How to save money with the motors you already have

The high cost of energy to industry is ensuring that companies are taking energy saving far more seriously than previously. Everywhere there are examples of companies replacing standard electric motors with high-efficiency types. However, even if this replacement movement really became a boom, the vast majority of electric motors in industry would be standard types for years to come. Against this backdrop, a relevant question to ask is "What can be done to save energy on the existing motor stock."

The obvious answer, of course, is to use AC drives more widely. However, this answer is flawed, as the vast majority of AC induction motors presently used by UK industry are operated at fixed speeds. Moreover, because they are operated at fixed speeds, the potential for saving energy with these units has been largely ignored.

With energy consumption in industry having grown by a factor of three compared with domestic consumption since the 1970s, ignoring the role of energy saving on standard motors is no longer an option. It has been calculated that the annual cost of energy to run an electric motor can amount to ten times its purchase cost. To illustrate this point, typical running costs of fully loaded motors are in the range 1100 year for a 2.2kW motor to 18,000 year for a 37kW motor.

The scale of these figures gives some idea of the scope for possible savings by companies with multiple motor installations. However, there is no magic formula to achieve this, just a few simple questions, rules and procedures, which, if asked regularly and strictly adhered to, can make a substantial difference to the amount of energy being consumed by a company's motor stock.

Switch them off

The simplest rule of all is to switch motors off when they are not in use. Although this may seem to be an obvious point, good housekeeping routines to ensure motors are turned off when not required can result in significant savings.

In the past, there have been reservations about switching off because of the stresses and strains that this puts on power transmission systems at start-up. Today these reservations are no longer as relevant as they were, due to the wide availability of soft starters.

The more gentle start provided by soft starters allows a much higher frequency of starting, which means that idling machinery can be switched on and off more frequently. To facilitate this operation, simple automatic controllers are available that, when combined with a soft starter, can convert conveyors, for example, to a quiescent mode whenever material is not being processed.

Depending on the operational conditions, hours run, etc, very acceptable payback periods can be assured from controlling fixed-speed motor applications in this way. With the relatively high no-load power requirements of much of the industry's machinery, 20-40kWh or more can easily be saved each hour by the simple act of switching off when no material is being processed

Improve the efficiency

Having identified the opportunities for switching-off motor stock, the next stage of an energy conservation programme is to look at how motor efficiency can be improved. The output of a motor, as detailed on its rating plate, is mechanical power stated in kW. Some of the electrical input to a motor disappears as 'losses'. Motor efficiency indicates the output power obtained for a given electrical input power [for example, a 3kW motor with 82 per cent efficiency will have an electrical input of 3.66kW (3/0.82). Clearly, then, the higher the motor efficiency, the less input kW required - and the lower the running costs.

Measures that can be taken to ensure maximum motor efficiency include maintaining good power quality. Motors are designed to operate with a frequency of 50Hz and a sinusoidal waveform; using power with distorted waveforms will only degrade motor efficiency.

Efficiency is also lost when motors are run either above or below their design voltages. The result of over-voltage is a lower power factor, which reduces overall motor effectiveness. The same is true of motors that are operated at less than 95 per cent of their design voltage. They typically lose two to four points of efficiency, and also suffer service temperature increases of up to 7degC, greatly reducing motor insulation life and impairing reliability. In addition, heat also affects motor efficiency, so motors should be shaded from the sun; they should be located in well- ventilated areas and, since dirt acts as an insulator, they should also be kept clean.

Run motors slower

Having considered supply considerations and cooling, the next relevant question to ask is "Can the speed of my motors be reduced?" It is important to investigate all opportunities for reducing operating speeds, as this has a big impact on running costs. Fans are very often running below their rated output and this is one of the most obvious places for making immediate savings. With fans and pumps, if the speed reduces by half, the power reduces by 87.5 per cent. Variable speed drives (VSDs) are the tool to achieve this power saving; they allow speed to be matched to the demands of the process, delivering enhanced energy efficiency. In contrast, where a fixed single speed is required, this can be achieved either by changing the shaft pulley ratio on a belt drive or by changing the ratio on a gearbox.

Optimise transmission efficiency

Complementing a speed reduction strategy, motor engineers should seek to optimise transmission efficiency by ensuring that bearings shafts, belts, chains and gears are properly installed and maintained and are not subject to wear, noise and vibration, all of which can result in running losses. In addition, the installation of the motor itself must be checked to ensure that there is no misalignment. This can cause additional unnecessary loading on motors and, hence, increased energy consumption plus reduced plant life through additional mechanical loading.

Preventative maintenance

With all the key areas covered for improvement in motor efficiency, the next step is to instigate a series of periodic checks on the motor stock as part of a preventative maintenance programme.

Inspections should include all mechanical components, and daily or weekly noise, vibration and temperature checks. These should be complemented, approximately twice a year, by testing of the motor winding and the winding to earth resistance, in order to identify insulation problems.

Finally, the preventative maintenance programme should itself be part of a larger, more comprehensive Motor Management Policy (MMP). An MMP is a coherent structured approach to the purchase and repair of a company's motors to enable companies to keep strictly to an energy efficiency policy. It is designed so that the best economic decision is made each time a new piece of plant containing a motor is purchased, or a when a repair or replacement is necessary.

Some typical recommendation from an MMP include: specifying 'EFF1' or 'EFF2' motors for all new plant with long running periods; always use a qualified and accredited rewind shop; if a rewind cost exceeds 50 per cent to 60 per cent of the cost of a new energy-efficient motor, buy the new motor; always replace very badly damaged motors; if a motor should be replaced but is needed urgently, then repair, providing that action is justified and the reason for the repair is recorded; repair high-efficiency motors in accordance with the AEMT/EEBPP guide "The repair of induction motors".