# About the rated current of stepper motors

Hi All,

I knew it is a good idea to limit the current to the rated current of a stepper motor. I also knew that if I provide too much current to a stepper motor, most energy will become heat instead of mechanical power. However, I'm just wondering. What is the relationship between the rated current and the maximum torque of a stepper motor?

Is the rated current corresponds to the maximum torque already? Is it possible to squeeze more torque by over current a stepper motor (maybe with some heatsinks)? Though over current a stepper motor is not a good idea in general, it can be handy if it can provide more torque quick and dirty. Will this work?

Gary

You are on the right track, however there are two separate issues.

The first issue is that the steady state rated current is the limit at which the motor begins to overheat.

The second issue is that due to the inductance of the coil, it takes some time for the current to build up to any particular value (the time being dependent on the resistance in the circuit, the inductance of the coil and the applied voltage). So, the instant that you first apply voltage to the coil, zero current is flowing. Applying higher voltages gives you more current over a given period of time, and hence more torque, quicker. However, you can't exceed the steady state current limit on average.

To get high torque, use high voltages and a modern chopper motor driver, like the A4988 or DRV8825. Pololu's drivers are cheap, very well engineered and state of the art.

For theory, here is an excellent introduction and probably, more than you ever wanted to know about steppers: Jones on Stepping Motors

jremington:
...
To get high torque, use high voltages and a modern chopper motor driver, like the A4988 or DRV8825. Pololu's drivers are cheap, very well engineered and state of the art.
...

I am using a stepper drive board with A4988 on it already. However, even with A4988, you still need to tell A4988 the current limit, which is normal the rated current. I'm thinking of the current limit of A4988 when I said "current" in the original post. (Sorry for being not careful enough.)

It takes time for anything to heat up, but if on average you exceed the maximum current, you are taking a chance. If the electronics malfunction (e.g. stuck “on”) the motor is toast.

Unlike other electronics it is simply the excess heat that kills a motor.
If you can arrange to over current when the motor is running and drop it down when still you will get more useful torque. The current a motor draws is at its maximum when it is stopped, the faster it goes the less current it draws, that is why the torque drops off at speed..

Grumpy_Mike:
the faster it goes the less current it draws.

I wonder is this true of a stepper motor that is being supplied at a huge over-voltage that is managed by a driver like the A4988? I suspect the driver compensates for the tendency of the back emf to oppose the current.

...R

Robin2:

Grumpy_Mike:
the faster it goes the less current it draws.

I wonder is this true of a stepper motor that is being supplied at a huge over-voltage that is managed by a driver like the A4988? I suspect the driver compensates for the tendency of the back emf to oppose the current.

…R

Yes, a chopper-type driver such as the A4988 does do that - up to a point. The rate at which the driver can change the current in the stepper windings is limited by the supply voltage; so the torque and current will still drop off with increasing speed, but much more slowly than with a a non-chopping driver.