Motion System Design
Things you need to know about sizing and selecting Servomotors

Things you need to know about sizing and selecting Servomotors

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

Servo types

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

Positioning is commonly thought of as a servo requirement, which puts vector- controlled motors in a class with brush and brushless servos. Brush-type servos provide proven reliability and well known technology. Linear and predictable performance makes them easy to integrate in designs.

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

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

Performance plus

Acceleration capability determines how quickly the load can be positioned. Vectors acceleration capability is 150% of continuous. Brush and brushless servos, on the other hand, have at least twice the acceleration capabilities of continuous, more if you over-size the control. But dont forget about cost.

Bandwidth is a measure of system response, or how fast the motor reacts to changes in command, disturbances, and torque variations. 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.

Reading speed-torque curves

In constant-speed applications, motors are defined in terms of horsepower (which is torque at a base speed). In positioning applications, 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 (that won t overheat the motor) and peak torque (intermittent) thats essentially acceleration torque. The voltages necessary to reach various speeds and current required to deliver specific torque is provided in table form to help designers select a control.

For example, look at an application requiring a continuous torque (torque over the duty cycle) of 30 lb-in. at a speed of 3,750 rpm, and a peak torque (or acceleration torque) of 80 lb-in. The bus voltage required is 300 Vdc. The continuous and peak currents required are 7 and 18 A. This motor will operate successfully in the application because the applications continuous torque is in the motors continuous operation area.

Application tips

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 increases, oscillations tend to occur and it takes longer for the load to settle in position. To prevent this, the controls 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 applications transmission can be derived to show the mechanical resonant frequency.

where JL = load inertia

JM = motor inertia

K = transmission stiffness

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

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

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Analyzing energy for optimum versus non-optimum ratios indicates that energy requirements increase as the ratio increases. For a mismatch of 2:1, the energy requirement increases by about 50%. If, for example, on an ideal 1:1 ratio a 5-A control is used, then a 6.2-A control must be used for a 2:1 ratio. In this power relationship, current is the square root.

Thus system power dissipation is minimized with inertia matching. Typical inertia ratios by application are:

¥ 1:1 to 3:1 for robotic type

¥ 4:1 to 7:1 for machine tool, packaging, etc.

¥ 5:1 to 10:1 for other X-Y positioning applications

Maintenance tips

Once motors are put into commission, the most common servo problems are noisy bearings caused by high-side loads, brake disc wear during attempts to use a holding brake as a stopping brake, demagnetization due to overcurrent on ferrite motor designs, and shorted and grounded armatures from brush dust buildup in dc brush type motors.

Engineers often express concern that their motors are operating hot. When determining whether or not a motor is overheated, do not judge the temperature by touch. Instead, use a temperature measuring device. Servo motors are designed to operate hot, with a winding temperature of 155¡C. This relates to the range of 100 to 125¡C on the motor housing. Note that your hand becomes uncomfortable touching 80¡C. This aside, if you find that the motor is running too hot, you can most likely attribute it to excessive load, worn bearings (from excessive side loads), or an improperly tuned drive letting large ripple currents run through the motor.

Other application complaints, such as higher than normal speeds or lower output torque, owe to the demagnetization of ferrite magnet motors. Sometimes exceeding the maximum current of the motor can be attributed to improper setting of the controls current limit.


Users of vector-controlled induction motors have little problem locating replacements. All ac induction motors are manufactured to NEMA standards.

However, with servo motors this historically has not been the situation. Way back when, a startup servo company found a niche in the marketplace and produced a motor for specific mechanical requirements. The resultant growth in the servo market made for numerous companies whose standard product merely met their own standard.

Finally, in 1988, NEMA published a standard for servo motors in its MG7 - Motion/Position Control Motors and Controls. The only guideline on which the numerous servo companies working on standards could agree, however, was metric dimensioning for new products.

NEMA is in the process of updating these standards, intending to publish guidelines for metric and inch dimensions. Contact NEMA at 1300 N. 17th St., Suite 1847, Rosslyn, Va. 22209 for more information.

This month s handy tips are provided by John Mazurkiewicz, Product Manager, Baldor Electric, Fort Smith, Ark.

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