What maketh a motor?

With the Enhanced Capital Allowance scheme now in effect, businesses buying multi-phase induction motors can claim the capital cost of the motor plus any direct installation costs.

With the Enhanced Capital Allowance (ECA) scheme now in effect, businesses buying multi-phase induction motors as independent units can claim the capital cost of the motor plus any direct installation costs.

For ac motor energy cost savings, interest centres on eff1 and eff2 levels of efficiency, as determined by the European motor manufacturers organisation, CEMAP.

Mostly, achieving higher efficiency ratings has involved analysing the energy efficiency of existing motors and making design changes. In Europe and America motors have been modified to meet the new efficiency criteria. For Europe it is eef1 and eff2 classifications while premium and EPACT categories are specified in the US. A basic 3-phase induction motor is a relatively simple device with few component parts. So, what needs to be done to improve the overall performance? There are a number of energy loss areas in a typical 3-phase induction motor.

Brook Crompton (currently being sold by Invensys) is the only UK company to manufacture eff1 motors. Its WP motor range achieved high efficiency by changing the winding style and rotor slot geometry to increase the overall length of the stator pack and rotor, or a change in diameter of the stator pack. A complete redesign of the motor’s exterior on the smaller motor frame sizes optimised the thermal design of the stator frame and cooling fan.

American motor manufacturer Baldor sees stator resistance as one of the main culprits and, to reduce this loss, magnetic and electric modifications have to be made. Larger gauge copper wire is inserted in the slots of the stator without increasing the amperage load. Better magnetic quality copper is used: where once 50 W/lb loss was the norm it is now 6 W/lb loss. Insulation has also been improved and can withstand 1000V spikes for one second, giving the necessary reliability for variable-frequency inverter drive applications.

Another cause of energy loss is rotor resistance. Baldor reduces it by using a high-pressure die cast aluminium rotor to give increased conductor area. Rotor vibration, which can create stress in the bearings, has also been reduced and is now down to half the NEMA allowable vibration limits.

In Europe, Dutchi Motors’ Hemmie van Deelen says that his company’s 3-phase induction motors needed no changes to meet eff2 requirements. But for eff1 motors the stator and rotor had to be extended in length, within existing frame sizes.This led to reduced energy loss and reduced rise in motor temperature.

The accompanying squirrel cage rotor combines silicon steel and aluminium. The same steel is used for the stator with a core of cast iron. Copper wire and class F insulation is standard for the stator windings.

In a test run on Dutchi’s 3-phase 7.5 kW (400V, 50 Hz, 13.7 A) motor running at 2920 rpm, the internal temperature rose to 70°C on full load. In achieving 90% efficiency the motor bettered the eff1 requirement of 89.5%.

ABB’s eff1 explosion proof motors, designated M3000 Ex, attain 95% when fully loaded at 90 kW. The basic recipe for energy efficient motors is well known, says Jouni Ikäheimo, R&D Manager, at ABB. You use more and better materials in the conducting parts. Specifically, more copper and better quality electrical steel is used in the new motors. It also uses die-cast rotors, which can be manufactured in a more optimised way than traditional copper bar rotors. This leads to lower losses and reduced temperature. The winding temperature is 5-10C° lower, extending winding lifetime by 50%. The bearing temperature is also down, by 10C°, which will extend re-greasing intervals by 50%.

Geared motor manufacturer Flender Power Transmission, which utilises high efficiency motors from WEG or Siemens, can achieve 75% efficiency when using a helical/worm box. The company’s P. Dudley, in praising the energy saving quality of these motors, claims that for smaller units, when fitted with a helical/bevel combination, it is possible to reach 97% efficiency.

As magnetic forces cycle within a motor, hysteresis and eddy current losses arise. Hysteresis loss comes through the constant re-orientation of magnetic forces within the steel, creating electrical currents. These currents circulate on themselves and produce heat without contributing to the output of the motor. These losses occur in both the stationary and rotating part of the motor, but the largest share occur in the stator.

Baldor tackles hysteresis losses through the use of improved steels and by limiting the intensity of the magnetic field. Eddy current losses are tackled by introducing thinner laminations for the stator and rotor and insulating them more effectively from each other.

Bearing friction is also the enemy of efficiency, as is windage which is partially due to the rotor’s rotational speed – the faster it spins the greater the drag created by surrounding air. A fan, although a necessity, also adds to the problem as it takes up energy.

Baldor and Dutchi have thus reduced the weight of their motor fans. Baldor’s external fan is moulded from glass-filled polypropylene. Dutchi offers aluminium as an option.Friction losses relate to a motor’s bearings and the lubricant. Depending on motor size, ball race bearings can be sealed for life or allow access for re-lubrication. For larger assemblies, ABB fits re-greasing valves whilst, for smaller frame sizes, sealed for life bearings are preferred.

For motor sizes 56 to 250, Dutchi fits sealed bearings packed with lithium grease, suitable for temperatures between -30° and 130°C. For larger 280 to 400 sizes with open bearings and a re-grease facility, an alternative high temperature grease is used. Exxon’s ‘Polyrex EM’ is chosen by Baldor for its greater shear stability and resistance to washout, rust and corrosion.

Eff1 motor manufacturers like to include temperature sensors to give warning of any approaching problem, bearings and windings being typical locations.

Information about the scheme, how to claim for ECAs, the required efficiency levels for a range of power ratings, plus rebate values that apply when purchasing a motor as part of a larger item of plant or machinery, can be found on www.eca.gov.uk.