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November 10, 2016

10 Interview Questions and Answers on Motors - Part-2

Q1: What causes a motor to turn?

A1: There are two parts to a motor: a stator, or field, and a rotor, or armature. Around one part there exists a magnetic field from the line current, and in the other part there is an induced current that causes a magnetic field of opposite polarity. These magnetic fields repel one another, thereby causing the rotating member to turn.

Q2. Why would you use a phase splitter and a three-phase motor instead of a single-phase motor?

A2: It is possible that at the moment single-phase power is all that is available, but in the future three-phase power is expected. Therefore, if you purchase the three-phase motor and a phase splitter, the wiring will be in place, the motor will be at the desired location, and the expenses will be cut down. There is also less maintenance on three-phase motors; this one fact will often influence the use of a phase splitter.

Q3. How is regulation obtained in a motor?

A3: The motor at start is similar to a transformer with a shorted secondary. The current in the rotor and stator will be high. As the motor approaches its rated speed, the rotor induces a voltage into the stator in opposition to the line voltage; this is called counter emf. The line current is then reduced in proportion to the speed.

Q4. Does an ac motor (other than a synchronous motor) run at synchronous speed?

A4: No. It must slip below synchronous speed so that an effective voltage will be produced.

Q5. What is a synchronous motor?

A5: A synchronous motor is almost exactly the same as an alternator. The field must be excited by direct current. The motor runs at the same speed or at a fixed multiple of the speed of the alternator supplying the current for its operation. Should it slipthe motor will pull out and stop since it must run pole for with pole the alternator.

Q6. What is a synchronous capacitor?

A6: It is a synchronous motor running without mechanical load on the line, with its field overexcited for power-factor correction.

Q7. If the field on a dc motor were opened, what would happen?

A7: The motor would try to run away with itself, or, in other words, the motor would reach a very high speed and might destroy itself.

Q8. What is a universal motor?

A8: This is a motor built like a series dc motor. However, the stator and armature are both laminated, designed for high speeds, and may be used on either ac or dc, although the speed and power will be greater on dc.

Q9. How can the direction of rotation of a universal motor be changed?

A9: By reversing either the field leads or the armature leads, but not both.

Q10. How can the direction of rotation of a dc motor be changed?

A10: By reversing either the field leads or the armature leads, but not both.

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