FEATURE DRIVES, CONTROLS & MOTORS
UTILISING WASTE HEAT generated in electric motors
Electric motors are industry’s biggest users of energy which is why efficiency is high on the agenda of government authorities and large corporations, says Welkon
T
he IEC 60034-30-1 standard published in March 2014 defines the
current highest IE4 efficiency class for electric motors, with the ensuing IE5 class currently under development. These are incremental improvements and soon we will hit the limit when it will not matter how much more steel lamination and copper we add to motors – the improvement in efficiency is bound to become disproportionate to its cost. It is therefore logical that, rather than
pushing the electrical efficiency higher and higher, some large users will look at utilisation of the waste heat originating from inside the electric motors.
NATURALLY OCCURRING LOSSES These have been categorised into Joule losses caused by the electric current passing through copper conductors in the stator winding. In squirrel cage motors Joule losses are caused by the induced current passing through the rotor cage. Losses originating in a motor’s steel lamination are classed as magnetising losses. Mechanical losses mainly include friction losses in bearings and shaft seals, whereas ventilation losses are caused by the motor's cooling fan as well as air resistance and turbulence caused by the rotor as it spins inside the stator. In addition, there are stray electro-
magnetic losses, the test and calculation method of which has changed a few times in the past 35 years. Eventually, almost all losses transform
into heat, which needs dissipating through the motor's ribbed frame into the surrounding environment. This heat is usually considered waste heat. However, some large motor users have now started to consider the possibility of utilising the waste heat originating in electric motors in order to use it in their manufacturing processes or to simply generate hot water for their buildings.
WATER JACKET COOLED MOTORS However, this is difficult to achieve when standard air-cooled motors with ribbed frames are being used. People are
TORQUE MONITORING IMPROVES EFFICIENCY
Monitoring torque in a drive shaft is one of the best ways of assessing the performance of plant and machinery. However because drive shafts rotate, hard wiring a sensor into place usually requires the use of a delicate slip ring. An alternative solution is to use a non-contact radio frequency detector to monitor Surface Acoustic Waves (SAWs). Torque imparts a small degree of twist into a driven shaft which will distort SAW devices (small quartz combs) affixed to the shaft. This deformation causes a change in the resonant frequency of the combs which can be measured via a non-contact radio frequency (RF) pick-up mounted close to the shaft. The pick-up emits an RF signal towards the shaft which is reflected back by the combs with its frequency changed in proportion to the distortion of the combs. Electronic processing and calibration of the returned signal generates a precise, real time
indication of the torque being transmitted by the shaft. A SAW transducer is able to sense torque in both directions and provides fast mechanical and electrical responses. As the method is non-contact it offers complete freedom from slip rings, brushes and/or complex electronics, which are often found in traditional torque measurement systems. SAW devices have a high immunity to magnetic forces allowing their use in, for example, motors where other analogue technologies are very susceptible to electronic interference. As industrial engineers automate manufacturing and processing operations they are increasingly turning to torque monitoring to generate the vital operating and production data that maintains production and efficiency.
Sensor Technology
www.sensors.co.uk
Because water is a much better thermal conductor than air, water jacket cooled motors are better cooled and substantially smaller than air-cooled motors of the same output and speed.
therefore increasingly considering water jacket cooled motors as another option. Water is a much better thermal conductor than air. As a result, water jacket cooled motors are better cooled and substantially smaller than air-cooled motors of the same output and speed. A direct comparison between a few examples of air-cooled and water jacket cooled motors can be seen in the table below.
Motor Cooling specification Water Air
90 kW 2 pole 110 kW 2 pole
200L 280M 225M 315S
132 kW 2 pole 250M 315M 160 kW 2 pole 280M 315L
A small circulation pump is needed to keep the cooling water moving through the motor's cooling jacket and the heat exchanger where the otherwise wasted heat gets transferred over for further utilisation. The overall energy efficiency improvement when the heat is fully utilised more than offsets the small additional electricity usage. The use of relatively small industrial squirrel cage motors in water jacket cooled execution for energy saving is not normally practical. However, if we consider big pumping or
compressor stations where several very large electric motors run side by side, then this whole concept starts to make sense. If we look at a 200kW 2pole IE4 MEZ motor its efficiency is 96.5% with waste heat of around 7kW. If only 5kW of this heat could be utilised
for heating buildings or pre-heating various process fluids, the potential savings are equal to more than two oil- filled radiators, the heating power of which is usually around 2kW each. However, IE4 motors are currently the most efficient on the market and in reality IE3, IE2 or even IE1 motors are still operating at most of the sites which in comparison create far more waste heat that could potentially be reused.
Welkon T: 07802 243668
www.welkon.net
16 FEBRUARY/MARCH 2018 |IRISH MANUFACTURING CONNECTINGINDUSTRY / IRISHMANUFACTURING
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