The most conventional unit of measuring the speed of a motor is RPM. This stands for revolutions per minute, or the number of times the motor’s shaft will rotate in one minute.
Speed Of A DC Motor
Back emf Eb of a DC motor is nothing but the induced emf in armature conductors due to rotation of the armature in magnetic field. Thus, the magnitude of Eb can be given by EMF equation of a DC generator.
Eb = PØNZ/60A
(where, P = no. of poles, Ø = flux/pole, N = speed in rpm, Z = no. of armature conductors, A = parallel paths)
Eb can also be given as,
Eb = V- IaRa
thus, from the above equations
N = Eb 60A/PØZ
but, for a DC motor A, P and Z are constants
Therefore, N ∝ K Eb/Ø (where, K=constant)
This shows the speed of a dc motor is directly proportional to the back emf and inversely proportional to the flux per pole.
Speed Regulation of DC Motor
The speed regulation is defined as the change in speed from no load to full load, expressed as a fraction or percentage of full load speed. A motor that has nearly constant speed at all loads below full rated load, has good speed regulation.
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