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9.6 Rotating Field in Asynchronous Machines

练习题

In a three-phase asynchronous machine, the rotating magnetic field is produced by:

A. A single-phase AC current flowing through the rotor winding

B. Three-phase AC currents displaced by 120° electrical degrees flowing through stator windings displaced by 120° spatial degrees

C. DC currents flowing through the stator windings with sequential switching

D. A permanent magnet mounted on the rotor rotating at synchronous speed

The synchronous speed $n_{1}$ of the rotating field in an asynchronous machine is given by $n_{1} = \frac{f _{1}}{p}$ , where $f_{1}$ is the supply frequency and $p$ is the number of pole pairs. If a machine has 4 poles and is connected to a 50 Hz supply, what is the synchronous speed in revolutions per second (rps)?

A. 50 rps

B. 25 rps

C. 12.5 rps

D. 100 rps

Which of the following factors directly determine the speed of the rotating magnetic field in an asynchronous machine?

A. The supply frequency

f_{1}

B. The number of poles in the stator winding

C. The rotor speed

n_{2}

D. The number of pole pairs

p

The direction of rotation of the magnetic field in a three-phase asynchronous machine can be reversed by interchanging any two of the three supply phases.

In an asynchronous machine, the rotor rotates at exactly the same speed as the rotating magnetic field under normal motor operation.

The speed at which the rotating magnetic field travels around the stator is called the ___ speed.

Explain why the rotor of an asynchronous machine cannot rotate at synchronous speed during normal motor operation.

A three-phase induction motor operates with a slip $s = 0.04$ (4%) when supplied from a 60 Hz source. According to the relationship $f_{s} = s \cdot f_{1}$ , what is the frequency of the rotor currents?

A. 60 Hz

B. 57.6 Hz

C. 2.4 Hz

D. 15 Hz

The stator winding in an asynchronous machine acts as the inductor, meaning it produces the rotating magnetic field that induces currents in the rotor.

Which of the following statements correctly describe the rotating magnetic field in a three-phase asynchronous machine?

A. The rotating field has constant magnitude at all times

B. The rotating field speed depends on the load on the motor shaft

C. The rotating field is produced by the stator winding

D. The rotating field induces voltages and currents in the rotor conductors

In an asynchronous machine, the rotating field is produced by the stator winding and rotates at synchronous speed $n_{1}$ . If the rotor rotates at speed $n_{2}$ , which formula correctly expresses the rotor frequency $f_{s}$ in terms of the number of pole pairs $p$ ?

f_{s} = p \cdot (n_{1} + n_{2})

f_{s} = p \cdot (n_{1} - n_{2})

f_{s} = p \cdot n_{1} \cdot n_{2}

f_{s} = \frac{p \cdot n _{1}}{n _{2}}

Which of the following statements correctly describe the relationship between the rotating field and the structural components of an asynchronous machine?

A. The stator winding acts as the inductor and produces the rotating magnetic field

B. The wound rotor must have the same number of poles as the stator to properly interact with the rotating field

C. The air gap forms part of the magnetic circuit through which the rotating field passes

D. The squirrel cage rotor automatically adapts to have the same number of poles as the stator's rotating field

E. The housing directly influences the speed of the rotating field

In an asynchronous machine, the rotor frequency $f_{s}$ equals the product of slip $s$ and stator frequency $f_{1}$ ( $f_{s} = s \cdot f_{1}$ ) because the rotating field's relative speed with respect to the rotor determines the frequency of induced currents.

A three-phase asynchronous machine has a stator that produces a rotating field at 1500 rpm (25 rps) with 2 pole pairs. If the rotor rotates at 1440 rpm (24 rps), calculate the rotor frequency in Hz. Show your reasoning.

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9.6 Rotating Field in Asynchronous Machines