TPC 208 Three-Phase Systems

Specifically, the core, the conductors, the end rings, and the shaft are the main parts of the squirrel-cage motor rotor stator yoke

rotor

Plants where considerable moisture exists often select a 240 V distribution system because it is considered ____________ than a 480 V system. easier to insulate easier to work with more robust safer to use

safer to use

If an induction motor ran at synchronous speed, it would develop zero torque because of the lack of control end play power slip

slip

Wound-rotor motors are used most often in applications that require high starting torque and low starting current, where the driven machine starts frequently infrequently quickly slowly

slowly

A synchronous motor using an amortisseur winding for starting is electrically the same as a ____________ motor. servo-based squirrel-cage induction universal variable-speed dc

squirrel-cage induction

40°C is the maximum temperature at which bearings may be safely operated standard temperature for measuring insulation resistance temperature at which oil oxidizes temperature at which solvent evaporates

standard temperature for measuring insulation resistance

An alternator designed to be driven directly by a low-speed prime mover must have many poles to generate power at 50 or 60 Hz. Alternators directly connected to ____________ often have just two or four poles. diesel engines steam engines steam turbines water turbines

steam turbines

Slipping a pole causes a(n) interruption of current in the ac line reduction of current in the dc line reduction of current in the ac line surge of current in the dc line

surge of current in the dc line

The speed at which the magnetic field of an induction motor rotates is called the frequency power slip speed synchronous speed

synchronous speed

Although alternators can be built with any number of phases, there is little advantage in using more than one two three six

three

High-resistance grounding is used where the continuity of service is vital, even when a ground fault occurs. High-resistance grounding limits the ground current to usually well below 0.1% of the ____________ fault current. four-way single-phase three-phase two-wire

three-phase

A completely automatic auxiliary generator set consists of an automatic ____________ with control relays, automatic engine starting controls, and a generator set with appropriate starting equipment. governor multiconductor transfer switch wattmeter

transfer switch

When considering power factor, remember that the induction motor is electrically equivalent to a ____________ with a short-circuited secondary winding. servo system transformer universal motor wound rotor

transformer

A four-pole, three-phase motor has _____ coils. three four eight twelve

twelve

When removing abrasive dust and particles from the windings of a large motor, a ____________ works better than compressed air. good hand towel spray bottle of kerosene vacuum cleaner wet sponge

vacuum cleaner

When you reconnect the coils of an induction motor designed to run on a potential difference of 240 V so that it will run on 480 V, the insulation is not a concern the potential across each coil is 240 V you reconnect the coils in parallel you reconnect the coils in series

you reconnect the coils in series

In a squirrel-cage induction motor, motor conductor bars inserted in the slots of the rotor core are not insulated from the core because the voltage across each bar is seldom more than ________ V. 0.1 1 10 50

10

A motor rated at 2 hp at full load can develop __________ hp if it delivers its maximum torque. 2 3 to 4.5 4 to 5.5 5 to 6

3 to 4.5

When using compressed air to blow out dust from motor windings, the pressure at the nozzle tip must be less than ________ psi. 20 30 50 90

30

Low-voltage distribution systems are usually classified as ____________ V or less. 240 480 600 4160

600

In most automatic installations, power failure relays act if the potential difference drops to __________ of the rated value. 90% or less 80% or less 70% or less 60% or less

70% or less

In automatic transfer system installations that back up critical electronic gear, action is initiated when the potential difference of any phase drops near ____________%. 50 70 90 99.7

90

In which of the following motors does torque vary inversely with speed? Constant horsepower Constant torque Variable horsepower Variable torque

Constant horsepower

Which of the following is a disadvantage of the Y-delta starter? It produces low starting torque It provides closed transition from starting to running It requires an external transformer It uses a complex motor connection

NOT It requires an external transformer Course 208, Lesson 6; Textbook Reference: 6.55 CBT Reference: Types of Starters

There is no current in the armature of an alternator when the armature is connected to the load field is excited regulator is disconnected rotor is turning

NOT field is excited Course 208, Lesson 8; Textbook Reference: 8.57 CBT Reference: Effect of Current in the Armature

If the load driven by an alternator is purely ____________, the current is in phase with the potential difference. capacitive inductive resistive resonant

NOT inductive Course 208, Lesson 8; Textbook Reference: 8.74 CBT Reference: Load Characteristics and Effects

Jogging, also called ____________, is a series of brief starts that enables a motor to provide small movements of the driven equipment. inching kneeing pausing start-stop

NOT start-stop Course 208, Lesson 7; Textbook Reference: 7.36 CBT Reference: Jogging

When interlocks are used in motor-starting circuits to overcome voltage-drop problems, which motor in the circuit is started first? The largest The smallest The last in the line on the circuit The first in line on the circuit

