BLDC Generator Back Current Protection

[Kirubakaran V]

Dear Experts:
Am working on engine driven BLDC generator project. To start the engine am using generator itself as a starter, through Electronic Speed Controller (ESC) a PWM signal is given to start the engine. Once the engine started, the generator starts to produce current, as the starter and load are connected in parallel to the generator, these generated current goes to load as well as to the ESC. This back current from generator spoils the ESC. So, its mandatory to have back current protection between the generator and ESC, which allows the one-way current passage from ESC to Generator and resist the current flow from generator to ESC. Please help me to build back current protector circuit. I have attached the block diagram of the system for your kind reference.

Thank you

 

[Tony Stewart]

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.

 

[aliarifat794]

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.

 

[rjenkinsgb]

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).

 

[Kirubakaran V]

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.

Dear Tony Stewart:
Thank you for your details input,
1) Yes your right, we have ESC acceleration limits.
2) Yes, we have three phase AC rectifier between generator and battery.
3) Because of back current between generator to ESC, ESC keep failing.
Please clarify, by keeping diode between generator to ESC will heat up the diode because of back current. if possible please recommend suitable diode for this setup.
Have a great day..!
thank you

 

[Kirubakaran V]

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.

Dear Aliarifat,
Thank you for your input.
Please clarify, if we keep diode in series between the generator output and the ESC input, on back current those diode will heat up?
Have a great day..!
Thank you

 

[Kirubakaran V]

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).

Dear rjenkinsgb:
Thank you for your detailed input,
if possible detail the circuit schematic,
Have a great day,
Thank you

 

[Tony Stewart]

I need to know which p/n is failing with a schematic diagram or service manual.

All diodes block current in reverse within specs.