Before analysing the various characteristics of motors, let us revise the torque and speed equations are applied to various types of motors.
... T α Φ Ia from torque equation.
This is because, 0.159(PZ)/A is a constant for a given motor.
Now Φ is the flux produced by the field winding and is proportional to the current passing through the field winding.
Φ α Ifield
But for various types of motors, current through the field winding is different. Accordingly torque equation must be modified.
For a d.c. shunt motor, Ish is constant as long as supply voltage is constant. Hence Φ flux is also constant.
... T α Ia for shunt motors
For a d.c. series motor, Ise is same as Ia. Hence flux Φ is proportional to the armature current Ia.
... T α Ia α Ia2 for series motors.
Similarly as Eb = (ΦPNZ)(60A), we can write the speed equation as,
Eb α Φ N
... N α Eb/Φ
But V = Eb + Ia Ra neglecting brush drop
... Eb = V – Ia Ra
... Speed equation becomes,
N α (V-Ia Ra)/Φ
So for shunt motor as flux is constant,
... N α V – Ia Ra
While for series motor, flux Φ is proportional to Ia.
These relations play an important role in understanding the various characteristics of different types of motors.
1.1 Speed Regulation
The speed regulation for a d.c. motor is defined as the ratio of change in speed corresponding to no load and full load condition to speed corresponding to full load.
Mathematically it is expressed as,