正在学习

9.10 Power and Torque in Three-Phase Asynchronous Motor

练习题

In a three-phase asynchronous motor, the air gap power ( $P_{a g}$ ) is 10 kW and the slip is 0.05. What is the rotor copper loss?

A. 500 W

B. 9500 W

C. 200 W

D. 10000 W

Which of the following statements correctly describe the power flow in a three-phase asynchronous motor?

A. Input power equals stator copper loss plus core loss plus air gap power

B. Air gap power equals rotor copper loss plus mechanical power developed

C. Mechanical power developed equals output power plus rotational losses

D. Rotor copper loss equals slip times air gap power

E. Output power is always greater than air gap power

The electromagnetic torque developed in an asynchronous motor is proportional to the square of the rotor current and depends on the rotor power factor.

In a three-phase asynchronous motor, the mechanical power developed ( $P_{m ec h}$ ) can be expressed in terms of air gap power ( $P_{a g}$ ) and slip (s) as: $P_{m ec h} =$ ___ $\times P_{a g}$ .

Explain why the rotor copper losses in an asynchronous motor increase with increasing slip, and what this means for motor efficiency at high slip values.

An asynchronous motor operates with a slip of 0.04. If the air gap power is 25 kW, what is the mechanical power developed?

A. 1 kW

B. 24 kW

C. 25 kW

D. 26 kW

At start-up, when the slip equals 1, the electromagnetic torque developed by an asynchronous motor is zero because there is no relative motion between the rotor and the stator field.

Which of the following factors directly affect the electromagnetic torque produced by a three-phase asynchronous motor?

A. Magnetic flux per pole (

Φ

)

B. Rotor current (

I_{2}

)

C. Rotor power factor (

cos φ_{2}

)

D. Stator supply frequency (

f_{1}

)

In a three-phase asynchronous motor, the electromagnetic torque is produced by the interaction between rotor currents and the magnetic field. According to Laplace's law, if the rotor speed approaches synchronous speed ( $n_{2} \to n_{1}$ ), what happens to the rotor induced EMF and consequently the electromagnetic torque?

A. Rotor EMF increases, torque increases

B. Rotor EMF decreases toward zero, torque decreases toward zero

C. Rotor EMF remains constant, torque remains constant

D. Rotor EMF decreases, but torque increases due to higher speed

At motor start-up ( $n_{2} = 0$ , $s = 1$ ), the rotor frequency equals the stator supply frequency ( $f_{S} = f_{1}$ ), resulting in maximum rotor induced EMF and consequently high starting currents similar to a transformer short-circuit condition.

During no-load operation of an asynchronous motor, the rotor speed is nearly equal to the synchronous speed, causing the slip to approach ___ and the rotor iron losses to be reduced due to low rotor current frequency.

Explain why the rotor of an asynchronous motor rotates in the same direction as the stator rotating field, and describe what happens to the electromagnetic torque when a mechanical load is applied to the shaft.

登录后解锁笔记、知识点解析、AI 问答

立即登录

9.10 Power and Torque in Three-Phase Asynchronous Motor