Servo motors are now sufficiently common throughout many sectors of industry that engineers ought to know the basics of specifying them for new applications and maintaining/replacing them in existing machinery.

The four cardinal conditions for optimum servo motor performance are RMS torque, peak torque, Motor Speed and most importantly Inertia Ratio. You should select a selected motor should sit comfortably within these four characteristics; try not be too close to the upper limit for any of them.

Be careful when reading the manufacturer’s rating data and do not fall into the ‘specmanship’ trap between manufacturers. A motor’s continuous stall torque is the rating at zero speed and may not be the continuous rating torque of the motor at your operating speed. Invariably there is a torque roll off, which can be quite dramatic, as the motor speed increases. This can be a function of the motor/amplifier combination and can be seen from the combination torque curves published by the manufacturer. Remember; if you are combining motors and amplifiers from different manufacturers then the ratings shown for the motor could be limited by the amplifier performance.

Be careful when considering the motor peak torque. Normally the ratings given are for the maximum capacity. This can have a duration of a second or less. However, if you are using less than the rated peak torque then it may be possible to use the lower value for longer periods. Check the manufacturer’s data, and if necessary seek advice.

It is not uncommon to regularly suffer overload tripping during acceleration. Check that the rate of acceleration set in the motion controller is the equivalent of the value used in your selection criteria – that is the problem nine times out of 10.

Inertia ratio is one of the critical considerations when selecting a servo system. The Inertia Ratio is the ratio of the total load, actuator, gearbox, coupling etc reflected to the motorshaft inertia. Manufacturers talk about ratios of anywhere between 10:1 and 100:1, or higher, being manageable by their systems. However, as a rule of thumb the more dynamic the system the closer to unity the inertia ratio needs to be. The higher inertia ratios that may be achievable will probably not be tolerant of any significant load change in the duty cycle and could have settling issues or even become totally unstable. If you are not sure ask the manufacturer to confirm your selection or suggest an alternative. It is important to remember that the actuators and load bearing dynamic components of the driven mechanism, in addition to the payload, have to be driven by the servo motor. In some cases the characteristics of these items can demand more torque than the payload. Therefore; the more information you can obtain regarding the total mechanism then the less potential there is for the system to fail.

On vertical load applications ensure that the regeneration capacity of the system is not overloaded when decelerating in the downwards direction. This also applies to any load, horizontal or vertical, that can try to overrun the motor. Normally regenerative capacities are taken into consideration in manufacturers’ selection software or the method of calculation should be available in the manufacturer’s literature.

Remember the saying ‘If you can hold your hand on a servo motor while it is running then its oversized!’ Correctly specified servo motors can run with high case temperatures.

Consequently make sure there is a sufficient thermal management programme to dissipate the generated heat and prevent thermal tripping.

Take precautions to ensure that no physical harm can be caused to personnel by the heat being dissipated by the motor through the case. Burns caused this way can be quite severe and law suits very alarming

When replacing a motor so not automatically offer an equivalent servo to the one already used on a machine. You have to ask why the motor is being replaced. Will an equivalent motor experience the same issues as the original motor? Always select your alternative servo motor from the basic application parameters.

If you are not sure how to select a servo motor and amplifier, or are not sure of the system offered to you, then take further advice.

An incorrect specification can be more expensive than changing the motor, take into consideration the potential changes to gearboxes, mounting arrangements and lack of performance plus the time delays.

Graham Kalko is with Servo Sizing Solutions, Great Oakley, Northants, UK. www.servosizingsolutions.co.uk