Motors = AC/DC

Question 1

Are AC motors used where speed control is required?

Answer 1

No, AC motors aren’t used where close speed control is required (DC is)

Question 2

What are the benefits of using an AC motor?

Answer 2

They require less maintenance.
They are more efficient than an DC motive of the same physical size.
They are cheaper and more compact.

Question 3

What are the disadvantages of AC motors?

Answer 3

They have little speed variation - they need specialist equipment to allow them to do this.

Question 4

what are the two main components of an AC Induction Motor?

Answer 4

Stator and Rotator

Question 5

What is a stator made out of and why?

Answer 5

A stator is made out of laminated steel and is formed with slots inside so that could can be wound to produce a magnetic field.

Question 6

What are the characteristics of a Stator?

Answer 6

They remain stationary and are connected to an AC supply, which is usually 440volts, 60Hz and 3 phase.

Question 7

What is a Rotor made of?

Answer 7

Made of thin strips of steel, with a copper ring at each end. these rings are joined together by copper bars embedded into the steel cylinder.

Question 8

What is the most commonly used rotor in the fleet?

Answer 8

Squirrel Cage Induction Motor

Question 9

How does the Rotor rotate?

Answer 9

a rotating magnetic field invites a current into the copper bars, which establishes a magnetic field around the motor. this field is set up in opposite polarity to the statist magnetic field, which causes the rotor to follow the rotating field.

Question 10

Does a Rotor have an electrical connection?

Answer 10

No, so it also doesn’t need brushes or slip rings.

Question 11

How are 3 phase motors connected?

Answer 11

3 phase motors can either be connected in the Star or Delta formations, depending on the motored operating conditions.

Question 12

How is a DC motor structured?

Answer 12

A DC motor is made up of coils and wires, unlike a squirrel cage induction motor.

Question 13

What are the principles used in DC motors?

Answer 13

Maxwells corkscrew rule and Flemings left hand rule.

Question 14

What is the idea for the motion of a DC Motor?

Answer 14

if a conductor is placed in a magnetic field it will follow a certain direction.

Question 15

Describe an elementary DC Motor.

Answer 15

a loop of wire placed between two poles of a magnet. The ends of the loop connect to copper segments, which in turn make contact with carbon brushes, which connect to a DC voltage.

Question 16

Describe Back EMF.

Answer 16

the conductors movement causes it to cut the magnetic field of the magnet and causes an EMF to be induced into the conductor, so it will end up opposing the motion producing it.

Question 17

What are the 3 classifications of a DC motor?

Answer 17

Series, Shunt and Compound.

Question 18

Describe a series classification.

Answer 18

Field coils connected in series with the armature. They only have a few turns of heavy gauge wire. Good starting torque - poor speed control/regulation. Heavy loads = slow and Light loads = rapidly. Removing load will cause the motor to overspeed. Drives machines such as fans,hoists and reactions where loads can’t be removed.

Question 19

Describe a Shunt classification.

Answer 19

Field coils connected directly across the voltage source and in parallel with the armature. They have many turns of thin gauge wire. poor starting torque but good speed regulation under all loads. The field is independent of variations in load and armature current, so torque developed varies directly with armature current. Since the field current is practically constant the shunt motor runs at an almost constant speed which makes it suitable for driving workshop machinery and constant speed pumps.

Question 20

Describe a Compound classification.

Answer 20

A combination of the series and shunt motors. It has a series field that is connected in series with armature and a shunt field that is parallel with the armature. The combination in series and shunt windings allows the motor to have the torque characteristics of the series motor and regulated speed characteristics of the shunt motor.

Question 21

Describe the Geometric Neutral Axis.

Answer 21

A physical position that is centred between the two opposite field windings, this position cannot be changed.

Question 22

Describe a Magnetic Neutral Axis.

Answer 22

A magnetic position that is centred in the magnetic field between the opposite field windings. This acid can be changed and is done so by distortion of the field winding magnetic field.

Question 23

Describe the Carbon Brush Position.

Answer 23

Must be fixed in the same position as the magnetic neutral axis. This ensures that there is no back EMF in the coil when the gap between the two halves of the split ring (commutator segments) passes the brush. This means there will be no sparking at the brushes.

Question 24

What is Armature Reaction?

Answer 24

As current flows through the coils, a magnetic field is created around the armature. The armature magnetic field will distort the field winding magnetic field, meaning the motor has armature reaction.