The first topic is Volts per Hertz (V/Hz). Many Variable Speed Drives (VSD’s) are called V/Hz drives. They may also be called scalar drives. In general these scalar or V/Hz drives are unsuitable for web handling applications, but are a starting point for the discussion of more capable drives.
AC induction motors are made with wire coils in the stator and conducting bars in the shape of a “Squirrel Cage” in the rotor. These coils and conductors have inductance and resistance. At AC frequencies, the inductance appears as reactance (similar to resistance). Thus the total reactance of the motor is a combination of the resistance and inductance of the stator and rotor. At low frequencies, the inductance decreases, lowering the total reactance and permitting more current to flow. In fact, with zero frequency (Direct Current), the reactance of the inductors is zero.
Current flowing through the resistance of a motor results in electrical losses (heat).
Because reactance decreases at low frequencies, current increases as does the heating. A very practical method of keeping the heating in check at low frequencies is to reduce the voltage with frequency. It is impractical to reduce the current to the motor at low frequencies.
Another way of looking at the requirement for Volts per Hertz is to consider the power a motor produces. Power is a scalar value in that calculations involving power use magnitudes only. Scalar calculations are simpler than vector equations which are concerned with the direction of the forces. The mechanical and electrical power in a motor must be equal. I will present a simplified equation equating mechanical and electrical power which ignores units for the mechanical power and ignores 3 phases and phase angle for the electrical power.
Power = RPM * Torque = Volts * Amps
Amps produce torque and we need torque even at low speeds. Therefore at low RPM (frequency), we need to reduce the volts to keep the mechanical and electrical power equal.
V/Hz applies to all ac motors and drives!