DC Motors
Application criteria for DC motors are discussed next.
Service Conditions
Similar to AC motors, the DC motors should be selected with regard to their environmental conditions. The service conditions may be usual or unusual and may involve environmental as well as operating conditions. The service conditions listed for AC motors also apply to the application of DC motors.
Operation of DC Motor on Rectified AC
The performance of a DC motor operating on rectified AC is different than if it were operating on a DC source having the same effective value of voltage. The reason for this is due to the continuous ripple or pulsation of the output voltage from the rectified AC voltage source. The ripple effect appears in the motor armature current and thus affects its performance. The effect of the rectified voltage on the motor armature current becomes significant when the rectifier pulse number is less than 6 or the amount of phase control is more than 15%. Also, when a DC motor is operated from an unfiltered rectifier power supply, bearing currents may result. Ripple currents may flow to ground by means of capacitive coupling between rotor winding and core. Even though these currents are small in magnitude, they may cause damage to antifriction and sleeve bearings under certain circumstances. Measures should be taken to minimize these currents to avoid damage to the motor.
Operation of the DC Motor below Base Speed
When a DC motor is operated below base speed by means of reduced armature voltage, the motor will heat up if rated full-load torque is maintained. To avoid overheating of the motor, reduce the load to compensate for the overheating of the motor. The speed of the DC motor is directly proportional to the armature voltage, and the torque is directly proportional to the armature current. Overheating can result due to the insufficient heat dissipating ability of the motor at these speeds.
Operation of the DC Motor above Base Speed
DC motors are built so that in case of an emergency they can withstand an overspeed of 25% above rated full-load speed without mechanical injury.
Overload Capability
The general industrial motors of open, forced ventilation, and totally enclosed water–air-cooled types are capable of carrying, with successful commutation, 115% of rated horsepower load continuously at rated voltage through- out their speed range. Refer to NEMA standard MG1, Sections 23.10, 23.11, and 23.28 for overload capability, momentary load capability, and rate of change of load current, respectively, on these types of motors.