Motor Characteristics
The characteristic curves of a motor are those curves which show relationships between the following quantities.
1. Torque and armature current i.e. Ta/Ia characteristic. It is known as electrical characteristic.
2. Speed and armature current i.e. N/Ia characteristic.
3. Speed and torque i.e. N/Ta characteristic. It is also known as mechanical characteristic. It can be found from (1) and (2) above.
While discussing motor characteristics, the following two relations should always be kept in mind :
Characteristics of Series Motors
1. Ta/Ia Characteristic. We have seen that Ta µ FIa. In this case, as field windings also carry the armature current, F µ Ia up to the point of magnetic saturation. Hence, before saturation,
At light loads, Ia and hence F is small. But as Ia increases, Ta increases as the square of the current. Hence, Ta/Ia curve is a parabola as shown in Fig. 29.14. After saturation, F is almost independent of Ia hence Ta µ Ia only. So the characteristic becomes a straight line. The shaft torque Tsh is less than armature torque due to stray losses. It is shown dotted in the figure. So we conclude that (prior to magnetic saturation) on heavy loads, a series motor exerts a torque proportional to the square of armature current. Hence, in cases where huge starting torque is required for accelerating heavy masses quickly as in hoists and electric trains etc., series motors are used.
Change in Eb, for various load currents is small and hence may be neglected for the time being. With increased Ia, F also increases. Hence, speed varies inversely as armature current as shown in Fig. 29.15.
When load is heavy, Ia is large. Hence, speed is low (this decreases Eb and allows more armature current to flow). But when load current and hence Ia falls to a small value, speed becomes dangerously high. Hence, a series motor should never be started without some mechanical (not belt-driven)
load on it otherwise it may develop excessive speed and get damaged due to heavy centrifugal forces so produced. It should be noted that series motor is a variable speed motor.
3. N/Ta or mechanical characteristic. It is found from above that when speed is high, torque is low and vice-versa. The relation between the two is as shown in Fig. 29.16.
Characteristics of Shunt Motors
1. Ta/Ia Characteristic
Assuming F to be practically constant (though at heavy loads, f decreases somewhat due to increased armature reaction) we find that Ta µ Ia.
Hence, the electrical characteristic as shown in Fig. 29.17, is practically a straight line through
the origin. Shaft torque is shown dotted. Since a heavy starting load will need a heavy starting current, shunt motor should never be started on (heavy) load.
2. N/Ia Characteristic
If F is assumed constant, then N µ Eb. As Eb is also practically constant, speed is, for most purposes, constant (Fig. 29.18).
But strictly speaking, both Eb and F decrease with increasing load. However, Eb decreases slightly more than f so that on the whole, there is some decrease in speed. The drop varies from 5 to 15% of full-load speed, being dependent on saturation, armature reaction and brush position.
Hence, the actual speed curve is slightly drooping as shown by the dotted line in Fig. 29.18. But, for all practical purposes, shunt motor is taken as a constant-speed motor.
Because there is no appreciable change in the speed of a shunt motor from no-load to full- load, it may be connected to loads which are totally and suddenly thrown off without any fear of excessive speed resulting. Due to the constancy of their speed, shunt motors are suitable for driving shafting, machine tools, lathes, wood-working machines and for all other purposes where an approximately constant speed is required.
3. N/Ta Characteristic can be deduced from (1) and (2) above and is shown in Fig. 29.19.