Adjustable Speed Torque and Horsepower Characteristics
The field flux is constant for an adjustable speed shunt motor operating in the voltage range. The applied armature voltage is being varied in such a way that the armature current is being held essentially constant up to the point where rated voltage is applied. Since torque is proportional to armature current times field flux, and both of these quantities are constant through the voltage range, torque is also constant. However, power is equal to voltage times current, and since the voltage is varying, the power is also varying. The voltage speed range of a DC motor is, therefore, known as the “constant torque” range. In the field weakened range, the armature voltage and current remain constant and, therefore, the power is constant. However, the flux is being reduced, and the torque is therefore being reduced. The field weakened speed range is known as the “constant horsepower” range. These speed range characteristics are very important in the application of adjustable speed motors.
Base motor speed is at maximum O.D. coil. Maximum motor speed is at gear-in-speed diameter. For most recoiler applications, a 3 or 4: 1 constant HP range is used. For example, 650/1950 RPM motor would be geared in so 650 RPM = rated line speed at 72″ coil diameter and 1950 RPM = same line speed at 24″ coil diameter. Because motor torque is at its highest level at base speed, the gear reducer it is coupled to must have HP rating at this RPM input. If the recoiler has a 20″ drum diameter, coil build-up mode would be used until coil O.D. reaches 24″. Constant line speed and strip tension would then be maintained as coil grew in O.D. to 72″.
To eliminate the need for a transmission, Chicago Slitter often uses a 6:1 constant HP range for the recoiler drive motor. Line speed potentials between low and high limits are then infinitely variable to suit load conditions for each slitting job.