In modern lift, which type of motor is used?

Question

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Option D: A and B both

Explanation & Concepts:
Modern lifts commonly utilize 3-phase induction motors because they are robust, require minimal maintenance, and offer high efficiency. Slip-ring (wound rotor) induction motors are specifically chosen when high starting torque and speed control are required, whereas squirrel cage induction motors are used for standard applications.

Working Principle: The induction motor operates on the principle of electromagnetic induction where a rotating magnetic field in the stator induces a current in the rotor. In slip-ring motors, external resistors are connected to the rotor circuit via slip rings to increase starting torque and limit starting current, which is critical for smooth lift acceleration.

Key Formulas: $T_s \propto \frac{s E_2^2 R_2}{R_2^2 + (s X_2)^2}$ — starting torque formula for an induction motor; $N_s = \frac{120 f}{P}$ — synchronous speed equation where f is frequency and P is poles

Key Points: 3-phase induction motors are preferred for their simple and rugged construction.; Slip-ring induction motors allow for external resistance control to achieve high starting torque.; Modern high-speed elevators often use Variable Voltage Variable Frequency (VVVF) drives with squirrel cage motors.; DC series motors were used in older systems but are largely replaced due to high maintenance of the commutator and brushes.

Advantages: High reliability and longevity; Easy speed control using electronic drives; High starting torque capability (slip-ring type)

Disadvantages: Requires complex drive electronics for smooth operation; Slip-ring variants require periodic maintenance of brushes and slip rings

Applications: Passenger elevators; Freight lifts; Industrial hoists

Additional Info: Option C (DC series motor) is outdated for modern passenger lifts due to the high maintenance needs of the brush-commutator assembly.. VVVF drive systems allow squirrel cage motors to match the performance formerly associated only with DC or slip-ring motors.

Answer: D is correct — Both standard 3-phase induction motors and slip-ring induction motors are used in various types of modern lift systems based on torque and control requirements.

Exam Tip: Always relate lift motor requirements to the necessary starting torque and the efficiency of the speed control method employed.

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Option A: 3 phase induction motor

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Option B: 3 phase slip-ring motor

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Option C: DC series motor

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