Pull away voltage (energy) from a DC motor driver board when it does regenerative braking

Question

I have 200 BLDC motor (six-step) driver boards. They work fairly well but, when I tested them on a heavy load (2500 kg), I discovered that they struggle when braking the motor. The PID algorithm can be set to slow down gently (low deceleration), but sometimes (in case of emergency, for example), a bigger deceleration is desirable. When I try a more steep deceleration, the boards get broken.

Investigating the problem, I discovered that the culprit is the voltage raising, due to the regenerative braking of the H-bridge.

The nominal power supply for the board is 48V rectified: a diodes bridge is outside the board, and the rectified voltage is fed to the board, which has a bank of capacitors.

When the motor is rotating at high speed (say, 2500 RPM), and a deceleration is wanted, the PWM for the motor is decreased. This causes a rise in the power supply which, if it is not too high, is tolerated. But if the rate of PWM decreasing is too high (big deceleration), the power supply (Vbus) rises too much, and a few critical components on the board do not tolerate it (for example, the mosfet drivers, and other voltage regulators for +15V and so on).

The electronic engineer proposes to deploy a resistor to dissipate the overvoltage; this resistor would be inserted between Vbus and GND when needed (the CPU has some spare pin to control the resistor insertion, but also an automatic insertion can be done, based on on a threshold, for example Vbus > 50 volts).

I don't like the idea of the resistor, because it would heat onboard, and because I thought what follows: why add a resistor, when I have a 350VA transformer which can dissipate energy much better? If only I could short the rectifier bridge when needed, I could discharge the extra voltage into the transformer. The rectifier bridge has exactly the task of impeding a current return, but in this case, sometimes, it is desirable.

I think it could work: a mosfet or a transistor to short (bypass) the bridge, activated when Vbus trespasses a certain threshold. Normally Vbus stays at 48-49 volts - only when braking it raises: if it stays under 58-60 volts there is no problem; if it raises more, the board gets broken. I implemented a software check: the software monitors Vbus and, if it raises too much when braking, the deceleration is reduced accordingly. The result is that the board does not melt down anymore, but I would like some more braking.

The schematic, in principle, is the following:

^{simulate this circuit – Schematic created using CircuitLab}

(Sorry for the crude schematic, I am not very expert and it took a lot of time...)

Answer 1

Normally Vbus stays at 48-49 volts - only when braking it raises: if it stays under 58-60 volts there is no problem; if it raises more, the board gets broken.

The obvious solution is a clamp that conducts if the voltage tries to go above 50 V. Something like this:-

^{simulate this circuit – Schematic created using CircuitLab}

why add a resistor, when I have a 350VA transformer which can dissipate energy much better? If only I could short the rectifier bridge when needed, I could discharge the extra voltage into the transformer.

You can't do that because the voltage at that point is a mains frequency sine wave going from ~0 V to +50 V. To feed power back into the transformer you would have to convert the DC voltage into AC with the correct voltage and phase.

Answer 2

Transformers are not designed to dissipate energy any more that the board components that are failing. You could put energy back into the AC source, but that requires an inverter, not a rectifier.

The advice that you have already been given is the best advice for this situation. The resistor should not be on the board, it should be external. Rather than switch in the resistor based on a threshold voltage, you could detect the voltage rising above the rectified voltage and switch based on a differential.

Answer 3

OK if the switch is actively controlled synchronous with the incoming AC waveform, (which was not very clear from the question) you could in principle get regenerative braking.

But you need a second switch from DC bus voltage V2 to the other secondary terminal, to make the waveform symmetrical and approximately the "modified sinewave" (three level step, +V, 0,-V) from cheap AC inverters. Otherwise there is a huge net DC component across the transformer secondary, causing magnetisation and saturation problems.

You have to take care with the switch timing; because when the switch is on, you modify V1 which affects your switch off timings.

Depending on power level and regulatory environment, you may not be allowed to do such obscene things to the incoming AC mains waveform : you may need to add "power factor correction", driving the switches with PWM and modifying the duty cycle to output sinusoidal currents on the mains.

If you use power MOSFETs for these two switches, you orient them such that their body diodes form part of the bridge rectifier : the next hack is to use 4 MOSFETs instead of the bridge, and control them to provide synchronous rectificaion eliminating the diode losses.

It goes without saying that any of these approaches transmits 100Hz torque ripple to the motor shaft during braking, which will be noisy and uncomfortable for the passengers.

Given a 3 phase supply you can make this ripple free : at this stage you have something much more complex than the current schematic, and it's pretty standard practice at that point.

Resistive braking is simple and has none of these issues.