Sizing and selecting servomotors

Selecting the right motor for a positioning application requires review of the application's speed (and speed range), inertia (and inertia ratio), acceleration (as it relates to overload or peak capacity), and package size.

Vector control technology combines an induction motor with a feedback device, such as an encoder. With feedback, the motor rpm can be controlled down to zero speed. And a programmable controller lets users command position.

Positioning is commonly thought of as a 'servo' requirement, so this puts vector controlled motors in a class with brush-type and brushless servos.

Brush type servomotors provide proven reliability and well-known technology (they have been around forever). Their linear and predictable performance makes them easy to integrate in designs.

Brushless servos provide higher speed capability, higher torques in smaller packages, lower inertia for quicker positioning, and a long, reliable and maintenance-free life.

Once the proper technology is determined, the next step is motor selection. This involves analysing the mechanics of the application, the inertia that the motor sees, and determining the torque levels for each section of the duty cycle.

Acceleration capability determines how fast the load can be positioned. A vector motor's acceleration capability is 150 per cent of continuous. Brush and brushless servomotors, on the other hand, have at least twice the acceleration capabilities of continuous, or more if you over-size the drive.

Bandwidth is a measure of system response, or how fast the technology reacts to a change in command, or disturbance, or torque changes. Velocity loop bandwidth is a measure of how fast a drive reacts to speed commands. Position loop bandwidth is usually dominated and determined by the load.

[image] Speed-torque curves help select the right motor and drive.

In constant speed applications, motors are defined in terms of kilowatt or horsepower ratings (which are torque at a 'base' speed). In positioning applications, the motors normally operate over a wide range, not simply at 'base' speed, so they are typically not 'rated' at 'base' conditions.

Speed-torque curves, therefore, display continuous torque (defined as torque which will not overheat the motor), and peak torque (intermittent) that's essentially acceleration torque.

Compare an application requiring a continuous torque (torque over the duty cycle) of 3 Nm at a speed of 2500 rpm and a peak torque, or acceleration torque, of 7 Nm. This can be accomplished with a voltage of 160 VDC. The continuous and peak currents required are 7 and 16 A. This motor will operate successfully in the application since the application's continuous torque is in the motor's continuous operation area.

The load and motor inertia should be compared. A maximum ratio of 10 (load) to 1 (motor) is recommended. This ratio affects response, resonance and power dissipation.

As the inertia mismatch is increased, oscillations tend to occur, and it takes longer to get the load to settle in position. To prevent this, the drive's gain is reduced. However, this extends settling time and leads to lower acceleration and slower positioning, and may not be acceptable for some applications.

An equation for the load, motor, and the application's transmission can be derived to show the mechanical resonant frequency. This is of the form [image].

[image] An ideal motor-load inertial match (red), compared with a large inertial mis-match (blue) - which causes oscillation. To eliminate oscillation, time to speed is lengthened (green) - reducing machine cycle time.

This equation indicates that the mechanical resonant frequency depends on the transmission stiffness and that it is lower for high inertia loads.

For best response, the resonant frequency should be outside the system bandwidth, typically 5-10 times the servo loop bandwidth. The easiest, quickest, and least expensive methods are to use gearing or a larger motor with more inertia.

Finally, analysing energy for optimum versus non-optimum ratios indicates that energy requirements increase as the mismatch ratio increases. For a mismatch of 5, the additional power dissipation is 6.7 times larger than the optimum situation; and a mismatch of 10 has additional power dissipation of 25.5 times larger than the optimum situation. Thus, system power dissipation is minimised with inertia matching.

Once motors are put into commission, the most common servo problems are noisy bearings caused by high side loads, brake disk wear during attempts to use a holding brake as a stopping brake, de-magnetisation due to overcurrent on ferrite motor designs, and shorted and grounded armature from brush dust build-up in DC brush type motors.