Chapter 5 DC Motors

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Basic Motor Construction

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

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

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)

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

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