gspir:
but this theoritical analysis cannot asnwer if a market PWM speed driver affect the torque or not . My first question in this topic is still unanswered i think.
Sometimes the answer to a question is "it depends". Say I ask you a very simple question: "Does the power dissipation in a resistor increase or decrease as the resistance value increases?"
If you answer an unequivocal "it increases" you are wrong, and similarly if you answer "it decreases" you are also wrong. Without seeing the whole circuit, the only correct answer is "It depends. Show me the circuit".
In some sense both of the things that you had been told (prior to asking here) about the effect of PWM on torque are true. I tried to summarize this in my first reply #2 above, but let me repeat it.
1. For any given operating conditions, if you reduce the voltage (for example by using PWM) you will reduce the current and hence the torque.
2. HOWEVER, as the speed is reduced you need less voltage to produce the same current (and hence torque).
Perhaps it will help to give a concrete example.
Say I had a DC motor operating with a steady 48 volts DC, and it was running at 2000 RPM with a 10 amp draw and had back EMF of 40 volts.
At this operating point the output power is approx 400 Watts, the torque approx 1.9 Nm and we can deduce that the overall series resistance (windings brushes etc) is 0.8 ohms. Further, lets assume that the load torque is constant (with speed) just to keep things simple.
Now say that I suddenly change from a steady 48 DC to PWM with 60% duty cycle (though still 48 volts peak) and assume a simple switch and freewheeling diode circuit.
The immediate effect (not exactly instantaneous because of inductance, but relatively quickly) is that the current and torque will decrease. And as the torque is now decreased, the motor will begin to slow down.
As the motor slows down however, it's internally generated voltage (the back EMF) decreases accordingly, and this causes the current to increase again. Eventually it slows to the point where the current rises back to 10 amps and the original torque value is restored.
As 60% duty cycle gives us about 28 volts, and the series drop at 10A is still 8 volts (as per original operation point), we can deduce that the motor will settle at whatever new speed gives only 20 volts back EMF. Normally this would be half the speed for 40 volts (the original operation point), so the motor will slow to 1000 RPM in this example, and still maintain full load torque.
