GARYLAND M/C QUIZ #7

When normal excitation has been achieved, the power factor will be ____ percent.

100

Give an overview of alternator control.

Alternators are often used to control the speed of several induction motors that require the same change in speed, such as motors on a conveyer line. The alternator is turned by a direct current motor or an AC motor coupled to an eddy current clutch. The output frequency of the alternator is determined by the speed of the rotor. The output voltage of the alternator is determined by the amount of DC excitation current applied to the rotor. Since the output voltage must change with a change of frequency, a variable voltage DC supply is used to provide excitation current. Most controls of this type employ some method of sensing alternator speed and make automatic adjustments to the excitation current.

A special-squirrel cage winding, called the ____________________ winding, is used to start a synchronous motor.

Amortisseur

When a generator produces electrical power, it produces ____ torque.

Counter

Mechanical brakes are available in two basic types: drum and

Disc

The alternator is turned by a direct current motor or an AC motor coupled to a(n) ____________________ current clutch.

Eddy

The field contactor contains a DC coil and is energized by the ____________________ current of the rotor.

Excitation

Many small single-phase motors change speed by varying the amount of voltage applied to the motor. This method changes the speed of the rotating magnetic field of the motor.

False

Starting circuits generally employ resistors of a higher wattage value than circuits that are intended for speed control.

False

Synchronous motors are able to operate at the speed of the rotating magnetic field because they are induction motors.

False

Variable frequency drives intended to control motors under 500 horsepower generally use SCRs or GTOs.

False

How does frequency control operate?

Frequency control operates on the principle that the frequency of the induced voltage in the motor secondary (rotor) will decrease as the speed of the rotor increases.

Maximum torque is developed when the magnetic fields of the stator and rotor are ____ with each other.

In Phase

Many transistor controlled variable frequency drives now employ a special type of transistor called an ____ Bipolar Transistor

Insulated Gate

Due to the problem of excessive voltage spikes caused by IGBT drives, some manufacturers produce a motor that is ____________________ rated.

Inverter

In a variable frequency drive, the ____ control sets the maximum speed of the motor.

Maximum hertz

What is phase paper?

Phase paper is insulating paper added to the terminal leads that exit the motor. Inverter rated motors generally have phase paper added to the terminal leads. The high voltage spikes affect the beginning lead of a coil much more than the wire inside the coil. The coil is an inductor that naturally opposes a change of current. Most of the insulation stress caused by high voltage spikes occurs at the beginning of a winding.

____________________ is defined by NEMA as a system of braking in which the motor connections are reversed so that the motor develops a counter torque that acts as a retarding force.

Plugging

The starting current and speed of a wound rotor induction motor is controlled by adding or subtracting the amount of ____________________ connected in the rotor circuit.

Resistance

Single-phase motors that use a centrifugal switch to disconnect the start windings cannot be used with variable voltage control. This limits the type of induction motors to capacitor-start capacitor-run and ____ motors.

Shaded Pole

There are different methods of obtaining a variable AC voltage. One method is with the use of an autotransformer with a ____ tap.

Sliding

The difference between rotor speed and synchronous speed (speed of the rotating magnetic field) is called ____ and is measured as a percentage.

Slip

The wound rotor induction motor is often called the ____________________ ring motor.

Slip

What is a brushless exciter?

The brushless exciter is basically a small three-phase alternator winding and three-phase rectifier located on the shaft of the rotor. One method of supplying excitation current to the rotor is with a brushless exciter. The brushless exciter has an advantage in that there are no brushes or slip rings to wear.

What is the main advantage of an eddy current clutch?

The main advantage of an eddy current clutch is that there is no mechanical connection between the rotor and drum. Since there is no mechanical connection, there is no friction to produce excessive heat and there is no wear as is the case with mechanical clutches.

A(n) ____________________ is a solid-state device similar to a silicon controlled rectifier (SCR), except that it will conduct both the positive and negative portions of a waveform.

Triac

A decrease in frequency produces a decrease in inductive reactance.

True

A direct-current machine can be used as either a motor or generator.

True

A synchronous motor should never be started with excitation applied to the rotor.

True

Magnetic clutches are used in applications where it is desirable to permit a motor to reach full speed before load is applied.

True

The wound rotor motor was the first alternating-current motor that permitted speed control.

True

When plugging, as a general rule, the reversing contactor is of a larger size than the forward contactor because of the increased plugging current.

True

Small synchronous motors generally contain ____ slip rings on the rotor shaft.

Two

One type of alternating current motor that can use variable voltage for speed control is the ____________________ or AC series motor.

Universal

Give an overview of universal motors.

Universal motors are commonly used in devices such as power drills, skill saws, vacuum cleaners, household mixers, and many other appliances. They can generally be recognized by the fact that they contain a commutator and brushes similar to a direct current motor. Universal motors are so named because they can operate on AC or DC voltage.

The most accurate method of plug stopping a motor is with a ____.

Zero Speed Switch

Name three methods that can be used to provide braking for a motor.

• Mechanical brakes • Dynamic braking • Plugging

What are the three factors that determine induced voltage?

• Strength of magnetic field (In this instance, the flux density of the pole pieces) • Length of conductor (Also stated as number of turns of wire. In this instance, it is the number of turns of wire in the armature winding.) • Speed of the cutting action (Armature speed)

What are the three factors that determine the amount of torque developed by a three-phase induction motor?

• Strength of the magnetic field of the stator • Strength of the magnetic field of the rotor • Phase angle difference between rotor and stator flux

How do synchronous motors differ from squirrel-cage and wound rotor induction motors?

• They can operate at synchronous speed. • They operate at a constant speed from no load to full load. Synchronous motors will either operate at synchronous speed or they will stall and stop running. • They can produce a leading power factor. • They are sometimes operated without load to help correct plant power factor. In this mode of operation, they are called synchronous condensers. • The rotor must be excited with an external source of direct current. • They contain a special squirrel cage winding called the amortisseur winding that is used to start the motor.