Machines; Direct Current Machines Objective 3

Question 1

What does “shunt” wound mean?

Answer 1

Windings connected in parallel

Question 2

What does “compound” wound mean?

Answer 2

Series and parallel

Question 3

True or false: basic field and armature winding connections are the same between dc motors and dc generators

Answer 3

True

Question 4

What are the physical characteristics of a shunt field winding?

Answer 4

Many turns of fine wire

Question 5

What is a shunt field winding resistance compared to resistance of the armature conductors?

Answer 5

Much higher.

Question 6

Where is the largest current in a shunt motor? Why?

Answer 6

Through the armature. Because parallel circuit, current splits based on resistance (same voltage in each branch). Resistance lowest in armature therefore current highest.

Question 7

How does shunt current compare to armature current in a shunt motor?

Answer 7

Shunt current can be as low as 1% of the armature current.

Question 8

What is the relationship between speed and torque in a shunt motor?

Answer 8

Small decrease in speed = substantial increase in torque

Question 9

What is the relationship between armature current and torque?

Answer 9

Proportional

Question 10

What is the relationship between speed and armature current?

Answer 10

Decreased speed = increased armature current (due to decrease in counter emf)

Question 11

What can be done to reduce high starting current of a shunt motor?

Answer 11

Add resistor in series with armature circuit at start.

Question 12

How do we control the speed of a shunt motor?

Answer 12

Vary shunt field resistance

Question 13

What is motor “runaway”?

Answer 13

High armature current due to not enough cemf

Weak cemf can be due to high field resistance (low field current; weak magnetic shunt-field)

Question 14

What can be done to prevent motor runaway in a shunt motor?

Answer 14

Add resistor in series with armature during starting

Question 15

What are the physical characteristics of a series field winding? Why is it constructed as such?

Answer 15

Few turns of heavy wire. Being in series with armature, it needs to carry the same current

Question 16

What is the relationship between torque and current in a series motor?

Answer 16

Torque is proportional to I²

Question 17

How does increasing armature current affect both series motors and shunt motors’ magnetic field intensity?

??? Pg.32

Answer 17

Series= current increases in both windings

Shunt = current only increases magnetic field intensity around armature winding conductors.

Question 18

Which aspects of the fundamental motor equation remain constant?

Answer 18

V(arm) = cemf + (I(arm) × R(arm))

V(arm) remains constant
R(arm) remains constant

Question 19

How do certain aspects of the fundamental motor equation change with a decreased load?

Answer 19

V(arm) = cemf + (I(arm) × R(arm))

I(arm) decreases with less load
Cemf increases to maintain electrical balance

Question 20

What precaution must be taken with a series motor?

Answer 20

Dangerously high speeds under light load

Question 21

How does starting torque compare to full load torque on a series motor?

Answer 21

Starting torque can be as much as 500% higher than full load torque.

Question 22

How do series and shunt windings compare? Physical characteristics and location?

Answer 22

Series - few turns, heavy wire, on top

Shunt - many turns, fine wire, on bottom

Question 23

What are two ways the field windings of a compound motor may be connected?

Answer 23

1) cumulative-compounded

2) differentially compounded

Question 24

How would you wire field windings on a compound motor if you wanted the magnetic fields to aid each other? Oppose?

Answer 24

Aid = cumulative compounded 
Oppose= differentially compounded

Question 25

What is the difference between a long shunt and a short shunt?

Answer 25

Little difference in actual operation Long shunt in parallel with series AND armature windings Short shunt in parallel with armature winding only.

Question 26

True or false: when load is added to cumulative compounded motor, shunt field strength remains constant.

Answer 26

True

Question 27

What would happen to a compound motor if the shunt winding disconnected and light load was connected?

Answer 27

Likely runaway, as it is now a series motor.

Question 28

Which type of motor is considered stable? What does this mean?

Answer 28

Cumulative-compound motor. It can handle a load that suddenly increases or decreases.

Question 29

When is a differentially compounded motor used?

Answer 29

When a nearly constant speed is required.

Question 30

How does a differentially compounded motor differ from the cumulative-compounded motors?

Answer 30

Current in series field winding flows in opposite direction to that of shunt field.

Question 31

What is the purpose of having a series field winding with opposite current flow?

Answer 31

To oppose shunt field flux to weaken total field flux so motor can speed up.

Question 32

What happens to a differentially compounded motor as load increases? What is an inherent risk with this?

Answer 32

Subtracting effect of series field winding lowers cemf thereby increasing series and armature currents. If this series current raises above shunt current the motor can reverse with damaging effects.

Question 33

What is the degree of compounding of a given machine dependent upon?

Answer 33

Strength of flux produced by series winding compared to that produced by shunt winding. Increasing series turns (compared to shunt turns) increases compounding

Question 34

Describe the relationship between speed and torque for: 1) shunt motor 2) series motor 3) cumulative-compound motor

Answer 34

Speed is inversely proportional to torque. 1) small drop in speed with increased torque 2) large drop in speed with increased torque 3) combined chanarcyeristics of above

Question 35

What does motor speed regulation compare?

Answer 35

Motor speed and load

Question 36

Which type of motor has the highest starting torque?

Answer 36

Series

Question 37

Why is the torque vs. Current curve a straight line for a shunt motor?

Answer 37

Torque produced varies only with changing armature current

Question 38

Why is the torque vs. Current curve curved for a series motor?

Answer 38

Torque produced with armature current flux AND series field flux

Question 39

What is the formula for speed regulation?

Answer 39

%n(r) = ((no load) - (full load) ÷ full load) × 100

Question 40

Which type of motor has poor speed control?

Answer 40

Series