When does a rotating load require a drive or a servo? I run into this issue on a weekly basis. Everyone has their own answer. As much as this may be a matter of opinion for most people, there are some guidelines that can help make this question more straightforward.
Some people define servo’s as closed loop versus drives which are open loop. The term servo does require that there is a feedback device to provide the loop closure. But there are many AC drive vendors making closed loop inverters to enhance the performance of the motor. AC drives with feedback are generally used where positioning is required. So the feedback element is not the determining factor for defining if an application is AC or DC.
The overall power level may define one versus the other, but not always. Brushless servo motors are generally limited to 7″ or 8″ diameter and an equivalent horsepower rating of 20-30 horsepower. There are frameless motors with even higher horsepower ratings. But the size and power rating are strictly a function of manufacturing and marketing constraints. For a major manufacturer, the question is really, how many motors of a given size are we going to sell? Based on the high cost of Neodymium permanent magnets, a larger servomotor is going to be very expensive.
But overall power ratings are not limited when you consider products from specialty companies like Powertec. Powertec takes standard AC motor frame designs and increases the power density by adding embedded permanent magnets on the rotor. Since the magnets are Ferrite, which aren’t as expensive, they are much more economical and allow designs as big as 400HP. So power level by itself doesn’t determine what technology to use.
The real answer is in the load conditions. What is the dynamic response required for the target application? The rate of change of the load is the key. Most AC drives are specified in terms of the frequency response or dynamic response of the power electronics. This important parameter is expressed in Hertz.
Dynamic response is the ability of the drive to regulate speed when the load varies. The load torque can change significantly, usually 90-100%, and the drive will recover the set speed within the time defined by the dynamic response. Typically, an open loop AC drive has a 10 hertz dynamic response, which means that it will regulate to 1/10th of a second.
AC drive technology has improved to the point where dynamic response can reach 200 Hertz when a rotary encoder is added to the motor. This means the drive can regulate load variations wiithin 5 milliseconds. Which is pretty fast when the load mass is high enough to require a motor of 25 horsepower or larger.
The basic physics are simply that the bigger the load, the slower the dynamic response. You just can’t make a ton of rotating mass change speed really quickly. And that’s how the controls should respond.