8.10. Losses and Efficiencies

The output of a dc machine, be it a generator or a motor, is always less than the input because some amount of energy supplied to the machine is lost as heat. Power losses sum over the three main com­ponents: iron losses (core losses), copper losses, and friction losses. Iron losses and friction losses can be put into a single group called mechanical losses, or stray-power losses. Copper losses are resistance losses that form a group of electrical losses.

Iron losses P_ir are the sum of the hysteresis loss and eddy-current loss. When the machine armature revolves in the field, the steel core of the armature periodically undergoes the reversal of magne­tization. The power required per cycle of magnetization, called the hysteresis loss, appears as heat in the iron. At the same time, the emfs generated in the armature core as it revolves in the field cause the eddy currents to circulate in the steel, which give rise to heat loss called eddy-current loss.

Iron losses depend on the magnetic flux density and the rate of magnetization reversal (the frequency of cyclic variation in the core flux). The flux density determines the emf of a machine, or its voltage, and the frequency of cyclic variation in flux is proportion­al to the speed. Therefore, in the machine running as a generator or as a motor, the iron loss will be constant and independent of the load if the terminal armature voltage and speed are constant.

Copper losses, or I²R losses, are the power lost as heat and expend­ed in the conductors of field and armature windings on overcoming the resistance offered by the conductors to the current flow.

The losses in the armature circuit made up of the armature wind­ing itself, brushes, brush contacts, and commutator bars depend on the armature current, i.e. they vary with the load.

Friction losses P_f are the losses of power expended on overcoming various frictional forces when the machine is running. They include the friction losses in the bearings, at the surface of the commutator due to the rubbing of the brushes, and in the armature core due to its fanning action, the latter power loss being referred to as the wind­age loss. These losses depend on the speed but are independent of the load on the machine.

The efficiency of a machine, expressed in percentage, is 100 times the ratio of the power output P₂ (useful power) to the power input P₁ (consumed power): η= (Р₂/Р_г) 100.

If the machine is a generator, its electrical output is found as the product of the terminal voltage V and the load current I, namely, P₂ = VI. The power input is P₁ = VI + P_ir + P_c + P_f, and the efficiency is then given as

If the machine runs as a motor, its electrical input is P₁ = VI. The power output is P₂ = VI – P_ir – P_c – Pf and the efficiency is then

8.11. Universal Commutator Motors

Any dc motor can in principle be run off an ac line because the motor torque that is dependent on the product of the armature cur­rent and the air-gap field flux does not reverses direction when the directions of the armature current and field flux reverse simultaneously.

For a large torque to be develop­ed, it is necessary that the armature current and field flux should

reverse direction simultaneously, Fig. 8.18. Wiring diagram of a i.e. they should be in phase. In a universal motor shunt motor, such a phase coincidence is impossible to reach because the flux produced by the field winding lags the impressed volt­age by about 90°. In a series motor the armature current is at the same time the field current. Neglecting the phase shift between the field current and magnetic flux, the armature current and flux may be regarded to be in phase.

Small commutator motors are made universal, i.e. adapted for operation from a dc and an ac power line. This type of motor is usually built without a compensating winding. The motor has dc terminals labeled 0 and =, and ac terminals 0 and ~ for its connec­tion to the dc and the ac supply respectively (Fig. 8.18). Thus, when the motor runs off the ac line, the number of turns of the field winding is much smaller than when the motor operates on dc, with the result that the power factor proves rather high despite the ab­sence of the compensating winding.

The characteristics of a universal commutator motor operated from the ac supply are similar to those of a dc series-wound motor. Single-phase small-power commutator motors find use in automat­ic, communication, and domestic facilities. They differ substantially from dc motors in design. The magnetic circuit of the stator for this type of motor is built up of steel sheets to minimize the eddy-current loss. The flux due to armature reaction produces the emf of self-induction which heavily reduces the power factor. To avoid the effect of armature reaction, the stator is fitted with a compensating winding whose flux opposes the armature flux. This winding can be placed in series with the armature or coupled inductively with it, or wound on the stator so as to perform the functions of both the field winding and the compensating winding simultaneously.

Use is sometimes made of the motors with a transformer coupling between stator and roto_r. These motors, called induction, or repul­sion, commutator motors, are mainly designed for household faci­lities and operated directly from the ac line. In such a motor, the winding connected across the line acts as the field and the compen­sating winding, the armature winding being shorted out by the commutator brushes.

REVIEW QUESTIONS

1. Give the expression for the emf of a dc machine.

2. What is the purpose of commutating poles?

3. Describe the process of self-excitation of a generator.

4. Draw and explain the external characteristics of a generator.

5. What is to be done to reverse a dc motor?

6. What factors affect the torque and speed of a dc motor?

7. Plot and explain the characteristics of a dc motor.

8. How can the speed of a dc motor be controlled?