Characteristics of D.C. Series Motor

 Characteristics of D.C. Series Motor

AU: May-03, 06, 15, Dec.-06, 09, 11, 14

i) Torque-Armature current characteristics

In case of series motor the series field winding is carrying the entire armature current. So flux produced is proportional to the armature current.

ϕ α I_a

Hence T_a α ϕ I_a  α I_a²

Thus torque in case of series motor is proportional to the square of the armature current. This relation is parabolic in nature as shown in the Fig. 4.14.1.

As load increases, armature current increases and torque produced increases proportional to the square of the armature current upto a certain limit.

As the entire I_a passes through the series field, there is a property of an electromagnet called saturation, may occur. Saturation means though the current through the winding increases, the flux produced remains constant. Hence after saturation the characteristics take the shape of straight line as flux becomes constant, as shown. The difference between T_a and T_sh is loss torque T_f which is also shown in the Fig. 4.14.1.

At start as T α I_a², these types of motors can produce high torque for small amount of armature current hence the series motors are suitable for the applications which demand high starting torque.

ii) Speed-Armature current characteristics

From the speed equation we get,

N α E_b / ϕ α V-I_aR_a-I_a-R_se/I_a as Φ∞ I_a

in case of series motor

Now the values of R_a and R_se are so small that the effect of  change in I_a on speed overrides the effect of change in V-I_aR_a-I_a R_se on the speed.

Hence in the speed equation, E_b = V and can be assumed constant. So speed equation reduces to,

          N ∞ I /I_a

So speed-armature current rectangular hyperbola type as Fig. 4.14.2.

iii) Speed-Torque characteristics

In case of series motors, T ∝ I_a² and N∞ I / I_a

Hence we can write, Να 1 / √T

• Thus as torque increases when load increases, the speed decreases. On no load, torque is very less and hence speed increases to dangerously high value. Thus the nature of the speed-torque characteristics is similar to the nature of the speed-armature current characteristics.

•The speed-torque characteristics of a series motor is shown in the Fig. 4.14.3.

1. Why Series Motor is Never Started on No Load ?

• It is seen earlier that motor armature current is decided by the load. On light load or no load, the armature current drawn by the motor is very small. In case of a d.c. series motor, ϕ ∝I_a and on no load as I is small hence flux produced is also very small.

According to speed equation,

N∞ 1 / Φ as E_b is almost constant.

• So on very light load or no load as flux is very small, the motor tries to run at dangerously high speed which may damage the motor mechanically. This can be seen from the speed-armature current and the speed-torque characteristics that on low armature current and low torque condition motor shows a tendency to rotate with dangerously high speed.

• This is the reason why series motor should never be started on light loads or no load conditions. For this reason it is not selected for belt drives as breaking or slipping of belt causes to throw the entire load off on the motor and made to run motor with no load which is dangerous.

Review Questions

1. Draw and explain the characteristics of d.c. series ada motor. AU: May-06, 15, Dec.-06, 09, 11, 14, Marks 6

2. Why a d.c. series motor cannot be started on no load? AU: May-03, Marks 4