I would like to know how to calculate or select DC motor parameters when knows some parameters.

Knowing parameters:

  • Speed range: -2000 to 2000 rpm
  • Torque range: -65 to 80 Nm
  • Expected friction torque: 2 Nm max
  • Total moment of inertia of the load: 0.8 kgm2 or less

How to calculate or select following parameters?

  • Va- armature voltage
  • Ra - armature resistance
  • La - armature inductance
  • K - motor constant
  • \$\begingroup\$ I tidied up your post but didn't know what you were trying to convey with the last line of text. Do you mean, "I am considering constant flux"? Add a question mark somewhere as you are supposed to be asking something. \$\endgroup\$
    – Transistor
    Commented Feb 28, 2020 at 17:52
  • \$\begingroup\$ @Transistor I removed about flux thing. \$\endgroup\$
    – Ind
    Commented Feb 28, 2020 at 18:04
  • \$\begingroup\$ "How to calculate or select following parameters?" - You can't calculate them from the information given. You must first choose a supply voltage. \$\endgroup\$ Commented Feb 28, 2020 at 23:23
  • \$\begingroup\$ I'm voting to close this question as off-topic because asking for a complete motor design procedure is much too complex for this forum. \$\endgroup\$
    – user80875
    Commented Feb 29, 2020 at 13:34

2 Answers 2


It is not clear if you are trying to design a motor or select one. The friction torque seems a little high for bearing friction, but quite low for a load unless you are just accelerating inertia and overcoming aerodynamic drag.

80 N-m at 2000 RPM is 17 kW. If the motor has an efficiency of 80%, you will need about 21 kW of electrical power. At 48 volts, you would need 440 amps. That is not a very convenient current level to work with. The first thing you need to do is decide what voltage and current levels you want to work with. Then you can determine the motor constant as described in another answer.

  • \$\begingroup\$ 17kW means power develop right? Then it can find Back emf voltage. \$\endgroup\$
    – Ind
    Commented Feb 29, 2020 at 1:22
  • \$\begingroup\$ How to get - values like -2000 speed and -65 torque. Based on that how to calculate motor details? \$\endgroup\$
    – Ind
    Commented Feb 29, 2020 at 1:24
  • \$\begingroup\$ How to get power loss when have friction torque 2Nm loss? \$\endgroup\$
    – Ind
    Commented Feb 29, 2020 at 1:28
  • \$\begingroup\$ Are you asking how to design a motor or how to select a motor to buy? \$\endgroup\$
    – user80875
    Commented Feb 29, 2020 at 3:09
  • 1
    \$\begingroup\$ The negative values of speed and torque indicate reverse direction of rotation and braking torque. Those are obtained by selecting or designing a electronic controller that has that capability. You have guidelines in the answers for determine the motor constant. Determining the number of rotor poles, diameter, length, magnetic circuit design, winding configuration, number of turns, wire size, commutator design, field magnet selection and structure is much to complex to be described here. It requires about 100 textbook pages and close to 100 data items. \$\endgroup\$
    – user80875
    Commented Feb 29, 2020 at 13:32

Given a speed/torque requirement, you're going to want to pick a motor out based on K, the maximum rated voltage, and the maximum rated current/stall current. The equations you care about:

Max motor speed (in rpm, steady state, no load): $$\dot{\theta}_{max} = V_{max}K $$

Max motor torque (in Nm): $$ \tau_{max} = I_{max} \frac{60}{2\pi K} $$

The friction torque and load inertia are both only relevant to the torque specification of your motor. Also note that you'll probably need a small (possibly single stage) gear reduction to hit your speed/torque requirements, and a 10s of kW motor controller (~20kW). Sounds like a motor you might find in an electric bike or low power electric motorcycle.

  • \$\begingroup\$ There is friction torque then T max is 80Nm or 82nm ? \$\endgroup\$
    – Ind
    Commented Feb 29, 2020 at 1:26
  • \$\begingroup\$ My answer describes how to do motor selection given a set of mechanical requirements. I can't with confidence tell you more things about what your mechanical requirements are \$\endgroup\$
    – Ocanath
    Commented Feb 29, 2020 at 3:21

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