Motor switching07 October 2019

Last year, a guide from the Carbon Trust into the energy efficiency of plants suggested many ways to improve environmental efficiency. One suggestion was to switch off continuous motors when they are not used

When it comes to optimising energy consumption and efficiency, it’s easy to disregard electric motors, because the basic technology is pretty well established – whether you’re talking about DC or AC, brush or induction, single-phase or three-phase. But the cost of running a motor can be considerable: the Carbon Trust points out that, in a single year, a motor can cost up to 10 times its purchase cost in energy.

According to the Carbon Trust’s guidance document on Motors and Drives (, this means that ‘typical running costs for a fully loaded motor are in the range of between £2,300/year for a 2.2 kW motor to £39,000/year for one rated at 37kW (assuming an energy price of £0.12/kWh)’. The same document states that ‘electric motor-driven systems’ account for 70% of all electricity consumption in industry, and it says ‘the cost savings made from implementing energy saving initiatives across multiple motors can be huge’.

It’s important that the correct size of motor is used: “Operating a motor with a load below 40% of its rated capacity is likely to result in some loss of efficiency,” says the Carbon Trust. “For efficient operation, a motor should be loaded from 50% of its rated capacity.” However, smaller motors are sometimes less efficient than larger units, and voltage optimisers can be a way to improve efficiency.

And timely maintenance is equally worthwhile. In fact, the Carbon Trust devotes a separate document to a maintenance checklist for electric motors ( This covers not just the usual checks on belt tension and alignment, lubrication, cooling and bearing wear, but also suggests that you measure the supply voltage (to check that it is within 10% of the nominal rated value) and confirm that the line voltages are balanced to within 1%. The document also recommends thermographic imaging of motors, switchgear and transmissions to identify hot spots and problem areas.


But one of the most effective ways to reduce the energy consumption of motors is also the simplest: turning the motor off. But there are considerations when it comes to turning the motor back on again.

During the acceleration period, when a motor is started, it requires extra torque and draws extra current – usually reckoned to be somewhere from four to eight times the standard rated current, albeit for a short period. Confusingly, this may be called the starting current, inrush current, input surge current or locked-rotor current.

This value may be shown on a motor data sheet (although not on the nameplate mounted to the motor), as something like %FLA – for example, the starting current as a percentage of full-load current: if the figure is 700, the starting current is 700% or seven times the full-load current. Similarly, starting torque might be given as a percentage of full-load torque – typically somewhere from 150-250%. However, the precise starting current varies according to factors such as the motor temperature, ambient temperature and of course load. This high current produces large stresses in the windings and inevitably generates more heat; this can increase motor mechanical wear and raise the likelihood of earthing and short-circuit faults.

The motor will probably also have a rated maximum number of starts per hour. This figure is not always easy to find; many data sheets, particularly for lower-cost motors, do not include this information, and when the author phoned the UK support line for a well-known electric motor manufacturer, they were unable to give the figures.

In any case, the number of permitted starts per hour – which is likely to be fewer for a larger motor – will vary according to the load and ambient temperature. If a motor is started from cold, it takes a while for the excess heat generated by the start to dissipate; if it is then stopped and started again soon after, the heat could build up rapidly. IEC standard 60034-12 ( states: ‘Motors shall be capable of withstanding two starts in succession (coasting to a rest between starts) from cold conditions, and one start from hot after running at rated conditions… In each case, a further start is permissible only if the motor temperature before starting does not exceed the steady temperature at rated load.’

The conventional sort of switch mechanism for an AC motor is a DOL (Direct On-Line) starter, but soft starters can reduce mechanical and thermal stress; it is important to realise, however, that they will probably not in themselves save energy.

A checklist is useful to determine the conditions for stopping/starting a motor, whether manually or automatically. The Carbon Trust’s suggestions include:

● Could it be switched off when not required?

● What is required to switch it off and how easy is that to implement?

● Are there health and safety risks, or the possibility of switching off the wrong systems?

If manual switch-off is feasible, a switch-off procedure should be devised and instructions put where an operator can see them. If an automated system is possible, there are several ways for it to work, such as a timer; an interlock, so that the motor only starts when a related device is switched on; and a load-sensing device, such as a thermostat, a level sensor or a tension control on a reel winder.

Clearly, you need to prevent a ‘flutter’ condition where the system is constantly being switched on and immediately off again. To do this, you build in a certain amount of hysteresis – a buffer region in which the switch is neutralised. An example would be a thermostat with different activation (on/off) points according to whether the temperature is rising or falling. For HVAC systems, an optimum start controller will learn the appropriate conditions to switch on and off.

BOX OUT: VSDs and voltage optimisers
Variable Speed Drives or Variable Frequency Drives (VSDs or VFDs) are an effective way to save energy by operating a motor at the correct load and speed without a loss in efficiency.

Variable-torque loads, such as centrifugal pumps and compressors, particularly benefit from lower speeds – earlier this year, OE featured a project to retrofit VSDs to water pumps at Alton Towers, which saved 1.4 million kWh a year (see

Another way to improve motor efficiency is to use a voltage optimiser. This reduces the input voltage to a level where devices can still operate but where they will use less energy: a motor running on partial load may benefit, and voltage optimisers can also be useful for other electrical devices, particularly if they have been designed for the typically lower mains voltage of other European countries. However, they will not help with voltage-independent devices, including motors driven by VFDs.

The traditional voltage optimiser is a transformer-based unit that reduces the voltage to a number of devices or even a whole building. However, a solid-state motor power optimiser or load sensing optimiser – which lowers the voltage when it senses that the load has reduced – can be hard-wired to an individual single- or three-phase motor; the Carbon Trust says that “on a motor running at less than 50% load, a power optimiser could save 5% to 15%”.

BOX OUT: Motor efficiency ratings
Some motors are more efficient than others, of course: the IEC (International Electrotechnical Commission) has efficiency standards (IEC60034-30) for AC motors from IE1 to IE5.

IE1 motors have been effectively prohibited from new installations in the EU since 2014, while IE2 motors can only be used with a VSD. IE3 (Premium Efficiency) and IE4 (Super Premium Efficiency) motors are available for most replacement applications, so there is little incentive to repair an older motor. IE5 motors are also starting to appear. Grundfos, for example, says that its MGE motors (with an integrated VSD) can save 10%, compared with a typical IE3 unit. Meanwhile, pictured is a Lafert SMPM (surface-mounted permanent magnet) motor that meets the IE4/IE5 efficiency ratings.

Toby Clark

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