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Author Topic: Steppers have no torque.  (Read 5850 times)

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Offline Tweakie.CNC

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Re: Steppers have no torque.
« Reply #10 on: February 05, 2018, 05:23:01 AM »
Quote
The old round stepper motors relied on ferrite magnets for their power. After 5 -10 years the ferrites lose their magnetisation and the stepper motor loses its torque. I have a large box of old round stepper motors of all sizes ...

I hope not.   ;)

All my steppers are round case and most, if not all, are now more than 10 years old.  :'(

Perhaps there are Steppers and there are Steppers (mine are Swiss made, Sonceboz).

Tweakie.
KEEP SAFE !

Offline rcaffin

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Re: Steppers have no torque.
« Reply #11 on: February 05, 2018, 05:50:46 AM »
Hi Tweakie

Do they have ferrite magnets?
Could it be worth while buying a new square one and comparing the drive?

Cheers
Roger

Offline Tweakie.CNC

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Re: Steppers have no torque.
« Reply #12 on: February 05, 2018, 06:01:49 AM »
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Do they have ferrite magnets?

Absolutely no idea.

I will contact Sonceboz and see what they have to say about the lifespan vs. torque of their motors - I think they only make round case types.

Tweakie.
KEEP SAFE !

Offline TPS

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Re: Steppers have no torque.
« Reply #13 on: February 05, 2018, 06:05:31 AM »
a first step would be to meassure the voltage,
for Motor supply.


anything is possible, just try to do it.
if you find some mistakes, in my bad bavarian english,they are yours.
Re: Steppers have no torque.
« Reply #14 on: February 05, 2018, 06:39:05 PM »
Hi,
another fairly simple test is to install an ammeter in one wire of the stepper.

The stepper driver will be using a sort of pulse width modulation technique to regulate the current. If the full voltage of the DC supply were hooked to the stepper
the current would be so great that the motor would be destroyed in short order. For the purposes of this test a good-old rugged analogue meter is best.
Note also that the current in any given wire will reverse direction with each step, so your meter will try to read backwards, provided the current is not too
high relative to the rating of the meter it will probably withstand being driven backwards.

Another couple of points should be made so that you can be sure that your measurement is accurate.

The first is that with microstepping, and just about all drives for CNC use it, the current builds up to its peak through each intervening microstep to its maximum
that occurs on the motors natural fullstep. I would recommend therefore to switch microstepping off for this measurement, that will means that any current
you measure will be the maximum the drive is capable of or configured for.

The second issue that it is common for stepper drives to reduce the current delivered to the stepper if the drive is sitting at idle for any length of time. So if
the stepper drives to a point and then sits there, still with its maximum current its going to get warm so if its stationary for more than a second or so the drive will
reduce its current by half to save the stepper from getting to hot. For this measurement you want to disable that current reduction feature so that you will
at all times get maximum current and therefore a realistic appreciation of the motor current.

Now you have to decide does the measured motor current correspond to the manufacturers rated current?

Craig
My wife left with my best friend...
     and I miss him!
Re: Steppers have no torque.
« Reply #15 on: February 05, 2018, 07:31:04 PM »
Hi,
there are other ways to measure the motor current.

I have a high bandwidth current clamp from DC to over 1MHz, very useful for such tasks but not usually in the realm of hobbyists kit.
You could put a low value resistance, say 0.1Ohm, in the wire and measure the voltage drop with an oscilloscope, should avoid the disadvantage of driving
an analogue meter in reverse. Also with the bandwidth available with an oscilloscope you should be able to see the signature of PWM that generated the current.

Another possibility is to find the shunt resistor on the driver circuit board. All the current to the motor will pass through a low value resistor and the circuit will
measure the voltage drop to control its PWM loop. If you can identify the shunt you could measure the voltage drop with an oscilloscope with the same advatages
as above. Will require that you study the board and trace out the circuit to the extent that you can identify the shunt resistor. That requires some knowledge
and experience, its part of my training and very much a part of my job which is why I mention it, you may have similar skills/experience and could therefore
adopt this very simple test.

Craig
My wife left with my best friend...
     and I miss him!
Re: Steppers have no torque.
« Reply #16 on: February 05, 2018, 09:48:58 PM »
Hi,
I have read that stepper magnets can be irrevocably demagnetised if the stepper is disassembled. It always makes me wonder how they make them I the  first place?
Do they assemble them real quick while the magnets are still hot and before all the goodness drains out of them?

Any permanent magnet can be demagnetized if a sufficient magnetizing force is applied contrary to its field, if memory serves the parameter which measures that threshold
is called coercivity. Ferrite magnets have vey useful coercivity, about 1/2-3/4 Tesla but rare earth magnets are better again 1-1.5 Tesla. You have to try that much
harder to demagnetize a rare earth magnet.

I think the risk comes when you disassemble a stepper the magnet is now dislocated from its soft iron magnetic circuit. It doesn't suddenly lose magnetism it just that
it doesn't have the easy magnetic conditions it is accustomed to. This is in effect a counter field equal to the remenance of its own field. In the case of ferrites
the difference between it remenance and its coercivity is fairly small, about 0.1 Tesla. Any additional demagnetizing flux due to a current in a winding could cause a
ferrite to demagnetize. Rare earth magnets the difference between remanance and coercivity is about 1/4-1/3 Tesla and so unlikely to encounter flux levels
sufficient to demagnetize certainly by accident.

My contention is that unless a ferrite is removed from its soft iron magnetic circuit or was deliberately spun in generator mode well in excess of it ratings that it
will not demagnetize and not therefore have a 'lifespan'. Whether my contention is correct is another matter!

Craig
My wife left with my best friend...
     and I miss him!

Offline rcaffin

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Re: Steppers have no torque.
« Reply #17 on: February 05, 2018, 09:51:15 PM »
I just put an analog ammeter in the supply line to the driver (all drivers). That tells me most of what is going on.

Cheers
Roger

Offline rcaffin

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Re: Steppers have no torque.
« Reply #18 on: February 05, 2018, 09:59:11 PM »
Hi Craig

Whether my contention is correct is another matter!
Just so.
It does seem to be accepted by many in the game (or trade) that the old round steppers with ferrite cores do lose their torque over time. For sure, I can report that my old (20 - 30 yrs) round steppers have lost most of their torque. They go around still, but I can stop even the big ones with my hand (Gecko 203V driver).

Cheers
Roger

Re: Steppers have no torque.
« Reply #19 on: February 05, 2018, 10:35:32 PM »
I wrecked a nice Nema 46 ooops, that's 42 stepper by disassembling it years ago.
Inquired about getting it re-magnetized and was told that they are magnetized while assembled. Some special machine.

The air gap around the rotor is critical. When the rotor went off center and was pulld out, the magnetism dropped buy over 60%. IIRC, the cost to repair was pretty steep. Was also about the time I was going with AC servos  .. so I scrapped it. Lesson learned.

I've also read that continuously overloading a stepper (servo) motor will degrade the magnetism.
« Last Edit: February 05, 2018, 11:23:17 PM by Overloaded »