Electrical Power Systems Technology

Mechanical Systems 357

1,885 W

= ———

746

horsepower = 2.53 HP

Another DC motor characteristic that we should discuss is armature

reaction. Armature reaction was discussed in relation to DC generators

in Chapter 7. This effect distorts the main magnetic field in DC motors,

as well as in DC generators. Similarly, the brushes of a DC motor can be

shifted to counteract the effect of armature reaction. However, interpoles

are ordinarily used to control armature reaction in DC motors.

The most desirable characteristic of DC motors is their speed-control

capability. Varying the applied DC voltage with a rheostat allows speed

to be varied from zero to the maximum rpm of the motor. Some types of

DC motors have more desirable speed characteristics than others. For this

reason, we can determine the comparative speed regulation for different

types of motors. Speed regulation is expressed as:

SNL – SFL × 100

%SR = ———————

SFL

where:

%SR = the percentage of speed regulation,

SNL = the no-load speed in rpm, and

SFL = the rated, full-load speed in rpm.

Sample Problem:

Given: the no-load speed of a universal motor is 6,250 r/min, and its

full-load speed is 5,000 r/min.

Find: the motor’s speed regulation.

Solution:

SNL – SFL × 100

%SR = ———————

SFL

6250 – 5000 × 100

= —————————

5000

= %SR = 25%