Should I keep the notation of PLECS documentation in PMSG?

Question

Hello, I'm currently designing a small scale Wind turbine using a direct drive with a permanent magnet syncronous generator (no gearbox) and the mechanical equation that's in the PLECS documentation is:

• d/dt (w_m) = 1/J (T_e - Fw_m - T_m)       (1)

where F is friction, w_m is the mechanical speed and, the electromagnetic torque (T_e) is positive and the mechanical torque (T_m) negative. In the literature I've read this equation is used with T_e negative and T_m positive, since it is used as a generator. 

• d/dt (w_m) = 1/J (T_m - Fw_m - T_e)       (2)

My question is it okay to change this terms and using the equation with the rest of my system? or should I keep just as PLECS documentation? is it worth to mention that in Mathcad 15 i used (2) with my calculations and the results i get are correct but the moment i go to simulate with PLECS the system just doesn't work. Thanks for reading this far. 

Answer 1

Renzo, which equation one chooses depends on the conventions used (motor vs. generator).  The conventions are somewhat arbitrary, but should be consistent in your model - does a positive T_e increase the speed or decrease the speed? One can convert between the conventions if needed.

It also depends on how one is sensing the torque and speed in the model.  For example, if you were to probe the PMSM then T_e will align with convention (1).  However, if you have an external torque source/sensor then it depends on the orientation of the source/sensor.