Frameless Motor Selection for Robotics - What do the Specifications Mean?

Search the web for frameless or kit motors and you will find many offerings to choose from. When looking at the motor specifications, there are many important parameters to consider such as rated speed, rated current, peak current, etc. What do all these things mean and why is it important to understand how the values are being presented?

A typical motor data sheet may look like this:

The motor electrical parameters detail the characteristics of the design and allow you to evaluate the best motor for your application. Since most vendors provide similar data sheets for their motors, it is important to pay attention to the units, tolerance, and temperature these items are listed at.

First, let’s look at the temperature at which K_m (motor constant) and K_t (torque sensitivity) are rated. This is usually somewhere between 20°C and 40°C. In the example above, K_t is rated at 25°C and at a “hot” value. The notes section states “hot” is, Winding temp = 155°C for K_t and K_b.  Parameters K_t and K_b will have higher values when rated at a lower temperature.  Be sure to take note of the temperature these values are rated at when evaluating a motor data sheet.

Peak torque, T_p or T_max, is another value to evaluate similarly. Most of the time, vendors will rate their peak torque in the linear range of the motor’s output capability, however it is important to check units and notes to make sure a proper comparison is being made. The peak torque output of a servo motor will become non-linear as you approach the absolute peak torque capability.  It may look good on paper but can be confusing when evaluating similar designs.

In general, motor manufactures provide the necessary information to make the selection process straight forward. As with most things, be mindful of the units and small print to ensure you are making solid evaluations between motor designs.