Fast running stepper motor

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Fast running stepper motor

Postby Viewmaster » Mon Feb 24, 2014 6:53 pm

I am using a 1.8 degree stepper motor, running at 60 RPM with a drive frequency of 200Hz into the motor driver board. (an Easydriver)

I need it to reverse run at least x16 faster......960RPM with 3.2kHz.

Until last week no matter what I did ( changing drive volts up to 35 and
trying various motors of different resistances ), I could not make it run faster than 240RPM with driving frequency of 800Hz.

But yesterday I knocked up a variable freq square wave generator and found that if I started the motor slow and increased the freq slowly it now runs at over 1000 RPM, giving me the x16 rewind.

What puzzles me is that is this normal for stepper motors, to make them run fast, the drive frequency has to be increased slowly rather that suddenly switched in?
Thanks
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Postby AncientBrit » Mon Feb 24, 2014 8:31 pm

Hi Albert,

I believe it is.

I've seen many references to 'ramping up' the speed if you are working beyond the normal stepping range.

Cheers,

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Re: Fast running stepper motor

Postby Harry Dalek » Mon Feb 24, 2014 10:11 pm

Viewmaster wrote:I am using a 1.8 degree stepper motor, running at 60 RPM with a drive frequency of 200Hz into the motor driver board. (an Easydriver)

I need it to reverse run at least x16 faster......960RPM with 3.2kHz.

Until last week no matter what I did ( changing drive volts up to 35 and
trying various motors of different resistances ), I could not make it run faster than 240RPM with driving frequency of 800Hz.

But yesterday I knocked up a variable freq square wave generator and found that if I started the motor slow and increased the freq slowly it now runs at over 1000 RPM, giving me the x16 rewind.

What puzzles me is that is this normal for stepper motors, to make them run fast, the drive frequency has to be increased slowly rather that suddenly switched in?
Thanks
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Hi Albert yes they are fussy buggers !
Did you try adjusting the pulse width i think the pulse width needs to change with the frequency increase some thing i wanted to try after the post on Hd motors i had a while back .
I want to do it at the same time bit hard with 2 pots but a dual pot will increase the frequency and change the pulse width same time .
They can run fast just need the correct pulse for the speed ...what i have seen on the net on high speed experiments its sure possible .
But i do think they are all the same and need a slow start up or they will stop, every one i have tried have to be run like that .
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Postby Viewmaster » Mon Feb 24, 2014 10:37 pm

Thanks Graham and Harry for 'stepping' up to the plate for me. :-)
I shall put in a permanent pot to wind up the freq when wishing to run fast.
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Postby Steve Anderson » Tue Feb 25, 2014 5:32 pm

I think it's safe to sat all motors have windings - OK, their may be exceptions like piezoelectric motors used in clocks and watches - but for now lets consider any motor with windings of copper.

This, even without any form of magnetic core is an inductance. It also has a series resistance presented by the copper windings - or any other conductive metal.

When you apply an instantaneous DC voltage to an inductance the current is zero. It ramps up in a linear fashion over time. If the windings were at absolute zero (a super-conductor) the current would rise without limit. In practical terms it limits at the DC resistance of the winding. The current rises much as the voltage does in a resistor-capacitor arrangement, it becomes asymptotic to the voltage divided by the resistance.

All wound motors depend on the current in the windings to produce torque which produces the output power on the shaft.

When you raise the drive frequency the I/t curve doesn't reach the required current to drive the motor from standstill or even a small mechanical load, much less than at its rated speed. Friction, air-drag and load balance will become limitations.

I'm no expert in this field, but that's as I understand it in broad terms.

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Postby Viewmaster » Tue Feb 25, 2014 11:36 pm

OK, Steve on all that, thanks.
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Postby gary » Wed Feb 26, 2014 9:39 am

Albert, all stepper motors exhibit a fall off of torque with speed .

Torque is proportional to the current in the windings and the rate of change of current in the windings is given by di/dt = V/L. Thus, providing you limit the current to a safe value you can increase the rate of change of current, and hence maximum speed, by increasing the voltage - you have done that, however, the torque of the stepper is still falling off with increase in speed.

Due to inertia and friction more torque is required to bring the motor up to speed than to maintain that speed once achieved.

Thus, if you try and bring the stepper instantly up to any given speed you run the risk of exceeding the torque that the motor can produce and it stalls and loses sync. Every stepper has a "pull-in torque" curve which indicates the maximum speed (read frequency) above which the motor will stall.

Ramping is essential in all but the lowest speed applications of stepper motors. Most ramping is linear for ease of implementation but would ideally be matched to the motor's pull in torque-speed curve.

On top of that there is also the mid-band resonance to be aware of. This is a mass-spring characteristic of a stepper and ramping through this prevents stalling (although doesn't help running at this speed so if your required speed is in this area you need other mechanisms).

You have done well to achieve 1000 RPM - I struggled to get 750 RPM.

Actually I documented my adventures with steppers somewhere on this site - you must have missed that otherwise you would have realised the need for ramping earlier.
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Postby Viewmaster » Wed Feb 26, 2014 6:54 pm

gary wrote:You have done well to achieve 1000 RPM - I struggled to get 750 RPM.
.


Thanks for reply, Gary. I just rechecked and had that wrong. I can get
about the same as you, not 1000. My driver board only delivers 750 m/a max. I daresay if I could go up to 2 amps the speed would be greater.
gary wrote:
Actually I documented my adventures with steppers somewhere on this site - you must have missed that otherwise you would have realised the need for ramping earlier.

I'll dig that out.

I started using the obsolete SAA1027 but that has a 12v limitation, so bought an Easydriver board.
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