DC MOTOR OPERATION
The amount of starting resistance necessary to limit starting current to a more desirable value is
calculated using Equation (6-10.
Rs = starting resistance
Et = terminal voltage
= desired armature starting current
Ra = armature resistance
If the full load current of the motor mentioned previously is 50 amps, and it is
desired to limit starting current to 125% of this value, find the required resistance
that must be added in series with the armature.
The starting resistors are used in a DC motor by placing them in the starting circuit of the motor
controller that is used to start the DC motor. Starting resistors are normally of variable
resistances, with the value of resistance in the circuit at any time being either manually or
automatically controlled. The maximum amount of resistance will always be inserted when the
motor is first started. As the speed of the motor increases, counter EMF will begin to increase,
decreasing armature current. The starting resistors may then be cut out, in successive steps, until
the motor reaches full running speed.
DC Motor Ratings
The nameplate ratings of a DC motor refer to the conditions of voltage, current, speed, and power
at which the motor is normally operated. The principal rating is known as the continuous rating,
which is the rating described on the nameplate of a motor. The continuous power rating is a
thermal rating. At this power, the motor can be operated for long periods of time without a large
rise in temperature and beyond the limits of the conductor insulating material, bearings and other
components, which are greatly affected by temperature.
The speed rating of a DC motor is often given on the nameplate. This speed is the upper limit
at which a motor can be operated without mechanical damage occurring.