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Hi,

We are doing a 400W  two-phase BLDC motor drive from an inverter Bridge.

The inverter is supplied by a 28VDC rail.

We have a back-to-back FET switch in the connection wire from the inverter to the BLDC motor.

Our contractor put this in.

The idea is that we simply switch this switch off in the event that the motor and inverter both go short circuit.

However, this would obviously be breaking an inductive current so I believe you would agree

that this is a bad idea?

I believe that this back to back FET switch should really be instead placed in the 28VDC rail supply line, just upstream of the 28VDC rail capacitor.

Would you agree?

Also, I believe it should be back-to-back IGBTs (with added diodes) rather than done with FETs ..because  FETs are too delicate to break

a short circuit current?

Also, our contractor has implemented the inverter with no current sensing or  limiting in the inverter bridge. This surely is

not the way to go? I have never heard of a BLDC inverter with no current sensing and thus no current limitation?

I cannot really give too much details of the application unfortunately. However, the motor will need to turn a handle

at certain times, and keep the handle in place, and quite a bit of torque will be required for this keeping the handle in place.

Everything that our  contractor has done is making me concerned because when the motor is holding the handle open, the motor would be static, and so would surely need to  implement

that algorithm which allows a motor to give maximum torque when static (not rotating). I dont see any of this algorithm in our contractors work.

(The control algorithm for BLDCs that allows for max torque at zero speed is called Field Oriented Control)

Also, i thought that  Field Oriented Control requires a 3 phase inverter, and our 4FET inverter (two phase BLDC drive) will surely not be enough to implement the algorithm for max torque at zero speed?

Also, its only 28VDC input to the inverter, but the hi side FETs dont have protection zeners from gate to source. Surely  this is not a good idea?

I mean, the customer has requested 100V FETs so there must be transients etc going on at the 28VDC rail?

What are your thoughts?