# Confused about peak torque of BLDC vs PMSM

Discussion in 'Electronics Resources' started by CircuitZord, May 19, 2014.

1. ### CircuitZord Thread Starter Member

Oct 8, 2012
59
2
I will keep it short and sweet.

BLDC (Brushless DC Motor) Vs PMSM (Permanent Magnet Synchronous Motor)

They are similar except for the way the windings are constructed such that PMSM provides a sinusoidal back-emf whereas BLDC a trapezoidal back-emf.

In general literature, as well as marketing it says that PMSM produces higher ripple free torque compared to BLDC. However looking at the equations:

[PMSM]
Torque = (3/2)*Ke*Ip

[BLDC]
Torque = 2*Ke*Ip

Where Ke is the sum of the back-emfs of each phase divided by the ω, the angular velocity of the rotor I'm guessing.

So Torque in the PMSM motor is 1.5 times the Ke*Ip and in BLDC it is twice.

2. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
What makes you think that Ke is the same for both motors?
Peak torque will occur at startup and very low speed. The running torque of a motor decreases with increasing speed.

3. ### CircuitZord Thread Starter Member

Oct 8, 2012
59
2
OK cool, that was my guess too but I wasn't sure as I didn't have any hard data to work with, just the derived equations.

You are right, and of course depending on the modulation strategy used in PMSM for example, the back-emf voltages could be higher or lower.

Thanks!