Speed Regulation of DC Motor

At fixed terminal voltage, adding shaft load to a DC motor usually increases armature current. The added internal voltage drop reduces back emf and speed unless a controller compensates for it. A smaller change between no-load and full-load speed means better speed regulation. A permanent magnet DC motor and a DC shunt motor have nearly constant field flux, so their open-loop speed normally falls moderately with load. A DC series motor changes speed much more and must not run without a coupled load or suitable overspeed protection. The behavior of a compound DC motor depends on its field connection and design. Fixed percentage claims are not universal; use the motor or drive manufacturer’s rated data at stated voltage, excitation and temperature.

Speed of a DC Motor

The generated-emf equation for a DC motor is

where
N = rotational speed in rpm.
P = number of poles.
A = number of parallel armature paths.
Z = total number of armature conductors.


For fixed construction, the speed of a DC motor is proportional to back emf and inversely proportional to the flux per pole (φ). Back emf is approximately the terminal voltage minus armature-resistance and brush voltage drops, so load current affects speed.

Speed Regulation of a DC Motor

Speed regulation compares steady no-load speed with steady full-load speed under the same stated supply and excitation conditions. This page uses full-load speed as the denominator.
With that convention, the per-unit (p.u.) speed regulation of DC motor is

The percentage speed regulation is

where
Therefore,
A smaller positive value means less speed droop from no load to full load. Some manufacturers use no-load or rated speed as the denominator, so compare values only after checking the stated convention and test conditions.