Applications of the Motor Effect

Question 1

What is an electric motor?

Answer 1

Devices that transform electrical potential energy into rotational kinetic energy.

Question 2

Recall what DC stands for?

Answer 2

Direct current

Question 3

Name the main components of a DC motor.

Answer 3

1. DC power supply
2. Split-ring Commutator
3. Brushes
4. Coil
5. Magnets

Question 4

What is the stationary part of the motor combined called?

Answer 4

The stator

Question 5

What is the rotating components called?

Answer 5

Rotor

Question 6

In a simple DC motor, what does the stator consist of?

Answer 6

Magnets (permanent) which are fixed to the casing of the motor - these provide an external magnetic field in which the coil rotates

Question 7

what is the role of the split ring commutator?

Answer 7

Rotates with the armature and coil - it reverses the direction of current every half-turn. It will flip direction of the forces allowing the coil to rotate in the same direction.

Question 8

What is the purpose of the brushes?

Answer 8

They are stationary parts of the system, that connect the coil to the DC power supply by brushing against the commutator.

Question 9

Describe how DC motors generate fast rotation using a direct current in a constant direction.
- RHGR
- No torque
- Flips the current

Answer 9

1. Initially, AB has a force upwards, and CD has a force down due to the motor effect, and confirmed by RHGR.

Question 10

Describe the effect of the coil when it rotates up to 90 degrees in a DC motor.

Answer 10

2. When the coil rotates up to 90, the forces no longer produce torque, but angular momentum keeps the coil rotating.

Question 11

What happens to the coil when it passes over 90 degrees?

Answer 11

3. When the coil passes 90, the opp. sides of commutator are in contact with the brushes, which reverses the current. The forces on AB and CD are flipped which keeps the coil rotating in the same direction.

Question 12

Justify the importance of a split-ring commutator and brushes in the operation of DC motor.

Answer 12

The Split-ring com. is very important for maintaining the same torque. We can flip the current, thus the force and keep the coil spinning in one direction. Stationary brushes allow the switch the current direction

Question 13

What is the formula for force in DC motors?

Answer 13

F = niLB

Question 14

What is the formula for torque?

Answer 14

torque = rFsin0

Question 15

What is the formula for net torque?

Answer 15

sub in force into torque
A = 2r x L = w * L
Hence, torque = nIABsin theta

Question 16

How is the area vector drawn?

Answer 16

Area vector is drawn perpendicularly to the surface of the area

Question 17

What is supply emf?

Answer 17

Supply emf is generated by the power supply which powers the current that rotates in a magnetic field

Question 18

Explain how back Emf is produced.

Answer 18

When the coil turns in the magnetic field, it experiences a changing flux. (resulted in an induced emf by Fara’s law. The back emf is the opposite in direction to the supply emf by Lenz’s law.

Question 19

What if the induced emf was in the same direction as supply emf?

Answer 19

The coil would rotate fasted, and the current would increase. However, this would be violating LOCOE.

Question 20

Describe the process of DC motors in terms of emf.
- flux
-rate of change in flux increases the rotation
- max speed where back is equal to the supply in magnitude

Answer 20

1. Coil is rot. slowly so there is slow change in flux. (Back Emf is low)
2. The rate of change in flux increases, thus the back emf increases as well. As the back emf opp. the supply emf, the net voltage is given by vnet = emf sup - emf back
3. eventually, it will reach max speed, where back will equal to supply.

Question 21

What is the formula for emf and resistance, and current?

Answer 21

I = V/R = (Esup. - Eback)//R

Question 22

Explain the importance of magnetic braking.

Answer 22

A part of the metal disk rotating will experience an increase in flux as it enters a magnetic field. The disk on the right will experience a decrease in flux, as it leaves the mag. field. Hence eddy currents are formed by Lenz’s law.

Question 23

Evaluate the usefulness of magnetic braking.

Answer 23

Adv:
- Do not require physical contact with the wheel, and hence do not wear out
- do not create noise/pollution/

Disadv:
- Huge amount of heat loss produced from eddy currents
- the slower the wheels spin, the less effect they have

Question 24

Suggest some ways to improve a DC motor to give ‘more greater/consistent torque’

Answer 24

1. Permanent magnets - electromagnets are a better way to control the current - changing the polarity
2. Soft Iron Core - high permeability it can strengthen the fields and increase the working of those machines.
3. Increase the torque by increasing the number of turns of the motor

Question 25

What is a radial magnetic field?

Answer 25

A magnetic radial field is where the magnets around the field are curved and the plane of the coil will be always parallel to the B field and torque will be constant.