Page 14 - Industrial Technology EXTRA - 10th August 2020

SERVO MOTORS...

When a servo motor that’s

big enough isn’t big enough

A servo motor, seemingly correctly sized, should be be enough the job, but somehow
it isn’t. We asked the experts at Motor Technology to explain exactly why it isn’t

t might seem a strange statement, but yes, you be selected on that basis. You would then select the
read that right – there are certain operating motor, the most common being 2 and 4 pole, the
conditions where servo motors that should be resulting speed of which, let’s say at 50Hz, would
Ibig enough for an application are actually too be approximately 2,850 and 1,425rpm
small. Too small that is in terms of their power respectively.
rating, which for most applications is a perfectly The available torque at the rated speed is what
acceptable way of selecting the right size motor. But provides the power rating of the motor. So, as the
not all. motors are typically operated at their rated speed,
For example, small, precision pumping using power as a way to specify a motor is
applications, such as dosing ingredients in the food appropriate. Servo motors can be operated over a
industry, would typically use an induction motor wide speed range, typically in the
and inverter. More often than not, the pumps region of 1:5000, and the
would be operating at a fixed speed and torque/speed characteristics
the motor-inverter combination would are very different. Let’s

August 2020 • INDUSTRIAL TECHNOLOGY EXTRA! • p14