Characteristics of DC Motors

CHARACTERISTICS OF DC MOTORS

The emf and torque equations are

Induced emf, E_a = 

E_a = K_a ϕN

Torque, T =

T = K_Tϕl_a

Speed, N = K (V - I_aR_a) /ϕ

The characteristics curve of a motor are

 (i) Torque and armature current characteristic (T_a/I_a) (Electrical characteristic).

(ii) Speed and armature current (N/I_a) characteristic.

(iii) Speed and torque characteristic (N/T_a) (Mechanical characteristic).

Τ_a α ϕΙ_a, Ν α E_b/ϕ

Characteristics of series motors

(i) T_a/I_a characteristic

In series motor

ϕ α Ι_a

Τ α ϕ Ι_a

 T α Ι_a²

If I_a increases T_a increases as the square of the current. It is shown in fig 3.32.

 (ii) N/I_a characteristics

Ν α E_b/ϕ

If I_a increases, ϕ also increases. Hence speed varies inversly as armature current

as shown in fig 3.33.

(iii) N/T_a characteristic

If speed is high, torque is low and vice-versa. This relation is shown in fig 3.34.

Characteristics of shunt motor

(i) T_a/I_a characteristic

Assuming ϕ is constant. Therefore T_a α I_a. Hence the electrical characteristic is practically a straight line through the origin is shown in fig 3.35.

 (ii) N/I_a characteristic

If o is assumed constant, then N α E_b. As E_b is practically constant and also speed constant for more purposes.

But increasing load, E_b and ϕ decreases so E_b decreases slightly and also some decrease in speed, it is shown in fig 3.36 by the dotted line.

 (iii) N/T_a characteristic

It is shown in fig 3.37

Characteristics of Compound Motor:

These motors have both series and shunt windings. If series excitation helps the shunt excitation (i.e) series flux is in the same direction then the motor is said to be cumulatively compounded. If series field opposes the shunt field then the motor is said to be differentially compounded. Both the motors shown in fig 3.38.

The characteristics of compound motor ie in between those of shunt and series

motors as shown in fig 3.39.