Chapter 17

DC Motors

Objectives

After studying this chapter, you will be able to:

•Explain the principles upon which DC motors operate

•Describe the construction of DC motors

•Discuss the different types of DC motors and their operating characteristics

Objectives (cont’d.)

• Describe basic motor maintenance procedures

Basic Motor Operation

• Motors generate rotating force (torque)

• Construction of a DC motor is similar to that of a DC generator– Frame, end bells, field poles, armature and

commutator

• Motor is different from a generator – Requires electrical energy for its operation– Converts electrical energy to mechanical

Basic Motor Operation (cont’d.)

• Right-hand rule for a motor– Thumb, index finger and middle finger at right

angles to one another– Point index finger in direction of main field flux– Middle finger in direction of current flow– Thumb points in direction of the force

• Force exerted on a conductor– Depends on strength of main field flux

Basic Motor Operation (cont’d.)

– Length of conductor within the field– Amount of current flowing in conductor

Basic Motor Operation (cont’d.)

• Torque and power– Torque may be calculated using Equation

17.2– Instrument designed to measure torque is

called a prony brake

Generator Action in a Motor

• Conductors rotate through a magnetic field, generating an emf

• Two voltages in a motor armature– Applied voltage from the source that causes

current to flow– Induced emf that opposes the current flow

• Counter electromotive force• Allows low armature resistance, high efficiency

Commutation

• Commutator – Rotating switch that periodically reverses

current direction to produce a constant torque

• Armature reaction in a motor– Armature current flows in the direction

opposite to the induced emf– The neutral plane shifts in the direction

opposite to direction of armature rotation

Commutation (cont’d.)

• Motor interpoles (commutating poles)– Means of compensating for armature reaction– Interpoles must have the same polarity as the

main poles directly behind them

• Compensating windings in motors– Costly, seldom used– Used in conjunction with interpoles in large

machines with heavy or varying loads

Motor Speed

• Speed is proportional to cemf

• A motor is rated at its maximum horsepower and speed at a constant load

• Speed regulation– Adjusting motor speed for changes in load

Types of DC Motors

• Series motors– Field connected in series with armature– Very high starting torque– Used in applications where variations in

speed with load are acceptable

• Shunt motors– Field connected directly across the line– Constant speed motors

Types of DC Motors (cont’d.)

• Compound motors– Combination of series and shunt motors– Two fields: one connected in series and the

other in parallel– Used to drive machines that require relatively

constant speed under varying loads

• Flywheel effect– Flywheel used to assist motor in driving load

Types of DC Motors (cont’d.)

• Permanent magnet motors– Main field flux is produced by permanent

magnets– Electromagnet used for secondary field flux

• Brushless DC motors– Electronic circuit is used to perform the

commutation of the applied DC– Optical or magnetic encoders used

Types of DC Motors (cont’d.)

• Stepping motors– Designed for jobs

requiring minimum torque

– Stators connected directly to power source

Motor Maintenance

• Failure to start– Causes: a ground, an open or short circuit,

incorrect connections, improper voltage, etc.

• Arcing at the brushes– Causes: insufficient brush contact, worn

brushes, low spring tension, dirty or off-center commutator, open or short in armature coil

Motor Maintenance (cont’d.)

• Vibration and pounding noises– Causes: worn bearings, loose parts, contact

between parts, improper motor alignment

• Overheating– Causes: overload, brush arcing, poor

ventilation, shorted armature or field coils

• Overheated bearings– Causes: problems with lubricant or fit

Summary

• Motors generate rotating force (torque)

• Motor construction is similar to generator construction but their operation is different

• Force on a conductor depends on conductor length, current and flux density

• There are two voltages in a motor armature, the applied voltage and induced emf which opposes the current flow

Summary (cont’d.)

• Interpoles and compensating windings are used to compensate for armature reaction

• Types of motors include series and shunt motors, compound motors, brushless DC motors, permanent magnet motors and stepping motors

• Periodic motor maintenance is important