Is Torque Output Enough to Select a Motor?

Torque output at the desired speed is the mechanical power required. This mechanical power output is widely used to define the requirements 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 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 must be examined to ensure sufficient power reaches the motor. This subject is usually not one that poses a problem in motor selection.

For the last 100 years, some form of 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, ESCs, 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 such as 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 could 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 that of another similar-sized 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 the details on how testing was conducted, and some critical information is typically left out if the supplier chooses to do so.

There is a practical way to address this. Every motor can be rated on a level playing field using information that is traditionally 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 the key parameter here as well. If you have a system where your temperature is limited, 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 rely on marketing claims touting the highest torque density, or axial flux motors are better, or proprietary EV motor technology. Without clear definition, such statements offer limited technical value. Everyone is looking for an advantage, you will even see people claiming motors are designed for robots. What that means is often unclear. If the Km of this motor is higher, then it can produce more torque under the thermal constraints.

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