Stepper motor rated voltage vs max voltage

[jackorocko]

I have been pondering on what type of stepper motors to buy for a project I have been putting together for the past couple of months. I found this website http://www.kelinginc.net/SMotorstock.html which has a ton of motors at what seems like reasonable prices. But I am a little confused on a few points.

At the top of the website it says in bold red letters, The Max. applied Voltage can be up to 20-25 times of Rated Voltage ***. What exactly does this mean and what does it mean exactly when the motors are listed with a voltage rating? Most of the motors on that website work with a rated voltage of < 12V. I thought I read somewhere that the more voltage applied to a stepper the faster the rpm's. Since motors are just big inductors, I assume that if I up the voltage then the current required would also reduce since the resistance of the coil won't change, but what about its reactance? I have always been rather thick headed when it comes to inductors and motors so if someone could clear the air a little I would appreciate it. My main concern is I bought a few drivers for the steppers and they have a max of 2.5A, most of the motors on that website are easily over that rating. The way the driver documentation puts it, 5A and up to 55VDC should handle any motor on the market up to NEMA 34

 

[BobK]

I am not an expert on this, but I think what they mean by the max voltage and current is what you would apply to hold the motor in position. If you are actually spinning the motor, the applied voltage would be much higher because of inductance and back EMF.

A stepper is not run by applying a constant voltage, you need to go thorugh 2 or more "phases", i.e. activate different coils, possibly alternating the polarity, to make it move one step, and one full rotation is many steps. To run continously you have to cycle through the phases at a rate depending on how many RPMs you want.

When you apply a voltage to an inductor, the current does not immediately go to a value, it builds up at a rate of:

dI/dt = V / L

So if you apply 1 volt to a 100mH inductor, the current would increase at a rate of 10A / second. So it would take 1/10 sec to reach 1 amp even if there was no resistance.

And as the rotor moves, it generates a voltage itself which is opposite to the voltage you apply to move it. The faster it is moving the higher this voltage is. So the actual voltage across the inductor becomes the applied voltage minus the back EMF.

Hopes this helps.

Bob

 

[jackorocko]

So the actual voltage across the inductor becomes the applied voltage minus the back EMF.

so is that then the rated voltage? How much voltage can I safely apply across the coil then? It just seems odd to me that most stepper drivers output higher voltages with less current. Why would the stepper motors be rated the opposite way. I think that is where I am confused the most at.

 

[Resqueline]

The way I see it is that Rated Voltage has to do with resistance & heating, whereas Max Applied Voltage has to do with insulation.
At DC (low speed) both will have to be the same, but as speed is increased the MAV can (& have to) increase over RV in order to attain max speed & torque.
RV & resistance translates to a current (which you want to keep constant - to keep torque constant - as speed increases).
So due to the inductance the MAV has to increase as frequency increases, but there's a limit to what the winding insulation can take.

 

[jackorocko]

So due to the inductance the MAV has to increase as frequency increases

So then that would concur with the other statement that I read, as voltage across the coil increases the faster the speed achieved. I know you can set the rpm of these motors, so if current must remain constant then voltage has to increase. Now, I guess the last question I have, is why then don't these steppers have a max voltage rating? There has to be a limit as resqueline has said, so what limit is that? I would assume this is on a motor by motor basis, but none of the datasheets I have looked at list a max voltage. I have only seen max voltage ratings on drivers.

 

[Resqueline]

I knew I had forgotten something..
I guess the winding insulation can take so much more than any drive circuit so it's normally no issue. I'd guess at a few hundred volts.