Which of the following 3-Phase Induction Motor Starter can not control Heavy Starting Current?

Question

Examoogle · Accepted Answer

Option C: Direct-on-line (DOL) starter

Explanation & Concepts:
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.

Working Principle: 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 ($I_{st} = V_{L} / Z_{st}$) until the rotor accelerates and builds up back-EMF.

Key Formulas: $I_{st} = I_{sc} = \frac{V_L}{\sqrt{3} Z_{st}}$ — Starting current in a Direct-On-Line (DOL) starter; $\frac{T_{st}}{T_{fl}} = \left(\frac{I_{st}}{I_{fl}}ight)^2 	imes s_{fl}$ — Ratio of starting torque to full-load torque for DOL starting

Key Points: 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.

Advantages: Simple construction, robust, and lowest cost among all starters.; Provides maximum starting torque ($T_{st}$) compared to reduced-voltage starting methods.; Easy to operate and maintain.

Disadvantages: 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.

Applications: 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.

Additional Info: Option A (Primary resistors/reactors): Reduces starting voltage to $V_{st} = x V_L$, limiting starting current to $x I_{sc}$.. Option B (Auto-transformer starter): Uses tapping factor $k$ to reduce starting voltage, limiting supply current to $k^2 I_{sc}$.. Option D (Star-delta starter): Reduces phase voltage by a factor of $1/\sqrt{3}$, reducing starting current from the mains to $1/3$ of the DOL starting current.

Answer: 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.

Exam Tip: Always remember that starting torque varies as the square of the applied voltage ($T_{st} \propto V^2$). Reduced voltage starters decrease starting current but significantly reduce starting torque.

Examoogle · Answer

Option A: Primary resistors (or rheostat) or reactors

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Option B: Auto-transformer (or auto starter)

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Option D: Star-delta starter

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