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PM motor technology for fan applications

04 January 2012

Since it was introduced in the 1990s, permanent magnet (PM) motor technology has revolutionised the fan industry. Geoff Lockwood discusses the ways various PM motor types and configurations can be used in the quest for energy efficient fan installations

Small PM motors reduce motor energy consumption by as much as 80 per cent because they have a wider efficiency curve compared with asynchronous ac induction motors, as well as better speed control characteristics. PM motors are available in different types: dc machines with separate drives, electronically commutated (EC) machines with integrated drives, single-core and multi-core motor windings that offer different properties and features for a variety of applications.

Understanding each of these configurations, and how they may be applied, leads to significant energy saving opportunities, whether it be for domestic appliances, homes, commercial buildings and retail facilities or in industrial engineering, IT and the telecommunications industry. Admittedly, there are potential issues with PM motor technology, such as acoustic noise and harmonic distortion, but these can be overcome, as will be demonstrated in this article.

Gert Haeussermann developed the first EC motor at the University of Stuttgart in 1985 when the pursuit of energy efficiency was not quite as fashionable as it is today. However, he foresaw the need for significant energy reductions in fan systems that is now encouraged and dictated through a number of incentives and regulations. The EC motor has now become the standard drive for high-efficiency, small and medium sized fans (5W to 5.5kW). The reason why becomes evident when one realises that the wide efficiency characteristic delivers an energy saving in the region of 80 per cent. EC motors are permanently excited motors with integral ac to dc power conversion and electronic commutation. Small to medium size units are manufactured in their millions across Europe each year.

So, how are higher efficiencies achieved with EC motors? Unlike asynchronous ac induction motors, the rotor of the EC motor does not have conductors, but instead contains permanent magnets. The resulting permanent-magnetic field induces currents in the stator windings, generating the necessary torque with no current loss in the rotor. The stators of EC motors have a reduced number of slots leading to simple windings. Moreover, they can have a winding that does not overlap, realising reduced winding ends and therefore less copper. Reduced copper means lower resistance losses. In all, this produces a lower power in the stator resulting in less stator iron losses.

EC technology became an internal code at ebm-papst to differentiate the new technology from ac induction and dc motors. It has since become a generic name used by many to describe a PM motor with integrated commutation and ac to dc conversion.

Where small fans are concerned, EC motors provide more energy savings when compared with small ac asynchronous induction motors due to the decades old requirement to control their speed. In ac induction motors the rotor resistance can be made ‘hard’ or ‘soft’. Hard rotors have a low resistance conductor that provides a steep torque characteristic. To meet the demand of simple speed control, small ac motors destined for use in fans were developed with soft rotors having high resistance rotor conductors. This provided a flatter torque curve.

The nominal speed of the motor is determined by the applied frequency and the number of poles. Asynchronous motors cannot rotate at synchronous speed and there is a slight loss of speed – referred to as ‘slip’. Soft rotors have more slip than hard rotors. The speed of soft rotor designs can be easily adjusted by varying the applied voltage. A reduction in the supply voltage increases the slip.

In contrast to the asynchronous motor the idle speed of the EC motor only depends on the applied voltage to the stator winding or the number of turns of the stator winding. Therefore the nominal speed is set by the motor/fan designer. The EC motor also has a much wider efficiency characteristic. The reduction in stator winding voltage reduces the rotational speed but as there are no slip losses it does not have the same significant losses in efficiency as the asynchronous motor.

An example that demonstrates the flat efficiency curve and more efficient speed characteristics of the EC motor is that of motors for fans in fan coil units (FCUs), which are compact local cooling and heating units in office environments. Most use one to five double inlet forward curved blowers. For acoustic reasons the speed is reduced to around 1,000rpm.

The speed reduction is achieved by reducing the supply voltage via a multi tapped transformer. Typical applied voltages are 130V, 160V and 190V. As the voltage to the asynchronous ac induction motor is reduced the slip increases resulting in a lower speed. However, the efficiency of the motor reduces as well. With a lower speed a lower torque is required to drive the impeller so the fan load moves down the torque curve and down the efficiency curve.

A typical asynchronous ac single phase induction motor used in a FCU is rated at 45 per cent. The EC alternative is 78 per cent. A large proportion of FCUs are twin fan units delivering 190 litre/s at 5Pa external pressure. Measurements show that 116W is a typical power input for the ac induction motor version. As the EC is nearly twice as efficient then it can be assumed that the power consumption would be half that of the ac version.

However, the reality is that the EC version consumes only 31W, some 73 per cent lower. The reason is the narrow peak efficiency of the ac motor and the low applied voltage (130V in this case) compared with the wider peak efficiency band and the minimum drop-off in efficiency with reduced speed of the EC motor.

Single-core or three-core?
EC motors for fan drives are either single-core or three-core machines. Single-core windings are simple to manufacture but a single-core winding machine can suffer from a ripple of the output torque. With fans this can lead to impeller vibration and noise. Mechanical isolation will ameliorate the problem, but only works with very small motors. The alternative is to use a three-core machine, which offers a smoother torque output. These have three windings, similar to a 3-phase ac machine where two windings are energised at any one time and the torque output of each overlaps.

As well as the ripple on the torque output affecting acoustics, the drive electronics can also be an issue. The rotating magnetic field is achieved by changing the direction of current flow through the stator many times per second. This is done by using electronic switching known as commutation. This switching can result in noise from the fan. The problem can be avoided by switching at a frequency above audible noise levels and the use of electronic filtration.

Like voltage phase chopping and variable frequency drives, EC motors will distort the mains supply resulting in poor power factor and interference with other equipment on the network. As with ac methods, this issue can be managed with either passive or active filtration.

The full benefits of EC technology are realised in air movement applications. The EC motor has an integral variable speed drive and so its inherent energy efficiency can be further enhanced as a result. For example, with FCUs the manufacturers are offering further energy saving by simply changing the speed based on room temperature from 100 per cent to 80 per cent speed. This small speed adjustment cuts the power input by half.

In another example involving the power consumption of fans serving a supermarket refrigeration plant, the ac backward curved fans were replaced with EC backward curved fans and the speed control strategy changed from sequential on-off to variable speed control. Initially the ac fan consumed 2.54kW. A change to EC dropped this to 1.22kW. However, a change in the speed control method to variable speed control dropped the power consumption to 0.55kW – a 1.99kW or 78 per cent saving.

This article is based on a presentation given by Geoff Lockwood at the MDS 2011 conference. Geoff Lockwood is with ebm-papst UK

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