Selecting DC Motors

This page is continued from DC Motors

DC Motor Rating

DC Motors are typically rated in terms of :

  • Rated voltage: The operating voltage on the input side of the motor.
  • Rated power: Power (in horsepower – hp or watts) that the motor is designed to deliver to the load (i.e., output power) for continuous operation.
    (Note that 1 hp = 746 W)
  • Rated speed: Speed (in revolutions per minute, denoted by r/min or rpm) for which the motor is designed to operate for continuous operation.
  • Rated load: The load which the motor is designed to carry for (theoretically) infinite period of time. “Full load” or “rated load” operating condition refers to the operation of motor when it is delivering rated power to the load.

Note: A motor may not always operate at its rated power and/or speed. Operation above these values is not advisable due to overloading. 

Selecting a DC motor to accurately meet a set of requirements requires careful attention. Having to choose between brush-type or brushless motors can complicate the selection. Even experienced designers may sometimes overlook critical motor parameters and find problems after the system is up and running. In the worst case, starting over may be the only alternative.

Since we only need to select a motor without learning its construction or working principle, we can use an expedient procedure select DC motors. This procedure is based upon an accurate definition of the target system parameters and designer experience.

DC motor selection parameters

Several motor parameters are the same for both brush-type and brushless DC motors. One of these is motor constant, Km. It is important but widely overlooked. It is used during motor sizing because it is a figure of merit of the motor power-to-torque ratio. Km is proportional to the ratio of peak torque, Tp, to peak power, Pp, at stall: Km = Tp / PpKm is also proportional to the ratio of torque sensitivity, Kt, to motor terminal resistance, Rm:Km = Kt / Rm.

After the required Km has been determined, a candidate motor with this value or greater is selected from a catalog. The motor is only a candidate at this point because other factors must be determined. As the design selection progresses, some trade-offs typically take place. For example, the motor must also satisfy physical size and inertia requirements. 


DC Motor (Separately excited or Permanent Magnet)

Drive type

Single Quadrant

Four Quadrant

DC Chopper

Converter Single or three phase fully (or half) controlled thyristor bridge Dual single/three phase fully controlled thyristor bridge DC Chopper
Torque/Speed Range Motoring in one direction (Braking in other direction) Motoring and braking in both directions 2 or 4 quadrant versions
Speed control Closed loop control of armature voltage with inner current control loop
Torque control Closed loop control of armature current
Ratings 10 W to 5 MW (Fractional HP to 7000 HP) 0.5 to 5 kW (0.7 to 7 HP) Traction > 500 kW
Max Power Available to multi-MW ratings, but motor limitations restrict the product of Power and Speed to 3 × 106 kW.rev/min
Min speed Good control down to standstill
Notable features Separately excited motors often used above base speed in constant-power mode DC to DC conversion
  Fast torque reversals Smooth torque possible
  • Low cost controller
  • Relatively simple technology
  • Good dynamic performance
  • Relatively simple technology
  • Brush gear maintenance
  • Possible failure on supply loss
  • Instability on fan/pump type loads
  • Low motor IP rating


General Application Considerations

  • Regenerative operation and braking
    All motors are inherently capable of regenerative operation, but in drives the basic power converter as used for the ‘bottom of the range’ version will not normally be capable of continuous regenerative operation. The cost of providing for fully regenerative operation is usually considerable, and users should always ask the question ‘do I really need it?’
    In most cases it is not the recovery of energy for its own sake which is of prime concern, but rather the need to achieve a specified dynamic performance. Where rapid reversal is called for, for example if kinetic energy has to be removed quickly, this implies that the energy is either returned to the supply (regenerative operation) or dissipated (usually in a braking resistor). An important point to bear in mind is that a non-regenerative drive will have an asymmetrical transient speed response, so that when a higher speed is demanded, the extra kinetic energy can be provided quickly, but if a lower speed is demanded, the drive can do no better than reduce the torque to zero and allow the speed to coast down.
  • Duty cycle and rating


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