Selecting dc motors

  Choosing a DC motor type and associated equipment for a given application requires consideration of several factors.
Speed range — The minimum and maximum speeds for an application determine the motor base speed. Allowable speed variation — Applications requiring constant speed at all torque values should use a shunt-wound motor. If speed changes with load and speed variation must be minimized to less than 2%, a regulator employing tachometer feedback

must be used.

  Torque requirements — The torque requirements at various operating speeds should be determined. Many applications are essentially constant torque, such as conveyors. Others, such as centrifugal blowers, require torque to vary as the square of the speed. In contrast, machine tools and center winders are constant horsepower, with torque decreasing

as speed increases. Thus, the speedtorque relationship determines the most economical motor.

  Reversing — This operation affects the power supply and control. When the motor cannot be stopped for switching series fields before reverse operation, compound and stabilizing windings should not be used if full load torque is needed in both directions. Bi-directional operation may also affect brush adjustments. Duty rating — DC motors carry
one of three ratings:
 • Continuous duty is applied to motors that will continuously dissipate all the heat generated by internal motor losses without exceeding rated temperature rise.
 • Definite time, intermittent duty motors will carry rated load for specified time without exceeding rated temperature rise. These motors must be allowed to cool to ambient before load is repeated.
 • Indefinite time, intermittent duty is usually associated with some RMS load of a duty-cycle operation. Peak torque — The peak torque that a dc motor delivers is limited by that load at which damaging commutation begins. Brush and commutator damage depends on sparking severity and duration. Therefore, peak torque depends on the duration and frequency of occurrence of the overload. Peak torque is often limited by the maximum current that the power supply can deliver.

  Motors can commutate greater loads at low speed without damage. NEMA standards specify that dc machines must deliver at least 150% rated current for one minute at any speed within rated range, but most motors exceed this requirement.

  Heating — The temperature of a dc motor is a function of ventilation and losses in the machine. Some
  losses — core, shunt-field, and brushfriction— are independent of load,and vary with speed and excitation. Several methods can predict operating temperature. The best method is to use thermal capability curves available from the manufacturer.