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Machine Riddles No.32 - Capacitors for PFC and for reducing starting current of induction motors
Is the rule of thumb that the size of capacitor required for PFC of an induction motor is sq rt of 3 x line volts x no load current x 0.8 correct? Otherwise give a more accurate formula. Is a similar formula available for sizing a capacitor for reducing the starting current  of an induction motor?
Author : Sinnadurai Sripadmanaban - From: Srilanka
Wed, January 26th, 2011 - 16:47
The magnetizing current of an induction motor can vary significantly depending on the design. For example, the high efficiency motors operate at lower flux density, and hence the magnetizing current will be less. A capacitor can be used to supply part of the magnetizing current.
Motors are classified by the National Electrical Manufacturers Association (NEMA) as Designs A, B, C, D, F, and wound rotor machines. Design A motors usually have low resistance rotors that provide good running characteristics at the expense of high starting current. A reduced voltage starter may be required for starting this type of motor.
Design B motors have a double cage motor and are used for full voltage starting. They have the same starting torque as Design A, but with only 75% of the starting current of a Design A motor. The applications are the same as Design A. Design B motors are more popular than Design A motors.
Design C motors have a double cage and deep bar construction, with higher rotor resistance than Design B. Design C motors have higher starting torque, but lower efficiency and somewhat greater slip than Design B motors.
These motors are suitable for constant speed loads, requiring fairly high starting torque while drawing relatively low starting current. Typical loads are compressors, conveyors, crushers, and reciprocating pumps.
Design D motors have the highest starting torque of all the designs. They are single cage motors that provide high starting torque but also have high slip with correspondingly lower efficiency. Design D motors are used for high inertia loads such as bulldozers, die-stamping machines, punch press, and shears.
Design F motors are usually high speed drives directly connected to loads that require low starting torques such as fans or centrifugal pumps. The rotor has low resistance, which produces low slip and correspondingly high efficiency but also low starting torque.
Usually the capacitor current should not exceed the motor no-load current. The desirable capacitor ratings for various motors are listed below for example.

Large squirrel-cage motors and industrial synchronous motors draw several times their full load current from the supply during starting. The power factor during the starting is usually in the range of 0.15–0.30 lagging. The actual shape and magnitude of the staring current curve depends on the motor design, the voltage at the motor terminals, and the speed torque characteristic of the mechanical load connected to the motor. The starting current through the system impedances can result in an unacceptable voltage drop that may be large enough to cause contactors to drop out and influence the ability of the motor to start.
Shunt capacitors are sometimes used to reduce the voltage dip when starting a large motor. Their effect is to reduce the reactive component of the input kVA. With this method, the high inductive component of the normal starting current is offset, at least partially, by the addition of capacitors to the motor bus during the starting period. The capacitor size needed for this purpose is usually 2–3 times the motor full load kVA rating. In order to control the voltage properly, the capacitor is usually switched out in steps as the motor accelerates. Due to the large kVAR size of these capacitors, they are usually in the circuit for only a few seconds during motor starting.
Author : Hamid - From: Iran- Firouzabad in Fars
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