Electricity I7

Question 1

The farraday motor

Answer 1

Faradays motor proved that electricity could be used to produce motion building the world’s 1st electric motor in 1821
It used the interaction of the magnetic field of a current carrying wire and a permanent magnet to achieve continuous motion of the wire around the magnet.

Question 2

The left hand rule

Answer 2

When a current carrying wire is held with the left hand the thumb points in the direction of the electron current the fingers will point in the direction of the magnetic lines of force

Question 3

What is torque

Answer 3

Torque is a twisting or turning force exerted on the loop that causes it to rotate its axis

Question 4

What are the 4 principal parts that make up the elementary DC motor

Answer 4

A magnetic field
A moveable conductor
A commutator
And brushes
Current supplied to a loop between two permanent magnets exciting the loop to motion where it gets locked in with the field magnetic pull. Comes to the neutral plane where the commutator utilizing a split ring, keeps the current flow the same to achieve a constant push/pull motion. The switch is achieved because once past the neutral plane the loop enters the other half of the stator and will receive a different force, if a commuator is not used the rotor will lock itself it the neutral plane.

Question 5

Starting of basic DC motor

Answer 5

The basic DC motor can be made to start itself by giving it 2 or more loops. It can self start because when one loop is the neutral plane the other develops maximus torque when power is connected.

Question 6

What is and Interpole

Answer 6

Interpole pieces are windings wired in series with the armature winding and are placed in line with the neutral plane and counteract the armature reaction of the motor operation. Wired in series this way self regulates and will apply the proportional amount of current for the winding and apply the perfect amount of correction of the flux lines nearing the neutral plane.

Question 7

What is the rotating element of the DC motor?

Answer 7

The armature or rotor

Question 8

Describe drum wound armature windings.

Answer 8

There are 2 basic types. Lap windings and wave windings. Lap windings are used for low voltage high current motors. The wave winding is used in motors with high voltage, low current requirements

Question 9

Where are armature leads connected?

Answer 9

Leads from the armature coils are connected to the raised portions of the commutator segments known as risers

Question 10

What are brushes made of?

Answer 10

Brushes are made of a soft carbon material containing a large amount of graphite.

Question 11

Describe motor brush service life.

Answer 11

Even though brushes are designed to be long lasting they are made to wear out faster than the commutator because it is cheaper and easier to replace brushes

Question 12

The stator field assembly consists of what?

Answer 12

It contains the pole pieces and the field coils.

Question 13

Most motor housings usually consist of how many pieces?

Answer 13

Motor housing usually have 3 pieces. A field frame, supports the field coils and the interpoles. Two end frames which support the protruding shaft and act as a cover on the other side.

Question 14

What is the rotor or armature core made of?

Answer 14

It is made up of soft steel to minimize Eddy currents (swirling action instead of straight line ) and hysteresis losses ( when a metallic retains magnetism causing the current switch to lag)

Question 15

Since CEMF opposes the line voltage it acts to cut down the short circuit current which the line voltage could potentially cause through the low resistance armature

Answer 15

Dido

Question 16

CEMF in basic DC motors

Answer 16

CMF in a DC motor acts as a regulator for motor speed. When more current is added, the speed increases. Flux lines are now being cut at faster rate, therefore a greater cemf is produced somewhat drawing the motor back to settle somewhere in between initial speed and anticipated speed gain.

Question 17

When a load is applied to a motor it tends to slow down reducing CMF, which increases the EMF supplied to the motor bringing it back to the right speed

Answer 17

When a load is removed from the motor, the motor speeds up, CEMF ghost up to reduce the EMF supplied to the motor to slow it back to the right speed

Question 18

What are some series motor no load characteristics?

Answer 18

When a series motor has no load, it speeds up in order to build-up CMF to reduce the armature current and thus the torque, unfortunately the motor will run until it self destroys.
* Shunt motor can operate with no load at a constant speed.

Question 19

Where are series motors useful?

Answer 19

Ceres motors are useful when sudden load changes are put on the motor. Series motors are used to do heavy work on cranes, Subway trains, electric locomotives, and similar applications

Question 20

How do you reverse the direction of rotation of a DC motor?

Answer 20

You must reverse the field leads or the armature leads but not both

Question 21

When would you use starters in a DC motor?

