Machines; Direct Current Machines Objective 2

Question 1

What is the difference between a DC generator and a DC motor?

Answer 1

The way they are used

Question 2

Torque in a motor rotor opposes _______________ applied by the load

Answer 2

Counter-rotational force (counter torque)

Question 3

What two magnetic fields interact to produce the force that causes a motor’s armature to rotate?

Answer 3

Magnetic field created by current in the:

1) armature winding and
2) field windings.

Question 4

Describe the Lorentz force

Answer 4

Pushes current-carrying conductor at right angles to a magnetic field.

Question 5

The right hand rule uses (conventional/ electron) flow to define current direction.

Answer 5

Electron

Question 6

Explain the different components of the right hand rule for motors

Answer 6

First finger = main field flux (points south)
Middle finger = direction of polarity of emf induced into the armature conductor
Thumb= direction of Lorentz force

Question 7

Describe how the concentration of flux contributes to Lorentz Force

Answer 7

Feild flux attempts to straighten out after being forced to bend around armature conductor magnetic field. This makes field flux concentrated on the side the armature flux is also concentrated.

Question 8

True or false Flux produced by the armature is perfectly symmetrical around the conductor

Answer 8

False. Armature field is compressed on one side. Opposite side of direction of force (see figure 21)

Question 9

What is the quantity of motor action compared to rotational force applied by the prime mover (in a generator)? Why?

Answer 9

Motor action < rotational force.

Because the armature current is due to a voltage induced by the rotation of the prime mover.

Question 10

What is the quantity of motor action compared to counter-rotational force applied by a mechanical load (in a motor)? Why?

Answer 10

Motor action = counter-rotational force

Because current is due to a voltage applied from an outside source.

Question 11

What is another name for induced voltage in a conductor?

Answer 11

Counter emf

Question 12

What is flemmings left hand rule for generators used to find?

Answer 12

Polarity of cemf

Question 13

What is the right-hand rule for motors used for?

Answer 13

Determine lorentz force on the conductor that causes a motor rotor to turn.

Question 14

What is the magnitude of output voltage dependent on in a DC generator?

Answer 14

1) speed of rotation

2) intensity of magnetic field.

Question 15

What is the equation used to find generated voltage of a generator?

Answer 15

E(g) = K × flux per pole × n

K - constant
n - r/min

Question 16

What is the equation used to find cemf of a motor?

Answer 16

Cemf = K × flux per pole × n

Question 17

Describe the state of the following at the instant a DC motor is started:

1) armature motion
2) generated cemf
3) properties limiting armature current

Answer 17

1) not turning
2) no Cemf generated
3) only resistance of armature winding (usually less than 1ohm)

Question 18

What is the formula for the fundamental motor equation?

Answer 18

V(A) = cemf + (I(arm) × R(arm))

Question 19

What happens to Cemf as a motor accelerates from ‘start’?

Answer 19

Cemf increases

Question 20

What is the main role of cemf in a motor?

Answer 20

Limits armature current by opposing applied voltage

Question 21

When does cemf stop increasing within an accelerating motor?

Answer 21

When applied voltage - cemf = voltage just sufficient to produce enough armature current to meet the torque requirements of the load.

Electrical balance

Question 22

What is the starting torque of a motor?
Compared to cemf
Compared to starting current

Answer 22

High compared to cemf

Proportional to starting current (both high at start).

Question 23

What happens to motor cemf when load increases? Why?

Answer 23

Cemf decreases because speed decreased

(Cemf = K × flux per pole × rotational frequency)

Question 24

What happens to armature current when load on a motor increases? Why?

Answer 24

Armature and torque both increase

(I(arm) = (V(arm) - V(cemf))÷ R(arm) (ohms law)
Since cemf decreases with increased load, this will effect current according to ohms law.

Torque and current are proportional.

Question 25

Having added load to a motor, it will stabilize at ___________(higher/lower) speed, with a(n) ______________(increase/decrease) in torque and a(n) _____________ (increase/decrease) in armature current.

Answer 25

Lower; higher; higher

Question 26

How can we reduce the starting current of a DC motor?

Answer 26

1) Placing a resistance in series with the armature at start. (Decrease resistance as speed /cemf increase)
2) apply less than rated voltage to the motor at the start. (Increase applied voltage as frequency increases)

Question 27

What component in a DC motor allows current to reverse at 180° in order to maintain rotation in the same direction (positive rotation)

Answer 27

Commutator

Question 28

Why are brushes located in the neutral plane?

Answer 28

At this location, they short out the commutator bars connected to a coil at a point where there is no or very low generated emf.

Question 29

What is armature reaction?

Answer 29

Distortion of the main feild flux pattern that causes brush sparking

Question 30

What is the result of main feild distortion due to armature reaction?

Answer 30

Causes neutral plane to shift

Question 31

What are two methods used to oppose the magnetic field produced by armature current (armature reaction)?

Answer 31

1) Interpoles wound with commutating windings

2) Compensating windings placed in main pole faces

Question 32

Which particular distorting effects do the compensating windings aim to oppose?

Answer 32

Distorting effects of armature current at THE POLES

Question 33

Which distorting effects do the commutating windings aim to oppose?

Answer 33

Distorting effects of armature current in the INTERPOLE area.

Question 34

What are three factors that affect the speed of a DC motor?

Answer 34

1) amount of load on the motor
2) intensity of the magnetic field
3) amount of armature voltage

Question 35

In order to regain equilibrium between torque and counter-torque, what happens to speed of motor when
1) generated torque is greater than counter-torque?

2)When counter-torque is greater than generated torque?

Answer 35

1) Motor accelerates until counter torque equals torque.

2) motor slows until developed torque equals counter torque.

Question 36

What happens to the following when load is added to a DC motor?

1) motor speed
2) cemf
3) armature current
4) torque

Answer 36

1) motor speed = decrease
2) cemf = increase
3) armature current = increase
4) torque = increase to equal counter torque

Cemf = V(arm) - (I(arm) × R(arm)) electrical balance

Question 37

What is a common method of motor speed control?

Answer 37

Adjusting strength of field current

Question 38

What happens to motor speed when field intensity is decreased? Why? Describe the process

Answer 38

Motor speeds up.
1) Armature cuts fewer lines of flux and therefore generates less cemf.
2) Less cemf means increased armature current
I(arm) = (V(arm) - cemf) ÷ R(arm)
3) increased I = increased torque (proportional)
4) motors developed torque is now greater than counter-torque of the load, so motor speeds up.

Question 39

According to ohms law, what is armature current proportional to?

Answer 39

Voltage

Question 40

According to ohms law, what is armature current inversely proportional to?

Answer 40

Resistance

Question 41

What is the process that occurs when we apply lower voltage to a DC motor?

Answer 41

1) armature current decreases proportionally
2) developed torque decreases due to lower I
3) speed decreases because of reduced torque
4) cemf decreases because of less lines of flux being cut
5) armature current stabilizes when electrical balance is achieved at lower speed.
V(arm) = cemf + (I(arm) × R(arm))