Is torque output enough to select a motor?  … definitely NOT

Well, that sounds like a problem because torque output at desired speed is the mechanical power required.  This mechanical power output is widely used to define the need for almost any motor driven system.

The torque output from any motor depends on three main things.

  1. Electrical power input
  2. Any controller or driver that may be in between the motor and the power source
  3. Thermal conditions and the mechanical mounting of the motor

Electrical power input cannot always be taken for granted. Different mains produce different voltage and within the standards there are many variations. If a power supply is used, its ratings and internal control methods need to be examined to insure enough power can get to the motor. This subject is usually not one that poses a problem in motor selection.

For the last 100 years, some form of control, controller, or driver has found its way between the motor and the power source. In the simplest form a soft start circuit is required, and it gets more complicated from there, covering inverters, drivers, ESC’s, servo drivers, and other power converters.  All electronically commutated motors need some type of power converter (driver) to adjust voltage and current going into the motor to control speed, torque, and position.  There is an efficiency loss in the converter, but it is usually small and well worth the benefit of the added control.

This leaves thermal conditions as the limiting factor. It is the most overlooked and most problem prone area in selecting a motor for your project.  Consumer, Commercial and Factory power transmission applications typically use AC motors (induction or Asynchronous). This class of motor has a very official name plate with ratings and each has been tested under well known controlled conditions to meet the ratings. This motor type also uses standard frame sizes from NEMA or IEC standards.

Newer technologies like Permanent Magnet AC motors, Brushless DC motors, PM Synchronous motors, Switched Reluctance motors, and others do not follow any industry standards for ratings and mounting. Because there are no certified tests and sizes, suppliers have the opportunity to do internal testing, or no testing, and claim their own rating on the motor.  If they allow the motor to get hot, or use a large heat sink to rate the motor, then it can carry a torque rating well above another similar size motor. There are no standards, therefore, torque ratings cannot be used to select motors.

Datasheets for the unregulated motor types are also lacking in consistency and information. They do not contain all of the details on how testing was conducted, and some critical information is typically left out if the supplier chooses to do so.

Ok, enough of the doom and gloom. There is a path through this mess.  Every motor can be rated on a level playing field using information that is traditional on the datasheet. Two simple parameters that every datasheet should have, Torque Constant, Kt, and Resistance, R.  Using the following equation the user can self calculate the Motor Constant Km. Km = Kt/SQRT(R). It is important that the units are correct and the measurements are done correctly, but this is typically available information.  Calculate the Km for every motor you are considering.

In a typical application, thermal conditions will govern what is possible. Again, Km is your ticket here also. If you have a system where your temperature is limited and the ability to dissipate heat is critical to success. Calculate (or model) the Watts dissipation available (heat flow) for the temperature rise you want. Km = Torque/SQRT(Watts).  You can then calculate what Km you will need and compare it to the Km of the motor you think will work.

It’s simple. One equation.

Do not believe any marketing hype touting the highest torque density, or Axial Flux motors are better, or we have proprietary EV motor technology… the list goes on. Everyone is looking for an advantage, you will even see people claiming motors are designed for robots. What does that mean, I am not sure, but if Km of this motor is higher, than it can produce more torque under the thermal constraints.

Just calculate the Km and see if the motor in question is really better.