Electric Motors

Question 1

What is the working principle of a DC Motor?

Answer 1

A magnetic field is supplied by two permanent or electro magnets. A rotor (armature) sits in the middle of the field and contains many current carrying wires.The force on the wire is:
F = BIL

Question 2

How do we arrange the wires in the armature of a DC motor?

Answer 2

Want it so that one half of the motor the current is going into the page and the other half of the motor the current is coming out of the page.

Question 3

Why do we have many coils of wire in the armature?

Answer 3

The force on each wire is so small so by combining many wires together will provide a greater force.

Question 4

Why do we need a commutator?

Answer 4

We need the current in each wire to be reversed every half turn otherwise we would not get rotation.

Question 5

What is the structure of a commutator?

Answer 5

It is a series of conductive segments. The coil of wires in the armature are connected to adjacent segments. The current is delivered to the armature through carbon brushes on adjacent sides of the commutator. Any segment that is in contact with the carbon brush will be shorted out.

Question 6

What is the overall torque produced by the motor?

Answer 6

T = BILnr
n is the number of wires that doesnt include the ones that are shorted out by the carbon brushes.
r is the radial distance from the central axis to the wires.

Question 7

How can a motor be used as an electric generator?

Answer 7

If the output shaft is rotated by some external system then the coils of wire would be generating an electromotive force.

Question 8

What is the magnitude of the electric force generated by a motor?

Answer 8

e = vBL where v is the velocity of the wire across the magnetic field.
This equation shows that the emf generated is proportional to the speed of the rotor.

Question 9

What is the back EMF?

Answer 9

When the current in the armature causes it to turn, the rotation will result in a back EMF that will reduce the armature current. The speed and output torque is then a balance of the applied voltage and the back EMF.

Question 10

What is the equation for the back EMF?

Answer 10

E = wrBLn, but since it is proportional to the rotational speed we can say that the back EMF, E, is E = KeW where Ke is the emf constant and W is the angular velocity.

Question 11

What are the characteristics of permanent DC motors?

Answer 11

In all motors the number of wires, their length and the radius at which they are positioned relative to the central axis is the same.

Question 12

What is the equation for the torque out of a motor?

Answer 12

T = (Kt)(Ia) since the magnetic flux density is nearly always constant. If we put this into the EMF equation E = wrBLn then we get
T = (Kt/Ra)(V - KeW)

Question 13

What happens to the torque as the speed increases?

Answer 13

it will decrease linearly with speed and the slope of this will be KtKe/Ra

Question 14

How do we control simple DC motors?

Answer 14

Change the voltage applied to the circuit. Initially the starting torque is greater than the load so the motor will accelerate. As the speed increases, the torque will decrease and the load will increase so the rate of acceleration will decrease until an equilibrium is reached.

Question 15

What is a limitation of permanent magnet DC motors?

Answer 15

Cannot change the size of the magnetic field so limits the speeds that the motor can reach.

Question 16

What are the characteristics of Electro magnetic DC motors?

Answer 16

The field strength of the magnetic field will be related to the current applied which in turn depends on how the circuit is connected.

Question 17

How do we normally connect the Field current to create the magnetic field?

Answer 17

Could have a separate power source for the magnetic field, which would give the motor the same characteristics as a permanent magnet motor. Otherwise connect the motor and the wires in parallel, series or a mixture of both.

Question 18

What is a Shunt wound motor?

Answer 18

When the motor and the magnetic field winding’s are connected in parallel. the field current is not affected by the back EMF in this configuration.

Question 19

What are the characteristics of a Shunt wound motor?

Answer 19

As it is not affected by the back EMF, it will have almost a constant speed of operation which is independent of voltage and is applicable over a wide range of loads.

Question 20

How is the current distributed in a Shunt Wound motor? And is it higher in the armature or the field windings?

Answer 20

It is split between the armature and the field windings, but both sections with receive the same amount of voltage.To produce a higher torque, the current must be higher in the armature, as torque is proportional to armature current.

Question 21

Why do we need a lot of field windings in a Shunt Wound motor?

Answer 21

To increase torque as torque is also proportional to the flux linkage, and the flux linkage will obviously be higher with more turns.

Question 22

Is the wire in the field of a Shunt Motor thicker or thinner than in the armature?

Answer 22

Thinner to increase the resistance so that the current through the field is smaller than the armature.

Question 23

What is a self excited motor?

Answer 23

A motor that has its field windings connected with the armature so that when this gets current so will the field.

Question 24

What is a series motor?

Answer 24

The armature and the field windings are connected in series with each other.

Question 25

How is the current distributed within a series motor?

Answer 25

It is evenly distributed amongst the armature and the field because they are connected in series.

Question 26

Why are the field wires made of a lot thicker wire than normal?

Answer 26

This is to provide less resistance and they also have fewer turns.

Question 27

What is the relationship between the applied current and the torque produced?

Answer 27

It is a linear relationship.

Question 28

Why do we use the series motor as a starter motor?

Answer 28

Because at low speeds we can generate a lot of torque for the motor speed that is able to overcome the initial inertia of large objects.

Question 29

Does the Series Motor have good or bad speed regulation?

Answer 29

Bad.

Question 30

What is a compound DC Motor?

Answer 30

A motor that has field windings connected in both series and parallel with the armature. This will give it the best properties of both series and shunt motors, but obviously its characteristics will never quite match that of the individual motor.

Question 31

What is the Ward Leonard method of speed control?

Answer 31

It is used to control the speed of DC motors. It does this by applying variable voltage across the armature of the motor.

Question 32

How does the Ward Leonard system vary the voltage?

Answer 32

It uses a motor generator set. This is a motor coupled with a generator that is then connected with the motor that we want to control the speed of. Nowadays these have been replaced with a transistor.