Feature Drives & motors motor technologies Re-examining
In commercial and industrial applications, attention is starting to shift towards permanent magnet synchronous motors. But what are the benefits and how easy are these to control? Stephen Takhar of Vacon UK comments
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nduction motors are inexpensive, robust, reliable and, with modern AC variable speed drive technology, easy to control. So what reason could there be for considering any other type of motor?
The answer is energy efficiency. It’s not that induction motors are particu- larly inefficient – in fact, modern motors in all but the smallest sizes will almost certainly have energy efficien- cies above 90%. However, so many motors are in use in commercial and industrial applications that achieving an efficiency improvement of just 1-2% on each one would both significantly
reduce CO2 emissions and save energy. The European Directive 2005/32/EC on Eco-Design Requirements for Energy- Using Products (the EuP Directive) man- dates 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. From January 2015, those 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. To add to this, there is provision in IEC 60034-30 for an even more efficient class of motor, which will be designated IE4. As a result, motor manufacturers and users are starting to re-examine other motor technologies, and permanent magnet synchronous motors offer a number of benefits. These require no magnetising current so can be made more efficient, they can be made smaller than standard induction motors for a given power rating, and have excellent torque characteristics at low speed, meaning that, in many applications, they eliminate the need for a gearbox.
Control
When it comes to speed control, the techniques of motor control employed
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in standard VSDs 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. This 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. Here, optimal current control can be achieved. 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.
It is also possible to extend the speed range of the motor by using field weakening, although the extra range is limited to around 20-50% above the motor’s nominal speed. In the field weakening range, the motor will draw negative reactive power and its output torque is limited because of the need to ensure that the maximum permissi- ble drive current is not exceeded. Further, under field weakening condi- tions, the back EMF of a synchronous motor can exceed the supply voltage. Field weakening should therefore be approached with caution.
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 solution with synchronous motors, in the same way that it does with stan- dard induction motors.
The limitations are also similar with both types of motor, in that sensor- less 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. If optimum results are to be achieved with sensor- less control of synchronous motors, control algorithms developed specifi- cally for this type of motor may be needed, although the drive hardware will typically be the same for both induction and synchronous motors.
The future
Permanent magnet synchronous motors offer a number of benefits
Permanent magnet synchronous motors are likely to be adopted much more widely in the future and, for the most part, are fully compatible with existing AC variable speed drive technology. Nevertheless, when selecting drives for use with synchronous motors, it is wise to work with a knowledgeable sup- plier to ensure that the best possible performance is obtained and that the potential of the new motors for energy efficient operation is fully realised.
Vacon T: 01455 611515
www.vacon.com
rotor, winding, magnets and bearings which enable it to survive temperatures as high as 240°C. It can also operate at 200°C and atmospheric pressures from high vacuum to 1700bar (25,000psi). Through laboratory and field trials, the 22mm motor was proven to perform despite vibration of up to 25grms, and
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resist impulse and impact forces of 100G: 50 times those undergone by the company's motors in jet fighters and Grand Prix cars. In addition, testing while fully submerged in oil revealed its initial 80-watt output rating in air could be trebled under such conditions. Now, in response to requests from engineers, the same modifications will be incorporated into 16mm, 32mm and 42mm versions of the HD motor. The next challenge is to broaden the range and hopefully push on towards 300°C, states the company. maxon motor
www.maxonmotor.co.uk Enter 238 OCTOBER 2011 Design Solutions 1971-2011 Enter 237 Developing a heavy duty motor range
eatures incorporated by maxon motor into a small brushless motor for operation in deep oil boreholes are now being used on a new heavy duty motor range. The resulting motor was the EC22 Heavy Duty, which has refinements to its
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