In our last post, we showed that you can get more torque at higher speeds if you operate a motor at a higher voltage. In the example below, the red curve, measured at 70 VDC, provides much better high speed torque than the orange (12V) curve. So why not always use 70 volts? Better still, why not apply 100 volts and really flatten out that curve?
And why stop there? Since we know that torque is directly proportional to current, why not increase the current setting so we can get more torque at low speeds, too? That way, we would never have a reason to buy a bigger motor.
Reality has a way of taking the fun out of things. If you’ve had much experience with step motors, you probably noticed they get warm, or even hot. That’s okay – if your motor is stone cold, you bought too much motor. As long as the outside doesn’t exceed 100 degrees C, the inside will be cool enough to prevent the coils from melting. Actually it’s the insulation on the magnet wire making up the coils that is the weak point. Most step motors are rated Class B, which means the internal temperature is limited to 130°C. The copper itself wouldn’t melt until 1085°C, but if there wasn’t any insulation, the coils wouldn’t be coils because all the turns would act like one big turn. So what causes heating in a step motor? Three things: Coil Resistance The magnet wire has resistance. That generates heat that is proportional to the resistance of the coils and the square of the applied current. So it you wanted to get twice as much torque by doubling the current, the motor would get four times as hot. Trust me when I say, “don’t try this at home”. Step motors have a rated current specification for a reason.
Hysteresis When the rotor spins, it causes the magnetic field in the stator steel to reverse each time a rotor tooth passes a stator tooth. That changing field makes the stator steel warm up. We call it hysteresis loss and you get it even if the motor is producing no output power. You can disconnect a step motor from the driver, spin it with another motor, and it will warm up. The faster the motor spins the more hysteresis loss you will get, regardless of applied current or voltage. Eddy Currents To make a motor produce output power, you must drive the stator coils with an alternating current. That alternating current causes an alternating magnetic field in the stator steel, which, in turn, induces an electric current in the steel itself, called an eddy current, as described by Ampere’s Law.
Since the steel has electrical resistance, eddy currents generate heat. That heat increases with motor speed and with applied voltage. The chart below illustrates how the sum of all three types of motor losses is affected by speed and voltage. And this is at the rated current. Increase the voltage and the current setting and you live dangerously.
The wise engineer properly sizes the motor for the load and operates it a safe current setting and appropriate power supply voltage. A good stepper drive user manual from a high quality step motor and drive manufacturer can help you get it right.