When a servo motor that's big enough isn't big enough. Andrew Fallows of Motor Technology issues a few words of wisdom for machine builders"¦
It might seem a strange statement, but believe it or not, there are certain operating conditions where servomotors that should be big enough for an application are actually too small. Too small that is, in terms of their power rating, which for most applications is a perfectly acceptable way of selecting the right size motor. But not for all. As an example, in small, precision pumping applications such as dosing ingredients in the food industry, one would typically use an induction motor and inverter. More often than not, the pumps would be operating at a fixed speed and the motor-inverter combination would be selected on that basis. You would then select the motor, the most common being 2 and 4 pole, the resulting speed of which, let's say at 50Hz, would be approximately 2850 and 1425rpm respectively.
The available torque at the rated speed is what provides the power rating of the motor. So, as the motors are typically operated at their rated speed, using power as a way to specify a motor is appropriate. Servo motors can be operated over a wide speed range, typically in the region of 1:5000, but the torque/speed characteristics are very different. Let's take a servo motor with a stall, or holding, torque of 7.5Nm and a rated torque of 4.8Nm at 6000rpm as an example. You could say that, based on the figures above, the power rating of the motor is: nominal power (Pn) = (nominal torque Mn × Nn nominal speed) / 9550 = (4.8Nm × 6000rpm) / 9550 = 3.02kW. However, you may be running the motor at 3,000 or 4,000rpm. What effect does this have on the power the motor is producing? At 4000rpm the motor will deliver approximately 5.7 and 6.15Nm at 3000rpm. This would equate to approximately 2.39kW and 1.93kW respectively.
There are a number of proven ways you can harness the full power of the motor by using a gearing mechanism such as belts and pulleys or gearboxes. But these mechanical devices may have restrictions on the operating speed. In the case of our pumping example, which shall we say is destined for a production cell operating under ATEX conditions, the gearboxes are limited to an input speed of 2300rpm. Even though the motor specification has rated power at 1.382 kW, the power available at 2000rpm is considerably less at 0.84 kW. We can operate the pumps well within the gearbox speed restriction, but this means that we are not utilising the full speed range of the motor, hence we need a higher kW rating than is actually required were it not for the ATEX operating restrictions.
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