1. Chapter 5
DC Motors

2. Principles of Operation
Conversion of electrical energy to mechanical energy is called motor action
Two requirements for motor action:
Current flow through a conductor
A force on the conductor develops
This force is produced when the conducting wire is placed inside the magnetic field formed between two magnetic poles

3. The Right-hand Rule
Shows the direction that conductor carrying current will be moved in a magnetic field

4. Rotary Motion
Current-carrying conductor in a magnetic field will tend to move at right angles to the field
The reaction of the wire to the field produces torque

5. Continuous Rotation
Achieved by reversing the direction of current flow in a wire
Current change is provided by a switching device called a commutator
The commutator and loop form the armature

6. DC Motor Operation
The illustration at the right demonstrates the switching action of the brushes and commutator in a process called commutation

7. Practical DC Motors
To overcome problems associated with a single loop in the armature of a motor, multiple loops and commutator segments are added

8. Control of Field Flux
Because magnetic flux lines have a tendency to repel each other, curved magnets are used at the ends of the poles

9. Counterelectromotive Force
Counterelectromotive Force, or CEMF, is a result of a conductor passing through a magnetic field
Electromotive force, or EMF, is generated the same way as voltage in a generator is produced
The amount of CEMF produced is proportional to three factors:
Physical properties of the armature
Strength of the magnetic field
Rotational speed of the armature

10. Armature Reaction
When the armature loop is at a right angle to the field flux lines, it is on the geometric neutral planes and therefore generates no CEMF
This shifting of the neutral plane is called armature reaction

11. Results of Armature Reaction
Arcing at the brushes results because of armature reaction and adversely affects the motor:
Reduces torque
Motor is less efficient
Brush life is shortened and the commutator is damaged

12. Interpoles
Interpoles are added to the armature to correct armature reaction
Also called commutating poles
Interpole windings are self-regulating

13. Motor Selection
Depending upon the application, different DC motors may be selected
Two characteristics used in motor selection are:
Speed regulation
Torque

14. Speed Regulation Motors are designed to operate at full load
Operation above full load is overload
Less than full load operation is referred to as partial load
Ability of a motor to maintain its speed under varying load conditions is called speed regulation

15. Torque Torque is a twisting action that causes a motor to rotate
Torque is measured by multiplying the force it will exert by the distance between the center of the shaft and the point where the force is being applied
T = F * r
F is magnetic force acting on the armature in pounds
r is the radius in feet
T is the rotary action exerted by the motor shaft in pound-feet

16. Work Is equal to distance × force W = D * F D = distance in feet
F = force in pounds
W = work in foot-pounds

17. Horsepower Power is equal to work over time
Motors are rated in horsepower on the nameplate

18. Motor Efficiency Copper losses
Armature I2R losses
Field losses
Mechanical losses
Iron losses
Eddy-current
Hysteresis
Friction losses
Bearing
Brush
Windage (air)
Some of the power applied to the motor is lost as heat
Two types of losses in motors are:
Copper losses
Mechanical losses

19. Basic Motor Construction

20. Motor Classification
Motors are generally classified by how their windings are connected to their DC power supply
There are three types of wound-field DC motors
Shunt
Series
Compound

21. Shunt Motor
The field winding is in parallel (or shunt) with the armature windings
Considered a constant-speed motor

22. Series Motor The field winding is in series with the armature
Under no-load conditions, the motor will run away (accelerate until it breaks apart)

23. Compound Motor Has both a series field and shunt field
Two types of compound motors:
Cumulative compound
Differential compound

24. Coil Terminal Identification
Shunt field is indicated as F1 and F2
Resistance = 100–500 Ohms
Series field is indicated as S1 and S2
Resistance = 1–5 Ohms
Armature winding is indicated as A1 and A2
Resistance = very low
Commutating (interpole) winding is indicated as C1 and C2