Speed of a D.C. Motor
Eb = V-IaRa
But Eb=PФZN/60A
PФZN/60A = V- IaRa
Or N = (V- IaRa)/ Ф × 60A/ PZ
Or N = K (V- IaRa)/ Ф
But V- IaRa = Ea
Therefore N= K Eb/ Ф
Or N α Eb/ Ф
Therefore, in a d.c. motor, speed is directly proportional to back e.m.f. Eb and inversely proportional to flux per pole Ф.
Speed Relations
If a d.c. motor has initial values of speed, flux per pole and back e.m.f. as N1 ,Ф1 and Eb1 respectively and the corresponding final values are N2 ,Ф2 and Eb2 then,
N1 α Eb1/ Ф1 and N2 α Eb2/ Ф2
Therefore N2/ N1 = (Eb2/ Eb1) ×( Ф1 / Ф2)
(i) For a shunt motor, flux practically remains constant so that Ф1 = Ф2.
therefore N2/ N1 = Eb2/ Eb1
(ii) For a series motor, Ф α Ia prior to saturation.
therefore N2/ N1 = (Eb2/ Eb1) × (Ia1/Ia2)
where Ia1 = initial armature current
Ia2 = final armature current
Speed Regulation
The speed regulation of a motor is the change in speed from full-load to no-load and is expressed as a percentage of the speed at full-load i.e.
% Speed regulation = [( N.L. speed - F.L.speed)/F.L.speed ] × 100
=[(No -N)/N] × 100
where No = No – load .speed
N = Full – load speed
Written by John on September 14th, 2009 with
1 comment.
Read more articles on DC Motor and Direct Current Machines and Electrical Machines.
#1. October 21st, 2009, at 10:07 AM.
niceeeeeeee