Constant Torque Applications
Armature voltage controlled DC drives provide constant torque. They can deliver the rated motor torque at any speed between zero and the base speed of the motor. Horsepower varies in direct proportion to speed. Full rated horsepower is developed only at base speed. Full motor torque is available from zero to 100 percent base speed
Constant horsepower applications
ARMATURE CONTROLLED DC DRIVES
Certain applications require constant horsepower over a specified speed range. Because an armature voltage controlled DC drive has constant torque characteristics, the drive must be oversized to handle these applications.
For example, a drive required to deliver constant horsepower over a 2:1 speed range must be rated at twice the required horsepower. This is because it only develops 50 percent of its rated horsepower at half speed.
The motor horsepower rating required for any constant torque drive operated in a constant horsepower application can be easily calculated by multiplying the desired horsepower by the ratio of the speed range over which horsepower must remain constant. If 5 HP is required over a 3:1 range, an armature-only controlled drive rated for 15 horsepower (5 x 3) would be required.
FIELD CONTROLLED DC DRIVE
In a shunt-wound DC motor, a reduction in field voltage to less than the design rating results in an increase in speed for a given armature voltage. This also results in a higher armature current for a given motor load. A simple method of accomplishing this type of control is by inserting a resistor in series with the field voltage source. This may be useful for achieving an ideal motor speed for the application.
A more sophisticated method uses a variable voltage field regulator. The variable regulator provides coordinated automatic armature and field voltage control for extended speed range in constant HP applications. The motor is armature voltage-controlled for constant torque, variable HP operation to base speed. The motor is then transferred to field control for constant HP-variable torque operation to maximum speed.