7.10. Motor Characteristics
The properties of all motors and, in particular, D.C. motors are defined as a totality of the following characteristics:
(ii) Operating and mechanical;
(iii) Braking; and
Starting characteristics. The starting characteristics determine the operation of starting the moment the motor begins running to the moment when steady-state operation is established include:
- The starting current Istart generally determined by the ratio Istart/Irun.
- The starting torque Tstart determined by the ratio Tstart/Trun
- The duration of starting tstart ;
- The economy of operation determined by the amount of energy consumed in starting; and
- The cost and reliability of the starting equipment.
Operating characteristics. The operating characteristics are those that give the relation between speed, torque and efficiency as functions of the useful power or the armature current for V = constant and constant resistances in the armature and field circuit.
Mechanical characteristics. Of major importance for industrial drive mechanisms are the mechanical characteristics, which are the relation N = f(T) (where N and T stand for speed and torque respectively) for conditions of constant voltage and resistances in the armature and field circuits. These also include the braking characteristics.
Regulation characteristics. These characteristics determine the properties of motors when their speed is controlled. These include:
- The regulation range determined by the ratio Nmax/Nmin.
- The efficiency of regulation from the point of view of the initial cost of the equipment and maintenance
- The nature of regulation -continuous or stepped; and
- The simplicity of the control apparatus and methods.
The D.C. motors possess versatile and diverse regulation characteristics, and for this reason are indispensable in installations where wide-range control of speed is necessary.
The characteristic curves of a motor are those curves which show relation between the following quantities:
- Torque and armature current i.e., Ta/Ia characteristic. This is also known as electrical characteristic.
- Speed and armature current i.e., N/Ia characteristic.
- Speed and torque i.e., N/Ta characteristic. This is also known as mechanical characteristic. This can be obtained from (1) and (2) above.
Following relations are worth keeping in mind while discussing motor characteristics:
7.10.1. Torque-current characteristics
- When running on no-load, a small armature current flows to supply the field and to drive the machine against the friction and other losses in it.
- As the load is applied to the motor, and is increased, the torque rises almost
proportionally to the increase in current. This is not quite true, because the flux has been assumed to be constant, whereas it decreases slightly owing to
armature reaction. The effect of this is to cause the top of the curve connecting torque and line current to bend over as shown in Fig. 65.
- The starting torque of a motor is determined by the starting resistance, which
in turn, governs the initial current through the machine when the main
switch is closed. At this moment the speed is zero, so that the back e.m.f is
zero and the starting current is given by I= V/R, where V is the supply voltage
and R is the total resistance, which includes the armature and starting resistance.
Fig. 65. Torque-current characteristic of a shunt motor.
If the starting current is limited by heating considerations to twice the full load current, then with normal supply voltage the starting torque of a shunt motor is twice the full load torque. If, however, the supply voltage is below normal, the flux is also less than twice full load torque. The importance of this will be appreciated when the starting torque of a series motor is compared with that of the shunt motor.
- In a series motor the torque (Ta=ɸIa) increases much more than does the armature current. This is because the flux itself increases with the armature current, though, owing to the magnetic saturation, the two are not strictly proportional. Nevertheless, for all but the heavy loads which tend to produce saturation of the field system, it may be said that the torque is approximately proportional to the square of the load current.
Fig. 66 shows the relationship between torque and current. Here the current commences at the no-load value, rises parabolic-ally at first, but increases more slowly as the effects of armature reaction and magnetic saturation becomes appreciable.
- ·Fig. 66. Torque-current characteristic of a series motor.
- This property of a series motor, by virtue of which a heavy current gives rise to a very high torque, also influences its starting characteristics. In a case of a shunt motor, it has already been seen, that the current at the moment of starting may be as high as twice the full-load value; if we allow for the armature of the magnetisation characteristic and for weakening effect of armature reaction and assume that the flux is increased to 1.5 times its full-load value, then it is obvious that the starting torque of a series motor is three times the full-load torque.
- Further more, if the supply voltage falls, the starting current may still be maintained at twice full-load value by cutting out some of the starting resistance, so that the high value of starting torque may still be maintained.
