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Motors > Introduction To Three Phase Motors > Flashcards

Introduction To Three Phase Motors Flashcards

1
Q

What is an induction motor?

A

A machine that gets its energy into the rotating part (Rotor) by mutual induction

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2
Q

What is torque

A

Result of interacting driving and restraining forces

Restraining forces may result from friction or other mechanical forces

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3
Q

What is the relationship between torque and counter torque when the motor is operating at a constant speed?

A

They are balanced

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4
Q

What happens when motor load increases?

A

Counter torque increases
Speed decreases
Motor current increases
Torque increases to match the new counter torque

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5
Q

What is efficiency?

A

The ratio of output energy over input energy

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6
Q

What are the 2 types of electrical losses?

A

Iron losses
Copper losses

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7
Q

What are iron losses?

A

Losses that occur in the stator (stationary iron) and the rotor (rotating iron)
Also known as core losses

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8
Q

What are copper losses?

A

Also occur in the stator and rotor although the rotor conductors may be made up of aluminum or brass

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9
Q

What are the 2 types of mechanical losses?

A

Friction
Windage

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10
Q

What are windage losses?

A

The energy lost due to the air movement produced by the rotor

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11
Q

What is included in the frame of a motor?

A

The end brackets or end bells
The bearings in the end brackets
The junction box

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12
Q

What is the purpose of the motor frame?

A

It protects the windings, bearings, and other parts from moisture, chemicals, mechanical damage and abrasion from grit

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13
Q

Describe the motor stator

A

The stationary part of an electrical machine
Comprised of the stationary iron and winding
It produces a magnetic field that intersects the rotor

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14
Q

What is included in the stator?

A

The laminated and slotted iron
The conductors
The conductor insulation
The protective sleeves and wedges for slots

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15
Q

What benefits do slot windings provide?

A

They distribute windings evenly through the stator, which cancels out some harmonics
Improves efficiency and and reduces interference with other equipment

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16
Q

Describe the stator core

A

Provides a high permeability path for magnetism
Constructed of thin silicon steel laminations designed to minimize eddy current and hysteresis losses

17
Q

What are the 2 slot windings methods on induction motors

A

Lap windings
Concentric windings

18
Q

Describe concentric windings

A

They have coils for each phase of each pole
They become progressively larger

19
Q

What is the term for a magnetic field with constant magnitude and is rotating at a constant speed

A

Synchronous speed

Nsync = Fx120/P

20
Q

Name two types of rotors

A

Squirrel cage induction motor (SCIM)
Wound rotor induction motor (WRIM)

21
Q

Describe a SCIM

A

Aluminum or copper bars placed in the slots on the periphery of the rotor
Rotor conductors are permanently shorted by the end rings
Rotor resistance is small and you can’t add external resistance
Can adapt to any number of pole pairs

22
Q

Why are the slots in a SCIM skewed instead of being parallel to each other

A

To prevent magnetic locking and create a uniform speed

23
Q

How is current induced in an induction motor

A

The difference between the rotor speed and the synchronous speed of the stators magnetic field

24
Q

What is slip speed

A

The difference between the rotor speed and synchronous speed

Nslip = Nsync-Nrotor

25
Q

What is the difference between a WRIM and a SCIM

A

The stator is identical but the rotor is constructed differently

26
Q

Describe the construction of a WRIM rotor

A

Conventional, insulated copper windings that terminate in slip rings.
Carbon brushes connect the rotor circuit to external, adjustable resistances.
An adjustable resistance increases the motors starting torque while decreasing the starting current.

27
Q

What are the three stages of operation in an induction motor

A
  1. AC power is supplied to the motors stator, a magnetic field that is constant in magnitude and rotates at constant speed
  2. The motor operates as a transformer with a rotating, short circuited secondary winding and the rotating magnetic field cuts the rotor conductors which induces voltage and current
  3. Once started, the speed by which the rotor slips behind the speed of the stators magnetic field is proportional to load counter torque
28
Q

Describe the left hand conductor rule

A

The left thumb points in the direction of electron flow, the 4 fingers show the polarity of the magnetic flux around the conductor

29
Q

Describe the left hand generator rule

A

The thumb points in the relative direction of the rotator conductors motion with respect to the external field. The middle finger points in the direction of any electron flow caused by the relative motion of the rotor conductor. The pointer finger points in the direction of the external magnetic field

30
Q

How is an induction motor like a transformer

A

The stator winding is the primary winding whereas the rotor winding is the secondary winding.
Unlike a transformer, the secondary winding of an induction motor is short circuited

31
Q

What controls secondary current

A

Slip speed
When load counter torque increases, so does secondary current and slip speed

32
Q

What can we do with the information on a motor nameplate

A
  1. Select overcurrent device type and size
  2. Select starter type and size
  3. Select overload device or setting
  4. Select wiring means and conductor type
  5. Troubleshoot the motor
33
Q

What are the NEMA standard variations

A
  1. Voltage may not exceed +/- 10%
  2. Frequency may not exceed +/-5%
  3. The combined variation may not exceed +/-10%
34
Q

Describe the 4 types of insulation classes

A
  1. Type H - used for motors that are required to run hot
  2. Type A - Rarely used on modern motors
  3. Type B and F - most commonly used types
35
Q

What is the duty rating

A

The amount of time a motor can run without a cooling period

36
Q

Describe the 4 types of rotor design

A
  1. Design A - normal starting torques, high starting currents, can handle brief periods of overload.
  2. Design B - common, normal starting torque, low starting current
  3. Design C - high starting torque, low starting current, designed for starting heavy loads.
  4. Design D - high starting torque, low starting current, suitable for high inertia starts, has poor speed regulation
37
Q

What direction does a properly wired motor rotate when viewed from the non drive end (front)

A

Counter clockwise

Motors (4 decks)

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