BLDC Motor - Operation with Phase Loss

[mramakrishnan]

Hi,
I am using a 3 phase BLDC Motor. The scenario is,
"When anyone of the phases to the BLDC motor fails, the hardware will disable the particular phase. My software should stop firing that phase."
Thereby the motor should run with only 2 phases. Is this practically possible to run a 3 phase motor by just exciting the 2 phases of the motor when the 3rd phase has failed.

 

[m.s.j]

The most common cause of unbalance for three-phase motors is the loss of phase resulting from an open fuse, connector, or conductor. Unbalances in other connected loads can also affect the motor. A voltage unbalance of 3.5 % can produce a 25 % or greater increase in motor temperature. This results primarily from negative sequence produced by the unbalance. This current produces flux in the motor air gap rotating in the opposite direct ion to the actual motor direction. The relative effect is essentially double-frequency current in the rotor. Skin effect results in higher resistance, negative-sequence impedance remains essentially at locked-rotor value. Thus, high current and high resistance compounds the heating effect.

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[mramakrishnan]

I agree with you for your statement. What i want to know is that, when one of the phases connected to the motor becomes Open, is it practically possible to run the motor with the other 2 phases alone.

I am having 6Mosfets which is switched ON and OFF to run the fire the 3 phases to the motor. The pattern would be something like this,
3 Mosfets at the top and botton for the 3 phases,

1. ATOP - BBOT
2. ATOP - CBOT
3. BTOP - CBOT
4. BTOP - ABOT
5. CTOP - ABOT
6. CTOP - BBOT, then back to 1.

This is how commutation is performed in the system. Suppose if Phase A is lost, i will stop the commutation pattern involving the phase A. i.e

I will have to commutate only 2 mosfets,

1. BTOP - CBOT
2. CTOP - BBOT.

In this case, whether the motor will run with firing these two phases alone. Reduced performance of the system is acceptable.

 

[m.s.j]

Starting of three phase induction motor with two phases from zero speed (stop position) is impossible, because average of electromagnetic starting torque in this case is zero. Of course running continuity of motor after one phase loss is possible for short time.