This type of motor (series motor) is superior to shunt motor for drives in which machines have to be started and accelerated from rest when fully-loaded, as is the case with traction equipment .
Differential compound motor. Refer Fig. 60. In this type of motor the two field windings (shunt and series) oppose each other. On light loads, such a machine runs as a shunt motor, since the series field winding, carrying only a small current, has relatively little effect.
On heavy loads, the series coils strengthen and since they are in opposition to the shunt winding, cause a reduction in the flux and a consequence decrease in torque.
On heavy overloads or when starting up on load. the series winding may become as strong as shunt, or it may even predominate, in which the torque will be reduced to zero or may even be reversed. In the latter case, the motor would tend to start-up in the wrong direction.
It is obvious that such characteristics may cause dangerous results, so the differential compound motors have only very limited applications in practice.
Fig. 68. Torque-current characteristic of a cumulative compound motor.
Cumulative compound motor. In this type of motor the two field windings assist each other as shown in Fig. 61. The flux on no-load is that due to the shunt-winding, while on load the flux and torque rise with the load current. The torque, therefore, increase more rapidly than in the case of the shunt machine, and on heavy loads it resembles the characteristic of the series motor.
Fig. 68 shows the torque-current characteristic of a cumulative compound motor.
The torque-current characteristics of shunt, series and compound (differential and cumulative) motors. are showing in Fig. 69 from which their properties, may be compared, as far as torque is concerned.
Fig. 69. Torque-current characteristics of shunt, series and compound motors.
7.10_2. Speed-current characteristics. The speed-current characteristics of various motors can be deduced from the following motor equation,
- In the shunt motor, the field circuit is connected to the supply terminals so that the exciting current remains constant as long the temperature of the machine is constant, and field regulator is not adjusted. Actually as the machine warms up, the field resistance increases and the exciting current decreases by about 4% for every 100C rise in temperature. Neglecting this effect and also due to armature reaction, it is seen that the speed of a shunt machine falls slightly as the load increases. The fall in speed is proportional to the volt drop IR in the armature circuit. If, however, we consider the effect of armature reaction, an increase of load causes a slight decrease in flux, unless the machine is fitted with com pensating windings. This weakening of the field tends to raise the speed, so that the actual fall in speed is less than that calculated by a consideration of the volt drop in the armature.
Fig. 70. Speed-current characteristic of a shunt motor.
- The speed of a shunt machine can be increased by inserting resistance in the field by means of a field regulator. This weakens the field and causes the motor to run faster in order to generate the necessary back e.m.f. of course, it is impossible to reduce the speed by this method below that at which it runs with no field resistance in the circuit.
- In case of a series motor the flux does not remain constant, or even approximately constant, because the field winding is in series with the load, so that as the load increases so also does the strength of the magnetic field. At first the flux increases approximately in proportion to the load, but as the field approaches saturation, owing to the heavier loads, the increase is not so rapid. The effects of temperature changes and of armature reaction may be neglected (in comparison with the above mentioned effect).
- It will be appreciated, while considering motor equation, that the back e.m.f. decreases as the armature current increases, as in shunt motor; in the latter, however, the decrease is due to the volt drop in the armature, while in the series machine the loss in volts occurs in the field as well as in the armature, since they are in series. The back e.m.f. in a series motor, therefore, decreases more rapidly than in a corresponding shunt machine. The speed, however, is proportional to the back e.m.f divided by the flux. the former decreases, while the latter increases with increasing load so that the speed decreases rapidly as the armature current increases. This property is a valuable feature in a drive of which the speed is required automatically to adjust to compensate for changes in load.
Fig. 71. (b) Speed-current characteristic of compound motors.
- On very low current, a series motor runs at very high speeds, or tends to race, as it is termed. This is dangerous, since the machine may be destroyed by the centrifugal forces set up in the rotating parts. For this reason, when installing a series motor it must be positively connected to its load by gearing or by direct connection and never by belting. Moreover, the minimum load should be great enough to keep the speed within safe limits, as is the case, for example, with railway motors, hoists and rolling mills.
- A compound motor runs on ‘no-load: at a speed determined by its shunt winding since the series field contributes little to the total flux in this condition.
