Geckos deliver only 67% of actual current?

[m8kingit]

i have a hypothetical question in referrence to geckos and other quality drives. I've noticed that gecko has you set the current to the actaul current rating of the motors, however, other manufacturers have you multiply the current rating by 1.4 to determine the current set limit.


All being said, there must be a reasonj that gecko has you set the current at 67% of the actual rating.
There are several manufacturers that have been in businesss longer than gecko, that have you calculate above the actaul current of the steppers, Intelligent motion was one of the manufacturers that came to mind.

Do you think it has something to do with geckos drives or is it just, them being in a safe place with the current?

[Mariss Freimanis]

If you have a 1A motor, you set the drive to 1A and if you measure the current, your ammeter will read 1A just as advertised.

The drive is a 10-microstep drive, so the currents will look like this for each of the 10 microsteps:

microstep 0, 0.078A, 0.997A
microstep 1, 0.233A, 0.972A
microstep 2, 0.383A, 0.924A
microstep 3, 0.522A, 0.852A
microstep 4, 0.649A, 0.760A
microstep 5, 0.760A, 0.649A
microstep 6, 0.852A, 0.522A
microstep 7, 0.924A, 0.383A
microstep 8, 0.972A, 0.233A
microstep 9, 0.997A, 0.078A

The current is a sine that has a peak value of 1A and an RMS value of 0.707A. So, why not jack-up the RMS current from 0.707A to 1A? You can do that by multiplying the motor's rated current by 1.41 and set the drive to 1.41A. Now the RMS current equals 1A; sounds good doesn't it? When one winding is near zero Amps the other winding is at 1.41A so everything should be even.

Well, it's not good unless you like a rough running motor. Step motor windings don't share a common magnetic flux path; you can prove that by trying to use the motor as a transformer.

You are putting in 141% of the motor's rated current, the iron will saturate and distort the motor's motion at low speeds. In other words the motor will vibrate at certain low speeds. You use a microstepping drive to eliminate vibration so this is going in the wrong direction.:-)

Finally, consider this:

You have a 100 oz-in holding torque motor. You drive it with a full-step drive and you get 100 in-oz of holding torque. The moment it begins to move, you get 65 in-oz of low-speed torque. Where did the missing 35 in-oz go? They are invested in vigorously vibrating your motor.

You don't like the teeth-rattling vibration of a full-step drive so you haul out a half-step drive and there is less vibration. But take a look at the half-step sequence; it's both windings 'on', one winding 'on', and so on. Alternating strong steps (100 in-oz), and weak steps (71 in-oz). If you overload the motor, it won't stall on the strong step. For all practical purposes you have a 71 in-oz motor now.

Take the same motor but use a microstep drive. You get 71 in-oz of holding torque. The moment the motor begins to move, you still get 71 in-oz of torque. Why? The motor isn't vibrating so no torque gets squandered.

Compare the drives: 65 in-oz for a full-step drive with terrific vibration, 71 in-oz for a half-step drive with less vibration and 71 in-oz for a microstep drive with almost no vibration at all.

Now jack-up the microstep drive current 1.41 times to get your 100 in-oz back. The moment the motor moves you will be investing some of that gained torque into vibrating the motor again.

Mariss

[m8kingit]

Mariss, Do other drive manufacturers make their motors different.

People like intelligent motion and a few others instruct you to set the current limit to 1.4 x the actual current.

I beleive in what you're saying, but is it possible that their drives are manufactured different.
For instance reading the specs on th im805 it tells to to use this method to set the current limit.
I know i shouldn't be asking you about some else's drive, but curiosity has got the best of me.

So what you are saying is that no matter who's drive i'm using the rms should be lower than the peak
Are there drives with special current reduction built in that would change all of this?

[Mariss Freimanis]

This is an interesting topic.

It isn't a matter of how the drive is built but rather how it is applied. My emphasis is on maximizing motor accuracy and smoothness so our application suggestion is to set the drive to the motor's rated current. You could just as well take the motor's rated current, multiply it by 1.41 and adjust the drive's current set to that value. Your motor will dissipate twice as much 'I-squared-R' heat at 141% of rated current so keep that in mind as another drawback.

My argument would be you give away smoothness for a marginal increase in low-speed torque; in my opinion the price you pay is too much. None of this matters at speeds above 300 RPM; our drives morph into full-step drives above that speed anyway.

Mariss

[m8kingit]

Thanks Mariss, I'll take your advise. I guess in my situtation was not knowing my motors rated cureent. i have a cnc masters mill, that i'm restructuring the controller to enable the use of mach3 and a larger power supply, and a paralell port nad to not mention getting rid of the daisy chaining, etc.

According the resistor they had in this controller, they are 6 amp motors, (235kohms)however, i was told by the genmtleman there that they were 5 amp motors, However, this guy did lie ans say he manufactured the controllers, they are gecko 201's in unmarked cans. I've already had your people look at them and verify them.
I guess the safe thing to do is to set my current at 5 amp since i'm not sure and there are no marking on the motors.
Thanks
again
Mark