# Finding current of stepper motor

Hi,
For the current project I am working on,I am using 2 or 3 Molon LMS 3000 series steppers. I am looking for a motor driver to use for those motors, but I do not know the current for it so I do not want to jump the gun and buy one too quickly. I found a page about the motor, but I do not know a lot about this stuff so I do not know what to make of it.

Motor webpage: http://www.molon.com/series3000.html

Usually the specifications include the coil resistance which, with the voltage, would allow you to determine the current. I think you will have to contact the company. It says 6, 12 or 24v so I presume there are three different models of the same motor. They may or may not have identical performance.

Those drivers should be OK if they can manage the motor current.

...R

Robin2:
Usually the specifications include the coil resistance which, with the voltage, would allow you to determine the current.

No, no , no!

Stepper motors are NOT light bulbs. High power steppers have a low resistance but are NOT* designed to have their current set by their internal resistance and the supply voltage. Being made of huge coils, they are very inductive, and therefore you need to apply a high voltage in order to make the current change rapidly (dI/dt=V/L). Since you need to invert the current in one coil to achieve a full step, the rate at which you can step is limited by your supply voltage and the motor's inductance. The voltage you have available does NOT define the current that the motor should run at.

The current limit is basically thermal, P=I^2*R losses. If the motor gets too hot, that's too much current.

You must use a feedback loop in your current chopper circuit to control the current. More load on the motor or higher operating speeds means greater PWM duty cycle is needed to reach the same current.

• the exception being low-power motors, of which this MIGHT be one, in which case the above post is kind of right but it's a really really bad approach in general. If your current limit is defined by your supply voltage and the motor's internal resistance, be aware that you will get very little torque and speed out of the motor because all the power is being dissipated in the motor's windings and there is basically none left over to actually provide any motive force.

polyglot:

Robin2:
Usually the specifications include the coil resistance which, with the voltage, would allow you to determine the current.

No, no , no!

I try to be tolerant of other people's contributions but this hysteria is a step too far.

The normal (only ?) way to know what steady state current will flow in a coil for a specified voltage (supplied in this case by the manufacturer) is also to know the coil resistance - in this case NOT provided by the manufacturer.

I never suggested that the OP should control the current in his motor like a "light bulb". The OP had identified suitable stepper drivers in his first post. But one needs to know the acceptable current for the motor in order to set the current limit on the driver board.

If you know an alternative way to work out the max coil current please tell.

...R

I don’t know what the acceptable current is, because it must be specified by the manufacturer or found experimentally from temperature. As such, I can’t really help the OP.

However, my point is that the resistance is irrelevant when you use a current-control chopper drive because the feedback loop therein will cause the correct current to flow regardless of the stepper’s resistance. If you’re depending on the resistance to set the current, then the driver circuit is very bad because it must run at an artificially low voltage and therefore will achieve a very low step rate and torque because it cannot achieve a reasonable dI/dt with the resistor soaking up the whole supply voltage.

However! For the purpose of buying a driver, 1A will be plenty. They’re pretty small motors by the look of it. You may need to run them at less than 1A to prevent overheat, but I don’t think you’ll need a driver capable of more than 1A, maybe 2A.

Robin2: I meant to be emphatic rather than insulting, my apologies if you took it that way.

And I should have been clearer that your post is correct for some specific (very low power) steppers. I meant that it’s a bad approach if taken as a generalisation for all stepper motors, e.g. where you might have a rated current of 3A, about 0.5R winding resistance and want to drive the stepper with about 48V. Clearly those numbers don’t add up if you just think of the stepper as a resistor.

polyglot:
where you might have a rated current of 3A, about 0.5R winding resistance and want to drive the stepper with about 48V. Clearly those numbers don't add up if you just think of the stepper as a resistor.

Apology accepted - I was mainly concerned that your long post would confuse the OP.

I am well aware of the stuff I have quoted above, and I had not explained this in my earlier reply because the OP was already planning to use a suitable driver.

There is no shortage of other newcomers trying to drive steppers with unsuitable drivers.

...R