In this article we will discuss about the characteristics and performance curves of a dc series motor.
Characteristics of a DC Series Motors:
1. Speed-Current Characteristic:
In case of a dc series motor, the mmf due to the exciting coils increases in direct proportion to the line or armature current, so (neglecting armature reaction effects) the value of flux varies with the load current according to the ordinary magnetization curve. Owing to armature reaction, the actual curve representing the useful flux falls below the open-circuit magnetization curve, as illustrated in Fig. 1.18. With larger currents the magnetic circuit gets saturated and flux φ tends to approach a constant value.
From the speed equation, it is obvious that speed is proportional to back emf Eb and inversely proportional to flux per pole φ. With the increase in armature current voltage drop in armature circuit and series field [I(Ra + Rse)] increases and, therefore, back emf Eb decreases, as shown in Fig. 1.18. However, under normal conditions I (Ra + Rse) drop is quite small and may be neglected.
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Thus if the applied voltage remains constant, speed N is inversely proportional to flux φ. If a curve is drawn between speed and input (or line) current I, it will be a rectangular hyperbola before magnetic saturation as up to saturation point the magnetisation curve is a straight line. In this region, the speed decreases abruptly with the increase in input current.
After magnetic saturation, the flux φ tends to become constant and speed-current characteristic becomes a straight line and speed decreases slightly due to voltage drop in armature and series field, as shown in Fig. 1.18. The speed becomes zero when the input current is the normal short-circuit current of the motor i.e., equal to applied voltage divided by the motor resistance (Ra + Rse). Normally this current is many times full-load value.
From speed-current characteristic curve it is obvious that the series motor is a variable speed motor i.e., speed varies with the variation in line current. With the decrease in load on a dc series motor, the speed increases and may become dangerously high at very light loads.
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Since on no load the speed is dangerously high, the machine may get damaged due to heavy centrifugal forces set up in the rotating parts. This is the reason that series motors are never started on no load.
To start a dc series motor, mechanical load is first put and then the motor is started.
Since on no load the series motor attains dangerously high speed, which causes heavy centrifugal force resulting in the damage of machine, series motors are not suitable for the services:
(i) Where the load may be entirely removed and
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(ii) For driving by means of belts because mishap to the belt would cause the motor run on no load.
The motors are suitable for gear drive, because gears provide some load on account of the frictional resistance of the gear teeth in case of sudden release of load.
However, very small series motors may be used with belts, since in case of mishap to belt the comparatively large frictional resistance would represent an appreciable load on it.
The minimum load on a dc series motor should be great enough (not below 15% of full load) to keep the speed of the motor within limits. In case the speed becomes dangerously high the motor must be disconnected from the supply main.
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2. Torque-Current Characteristic:
From the expression of mechanical torque T it is obvious that torque is directly proportional to the product of flux per pole φ and armature current Ia. Up to saturation point flux is proportional to field current and hence to the armature current, because Ia = If. Therefore, on light load mechanical torque T is proportional to the square of the armature current i.e., T α Ia2 and hence curve drawn between torque and armature current up to saturation point is a parabola, as shown in Fig. 1.18.
After saturation point, flux φ is almost independent of excitation current and so the torque is proportional to the armature current i.e., T α Ia. Hence the characteristic becomes a straight line. The useful (or shaft) torque is, of course, less than the total (or gross) torque developed. This is due to torque lost in iron and friction and windage losses.
From the torque-armature current curve it is evident that so long as the field of the motor is not saturated, the series motor exerts a torque proportional to the square of current i.e., starting torque is very high. Hence series motors are used where large starting torque is required for accelerating heavy masses quickly such as in hoists, electric railways, trolleys and electric vehicles.
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From torque-current characteristic it is evident that series motor develops large starting torque to accelerate the heavy masses and from speed-current characteristic it is also evident that speed falls as the load increases, so series motor is automatically relieved from heavy excessive load, therefore, series motor is best suited for electric traction work.
3. Speed-Torque Characteristic:
For low values of load current,
Where, C1 and D1 are two constants, i.e., the shape of the speed-torque characteristic will be hyperbolic.
For large values of load current, φ remains almost constant and therefore, speed will be given as-
N = C2 – D2T …(1.32)
Where, C2 and D2 are constants for the machine, i.e., speed- torque characteristic will be linear in nature.
Speed-torque characteristic is also known as mechanical characteristic. Speed sharply falls with the increase in torque for smaller values of load. But at higher loads, the speed drops linearly but slowly with increasing torque.
Hence series motors are best suited for services where the motor is directly coupled to the load such as fans whose speed falls with the increase in load torque.
Performance Curves of a DC Series Motor:
In Fig. 1.20, four important characteristics of a dc series motor, namely torque, speed, current and efficiency each plotted against useful output power are shown.
From the performance curves for a dc series wound motor it is noted that:
(i) The speed of series motor falls rapidly with the increase in load, so a series motor is not suitable for services requiring a substantially constant speed.
(ii) The efficiency increases rapidly in the beginning, reaches its maximum value and then decreases. This is due to the fact that at light loads the friction and iron losses are large compared with the load and the effect of these losses becomes less with the increase in load. The armature and field copper loss varies as the square of the current, so these losses increase rapidly with the increase in load. The efficiency becomes maximum when friction and iron losses are practically equal to the copper loss.
(iii) Series motor develops a starting torque comparatively greater than that developed by a shunt motor for a given current. Hence series motors are best suited where huge starting torque is required i.e., for streetcars, cranes, hoists and locomotives.
(iv) In addition to the large starting torque, there is another unique characteristic of series motors which makes them best suited for traction services. For instance let the streetcar ascend a gradient. In case the shunt wound motor is used for driving the above streetcar, it will maintain the speed of the car at approximately the same value that it had on the level ground and, therefore, will tend to draw an excessive current.
A series motor, on the other hand, automatically slows down on such a gradient because of increased current demand and, therefore, it develops more torque at reduced speed. This drop in speed allows the motor to develop a large starting torque with but a moderate increase of power. Hence under the same load conditions, the rating of a series motor would be less than for a shunt motor.