A polyphase induction motor of the slip ring or wound rotor type can be used ____________

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Examoogle · Accepted Answer

any of the mentioned

Explanation: A polyphase wound rotor (slip ring) induction motor is highly versatile because its rotor circuit is accessible via external slip rings and brushes. This design allows for the insertion of external resistance to control starting torque or, when driven at speeds other than synchronous, operation as a doubly-fed induction machine for frequency conversion.

Working Principle: The torque $T$ is proportional to the rotor resistance $R_2$ and slip $s$ as $T \propto \frac{s E_2^2 R_2}{R_2^2 + (s X_2)^2}$. By varying the external resistance connected to the slip rings, one can achieve maximum torque at standstill (high starting torque). Conversely, by connecting the rotor to an external AC supply or variable frequency drive, the machine can function as a frequency changer, where the output frequency is related to the mechanical speed and stator supply frequency.

Key Formulas: $T_{st} \propto \frac{R_2}{R_2^2 + X_2^2}$ — starting torque depends on external rotor resistance $R_2$; $f_r = |f_s \pm f_m|$ — rotor frequency relationship for frequency conversion

Key Points: Slip rings allow adding external resistance to the rotor circuit.; Higher starting torque is achieved by increasing initial $R_2$.; Speed control is possible by varying rotor resistance, though it reduces efficiency.; The machine can act as a frequency changer in specialized industrial applications.

Advantages: High starting torque without high starting current.; Smooth speed control capabilities.; Lower peak current compared to squirrel cage induction motors during start.

Disadvantages: Higher cost and maintenance due to brushes and slip rings.; Lower efficiency during speed control operation due to $I^2R$ losses in external resistors.; Less robust than squirrel cage motors.

Applications: Cranes and hoists; Conveyor systems; Compressors requiring high starting torque; Variable frequency drives (VFD) applications

Additional Info: The frequency conversion property is essentially based on the induction machine's ability to act as a frequency changer (or asynchronous generator) when driven at non-synchronous speeds.. Option A is a standard feature; Option B is a specialized utility of the wound rotor architecture.

Answer: C is correct — The wound rotor induction motor's design allows it to perform both high-starting-torque functions and act as a frequency converter due to external rotor access.

Exam Tip: Always remember that inserting resistance in a wound rotor circuit increases starting torque but drastically decreases efficiency; use this technique only for the starting phase!

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