Question 1

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

Accepted Answer

C

Question 2

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?

Accepted Answer

The trip current for a thermally protected continuous-duty motor of more than one horsepower can be up to 156% of the full-load current, according to the National Electrical Code (NEC) guidelines.

Question 3

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

Accepted Answer

Table 430.248 in the NEC lists the running currents for single-phase AC motors.

Question 4

How does temperature rating impact conductor size selection?

Accepted Answer

The answer of How does temperature rating impact conductor size...

Question 5

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?

Accepted Answer

The answer of A 100-horsepower, three-phase squirrel-cage induction motor is...

Question 6

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

Accepted Answer

Two-phase motors designed for low speed-high torque applications use the same rules for determining full-load currents as single-phase motors because they often operate under similar load conditions and require similar current calculations to ensure proper performance and efficiency.

Question 7

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

Accepted Answer

The voltages of a two-phase system are 90 degrees out of phase with each other.

Question 8

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.

Accepted Answer

The answer of The starting current of a squirrel-cage motor...

Question 9

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.

Accepted Answer

The answer of A 150-horsepower, three-phase squirrel-cage induction motor is...

Question 10

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

Accepted Answer

Synchronous motors can be made to have a leading power factor by overexciting the rotor, which allows them to supply reactive power to the grid.

Question 11

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

Accepted Answer

The motor overload size is determined using the current listed on the nameplate, as it provides the specific current rating for that particular motor.

Question 12

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?

Accepted Answer

The overload protection for a motor is typically set at 125% of the full-load current. For a 32-ampere motor, 125% of 32 is 40 amperes.

Question 13

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

Accepted Answer

In a two-phase, three-wire system, the neutral conductor carries the vector sum of the currents from the two phases. To handle this, the size of the neutral conductor must be increased by the square root of two to accommodate the increased current.

Question 14

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

Accepted Answer

The answer of What should be done if a dual-element...

Question 15

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

Accepted Answer

The answer of What is the relationship between the rotor...

Question 16

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

Accepted Answer

The answer of A one-horsepower motor will have a full-load...

Question 17

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

Accepted Answer

The answer of What is included in the NEC motor...

Question 18

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

Accepted Answer

The answer of Most common squirrel-cage motors used in industry...

Question 19

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

Accepted Answer

Inverse-time circuit breakers are commonly used to protect motor circuits because they provide both short-circuit and overload protection, similar to dual-element time-delay fuses.

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 ____.

Accepted Answer

When a synchronous motor operates at a power factor less than unity, the current increases to maintain the same power output. Therefore, if the power factor is 90%, the full-load current must be increased by 10% to compensate for the lower power factor.

Quiz 10: Motor Installation