06-02-2013, 04:11 PM
Motor Drives
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Electric Motor Drives
•DEFINITION: Electric drives for motor is used to draw electrical energy from the mains and supply the electrical energy to the motor at whatever voltage, current and frequency necessary to achieve the desired mechanical output.
DC Motor Drives
•Advantages of DC motor
–Ease of control
–Deliver high starting torque
–Near-linear performance
•Disadvantages:
–High maintenance
–Large and expensive (compared to induction motor)
–Not suitable for high-speed operation due to commutator and brushes
–Not suitable in explosive or very clean environment
•DC motor drives:
–The drive is relatively simple and cheap (compared to induction motor drives. But DC motor itself is more expensive
–Due to the numerous disadvantages of DC motor, it is getting less popular
Operation
•When a separately excited motor is excited by a field current of ifand an armature current of iaflows in the circuit, the motor develops a back emfand a torque to balance the load torque at a particular speed.
•The ifis independent of the ia.Each windings are supplied separately. Any change in the armature current has no effect on the field current.
Torque and speed control
•From the derivation, several important facts can be deduced for steady-state operation of DC motor.
•For a fixed field current, or flux (If) , the torque demand can be satisfied by varying the armature current (Ia).
•The motor speed can be varied by:
–controlling Va(voltage control)
–controlling Vf(field control)
•These observations leads to the application of variable DC voltage to control the speed and torque of DC motor.
DC Drive types
•For low cost, low power applications (up to about 10kW) a single phase rectifiers can be used.Low-power, economical drives can also be constructed using single phase half-wave rectifier with free-wheeling diodes.
•For higher power drives (up to MW range), three-phase supply with three-phase rectifier is normally employed.
•For low to medium power DC supplied drives (such as battery), a chopper (DC-DC converter) is used.
•Is is also common to find in some applications (especially locomotives), choppers are used in conjunction with uncontrolled bridge rectifiers. They are normally rated at medium power (100s of kW)