Curtis AC Controllers drive Surface and Internal PMAC

For many years Curtis have been known for designing extremely reliable and versatile AC motor controllers.

These controllers are periodically updated to meet and exceed European Safety requirements and the E and SE Series models are now capable of running Synchronous and Asynchronous AC motors. Their advanced vector control technology ensures that efficiency is optimised under all load conditions and temperatures.

LetÂ’s look at the motor types, as this can be confusing.

Induction motors are Asynchronous. This means that the rotating core of the motor, the rotor, runs faster or slower than the rotating AC magnetic field. The rotating magnetic field induces current into the rotor and sets up the opposing magnetic field. The AC induction motor is extremely robust as its solid metal rotor has no electro-mechanical brush gear. Fewer components and ease of manufacturing means lower cost. That and the fact that theyÂ’re virtually indestructible, makes the induction motor a popular choice. ItÂ’s so reliable that Tesla chose it for the original model S and itÂ’s used on over 90% of industrial trucks. But of course thereÂ’s always a trade-off, and with induction thatÂ’s a lower efficiency (up to base speed) and an increased size and weight.

Permanent magnet AC motors are Synchronous in that the rotating core of the motor turns at the same as the rotating magnetic field. The rotor has magnets either on the surface of the rotor or embedded into the rotor. These are known as Surface PMAC or Internal PMAC and both have different characteristics. PMAC are highly efficient at base speed and are effective electric generators too. Being smaller and lighter than induction makes them good for a variety of applications like AGV design or robotics.

Motors are designed to achieve peak power at a certain speed; the base speed of the motor. At this point the motor is absorbing as much electrical power as possible. To run the motor faster than base speed, a motor field has to be weakened electronically. The base speed is determined as part of the unique auto-characterisation process pioneered by Curtis at the turn of the century.

For an induction motor itÂ’s simple to reduce a motor field and extend the motor speed, this is energy efficient and one reason Tesla used this motor technology. This is very useful for traction applications. With a PMAC motor more energy is used to suppress the field of the permanent magnets. A Surface PMAC motor is less likely to be field weakened as the magnetic path is not as stable as an Internal PMAC motor. An Internal PMAC motor may be field weakened but extreme care has to be taken in order not to de-magnetise the magnets. This precise field weakening is determined by the motor model held within the controller.

PMAC motors offer higher efficiency up to the base speed and also make very efficient generators as no energy is used to produce the rotor field.