Servo Motor Selection Criteria

By Wilfred Voss,
Copperhill Technologies Corporation

The motor data needed to select a motor are rated speed, rated torque, intermittent torque, and rotor inertia. However, the best servo motor selection criteria is to use the motor's performance curve (torque over speed) and to verify it with the application requirements. Not all motor data sheets do provide such detailed information, since some manufacturers prefer to define the rated/intermittent torque and the rated speed of their motors in a more conservatively manner. Under certain conditions it is, however, possible to operate motors beyond their rated data.

Again, the following motor data are essential for the selection process:

• Rated Speed
  

• Rated Torque
  

• Max. (Peak) Torque
  

• Rotor Inertia

The following criteria have to be fulfilled:

• The motor’s rated speed is equal to or higher than the calculated (required) speed.
  

• The motor’s rated torque is equal to or higher than the calculated (System) RMS torque.
  

• The motor’s maximum torque is higher than the calculated (System) peak torque.
  

• The ratio of load to motor inertia does not exceed the user-defined safety factor.

In case of servo motors the ratio of load to motor inertia should not exceed 10:1. Otherwise the motor could start jerking.

Matters become a bit more complex when the motor's performance profile, i.e. torque over speed performance, is used to determine whether or not the requirements are met.

To start with the basics, we need to see how the rated speed and rated/intermittent( (peak) torque of a motor is being defined. Basically it is at each manufacturer's discretion how exactly they define the data. The following picture shows a sample of a motor performance profile:

In this example the rated speed (Vn) is defined as the max. possible speed where the motor still supports the continuous torque (Tcont). The max. speed of the motor is actually higher, but the torque will eventually go down to zero. The intermittent torque (Tmax) may be supported even at higher velocity than the rated speed, however, manufacturers tend to provide data that are on the safe side of the motor operation.

The previous profile may also be a simplified derivation from the actual motor performance. The following picture shows a more complex case:

One way to define the motor data would be to set rated speed, rated torque and intermittent (peak) torque at a safe point in the performance profile as shown in the next picture:

The red lines indicate an example to set the "official" motor data. Besides the point that this is a very coarse definition of the motor's capabilities, this example also shows that the motor would be rejected, since it would (theoretically) not provide the required peak torque.

The following example shows yet another case where the rated motor data would not meet the application requirements, however, the performance profile does support the requirements.

The rated speed of this sample motor has been defined at 2000 rpm, but the torque requirements are low enough that the motor can support the torque even higher than at 2000 rpm.

The motion control engineer must take all of these described circumstances into consideration:

1. If the motor data sheet does not provide the performance profile (torque over speed or vice versa) of a particular motor, the engineer must use the rated data and make sure the following criteria are met:

• The motor’s rated speed is equal to or higher than the calculated (required) speed.
  

• The motor’s rated torque is equal to or higher than the calculated (System) RMS torque.
  

• The motor’s maximum torque is higher than the calculated (System) peak torque.
  

• The ratio of load to motor inertia does not exceed the user-defined safety factor.

2. If the motor data sheet does provide the performance profile (torque over speed or vice versa) of a particular motor, the engineer should still use the rated motor data. The actual selection, however, is based on a comparison of the performance profile with the application requirements, i.e. the engineer verifies whether or not the performance profile supports each torque at the corresponding speed as defined in the duty cycle.