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Author Topic: Can someone explain the current draw of these stepper motors, please.  (Read 9659 times)

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Offline simpson36

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I put an ammeter on the power feed to a Gecko 20V. The current draw is unexpectedly less than half of what I anticipated, so there is something about this that I don't understand.


Motor is Keling 2.8 amp, 4.17v running off 36v power supply. It idles at about a half an amp and only draws about one amp at full stall.

Why is there such a difference?

I have a pair of 2.8 amp and a 6 amp motor running off an 8.8 amp power supply and I am adding a 4th axis motor (another 6 amps). I was about to replace the power supply with a larger one, but it seems the motors draw much less that I though so I may be OK with the PS that I have.

It is simply a question of higher voltage drawing less amps in the same motor?

Offline Hood

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Re: Can someone explain the current draw of these stepper motors, please.
« Reply #1 on: November 11, 2008, 12:01:20 PM »
It may depend on the way you have them wired and also what the rated current is, ie is it for series connection or parallel?
 Hood
Re: Can someone explain the current draw of these stepper motors, please.
« Reply #2 on: November 11, 2008, 12:05:39 PM »
You need to be looking at power, not current.  You will never be drawing full motor current, at full supply voltage. The motor is rated at 4.17V, 2.8A.  Power is voltage times current, so the max motor power is 4.17 x 2.8 = 11.7W.  your power supply is 36V, and you're measuring 0.5A, so the power is 36 * 0.5 = 18W.  Close enough.  You are running at, if not above, the max ratings for the motor.  Two of these motors would double the power, to 36W.  If your 6A motor is also rated 4.17V, then its power will be 25W.  Your power supply is rated 36V, 8.8A, which is 316W.  All three motors combined draw 18 + 18 + 25 = 61W, so your 8.8A supply should be more than adequate.

Regards,
Ray L.
Regards,
Ray L.

Offline simpson36

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Re: Can someone explain the current draw of these stepper motors, please.
« Reply #3 on: November 11, 2008, 12:49:42 PM »
Hood, the numbers I gave are at the motor's current configuration, which is parallel.

Ray, thanks for the explaination. I do understand the relationships you described.

What was confusing me was that the Gecko drives (203v) require a resistor rated relative to the motor amps without regard for the operating voltage . .  that started me thinking I must not understand something about these steppers.

Perhaps the Gecko knows the voltage coming in and does some type of internal adjusment for the load useing the resistor only as a ref.

The fact that one of the three new Gecjo 203v broke after only 2 months also had me fearing I did somehting wrong in my setup. They found a bad component and replaced the drive for me, so all is well now . . plus I now have a spare.


In any event, good to know I have enough juice to drive my rig including the new 4th axis motor.


Offline lemo

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Re: Can someone explain the current draw of these stepper motors, please.
« Reply #4 on: November 11, 2008, 01:06:11 PM »
The GEcko's reduce the current when not moving to prevent motor overheating which makes a look at the 'real' and the current while idling impossible. The system running cannot be measured as DC current as it's a pulsed system you are observing (AC). A syst4em where the frequency is not constant. The Gecko's also draw from the power source with something like 25KHz.... so measuring the current from the power supply to the gecko is difficult as well. There are very good explanations on the gecko site for all of this.
I measure the ac current of my power supply to the wall outlet and see how much energy goes into the entire system. When only one axis moves, I have an indication on how much energy is used by that component of the entire system. Still, estimated...
Cheers
Lemo
« Last Edit: November 11, 2008, 01:09:01 PM by lemo »
Cut five times and still to short...
Re: Can someone explain the current draw of these stepper motors, please.
« Reply #5 on: November 11, 2008, 01:28:13 PM »
Hood, the numbers I gave are at the motor's current configuration, which is parallel.

Ray, thanks for the explaination. I do understand the relationships you described.

What was confusing me was that the Gecko drives (203v) require a resistor rated relative to the motor amps without regard for the operating voltage . . that started me thinking I must not understand something about these steppers.

Perhaps the Gecko knows the voltage coming in and does some type of internal adjusment for the load useing the resistor only as a ref.

The fact that one of the three new Gecjo 203v broke after only 2 months also had me fearing I did somehting wrong in my setup. They found a bad component and replaced the drive for me, so all is well now . . plus I now have a spare.


In any event, good to know I have enough juice to drive my rig including the new 4th axis motor.




The Geckos modulate the current to achieve the specified average current.  They don't really know or care what the supply voltage is.  They connect the motor winding to the power supply.  Because of the winding inductance, the current does not instantly increase.  Instead, it starts to increase from zero at a rate determined by the supply voltage and the motor inductance.  When it eventually reaches the limit set by the resistor, the motor winding is disconnected from the supply.  Again, the motor current does not instanly go to zero, but starts ramping down.  When it reaches some lower threshold, which is some fraction of the current limit, the Gecko will re-connect it to the supply.  This cycle repeats for as long as the winding is energized. 

The average current seen by the stepper will be equal to the value set by the current limit resistor.  The average current seen by the supply will be far less, due to the much higher voltage.  The peak current seen by the supply will equal the peak current drawn by the motor, but it's only drawn for a very short period of time, so the average value you read with your DVM is far less.

