Introduction To Three Phase Motors

Question 1

What is an induction motor?

Answer 1

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

Question 2

What is torque

Answer 2

Result of interacting driving and restraining forces

Restraining forces may result from friction or other mechanical forces

Question 3

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

Answer 3

They are balanced

Question 4

What happens when motor load increases?

Answer 4

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

Question 5

What is efficiency?

Answer 5

The ratio of output energy over input energy

Question 6

What are the 2 types of electrical losses?

Answer 6

Iron losses
Copper losses

Question 7

What are iron losses?

Answer 7

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

Question 8

What are copper losses?

Answer 8

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

Question 9

What are the 2 types of mechanical losses?

Answer 9

Friction
Windage

Question 10

What are windage losses?

Answer 10

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

Question 11

What is included in the frame of a motor?

Answer 11

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

Question 12

What is the purpose of the motor frame?

Answer 12

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

Question 13

Describe the motor stator

Answer 13

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

Question 14

What is included in the stator?

Answer 14

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

Question 15

What benefits do slot windings provide?

Answer 15

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

Question 16

Describe the stator core

Answer 16

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

Question 17

What are the 2 slot windings methods on induction motors

Answer 17

Lap windings
Concentric windings

Question 18

Describe concentric windings

Answer 18

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

Question 19

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

Answer 19

Synchronous speed

Nsync = Fx120/P

Question 20

Name two types of rotors

Answer 20

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

Question 21

Describe a SCIM

Answer 21

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

Question 22

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

Answer 22

To prevent magnetic locking and create a uniform speed

Question 23

How is current induced in an induction motor

Answer 23

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

Question 24

What is slip speed

Answer 24

The difference between the rotor speed and synchronous speed

Nslip = Nsync-Nrotor

Question 25

What is the difference between a WRIM and a SCIM

Answer 25

The stator is identical but the rotor is constructed differently

Question 26

Describe the construction of a WRIM rotor

Answer 26

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.

Question 27

What are the three stages of operation in an induction motor

Answer 27

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

Question 28

Describe the left hand conductor rule

Answer 28

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

Question 29

Describe the left hand generator rule

Answer 29

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

Question 30

How is an induction motor like a transformer

Answer 30

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

Question 31

What controls secondary current

Answer 31

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

Question 32

What can we do with the information on a motor nameplate

Answer 32

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

Question 33

What are the NEMA standard variations

Answer 33

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

Question 34

Describe the 4 types of insulation classes

Answer 34

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

Question 35

What is the duty rating

Answer 35

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

Question 36

Describe the 4 types of rotor design

Answer 36

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

Question 37

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

Answer 37

Counter clockwise