Answer 21

Starters are needed to limit the armature current that flows when the motor is 1st turned on. The starter is used to bring the motor up to its normal speed and is then switch out of the circuit usually with a centrical switch

Question 22

What is a drum controller?

Answer 22

Drum controllers are used with motors that require very frequent starting, stopping, and changing of speed they also reverse them as needed

Question 23

What is synchronous speed

Answer 23

Synchronous speed is the natural rate of rotation of the magnetic field of the stator

Question 24

Synchronous speed equation

Answer 24

Synchronous speed or rpm = the AC frequency times 120 Divided by the number of Poles or phases

Question 25

Basic operation of a synchronous motor

Answer 25

In a basic synchronous motor the rotor is locked in position by the attractive force of the stator field and is turned at a synchronous speed with the stator rotating magnetic field

Question 26

Starting of the 3 phase synchronous motor

Answer 26

3 phase synchronous motors are usually started by a special exciter motor they can also be made self stqarting by providing them with a damper winding over the regular wound rotor

Question 27

A 2 pole motor that works on 60 Hz power has a synchronous speed of 3600 rpm

Answer 27

Dido

Question 28

Induction motors use electromagnetic induction to magnetise the rotors

Answer 28

Dido

Question 29

The stator and rotor act like the primary and secondary windings of a transformer

Answer 29

True!

Question 30

For the induction motor to work the rotor must slip and turn at a rate slightly less than that of the rotating magnetic field if it did not do this it would not cut any lines of flux and would not turn

Answer 30

True!+

Question 31

Double squirrel cage rotors provides higher starting torque than the conventional squirrel cage rotor. high resistance bars set in the rotor make this possible

Answer 31

True!

Question 32

The wound rotor motor differs from other induction motors in that it uses resistance in series with its rotor windings through a slip ring and brush assembly

Answer 32

True!

Question 33

What is the right hand rule for motors?

Answer 33

It shows the direction that a current carrying wire will moved in a magnetic field. Point hand like a gun, with forefinger or barrel pointed in the same direction as the magnetic field lines. thumb would be motion the conductor will move, and the middle finger points to the left and represents the electron current flow direction..

Question 34

What are hysteresis losses?

Answer 34

Hysteresis losses occur when metals retain or lose magnetism in a lagged effect according to actual current reversals. Can be avoided by using soft steel in the core material of the rotor.

Question 35

Power can be described in which two ways?

Answer 35

Power output is a measure of mechanical power delivered by the motor at full load and is given in horsepower.
Electrical power is the full consumption of the motor, expressed in watts P= IxE

Question 36

What is the starting torque of a 3 phase synchronous motor?

Answer 36

It is zero because the stator magnetic field wants to pull the tomorrow with it, but there is an attracting force in the exact opposite direction which leads to the rotor “locked up”

Question 37

Describe squirrel cage motors.

Answer 37

Squirrel cage is the most used of all. Metallic bars fit around rotor and are perpendicular to the lines of flux. They are shorted out at each ends of the rotor with an ammortisure ring. So when the stator magnetic field begins to rotate, the cage begins a push/ pull with the field for easy and efficiant starting of the motor.

Question 37

Describe squirrel cage motors.

Answer 37

Squirrel cage is the most used of all. Metallic bars fit around rotor and are perpendicular to the lines of flux. They are shorted out at each ends of the rotor with an ammortisure ring. So when the stator magnetic field begins to rotate, the cage begins a push/ pull with the field for easy and efficiant starting of the motor.

Question 38

Describe the hysteresis motor.

Answer 38

Hysterisis loses occur when the molecules of an iron bar remain aligned in an external magnetic field even though their physical position was changed with moving the bar. A large amount of power is consumed in the rotor due to HL, it also provides us with a very large amount of torque for operation. Uses cobalt steel in its rotor because of its retentivity and permeability ( ability to retain magnetism and ability for things to pass through)

Question 39

Describe the repulsion motor.

Answer 39

Resembles dc and induction motors but operates on magnetic repulsion. No torque developed at neutral plane or plane of pole pieces, anywhere in between though. Has high starting torque and can operate at high speeds.

Question 40

Describe the repulsion start induction rum motor.

Answer 40

A centrifugal switch is used in starting. Once the motor reaches certain rpm the switch makes up and cuts off flow of current to brushes, the motor is then in step with the rotating magnetic field where it then runs as an induction motor.