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4th Year Electrical > Machines; Direct Current Machines Objective 3 > Flashcards

Machines; Direct Current Machines Objective 3 Flashcards

1
Q

What does “shunt” wound mean?

A

Windings connected in parallel

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2
Q

What does “compound” wound mean?

A

Series and parallel

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3
Q

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

A

True

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4
Q

What are the physical characteristics of a shunt field winding?

A

Many turns of fine wire

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5
Q

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

A

Much higher.

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6
Q

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

A

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

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7
Q

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

A

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

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8
Q

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

A

Small decrease in speed = substantial increase in torque

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9
Q

What is the relationship between armature current and torque?

A

Proportional

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10
Q

What is the relationship between speed and armature current?

A

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

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11
Q

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

A

Add resistor in series with armature circuit at start.

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12
Q

How do we control the speed of a shunt motor?

A

Vary shunt field resistance

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13
Q

What is motor “runaway”?

A

High armature current due to not enough cemf

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

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14
Q

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

A

Add resistor in series with armature during starting

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15
Q

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

A

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

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16
Q

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

A

Torque is proportional to I²

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17
Q

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

??? Pg.32

A

Series= current increases in both windings

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

18
Q

Which aspects of the fundamental motor equation remain constant?

A

V(arm) = cemf + (I(arm) × R(arm))

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

19
Q

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

A

V(arm) = cemf + (I(arm) × R(arm))

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

20
Q

What precaution must be taken with a series motor?

A

Dangerously high speeds under light load

21
Q

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

A

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

22
Q

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

A

Series - few turns, heavy wire, on top

Shunt - many turns, fine wire, on bottom

23
Q

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

A

1) cumulative-compounded

2) differentially compounded

24
Q

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

A 
Aid = cumulative compounded 
Oppose= differentially compounded
25
Q

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

A

Little difference in actual operation

Long shunt in parallel with series AND armature windings

Short shunt in parallel with armature winding only.

26
Q

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

A

True

27
Q

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

A

Likely runaway, as it is now a series motor.

28
Q

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

A

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

29
Q

When is a differentially compounded motor used?

A

When a nearly constant speed is required.

30
Q

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

A

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

31
Q

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

A

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

32
Q

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

A

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.

33
Q

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

A

Strength of flux produced by series winding compared to that produced by shunt winding.

Increasing series turns (compared to shunt turns) increases compounding

34
Q

Describe the relationship between speed and torque for:

1) shunt motor
2) series motor
3) cumulative-compound motor

A

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

35
Q

What does motor speed regulation compare?

A

Motor speed and load

36
Q

Which type of motor has the highest starting torque?

A

Series

37
Q

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

A

Torque produced varies only with changing armature current

38
Q

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

A

Torque produced with armature current flux AND series field flux

39
Q

What is the formula for speed regulation?

A

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

40
Q

Which type of motor has poor speed control?

A

Series

4th Year Electrical (24 decks)

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