The whole point of running steppers on a supply voltage 10-20X higher than the rated voltage is the help the current in the windings to build faster, to enable higher speed operation.   Otherwise, at high RPMs, you'd never come close to full rated current, the torque would fall off very sharply.

Regards,
Ray L.
Regards,
Ray L.

Offline simpson36

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Re: Can someone explain the current draw of these stepper motors, please.
« Reply #6 on: November 11, 2008, 03:30:44 PM »
Lemo and Ray,

I am an ME and not relay good with electrons. I have some basic understanding of volt/amp/watt relationships and I understand how some common components; resistor, transistor, capacitor, diode, etc work.

One comment "the Gecko connects the motor to the power supply"  clicks. Would it be at all accurate to then consider a stepper controller as a 'smart relay' of sorts, or am I still in left field? I can see where a relay does not care what voltage is being passed thru it (so long as it is within the rating) as it s just a switch between the supply and the load.

I had it in mind that the Gecko itself was powering the stepper. Apparently this is not the case at all. I also get the pulsed DC effecting the ammeter.

Mysteriously, I also have an ammeter on the DC output of my PWM speed controller and it seems to read what I would expect. The controller is 5 amp continuous and has a limiter. The ammeter shows around 2 for normal operation and spikes to 6 when the motor is really stressing or is stalled. So . .  I'm still a little confused about that.

Why wouldn't the 120v DC PWM controller misread badly like the 36v DC Gecko input?  Output vs, input? Is it a frequency thing?

BTW, I have a little device called 'Kill-a-Watt' that measures AC draw, so I'll give that suggestion a try.

Thanks for the info and patience, guys. It's making more sense now . . . slowly.





Re: Can someone explain the current draw of these stepper motors, please.
« Reply #7 on: November 11, 2008, 05:58:32 PM »
The Gecko is doing nothing more than either connecting the motor winding to the power supply, or not, so I suppose you can think of it as a relay in that respect, albeit a VERY fast one.  Since you're an ME (as I am), think of it as more like a pressure regulator.  When the pressure on the load is too low, it opens a valve, allowing pressure from the compressed air source into the load.  Once the desired pressure is reached, the valve is shut off, and the load is isolated from the compressor.  In this case, current is the analog of pressure, and the Gecko takes the place of the valve, and the pressure sensing diaphragm that controls it.

Your PWM speed controller is a different animal, since you're not running it at 20X the motors rated voltage.  If you put an oscilloscope on the stepper motor winding, you'd see the voltage switching at a high rate between 0V and the supply voltage.  If you looked at the current, you'd see it modulating around the limit current, ramping up to slightly higher than the limit current when the voltage is equal to the supply voltage, and ramping down to slightly below the limit current when the voltage is near 0V.  But, you'd find it's actually connected to the supply a relatively small percentage of the time.  You will NEVER see the voltage applied to the winding for any significant length of time, even when the motor is running with a fairly heavy load.  You PWM controller will be runnig a significant on-time even at low motor speeds.  At max speed, it will be on 100% of the time.  If the stepper winding were connected to the supply for any length of time, the current would very quickly FAR exceed the motors ratings, and the windings would burn out.

This will teach you everything you ever need to know about steppers.  This topic is covered in the section on current limiting:  http://www.cs.uiowa.edu/~jones/step/

Regards,
RayL.
Regards,
Ray L.

Offline simpson36

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Re: Can someone explain the current draw of these stepper motors, please.
« Reply #8 on: November 12, 2008, 10:07:00 AM »
THX again, Ray

You have a way of using analogies to make things clear(er).

And thx for the link.

I did the retrofit on this little baby mill for a specific project. However, it turned out to be great fun. When that project is completed, I plan to sell the mill complete and go with a little larger mill (X3?) and do another retrofit using servo motors instead of steppers, so an in-depth study of steppers would probably not be benefitial other than as a point of interest.

Here again, I am no expert at this point, but servos seem to have a lot of advantages for not too much more money.
Re: Can someone explain the current draw of these stepper motors, please.
« Reply #9 on: November 12, 2008, 10:38:16 AM »
THX again, Ray

You have a way of using analogies to make things clear(er).

And thx for the link.

I did the retrofit on this little baby mill for a specific project. However, it turned out to be great fun. When that project is completed, I plan to sell the mill complete and go with a little larger mill (X3?) and do another retrofit using servo motors instead of steppers, so an in-depth study of steppers would probably not be benefitial other than as a point of interest.

Here again, I am no expert at this point, but servos seem to have a lot of advantages for not too much more money.


On a small machine like an X3, I don't think servos have any particular advantage.  You won't be able to make use of the potential higher speed, due to the still rather short travels.  For an X3, I think steppers will give you at least the same performance, for less money.  Per Mariss Freimaniss, the owner/designer of all Gecko drives:

Where a stepper looses:

When more than 200W mechanical is needed.
When there isn't a large ratio between feedrate and rapids.
When battery power is used instead of mains AC.

Where a servomotor looses:

When there is a large static load (moving the knee on a Bridgeport).
When the motor chronically turns less than 360 degrees.
When a low RPM load requires an uneconomically large reduction ratio.


Mariss's rule of thumb is, up to 100W, steppers are best.  Above 300W, servos are best.  Between 100W and 200W, either will work.  An X3 will certainly be well under 200W, probably under 100W.

Regards,
Ray L.
Regards,
Ray L.