Three Phase BLDC Motor Torque

Discussion in 'General Electronics Chat' started by jasonli1990, Aug 29, 2011.

1. jasonli1990 Thread Starter New Member

Apr 13, 2011
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Hi all

I was a little bit confused about torque and speed for a 3-phase BLDC Motor.

I know that torque is proportional to current and speed is proportional to voltage. Doesn't mean torque is proportional to voltage as well due to the fact that voltage applied to the motor is proportional to current (Ohm's Law)? If that is true, why people don't say torque is proporitonal to voltage?

Cheers

2. strantor AAC Fanatic!

Oct 3, 2010
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If you're talking "no load" then I understand where you are coming from. let's say you could run a DC motor at full speed (full voltage) and draw only 1mA. as soon as you start loading it down though, it is going to start drawing more current, and if it's current you're controlling (I.E. current limiting) then the voltage(speed) will drop to a level that ohm's law equation say it should be for given current limit.
Now, for a BLDC, since it's an AC motor, it's speed is controlled by frequency rather than voltage like in a DC motor. The motor control circuitry senses the speed that it's going, and varies the voltage/current.

3. jasonli1990 Thread Starter New Member

Apr 13, 2011
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Thanks for reply. But I still didn't quite get the torque thing.

Another thing I didn't agree with you is the speed control. I understand that the BLDC is basically like an AC motor. The voltage applied to each winding is actually controlled by the pwm signals that are fed into the half bridge circuits. So from my point of view, the speed of the motor is controlled by varying the applied voltage to the motor (which means varying the duty cycle of the pwm signals).

Please correct me if I misunderstood the whole thing.

Cheers,
Jason

4. t_n_k AAC Fanatic!

Mar 6, 2009
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Like a DC motor the BLDC will still produce a back emf related to its speed. The current flow required to match the combined load torque and combined losses is controlled by the applied voltage but there won't be a direct linear relationship between applied voltage and the current flow. Rather there will be some correspondence between the current flow and the difference between the applied voltage and the motor internal back emf.

5. strantor AAC Fanatic!

Oct 3, 2010
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The way I understand it is that PWM duty cycle % is what sets the average voltage level in a winding during the ON period of that winding. The windings are fired in 3 phases just like a 3phase AC motor, so speed is controlled by frequency and voltage is controlled by PWM. The circuitry would increase PWM duty cycle % in order to increase voltage in the winding to further increase current in the winding (torque) if it sensed an increase in the load.

jasonli1990 likes this.

Apr 13, 2011
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Thanks alot.

Cheers,
Jason