]

Have you suspected that the machine may have a dodgy ground rather than the motor?

[I.M. OUTAHEREParticipant @i-m-outahere]

ersonally i think Posted by Neil Wyatt on 01/04/2018 09:11:54:

Have you suspected that the machine may have a dodgy ground rather than the motor?

That is the reason i suggested disconnecting the main power from the machine to eliminate this issue .

Personally i think grounding the driver and power,supply to the lathe may be an option but where is the residual voltage coming from ? If you go back and do some of the voltage tests as i   have suggested and note the polarity indicated on the multimeter you can see where the voltage or potential  is coming from.

One thing i haven't asked is if there is only sparks when you are driving the stepper or is it as soon as you touch the stepper to the lathe ?

Try grounding the power supply ( negative ) to rhe lathe and see if it fixes the priblem ..

 

 

 

Edited By XD 351 on 01/04/2018 11:05:31

[MuzzerParticipant @muzzer]

If you have an RCD in the supply, any grounding issue that is capable of causing sparks would cause it to trip. Otherwise, as Neil suggests, you should ensure there is a good fat ground connection to the machine. Sounds like the first thing to check, as it's a safety issue rather than just something that stops your stepper from working.

Muirray

[John HaineParticipant @johnhaine32865]

Yes, you need a good ground connection, but ONLY ONE! I have found that having multiple earth connections can lead to strange earth loop problems. The trouble with CNC as opposed to audio is that you can't hear the hum (or its equivalent) so it can be hard to trace problems.

[Gordon WParticipant @gordonw]

I would have a good check on the house wiring, esp. earth. I had some similar problems for a year or more, finally borrowed a sensitive meter which showed earth faults. Traced to an earth, put in by a registered electrician, which was not connected properly. I renewed all the earth connections , had no trouble since. Of course it may well be something else but worth checking.

[Speedy Builder5Participant @speedybuilder5]

Is there a blocking diode somewhere in the circuit ? Ie when you check continuity, the diode blocks the path to earth, but if the current is applied in the opposite direction then there is current flow or something??
BobH

[SamsarandaParticipant @samsaranda]

Possibly parallel earths could be the cause, not unknown for them to build quite a potential between them, on larger equipments have been known to have lethal consequences, obviously not on circuitry with 36v.

Dave W

[blowlampParticipant @blowlamp]

I'm hoping this issue is now fixed.

As suggested in the replies, I did a few more checks today and was amazed when I put the meter between the motor body and a set of windings to find a dead short. I immediately repeated the test on my other new motor and found that to be OK, however, connecting this 'good' motor to the stepper driver on the lathe and gingerly earthing it to the lathe caused the driver to throw an error. "What to do now?", I asked myself.

Knowing for sure the first motor had a shorted winding, it seemed to be worth dismantling to try and find the cause. The first thing off was the cable clamp for the winding wires. For some reason, I tested again for a short circuit and was surprised that it was no longer present.

The long story short is that the cable clamp screws were too long by probably 2mm or so and had pushed their way through some fairly substantial plastic insulation to reach the windings and short them out. This was the case for BOTH of my motors.

To be sure of fixing the fault properly, I stripped off the encoder assembly along with the motor back plate to make sure everything was in order and that no permanent damage had been done. Thankfully, all that was needed was some reshaping of the plastic insulation with small pliers/tweezers and a little repositioning of two or three windings with a cocktail stick.

I haven't had time to rebuild the lathe yet, but a jump wire between motor body and lathe chassis now has no effect, although I can still measure about 14 VAC or 0.2 VDC between motor and lathe.

Thanks for all the helpful replies.

Martin.

[SillyOldDufferModerator @sillyoldduffer]

Posted by blowlamp on 01/04/2018 20:57:11:

…

Thanks for all the helpful replies.

Martin.

Good job Martin. Isn't it annoying when someone designs and builds something as technically demanding as a stepper motor and then, as in this case, screws up a minor detail?

Muzzer gets a gold-star for his early diagnosis. I'd have been really impressed if he'd identified the actual cause of the short as well, but that's asking too much!

Dave

[Author]

Posts