Question for Motor Controllers Expert

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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?


Re: Question for Motor Controllers Expert

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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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Re: Question for Motor Controllers Expert says...
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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.

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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

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