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AC machines > AC machines midterm > Flashcards

AC machines midterm Flashcards

1
Q

What is flux density

A

The amount of field lines in a given area

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2
Q

What is flux density unit measured in

A

Tesla’s (T)

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3
Q

What is flux unit measured in

A

Weber’s (B)

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4
Q

What is permeability

A

Ability of an object to be magnetized

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5
Q

What is Reluctance

A

Opposite of permeability,
Inability to be magnetized

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6
Q

What is residual magnetism

A

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

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7
Q

What is retentivity

A

Ability for a core material to retain magnetism

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8
Q

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

A

High permeability, low retentivity. Often made it silicon steel

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9
Q

Problems with core material

Saturation

A

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

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10
Q

Problems with core material

Hysteresis losses

A

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

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11
Q

Problems with core material

Eddy currents, how to reduce

A

Use laminated sheets to reduce losses in core

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12
Q

How to limit hysteresis loops

A

Use core material with low retentivity to reduce losses

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13
Q

Best material for a core

A

Silicon steel

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14
Q

What are Eddy currents

A

Circulating currents in a core that create heat and i²r losses

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15
Q

What is Faraday’s law

A

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

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16
Q

What is Lenz’s law

A

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

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17
Q

What material is a stator made of

A

Made of stacked silicon alloy steel laminations

18
Q

Right hand rule

A

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

19
Q

Left hand rule

A

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

20
Q

Formula for percent slip

A

(NS - NR) divided by NS

21
Q

Formula for speed regulation

A

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

22
Q

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

A

Electrolytic to shift start winding current

23
Q

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

A

High starting torque loads, and has less starting current

24
Q

What motor is not reversible

A

Shaded pole

25
Q

What is Lorentz force

A

A current carrying conductor in a field will experience a force

26
Q

What is Faraday’s law

A

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

27
Q

Four methods to disconnect start winding

A

Thermal sensor
Current sensor
Centrifugal switch
Voltage sensor

28
Q

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

A

AC universal

29
Q

Why are pole pieces constructed from laminated steel sheets?

A

To reduce eddy currents

30
Q

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

A

Centrifugal switch, start winding, electrolytic capacitor

31
Q

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

A

Cap start

32
Q

At what percentage of speed does a centrifugal switch open

A

75 to 85 percent

33
Q

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

A

Cap start and resistance start

34
Q

What happens if a centrifugal switch fails to reclose

A

It won’t restart

35
Q

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

A

Cap run

36
Q

What is the function of the centrifugal switch

A

Remove start winding

37
Q

What motor is best suited for refrigerator

A

Cap start hermetically sealed

38
Q

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

A

Series

39
Q

How can you reverse the direction of an AC universal motor

A

Change connections a1 and a2 or S1 and S2

40
Q

What is the typical application of a shaded pole motor

A

Small fans

41
Q

List three types of capacitors used with AC motors

A

Electrolytic, metal film, paper and oil

42
Q

Can a 2 lead motor be reversed

A

No

AC machines (1 decks)

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