Join 60,000+ competitive exam aspirants
or with email
Your Cart
Subtotal₹0
Total₹0
Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
Join 60,000+ competitive exam aspirants
Ratio of starting current to rated current of induction motor is
to 0.8
to 2.5
to 7
1
to 7
The starting current of a 3-phase induction motor is significantly higher than its rated full-load current because the slip is unity (s = 1) at standstill, causing the rotor circuit to act like a short-circuited transformer secondary. In commercial induction motors, this transient inrush current typically ranges from 3 to 7 times the full-load current.
The starting current of a 3-phase induction motor is significantly higher than its rated full-load current because the slip is unity (s = 1) at standstill, causing the rotor circuit to act like a short-circuited transformer secondary. In commercial induction motors, this transient inrush current typically ranges from 3 to 7 times the full-load current.
s=NsNs−Nr — Slip equation
Ist=ZeqV — Starting current based on standstill equivalent impedance
At the instant of switching (starting), the rotor speed is zero, resulting in maximum magnetic flux cutting the rotor bars. This induces high electromotive force (EMF) and high frequency current in the rotor, which is reflected back to the stator side, drawing a large current from the supply. As the motor accelerates, the relative speed between the rotating magnetic field and the rotor decreases, causing the current to drop to its normal rated value.
High starting current is due to the lack of back EMF at the instant of startup.
Induction motors often require starters (Star-Delta, Soft Starter) to limit this current.
The ratio is higher for larger motors and depends on motor design characteristics.
Starting current is independent of the load applied to the shaft.
Simplicity of construction
High reliability and ruggedness
Voltage dips in the power distribution network during startup
Thermal stress on motor windings due to high I2R losses
Industrial fans and pumps
Compressors
Conveyor systems
The starting current is equivalent to the blocked rotor current.
Option A (0.5 to 0.8) is incorrect as this would imply a current less than rated full load.
Option B (1.5 to 2.5) represents the typical current during acceleration or light load conditions.
C is correct — The ratio of the starting current to the rated current for a standard 3-phase induction motor is typically between 3 and 7.
Always remember that during startup, the induction motor behaves like a transformer with a short-circuited secondary, leading to a high magnitude of current.