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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Starting Line Current & Starting Motor Current is reduced by the factor ____________ & _________ respectively in Auto-transformer starter.
X & X²
X² & X
𝟏/√𝟑 & 𝟏/𝟑
X & X
X² & X
In an auto-transformer starter with a transformation ratio of x (or tapping ratio x), the motor starting voltage is reduced to xVL. Consequently, the starting motor current is reduced by a factor of x, and the starting line current drawn from the supply is reduced by a factor of x2.
In an auto-transformer starter with a transformation ratio of x (or tapping ratio x), the motor starting voltage is reduced to xVL. Consequently, the starting motor current is reduced by a factor of x, and the starting line current drawn from the supply is reduced by a factor of x2.
Ist,m=x⋅Isc — Starting motor current with auto-transformer
Ist,line=x2⋅Isc — Starting line current drawn from supply
Tst=x2⋅Tsc — Starting torque developed by the motor
When an auto-transformer with a tapping fraction x (where 0<x<1) is used, the voltage applied across the motor terminals is Vm=xVL. Since motor starting current is directly proportional to applied voltage (Ist,m=xIsc), the starting current in the motor circuit is reduced by a factor of x. The input power to the auto-transformer equals the output power (neglecting losses), so VLIst,line=(xVL)Ist,m, which yields Ist,line=xIst,m=x2Isc, reducing the starting line current by a factor of x2.
The tapping ratio x is typically set at 50%, 65%, or 80% of line voltage.
The ratio of starting torque to full-load torque is directly proportional to x2.
Auto-transformer starters are suitable for both Star-connected and Delta-connected induction motors.
Flexibility in adjusting starting voltage using different tapping ratios (e.g., 50%, 65%, 80%).
High torque per ampere of line current compared to primary resistor starters.
Expensive and bulky compared to Star-Delta starters.
Requires complex switching arrangement.
Starting large squirrel-cage induction motors where smooth acceleration with restricted line current is required.
Option A (x & x2) reverses the factors for line current and motor current.
Option C (1/3 & 1/3) corresponds to fixed ratios in Star-Delta starters rather than tapping factor x in auto-transformer starters.
Option D (x & x) fails to account for the transformer primary-to-secondary turns ratio affecting line current.
B is correct — The starting line current is reduced by x2 and the starting motor current is reduced by x in an auto-transformer starter.
Always remember that torque and supply line current both scale with the square of the tapping ratio (x2), while the motor winding current scales linearly with x.