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November 10, 2016

Question and Answers on Motor - 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.

October 31, 2016

Why is it preferable to generate alternating current rather than direct current?

Alternating current can be changed in voltage by means of transformers. This is necessary because to transmit power over any distance it must be at high voltage. Too much power is lost when transmitting at low voltage. Direct current cannot be changed in voltage without first changing it to alternating current and then raising the voltage; the operation must then be reversed at the receiving end.

On an alternator, does the dc part (the poles) or the ac part (the conductors) rotate?

It is immaterial which part rotates. However, the dc field is usually made the rotating part, and the stator is usually the ac part of the device. This is because the dc field excitation can be of relatively low voltage, and it is easier to insulate a rotating part for low voltage than for high voltage. The ac output is usually a much higher voltage, and it is much more practical to insulate the stator for the high voltage. Also, with this arrangement there are
no brushes required on the output side.

If transformers with different electrical characteristics are connected in parallel, what will happen?

They won’t distribute the load equally; one transformer will tend to assume more of the load than the other. This leads to overheating and, in severe cases, the destruction of the transformer(s).

What is an induction regulator?

This device is similar to a booster transformer. It has a primary and a secondary winding, which are wound on separate cores. The primary can be moved in either direction; this is usually done by an electric motor. In turning, the primary bucks or boosts the line voltage, as required. The amount of bucking or boosting is anticipated by the current being drawn by the line.

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