BLDC Generator Back Current Protection

Tony Stewart said:

Starters without adequate acceleration limiting will drawing about 10x the max rated current of the motor based on heat rise.

ESC's must have user-controlled acceleration limits to limit startup torque and current. However, if the load relies on RPM for cooling, a slow start can cause excessive motor thermal rise.

Both the "rectifier" for battery charge control and the ESC for supplying battery power to the motor load must have a 3 phase AC rectifier on the front end and thus blocker reverse current. The ESC must also have diode clamps to the battery with optional braking resistors for deceleration with high inertial loads.

If there are no braking resistors and the battery has high ESR from wear & tear, it can create over-voltage on the battery.

If the ESC or the Battery Charger cause back current and needs protection, I need to know which item is failing to get a schematic diagram or service manual. Each must have 3 ph rectifier bridge with either diodes or dual FET's with the commutation control IC to block reverse current.

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aliarifat794 said:

You can connect a diode in series between the generator output and the ESC input. This diode will allow current to flow from the ESC to the generator but not in the reverse direction.

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rjenkinsgb said:

Any full bridge driver should already have "flywheel" diodes from the outputs back to the supplies; they are essential for proper operation!

In effect, a full wave bridge that directs back EMF to the DC side.
If the internal device bridge is not adequately rated, you need to add a higher power high-speed bridge of suitable ratings.

You cannot add diodes between the ECS and generator! the signal there is AC and both directions of current flow are required.


Typical internal circuit for a three phase power stage; this example shows a split supply, but the centre tap is not essential.



The battery charging side needs regulation as well, you cannot only have a rectifier for that.

A good servo drive could be more suitable to control the motor. Servos typically have inputs for both speed and torque, and can actively brake the motor is the setpoint is lower than the actual speed, which returns power to the DC supply, via the internal diodes.

By switching to torque control once the engine started & with a zero-speed setpoint, the current produced by braking could be regulated with an additional feedback circuit (translating battery voltage to torque command) to provide a stable voltage at the batteries, to charge them.

The servo drive and motor would need to be rated to above the maximum possible engine speed (x motor coupling ratio).

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