How to Control a Permanent Magnet Servo Motor

Permanent magnet servo motors have the highest torque/size and power/size ratios in the industry. They are often used in a wide range of applications including electric vehicles, retrofit applications, and conveyors, mixers, grinders, pumps, and more. There are several important factors that should be considered when deciding between a PM servo motor and an induction motor for a given application. Factors include dynamic performance, torque density, inertia matching, and control schemes.

The main drawback to the permanent magnet servo motor is that it requires precise control systems equipment to operate properly. This increases upfront costs and can make this technology unfeasible in some applications. Despite this, many manufacturers continue to develop new designs and materials that reduce the cost of these systems.

PM servo motors require a system to monitor and control rotor position, speed, and output power. This is accomplished by using a feedback sensor such as a potentiometer to produce a voltage value that correlates with the shaft position of the motor. The feedback signal is sent to an error comparator amplifier which compares the voltage resulting from the potentiometer input to the desired or expected position of the motor shaft and produces a control output signal that either raises or lowers the current passing through the motor windings.

When the rotor is not accelerating or decelerating, the motor can be controlled with a low-speed observer. This method improves the accuracy of position signals at very low speeds and eliminates lag. However, the use of an observer can increase the initial starting currents of a PM motor which could cause it to overheat and damage its internal components.

In addition to the observer, a PM motor can be used with an encoder that provides accurate position information for a system. These sensors are available as either a rotary or linear version and provide feedback for position control of the motor shaft. They are available in a variety of resolutions and interfaces to fit different systems.

Using the observer method, the position of a PM servo motor can be accurately determined at very low speeds. This improves the accuracy of the position signal compared to other methods that use polynomial interpolation and can reduce the time needed for the observer to reach full stability. This method also increases the stability of the control system and error convergence.

Another limitation of a permanent magnet servo motor is the fact that it can lose its magnetic properties when exposed to certain conditions. This phenomenon is known as demagnetization and happens when the magnets reach their Curie temperature, a point where they lose their magnetism. This can occur if the permanent magnets are subject to high temperatures or vibration.

To prevent this from happening, a system can be designed to use different magnets for each axis of rotation. This will allow for a hedge against the volatility of rare earth material prices and availability. This is a common feature in high-speed PM servo motors that utilize neodymium or samarium magnets.