Motor Controller Design Considerations

Question

I'm working on developing a speed controller for a motor that is geared to rotate a mass. Currently, the motor in question has a step voltage applied to it and moves the mass at a relatively slow speed. As it has been explained to me, applying a step voltage to a motor would create a large inductive load due to a high value of dv/dt, but as a novice in motors, I have been unable to find any constitutive relationship defining this rule.

Additionally, if I am attempting to ramp from a slow wind-up speed, to a high operating speed, and finally to a slow wind-down speed - what are some things I should take into consideration when designing the controller for this device.

I am definitely a novice in the field of controller design and am just beginning to learn all of this. I will be using labview.

Thanks!

Answer 1

This would be best off asked in electronics.SE. But in short, there are two aspects that would be helpful to consider: DC resistance of the windings, and back-EMF. When the motor starts up (rotational velocity is zero), the current drawn will simply be determined by the DC resistance (If winding inductance is relatively low). As the motor speeds up, it begins to act like a generator; the back-EMF opposes the voltage driving it. This difference reduces to zero with no load when the motor reaches full speed. As long as your circuit can handle the start-up current, there aren't any concerns. Otherwise you'll need to step the voltage up in increments. Torque will need to be considered. A general rule is torque is proportional to current, and voltage proportional to speed, but that is where my knowledge gets hazy. But that is why current is largest when first starting the motor, and why increasing voltage in steps reduces peak current.

It's a vaguely similar process when slowing down but with added complications. If the driving circuit has infinite impedance when it removes power from the motor, the windings act as an inductor with stored energy. And since inductors oppose changes in current, it will create a very high voltage spike (theoretically infinite) to "try" to keep the current flowing: plus the effects of back-EMF. This can damage the circuitry, but for DC motors this is easily solved with diodes.

There are plenty of circuits in Google images to get ideas from. But without more information, that's about all I can say.

Answer 2

Not sure how much this might help, but I recently finished a similar project, designing a fixture to spin a 350g mass at a radius of about 80mm.

I used an arduino, but also created a circuit to switch 12v into the stepper motor using pwm. One of the issues I faced was reading a correct value from the encoder. It turned out that the large current through the motor, ~1 amp, was affecting the signal on my encoder line. I had wires crossing close by each other, so when I designed a new pcb with noise consideration in mind, separating the power from the signal as much as possible, I was able to get it working perfectly.

In terms of the large inductive load from stepping a motor, this is something one of the electrical engineers was telling me might be a concern for my board. One of the solutions which we added was a Schottky Diode across the P-FET, but I am not familiar with why exactly that would help. (mechanical engineering student)