- In a cumulative compound motor at ‘no-load’, the series field strengthens the shunt winding so that the speed falls as in a series machine. Since the flux at any load is equal to the shunt and series fluxes, the speed is less than it would be if running on either field alone. The speed-current characteristic of such a machine is shown in Fig. 71 (b). Such a characteristic has two important advantages. These are:
(i) The machine has the compensating action of reducing its speed on heavy loads, as is the case with series machine.
(ii) The maximum speed on no-load is limited by the shunt winding, since this produces a maximum flux even on no-load.
This type of motor, therefore, is suitable for driving machines which operate on a cycle consisting of a power or working stroke followed by a return or idle stroke. The series winding produces a fall in speed on the working stroke, while the shunt winding permits the return stroke to be completed at a high, but safe, speed. A fly-wheel is also provided to act as a load equilizer in such. a drive.
In case of a differential compound motor, since the series winding opposes the shunt, the resultant flux decreases as the load increases; thus the machine runs at a higher speed than it would do as a shunt motor. If the series windings were relatively weak, this reduction in flux might be just sufficient for the fall in speed, brought about by the volt drop in the machine. Such a motor would have a useful application in driving loads at a constant speed. If the series field were strong, however, an increase in load would result in a decrease in the magnetic flux and a rise in speed would take place as shown in Fig. 49, the heavier the load, the faster would the motor tend to run. This is the property which may have dangerous consequences, since a heavy overload would result in such a high speed that the motor would destroy itself.
7.10.3. Speed-Torque (or Mechanical) characteristics. The speed-torque characteristics of the four types, i.e. shunt, series, cumulative and differential of motors drawn on the same diagram are shown in Fig. 72 for the purpose of comparison.
The main properties of individual motors, from this diagram, may be summarised as under:
- Shunt motor. As the load torque increases the speed falls somewhat, but the machine may be regarded as an approximately constant speed motor.
The shunt motor is used:
- When the speed is required to remain approximately constant from no-load to full-load.
- When the load has to be driven at a number of speeds, anyone of which is required to remain approximately constant.
- Series motor. As the load torque increases the speed falls rapidly. At low torque the speed becomes very high and machine tends to race.
The series motors are used:
- When large starting torque is required (as in traction motors).
- When the load is subject to heavy fluctuations, and a reduced speed is desired to compensate for the high torque, provided that there is no possibility of the machine ‘losing’ its load.
Fig. 72. Speed-torque characteristics of D.C. motors.
- Cumulative compound motor. In this type of motor the speed falls appreciably as the torque increases, but on low torques the maximum speed is limited to a safe value. These motors are used:
- When a large starting torque is required but when the load may fall so low that a series motor would race.
- When the load is of a fluctuating nature and a reduced speed is desirable on the heavy loads.
In such a case a flywheel is usually fitted so that when speed is so reduced the kinetic energy stored in the flywheel at high speeds is given up to assist the motor in driving the heavy load.
- When the supply voltage is subject to fluctuations (as in traction systems).
- Differential compound motor. The speed at low torque is limited by the shunt winding, as in the cumulative compound machine. At high torques, the speed may be arranged to remain constant or, with a stronger series field, the speed may rise with increasing load.
On very heavy loads the machine may tend to race.
Its use is usually restricted to applications which require a constant speed.
Industrial applications of D.C. Motors:
1. Shunt motors:
- Drills and milling machines
- Line-shaft drives
- Boring mills
- Grinders and shapers
- Spinning and weaving machines
- Wood working machines
- Small printing presses
- Light machine tools generally.
2. Series motors:
- Traction drives generally
- Tram cars and railway cars
- Cranes, derricks, hoists, elevators and winches
- Fans and air compressors
- Vacuum cleaners, hair driers, sewing machines
- Universal machines generally.
3.Cumulative compound motors:
(i) Punching, shearing and planing machines
(ii) Lifts, haulage gears and mine hoists
(iii) Pumps and power fans
(iv) Rolling mills, stamping presses and large printing presses
(v) Trolley buses.
4.Differential compound motors:
(i) Battery boosters
(ii) Experimental and research work.