High-efficiency motors explained

Andy Glover attempts to unravel what constitutes a high-efficiency electric motor.

CEMEP, the European Committee of Manufacturers of Electrical Machines and Power Electronics, classifies high efficiency electric motors into three categories: Eff 3 machines, which are basically standard motors; Eff 2 types that provide improved efficiency; and Eff 1 motors that provide premium efficiency.

In addition, some companies, WEG among them, also provide Top Premium Efficiency machines (Eff 1+) that exceed all the requirements of CEMEP's Eff 1 level.

The motors included in the CEMEP categories are designated as two- or four-pole three-phase AC squirrel cage induction machines; totally enclosed and fan ventilated in the range 1.1 to 90kW.

Above the 90kW figure, in the UK the Water Industry Mechanical and Electrical Specification (WIMES 3.03) has been adopted as the benchmark against which qualifying products under the Government's Enhanced Capital Allowance (ECA) scheme must comply.

The WIMES specification lays down minimum full load efficiencies for two- and four-pole electric motors in the ranges 110-400kW and six- and eight-pole motors in the range 5.5-315kW as well as minimum requirements for power factors and three-quarters load efficiency values.

Under the CEMEP classifications, motors rated as Eff 1 offer the highest efficiencies: in the mid 90% range (WEG's W21 Top Premium line provides efficiencies up to 96.8%).

These motors will, on average, reduce energy losses by up to 40%, and provide a payback on investment within one year where high operating hours are experienced.

The lower classification Eff 2 motors also produce savings, of around 20% per year, providing satisfactory efficiency with a minimum price premium.

The remaining, Eff 3 category motors, are the motors which Eff 1 and Eff 2 motors are designed to replace.

They offer very low levels of efficiency and do not represent a good long- term investment.

It has been calculated that replacing Eff 3 motors with Eff 2 could save Europe Eur 300 million a year.

The labels appearing on motor rating plates denote the efficiency rating of a motor; however, there is some conjecture among manufacturers as to the veracity of some claims to high efficiency.

In effect, what constitutes a high efficiency motor?.

The definition of a high efficiency motor is one that accomplishes more work per unit of electricity consumed than a standard motor.

What makes this possible is a combination of improved design, better materials and improved construction.

Specifically, high efficiency motors have higher quality and thinner steel laminations in their stators, more copper in their windings, better quality insulation, reduced fan losses and closer machining tolerances.

They save energy by reducing a number of motor losses, including stator and rotor resistance, friction (in bearings and brushes), windage (in fans or auxiliary machines) and load losses.

In Europe the recognised motor efficiency testing protocol is IEC60034-2.

According to this, there are two ways of determining the efficiency of an AC electric motor: by summation of losses; or by total loss measurement.

To calculate the efficiency of a motor by the summation of losses test, each of the constant and load losses has to be measured.

The additional losses are assumed to be 0.5% of the rated input for motors.

The%age efficiency is then calculated as input losses divided by input multiplied by 100%.

On the other hand, the total loss test is based on the measurement of input and output power and the efficiency is calculated as output divided by input multiplied by 100% IEC60034-2 states that the choice of test to be made depends on the information required, the accuracy required and the type and sise of the machine involved.

Unless otherwise specified, the guaranteed efficiency of a machine is that which is based on the determination of separate losses.

The input and output power test is highly sensitive to the equipment used.

In fact, any inaccuracy in these measurements appears as a direct error in the efficiency, eg with an accuracy of power measurement not better than 1% the efficiency can be 2% in error.

This is not so critical for smaller motors, but can lead to high inaccuracies for larger machines.

In view of this, IEC60034-2 states that the preferred test for polyphase induction machines is the method of summation of losses.

The latter test costs more for the motor manufacturer, as it demands more time to carry out.

Hence, some manufacturers tend to standardise on the input and output test.

In addition to measuring motor efficiency it is also important from an end-user standpoint to address the external factors, which may influence motor efficiency in-situ.

These can be quantified as follows.

Core losses in an electric motor vary with the voltage and frequency.

Therefore, when a motor is fed by a supply with some harmonic content (ie motors fed by inverters) its iron losses will increase.

A variation in the supply voltage, quite common in some types of industry, also affects motor efficiency by changing iron losses.

If a motor is required to operate at different speeds its losses due to friction will vary.

If bearings are overgreased, these losses will increase.

Misalignment can also increase bearing friction and therefore affect motor efficiency.

Rubber sealed bearings have higher friction losses than open bearings and should be avoided for optimum efficiency.

A poor fan design can also reduce motor efficiency.

In a nonsinusoidal supply, the harmonic distortion causes an increase in iron losses due to the high frequency of the harmonics.

However, these harmonics can also increase the winding's losses, but in a smaller proportion once they depend on the squared current.

For example, a harmonic of current equal to 10% of the fundamental generates only 1% of the winding's losses.

These additional losses are responsible for an extra heat on the motor, resulting in an increase in the winding's electrical resistance.

An excessive ambient temperature or improper cooling will have a similar effect.

To quantify the effect of the higher electrical resistance take, for instance, a 150kW two-pole 400V 50Hz induction motor with a winding resistance of 0.0108ohm at 21.6C temperature.

Assuming this motor has a temperature rise of 88.2C at rated load, its winding resistance at this temperature is 0.01526ohm.

Hence, at working temperature, its resistance is increased by 41.3% - or the I2R losses increase by 41.3% at rated temperature.

This makes it clear that the efficiency must be measured at the motor's working temperature.

WEG offers one of the widest ranges of high efficiency motors for use in all types of environments: normal industrial, offshore, and hazardous, where combustible dust and gases are present.

WEG's W21 Line of motors is one of the most energy efficient ranges on the market today.

It both complies with and, in many cases, exceeds, the demands of CEMEP's Eff 1 and Eff 2 classifications, providing users with the best possible solution to their energy saving requirements.

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