Sunday, July 14, 2013

Why motors have large starting current

Reasons why motors have big starting current or inrush current:

- running motors produce back emf (electromotive force) in their coils

- back emf is caused by magnetic field which resists current (flow of electrons) through the armature winding

- back emf functions like a resistor which prevents current flow

- no back emf exists when motor is not running

- no back emf when motor is at rest

- at start up, resistance is zero (which is a short circuit condition), therefore current is large because no resistance to flow of current

- when a motor is running, the armature windings produce a resisting voltage (pressure) which opposes current flow as long as it is running

- a motor at rest acts like a transformer having secondary winding short circuited because the rotor windings in a squirrel cage induction motor is short circuited (closed circuit)

- starting currents (in-rush currents) for induction motors generally are 8 to 10 times the rated full load current

- a motor at rest has large inertia, therefore larger current is required to start it (this is similar to pushing a cart from stationary position, it's harder to push at the beginning, you need to exert a larger force and power, but as soon as the wheels start rolling, it is easier to push the car once it is moving)

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