Deck 10: Motor Installation

Question 1

What should be done if a dual-element time-delay fuse will not let the motor start?

Answer 1

If for some reason this fuse will not permit the motor to start, it is stated that the rating of a dual-element time-delay fuse may be increased to a maximum of 225 percent of the full-load motor current. It should be noted that since this is a maximum value, the fuse size cannot be increased to the next higher value, but must be reduced to the closest standard value without exceeding the maximum value. For example, for a 248 ampere motor, 248 amperes * 2.25 (225%) = 558 amperes. In this case, a 500-ampere fuse would be employed.

Question 2

The starting current of a squirrel-cage motor can be determined by multiplying the kVA rating by the ____ rating and then dividing by the applied voltage.

A) nameplate
B) horsepower
C) locked-rotor
D) resistance

Answer 2

B

Question 3

Most common squirrel-cage motors used in industry fall into the design ____ classification.

A) A
B) B
C) C
D) D

Answer 3

B

Question 4

The temperature ratings of devices such as circuit breakers, fuses, and terminals are found ____.

A) on the nameplate
B) in the NEC tables
C) in UL product directories
D) through individual testing

Question 5

Two-phase motors that are especially designed for ____ applications use the same rules for determining full-load currents as single-phase motors.

A) high speed-high torque
B) low speed-high torque
C) high speed-low torque
D) low speed-low torque

Question 6

A 150-horsepower, three-phase squirrel-cage induction motor is connected to 440 volts. The motor nameplate lists the following information: Amps 175 SF 1.25  NEMA code B The conductors are to be copper with type THHN insulation. The short-circuit protective device is to be an inverse-time circuit breaker. The termination temperature rating is not known. Determine the conductor size and overload size.

Question 7

What is the relationship between the rotor bars of a squirrel-cage motor and the starting current?

Question 8

The voltages of a two-phase system are ____ degrees out of phase with each other.

A) 90
B) 120
C) 150
D) 180

Question 9

Most motor circuits are protected by dual-element time-delay fuses or ____.

A) dual-time circuit breakers
B) instantaneous trip circuit breakers
C) nontime delay fuses
D) inverse-time circuit breakers

Question 10

Which NEC table lists the running currents for single-phase ac motors?

A) 430.247
B) 430.248
C) 430.249
D) 430.250

Question 11

What is included in the NEC motor tables, and how is this information used?

Question 12

What percentage of the full-load current of a thermally protected continuous-duty motor of more than one hp can the trip current be, if the full-load current is 15 amperes?

A) 122
B) 140
C) 156
D) 170

Question 13

When a two-phase, three-wire system is used, the size of the neutral conductor must be increased by ____.

A) the square root of two
B) two
C) the square root of three
D) three

Question 14

How does temperature rating impact conductor size selection?

Question 15

A one-horsepower motor will have a full-load current of ____ amperes when connected to 90 volts dc.

A) 6.8
B) 9.5
C) 9.6
D) 12.2

Question 16

____ motors are often made to have a leading power factor by overexcitation of the rotor current.

A) Unity
B) Squirrel-cage
C) Wound-rotor
D) Synchronous

Question 17

A 100-horsepower, three-phase squirrel-cage induction motor is connected to a 240-volt line. A dual-element time-delay fuse is to be used as the short-circuit protective device. What is the correct fuse rating?

A) 300
B) 350
C) 400
D) 450

Question 18

The motor overload size is to be determined by using the current listed ____.

A) on the nameplate
B) in the NEC tables
C) in the UL tables
D) on the stator

Question 19

A 25-horsepower, three-phase induction motor has a nameplate rating of 32 amperes. The nameplate also shows a temperature rise of 30 degrees C. What is the overload size for this motor in amperes?

A) 20
B) 30
C) 40
D) 50

Question 20

If a synchronous-type motor is operated at 90 percent power factor, the rated full-load current in the table is to be ____.

A) decreased by 90%
B) decreased by 10%
C) increased by 10%
D) increased by 90%