CEP Motor Control & Drive Systems 3

Question 1

What is the primary energy-saving advantage of using a VFD for centrifugal pump and fan applications?

Accepted Answer

Significant energy savings by reducing speed to match actual load demand, since power varies as the cube of speed

Answer

Elimination of all motor maintenance requirements

Answer

Higher starting torque than across-the-line starting for large loads

Answer

Automatic correction of power factor to unity at all loads

Question 2

In motor control terminology, 'jogging' (or 'inching') refers to:

Accepted Answer

Momentarily energizing the motor to move a load by small increments without engaging the run circuit

Answer

Running the motor continuously at a reduced speed set point

Answer

Switching the motor between forward and reverse directions automatically

Answer

Starting the motor at half voltage to reduce inrush current

Question 3

According to NEC Article 430.22, motor branch circuit conductors must be rated at not less than what percentage of the motor's full-load current (FLC)?

Accepted Answer

125% of FLC

Answer

100% of FLC

Answer

115% of FLC

Answer

150% of FLC

Question 4

A motor nameplate shows a service factor (SF) of 1.15. This means the motor can:

Accepted Answer

Handle 115% of its rated horsepower load on a continuous basis without damaging overheating

Answer

Operate continuously at 115% of its rated speed without damage

Answer

Require 15% less maintenance than a motor with SF of 1.0

Answer

Achieve 15% greater efficiency than a standard motor under rated load

Question 5

A 9-lead dual-voltage motor can be reconnected for high-voltage operation by wiring the windings in:

Accepted Answer

Series, increasing the voltage across each winding group

Answer

Parallel, reducing current per winding

Answer

Star (wye) configuration with all leads tied together

Answer

Delta configuration with center taps connected

Question 6

According to NEC Article 430, motors rated over 1 horsepower that are not continuously attended must be provided with:

Accepted Answer

Running overload protection that automatically disconnects the motor if it overheats

Answer

A ground fault circuit interrupter (GFCI) on the branch circuit

Answer

An automatic restarting device after overload trip

Answer

A phase-sequence relay to prevent reverse rotation

Question 7

The locked rotor current (LRC) of a typical NEMA Design B AC induction motor is approximately how many times the motor's full-load current (FLC)?

Accepted Answer

6 to 8 times FLC

Answer

1 to 2 times FLC

Answer

2 to 4 times FLC

Answer

10 to 12 times FLC