To understand the application of DC motors, one needs a clear idea of operating characteristics of the types of DC motors available. These are series motors, shunt motors and compound motors. Each type provides specific characteristics and based on it the application of the motor is determined.

Series Motors
The fundamental operating components of these motors include an armature and a field. The same current flown through the armature and the field which are connected in series. Usually the field coil is made up of a few turns of heavy gauge (thick) wires to enable large currents to flow. This enables the motor to develop high staring torques well over twice the rated load torque. The armatures are usually lap wound, which is good for high currant, and the lap winding provides parallel path for current flow at low voltages.

Unfortunately series motors have poor speed control, running at low speeds with heavy load and at high seeds with light load. Because of this characteristic, series motors should not drive equipment with a belt. Should the belt break, which is quite possible, the equipment load is gone. The motor could then over speed dangerously and destroy itself in a very short time. Hence a V or flat belt or even a chain drive is not advisable. Series motors should be directly coupled or through a gear box. Electrically an over speed trip is a must.

The high torque capability provides the motor to generate the high load required for lifting heavy weights in large cranes or drive heavy electric driven trains from rest, and climb grades without stalling. It is this characteristic that starts a car engine overcoming even extreme cold temperature and the associated viscous lubricants in the car engine and get it running. This is a common everyday aspect that is taken for granted by all of us.

Shunt Motor
It is probably the most commonly used DC motor. Here the shunt field is parallel to the armature winding. The shunt field coil has large number of turns using thinner wire resulting in low shunt field current and moderate armature currents. This motor provides better speed regulation by controlling the shunt field voltage. If the motor looses it‘s field, it will accelerate to some extent until the torque producing current is shut off. This motor will therefore not over speed to destruction like the series motor. It could be used for machine shop equipment and process line drives where speed and tension control characteristic between drives is important, and starting loads are not very high; beyond motor rating.

Computer Motors
Characteristics of the series and shunt motors of having higher torque and speed control characteristics are combined together in the compound motor. It has both a series field and a shut field winding on the same pole.

This motor responds much better to heavy load swings than a shunt motor because of higher current flowing through the series field coils, boosting field strength and resulting in higher torque and speeds.

With the addition of the shunt coil; at lighter loads unlike the series motor the added shunt field flux limits the top speed, and prevents the destruction like that of a series motor. Common uses for compound motors are for elevators, air compressors, conveyors, presses and shears.

Maintenance
Proper maintenance of DC motors includes regular visual inspection and electrical testing of armature and field. This provides a reliable understanding of the motor‘s condition. The inspection and testing should be zone wise which are armature, commutator, brushes and field coils. The insulation to ground test of DC motors is a proper evaluation of insulation which is particularly effected through ingress of dirt and moisture.

Inspection and checking of brushes and commutator are important part of motor maintenance. The condition of the brush contact face, and commutator surface and uniform spring pressure of brush on commutator are important for good motor performance, on load.

Armature inspection should highlight cracked or brittle insulation, broken banding, dirt or oil contamination. Ground leakage is an indicator of the relative insulation status. Bar to bar resistance check could indicate shorted winding or defects in riser soldering. Additionally infrared inspection can reveal overheating of brushes, commutator and any loose or hot connection on risers.

Field coil inspection may reveal cracks, or insulation brittleness. Usually winding shorts are checked by performing drop test applying line AC voltage to field leads, Voltage drop across each field pole is measured, which should be practically same for all. Any marked variation is an indication of winding short.

Commutator may develop very thin films caused by chemical action between the copper and carbon rubbing surfaces accelerated by high sparking which could be caused by uneven brush contact pressure. This could lead to poor commutation resulting in excessive wear of brush and commutator.

Commutator bars may rise in slots caused by the wedge or wedge ring holding the bars coming loose, leading to brush chatter. Similarly insulation mica layers between commutator segments way become loose and extend above the commutator bars, also causing brush chatter. These should be wedged in properly for rectification.

DC motors are expensive equipment for which regular and proper maintenance is necessary. Regular and proper care of the motor components as outlined in the earlier paragraphs will unsure better service life between repairs for DC motors.

S.A. Mansoor: Director Engineering, Partex Group