Control for permanent magnet motors

Induction motors are inexpensive, robust, reliable and, with modern AC variable speed drive technology, they are easy to control. So what possible reason could there be for considering the use of any other type of motor? The answer to this question is easily provided in just two words - energy efficiency.

It's not that induction motors are particularly inefficient. In fact, modern motors in all but the smallest sizes will almost certainly have energy efficiencies well above 90 per cent. However, so many motors are in use in commercial and industrial applications that achieving an efficiency improvement of just a per cent or two on each one would, overall, save a lot of energy and significantly reduce CO2 emissions.

For this reason, European Directive 2005/32/EC on Eco-Design Requirements for Energy-Using Products - which is more usually known as the EuP Directive - mandates a phased introduction of more and more efficient motors. For example, from June 2011, all motors placed on the market must, as a minimum, reach the requirements for the IE2 efficiency class, as defined in IEC 60034-30. And from January 2015, motors rated 7.5kW to 375kW will have to be class IE3, or IE2 if fitted with a variable speed drive. This requirement will be extended to motors below 7.5kW from 2017.

These are excellent aspirations, but it is becoming increasingly difficult for manufacturers to develop and manufacture cost-effective induction motors that satisfy the requirements for the higher efficiency levels. And the situation is only going to get worse, as there is provision in IEC 60034-30 for an even more efficient class of motor, which will be designated IE4.

Because producing standard induction motors with sufficiently high efficiency is problematic, it is no surprise that motor manufacturers - and users - are starting to re-examine other motor technologies. This has led to the realisation that permanent magnet synchronous motors have a lot to offer.

In particular, because they require no magnetising current, they can be made more efficient than standard induction motors which means that they may offer the best opportunity of meeting IE4 requirements when these are ultimately defined. In addition, permanent magnet synchronous motors can be made smaller than standard induction motors for a given power rating, and they have excellent torque characteristics at low speed, which means that, in many applications, they eliminate the need for a gearbox.

Speed control for synchronous motors

But what about speed control for synchronous motors, bearing in mind that today's AC variable speed drives were developed primarily to control standard induction motors? Fortunately, the techniques of motor control employed in standard variable speed drives can be adapted so that the same drive can be used to control both induction motors and permanent magnet synchronous motors. The most basic method of control for the new motors is voltage/frequency (v/f) control with no speed or position sensor feedback. As is the case with induction motors, this form of control is most useful for less demanding applications.

The dynamic response of the motor is not particularly fast, but overall control is robust and speed accuracy is good. V/f control does not, however, offer the best possible efficiency from the motor/drive system, and varying the load torque affects not only the active power delivered to the motor but also the reactive power. For high-speed applications, such as turbo blowers and compressors, v/f control may be the only practical option.

In most applications, however, improved performance can be obtained by using closed loop speed/torque control with feedback from an encoder. With this arrangement, optimal current control can be achieved, which means that the required level of torque is achieved with the minimum possible current under all operating conditions. In addition, reactive current can be reduced to zero provided that field weakening is not used. Closed loop control can, therefore, deliver the highest possible overall efficiency when used with synchronous motors.

In addition, it is possible to extend the speed range of the motor by using field weakening, although the extra range is limited to around 20-50 per cent above the motor's nominal speed. In the field-weakening range, however, the motor will draw negative reactive power (that is, it behaves as a capacitive load on the supply) and its output torque is limited because of the need to ensure that the maximum permissible drive current is not exceeded.

Further, under field-weakening conditions, the back EMF of a synchronous motor can exceed the supply voltage, which can be problematic. Overall, therefore, field weakening, while undoubtedly valuable in some applications, should be approached with caution.

Sensorless flux vector control

While closed loop control with encoder feedback offers significant benefits, there are many applications where it is desirable to avoid the additional cost and inconvenience of fitting an encoder. In many of these cases, sensorless flux vector control offers an attractive option with synchronous motors, in the same way that it does with standard induction motors.

The limitations are also similar with the two types of motor, in that sensorless flux vector control delivers good dynamic response and excellent torque control, but motors with this form of control cannot deliver maximum torque at low speeds. Note also, that if optimum results are to be achieved with sensorless control of synchronous motors, control algorithms developed specifically for this type of motor may be needed, although the drive hardware will typically be the same for both induction and synchronous motors.

As we have seen, permanent magnet synchronous motors are likely to be adopted much more widely in the future for commercial and industrial applications. Fortunately, however, these new motors are for the most part fully compatible with existing AC variable speed drive technology.

Nevertheless, when selecting drives for use with synchronous motors, it is a wise decision to work with a supplier like Vacon that has expertise and experience in this field, to ensure that the best possible performance is obtained and that the potential of the new motors for energy efficient operation is fully realised. Visit the website at www.vacon.com to find out more.