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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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Which of the following 3-Phase Induction Motor Starter can not control Heavy Starting Current?
Primary resistors (or rheostat) or reactors
Auto-transformer (or auto starter)
Direct-on-line (DOL) starter
Star-delta starter
Direct-on-line (DOL) starter
A Direct-On-Line (DOL) starter connects the 3-phase induction motor directly across the full supply voltage. Because full line voltage is applied at startup, it cannot reduce or control the heavy starting current, which typically reaches 5 to 8 times the full-load current.
A Direct-On-Line (DOL) starter connects the 3-phase induction motor directly across the full supply voltage. Because full line voltage is applied at startup, it cannot reduce or control the heavy starting current, which typically reaches 5 to 8 times the full-load current.
Ist=Isc=3ZstVL — Starting current in a Direct-On-Line (DOL) starter
TflTst=(IflIst)2×sfl — Ratio of starting torque to full-load torque for DOL starting
At the instant of starting, an induction motor acts like a short-circuited transformer because its rotor is stationary (slip s=1). Applying full line voltage directly via a DOL starter results in a severe current surge (Ist=VL/Zst) until the rotor accelerates and builds up back-EMF.
A Direct-On-Line (DOL) starter applies 100% of the line voltage to the motor terminals at standstill.
It causes a heavy starting current surge of 5 to 8 times the full-load rated current.
DOL starters are generally restricted to low-power induction motors (typically up to 5 kW or 7.5 HP) to prevent line voltage dips.
Simple construction, robust, and lowest cost among all starters.
Provides maximum starting torque (Tst) compared to reduced-voltage starting methods.
Easy to operate and maintain.
High starting current surge causes severe voltage dips in the supply network.
Produces high mechanical stress on motor shafts and mechanical drives.
Causes rapid thermal heating of motor windings during frequent starts.
Small 3-phase squirrel cage induction motors (up to 5 kW).
Water pumps, small machine tools, compressors, and blowers where high starting current is permissible.
Option A (Primary resistors/reactors): Reduces starting voltage to Vst=xVL, limiting starting current to xIsc.
Option B (Auto-transformer starter): Uses tapping factor k to reduce starting voltage, limiting supply current to k2Isc.
Option D (Star-delta starter): Reduces phase voltage by a factor of 1/3, reducing starting current from the mains to 1/3 of the DOL starting current.
C is correct — Direct-on-line (DOL) starter applies full mains voltage directly to the motor terminals, making it incapable of limiting or controlling the heavy starting current.
Always remember that starting torque varies as the square of the applied voltage (Tst∝V2). Reduced voltage starters decrease starting current but significantly reduce starting torque.