Mechanical Systems 355
to consider each of the steps involved, in order to understand DC motor
operation. As the load on a motor is increased, its torque also increases as
the motor tries to meet the increased load requirement. However, the cur-
rent drawn by a motor also increases when load is increased.
The presence of a cemf to oppose armature current is very important
in motor operation. The lack of any cemf when a motor is being started
explains why motors draw a very large initial starting current, compared
to the running current they draw once full speed is reached. Maximum ar-
mature current flows when there is no cemf. As cemf increases, armature
current decreases. Thus, resistances in series with the armature circuit are
often used to compensate for the lack of cemf, and to reduce the starting
current of a motor. After a motor has reached full speed, these resistances
may be bypassed by automatic or manual switching systems, in order to
allow the motor to produce maximum torque. Keep in mind that the ar-
mature current, which directly affects torque, can be expressed as:VT – VC
IA = ————
RA
where:
IA = the armature current in amperes,
VT = the terminal voltage of the motor in volts,
VC = the cemf generated by the motor in volts, and
RA = the armature resistance in ohms.In determining the functional characteristics of a motor, the torque
developed can be expressed as:T = K Φ IA
where:
T = the torque in foot-pounds,
K = a constant based on physical characteristics
(conductor size, frame size, etc.),
Φ = the quantity of magnetic flux between poles, and
IA = the armature current in amperes.Torque can be measured by several types of motor analysis equip-
ment. The horsepower rating of a motor is based on the amount of torque
produced at the rated full-load values. Horsepower, which is the usual
method of rating motors, can be expressed mathematically as: