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Kumar Nishant asked tell me the working mechanism of speed control of motors ?
tell me the working mechanism of speed control of motors by using the variable voltage variable frequency drives?tell me in detail by using characteristic curves.
And got the following answer:
First some basic points: You are talking about induction motors where the stator is the field. The magnetic field rotates because of the AC supply and using more than one phase. Induction motors are made to suit the mains supply frequency and have a reasonably stable speed related to that with some slippage. For example, 50 Hz is 1500 RPM field and 1440 RPM motor. The motor generates a back EMF by virtue of its rotation speed, and the slip is required so the field that produces this back EMF can be induced in the rotor. At no load this back EMF almost equals the nominal supply so very little current flows. When a load is applied, the motor slows down a little, reducing the back EMF and so increasing the current, speeding the motor so a new balance of current and speed is obtained. The torque is proportional to current so it has increased. The speed is proportional to frequency. The voltage must be sufficient for the motor current to correct for the load according to the back EMF as described. Direction is reversed by reversing connections for one phase. The speed can be changed by varying the frequency. However the voltage needs to be changed at the same time to maintain that back EMF relationship with load and current. A concept called V/Hz is used. This is more or less a constant, and can be obtained from the nameplate V and Hz of the motor. At low frequencies and speeds the voltage is low and so on. The outcome of this is that an induction motor can be speed controlled. The maximum torque is the same for any proper speed, because the maximum current is constant determined by wire size. The power output varies, because the power is proportional to speed multiplied by torque. The VFD itself converts the supply to DC, then generates three separate phases using sine modulated steps, PWM etc. A microcontroller runs this. They are usually programmable for overcurrent, starting, maximum current, maximum torque, speed, and some have built in tacho feedback capability. The second link is a better description.