Rod Hower wrote: > > Victor, > And your point is? > A shunt motor control can achieve the speed torque > range of an AC motor, although I would think the AC system > is more efficient in this respect. I have designed brush DC, Sep-ex, > compound wound, AC induction, BLDC systems, so I am familiar with > the speed torque curves of all systems. My only point is the shunt > system can perform similar to the AC system (although at lower RPM to > a well designed AC motor that does not have a commutator). > As Lee has pointed out before, the operation of each motor is similar > although commutation of the DC system is mechanical while the AC motor > is electronic (the inverter controls V/F). > I'll admit I am a fan of AC systems, but the shunt motor is comparible > in performance if designed properly. > If I where to choose between the two I would definately take the AC system. > Rod
Rod, I'm not debating advantages or disadvantages of DC vs AC. You sure can get similar characteristics from SepEx motor by playing with the field. Since it was stated that the voltage defines top speed, my point was to highlight the difference: for DC motor it's enough to apply a voltage and it starts running (characteristics aside). In general - under some load - the more voltage, the faster it runs. In case of AC it's not enough to apply AC voltage it has to be > 0 Hz frequency as well. Also increasing [AC] voltage will not make AC motor run faster. *You* need to take care of frequency increase to achieve that. This is different from DC motors where the frequency converter (commutator) is part of the rotor and you have no control over that (cannot control frequency and voltage separately as you can for AC motors. High voltage and low frequency makes the motor "stiff", spinning with some RPM regardless of the load (almost like stepper motor), while lower voltage will make it "softer". You cannot do that with DC motor - higher load will inevitably lower its RPM. (closed loop controls aside here). Again, it's not better vs worse or choosing one over the other, it's to highlight the differences for a newbie. Victor Victor
