Total Material

Question 1

What are harmonics?

Answer 1

Distortions to the voltage and current waveforms from their normal sinusoidal shape

Question 2

Harmonics are similar to electrical noise (T/F)

Answer 2

False. Harmonics are not similar to electrical noise. They are a steady state problem.

Question 3

Harmonics are usually a steady state problem. What does this mean?

Answer 3

If harmonics are present, then they are always present.

Question 4

What are the 2 sources that harmonics come from?

Answer 4

1. Solid State devices
2. Lighting

Question 5

What type of solid state devices produce harmonics?

Answer 5

1.SCR
2. Thyristors

Question 6

What type of lighting produces harmonics?

Answer 6

Lighting equipped with ballasts

Question 7

What tends to magnify harmonics?

Answer 7

PFCE

Question 8

There are 2 ways of dealing with harmonics. What is the first?

Answer 8

Isolation

Question 9

There are 2 ways of dealing with harmonics. What is the second?

Answer 9

Improving power quality

Question 10

What are the methods of isolation to deal with harmonics?

Answer 10

Isolation transformers
Uninterruptible Power Supplies (UPS)

Question 11

What are the methods of improving power quality?

Answer 11

Using shunt and series filters.

Question 12

In the presence of harmonic currents, motors will run at a higher or lower temperature?

Answer 12

Higher.

Question 13

Why do harmonics have the ability to produce mechanical vibrations and noise?

Answer 13

Harmonics can produce torque pulsations.

Question 14

Electronic equipment often requires an accurate voltage and current waveform. How do harmonics affect these waveforms?

Answer 14

Harmonics distort the waveform by reducing its magnitude, and misshaping the wave.

Question 15

What is another word for shunt filter?

Answer 15

High pass filter

Question 16

What is another word for series filter?

Answer 16

Low pass filter

Question 17

How can we control both voltage and frequency over a large range?

Answer 17

Use a VFD

Question 18

VFD stands for:

Answer 18

Variable Frequency Drive

Question 19

In the VFD, the converter is a ________ that is either _________ or ___________

Answer 19

1. Rectifier
2. Uncontrolled using diodes
3. Controlled using SCRs

Question 20

A converter has 2 voltage types. These are ____ and _____

Answer 20

Fixed and Variable DC (Link) Voltage

Question 21

What is the purpose of the DC link in a VFD?

Answer 21

Filter out the DC link ripple voltage using an inductor or capacitor

Question 22

What is the purpose of the inverter in a VFD?

Answer 22

Uses the DC link voltage to provide a variable AC voltage and Frequency to the motor at a constant volts/hertz ratio

Question 23

What are 3 advantages of using VFDs?

Answer 23

1. Control speed over large range
2. Start the same load at different speeds safely
3. The motor will run cooler than if we reduce voltage only

Question 24

What are the disadvantages of VFDs?

Answer 24

Expensive
More components to maintain
More heat generation