AC machines midterm

Question 1

What is flux density

Answer 1

The amount of field lines in a given area

Question 2

What is flux density unit measured in

Answer 2

Tesla’s (T)

Question 3

What is flux unit measured in

Answer 3

Weber’s (B)

Question 4

What is permeability

Answer 4

Ability of an object to be magnetized

Question 5

What is Reluctance

Answer 5

Opposite of permeability,
Inability to be magnetized

Question 6

What is residual magnetism

Answer 6

After coil returns to 0 current, the magnetic domains become disorganized but some remain aligned creating residual magnetism

Question 7

What is retentivity

Answer 7

Ability for a core material to retain magnetism

Question 8

What is an ideal core material and what attributes would it have

Answer 8

High permeability, low retentivity. Often made it silicon steel

Question 9

Problems with core material

Saturation

Answer 9

Only use rated voltage for machines as additional voltage only creates heating

Question 10

Problems with core material

Hysteresis losses

Answer 10

Causes heating in core. Use silicon steel core to reduce losses

Question 11

Problems with core material

Eddy currents, how to reduce

Answer 11

Use laminated sheets to reduce losses in core

Question 12

How to limit hysteresis loops

Answer 12

Use core material with low retentivity to reduce losses

Question 13

Best material for a core

Answer 13

Silicon steel

Question 14

What are Eddy currents

Answer 14

Circulating currents in a core that create heat and i²r losses

Question 15

What is Faraday’s law

Answer 15

A moving or changing field will induce a voltage and current into an inductor

Question 16

What is Lenz’s law

Answer 16

The polarity of an induced current will always oppose the originating field

Question 17

What material is a stator made of

Answer 17

Made of stacked silicon alloy steel laminations

Question 18

Right hand rule

Answer 18

Generators,
Thumb - thrust
Pointer finger - direction of field North to South
Center finger- direction of current

Question 19

Left hand rule

Answer 19

Motors
Thumb- force on conductors
Pointer finger- field direction north to south
Center finger - direction of current

Question 20

Formula for percent slip

Answer 20

(NS - NR) divided by NS

Question 21

Formula for speed regulation

Answer 21

(Speed no load - speed full load) divided by speed full load

Question 22

What kind of cap does a cap start induction run motor use

Answer 22

Electrolytic to shift start winding current

Question 23

What is a cap start motor used for and how does it’s current draw compare to an inductive start

Answer 23

High starting torque loads, and has less starting current

Question 24

What motor is not reversible

Answer 24

Shaded pole

Question 25

What is Lorentz force

Answer 25

A current carrying conductor in a field will experience a force

Question 26

What is Faraday's law

Answer 26

A moving conductor through a field will have current induced or a moving or changing field will induce voltage and current in a conductor

Question 27

Four methods to disconnect start winding

Answer 27

Thermal sensor Current sensor Centrifugal switch Voltage sensor

Question 28

What type of AC motor does not use a squirrel cage rotor assembly

Answer 28

AC universal

Question 29

Why are pole pieces constructed from laminated steel sheets?

Answer 29

To reduce eddy currents

Question 30

What components does a cap start induction run split phase motor have in the start winding circuit

Answer 30

Centrifugal switch, start winding, electrolytic capacitor

Question 31

What's single phase AC motor has a high starting torque

Answer 31

Cap start

Question 32

At what percentage of speed does a centrifugal switch open

Answer 32

75 to 85 percent

Question 33

Which split phase motors have their start winding disconnected at 75% rated speed

Answer 33

Cap start and resistance start

Question 34

What happens if a centrifugal switch fails to reclose

Answer 34

It won't restart

Question 35

Which split phase motor has their run winding on at all times

Answer 35

Cap run

Question 36

What is the function of the centrifugal switch

Answer 36

Remove start winding

Question 37

What motor is best suited for refrigerator

Answer 37

Cap start hermetically sealed

Question 38

In a universal AC motor are the armature and field coils connected in series or parallel?

Answer 38

Series

Question 39

How can you reverse the direction of an AC universal motor

Answer 39

Change connections a1 and a2 or S1 and S2

Question 40

What is the typical application of a shaded pole motor

Answer 40

Small fans

Question 41

List three types of capacitors used with AC motors

Answer 41

Electrolytic, metal film, paper and oil

Question 42

Can a 2 lead motor be reversed

Answer 42

No