The largest

When dc power is applied to the rotor of a synchronous motor, a rotating magnetic field is produced a set of north and south poles is produced the motor acts as a squirrel-cage motor torque is produced

a set of north and south poles is produced

Because the rotor of a synchronous motor does not depend on induction to produce current, the ____________ can be larger than in an induction motor. air gap armature brushes rotor

air gap

When taking air-gap measurements on large motors, you should keep a record of the measurements and compare them regularly take measurements, about 90° apart take one measurement in the direction in which the load bears on the bearing all of the above

all of the above

Reduced-voltage starters for three-phase motors change the potential difference across the motor by means of autotransformers or resistors capacitors or autotransformers overload relays or capacitors resistors or overload relays

autotransformers or resistors

A synchronous motor requires only an ac power source only a dc power source both ac and dc power none of the above

both ac and dc power

Single-phase operation of three-phase induction motors on older circuits has caused many motor burnouts circuit-breaker trips fusing dilemmas restarts

burnouts

The supply winding in an alternator can be part of either the rotor or the stator is always part of the rotor is always part of the stator is part of neither the stator nor the rotor

can be part of either the rotor or the stator

Excessive greasing of motor bearings can cause grease to be forced into the windings, as well as ____________ of the grease. churning drying lumping thickening

churning

The two kinds of multispeed induction motors are the multiple-winding motor and the ____________ motor. capacitor-start consequent-pole polyphase six-pole

consequent-pole

In a dual-voltage motor, the potential difference across one phase in a Y-connected winding equals the potential difference across the line divided by 3 divided by the square root of 3 multiplied by 3 multiplied by the square root of 3

divided by the square root of 3

A variable-torque motor produces torque that ____________ and power output that ____________ when the speed doubles. doubles; doubles doubles; quadruples is halved; doubles is halved; remains the same

doubles; quadruples

A motor with a single winding can be connected for four-pole operation only either four-pole or eight-pole operation either six-pole or 12-pole operation four-pole, six-pole, eight-pole, or 12-pole operation

either four-pole or eight-pole operation

The permanent joining of motor frames, conduits, control boxes, and other metal parts not intended to carry current in normal operations is known as equipment bonding equipment grounding short circuiting system grounding

equipment bonding

A relay with inverse-time characteristics operates by induction only faster as the current increases for a specific time after the current reaches a certain value without intentional time delay

faster as the current increases

Induction motors can be connected directly across the line on starting. However, the starting current may be ____________ the rated (running) current. half double four to nine times fifteen times

four to nine times

A synchronous motor is unique in that its speed is controlled by the ____________ of the power supply and is not affected by variations in the ____________ on the motor. frequency; load frequency; torque power factor; load power factor; torque

frequency; load

The frequency of a generator set is controlled by a ____________, which controls engine speed. diac and SCR exciter governor throttle

governor

Certain kinds of steel and brass make excellent long-life slip rings if the metals are burnished lightly coated with Teflon heated slowly highly polished

highly polished

The ____________ motor gets its name from the fact that the rotor is not connected electrically to the source of power. armature induction slipping-pole synchronous

induction

Increasing the load on a motor beyond its rated capacity increases the motor's operating temperature and shortens the life of the fuses insulation overloads terminations

insulation

In a squirrel-cage motor, low starting torque is a result of a high power factor equal resistance and reactance high resistance and low reactance low resistance and high reactance

low resistance and high reactance

Which of the following stops a motor when the potential difference falls too low and automatically restarts the motor when the potential difference returns to normal? low-voltage release low-voltage protection mechanical interlock plugging

low-voltage release

To ensure that three-phase motor control equipment is in good operating condition, you should file the contacts to smooth them lubricate the contacts regularly lubricate the toggle switches regularly maintain a regular plan of inspections

maintain a regular plan of inspections

200°F (95°C) is the maximum temperature at which bearings may be safely operated standard temperature for measuring insulation resistance temperature at which oil oxidizes temperature at which solvent evaporates

maximum temperature at which bearings may be safely operated

On a magnetic across-the-line starter, the incoming power lines are connected to terminals L1, L2, and L3 on the top of the motor starter, and the ____________ leads are connected to terminals T1, T2, and T3 on the bottom of the motor starter. contactor motor overload relay transformer

motor

A lack of torque, a decrease in speed, and growling noises are all indications of a(n) high-resistance point in the rotor circuit open rotor circuit overheated end connection shorted rotor circuit

open rotor circuit

During plugging, the current in an induction motor is higher than the locked-rotor current, causing a(n) increase in power power loss reversal in the rotation of the motor shaft speed up in the rotation of the motor shaft

power loss

In establishing a schedule for inspecting generators, you should match the schedule to the maintenance schedule of the older plant equipment prime mover rest of the maintenance staff voltage regulator

prime mover

An inductor connected in series with the motor winding is found in a(n) autotransformer starter reactor starter resistance starter Y-delta starter

reactor starter

Reduced potential difference in a primary-resistance starter is obtained by connecting ____________ in series with each stator lead, then shunting them out of the circuit at high speed. capacitors overload relays resistors slip rings

resistors

The main difference between low-voltage release and low-voltage protection concerns how the motor is restarted reversed stopped transitioned

restarted