Question for Motor Controllers Expert

For review and resolution of such problems I normally ask a fee. I will, hopwever, suggest that you need to review your entire power distribution architecture, the reasons for the diodes in these different sub-modules (are they your design or are they bought in?)

many system developers utilise the fact that motors will provide a back voltage when free-running as a useful source of regenerative breaking (slowing the motor slightly and recharging the battery). Any system running from that battery bus needs to be capable of running with the elevated charging voltage without sufering because of it.

You should really do a detailed study of what the operating environment of your system is going to be and design in the necessry means of providing protection for the more sensative components.

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Paul E. Bennett ....................
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Paul E. Bennett
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I am not an expert of motor control and I am hoping that someone with some experience in this area can give me a hand. I have a custom motor controller that drives a dc brushless motor for an underwater vehicle. I am using a 2 quadrant pwm switching scheme. The battery is connected to the motor driver through a diode (in another subsystem) that protects the circuit from reverse current flow.

The motor works fine except that as the RPM rises above I start to see a voltage waveform across the diode. This voltage causes the bus voltage to other subsystem to rise up to 10 Volts, which is a problem. If there were no series diode in the line, the battery charges during these cycles and keeps the bus voltage nice and steady. Unfortunately, this series diode is in another subsystem and has to be there for other reasons.

I believe that a lot of motor controllers put a very large capacitor across the supply to smooth this. I can't do this because this system has to operate in an underwater environment under high pressure. Electrolytics and other large capacitors cannot tolerate pressure.

I don't want to put a zener in to clamp the voltage because it would have to dissipate a lot of power. If I put a series diode near the motor, this would block the voltage from appearing on the bus of the other systems. However, I would be dissipating a lot of power in the diode since the motor has peaks of 10 amps. The system is battery powered and power consumption is critical.

Is there an easy solution to this problem? Can someone point in the right direction? Would a different modulation scheme help?

Thanks, Bob

Reply to
Bob

The capacitors in motor controllers are usually not enough to have much affect on regen (you appear to be referring to the motor running faster than the requested drive voltage). They are there to decouple the controller from the DC power (battery). As such there ability to withstand ripple current is as important as the ability to source and sink current. The ripple current is usually the harder constraint to reach unless you using a slow PWM.

Without these caps you will have quite high voltages at high frequencies to contend with. Your power source will also have a high ripple current demanded from it.

I have heard of a scheme that switched varying numbers of cells to the motor to step up the speed (and cycled the cells used to balance the drain evenly). That would work but it sounds rather bulky and complex.

Robert Adsett

Reply to
R Adsett

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