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The power used by electric motors accounts for nearly two-thirds of the electricity consumed by industry in the United Kingdom and is responsible for a significant volume of greenhouse gas emissions. Even a small or medium sized business could have hundreds of electric motors on site.

The cost of running an electric motor is often underestimated. In a single year the cost of the energy needed to run a motor can be as much as ten times its purchase cost. Typically, the annual cost of running a small motor rated at 2.2 kilowatts can be around 拢2,300, rising to around 拢39,000 for a motor rated at 37 kilowatts, depending on electricity costs. With running costs this high, it is important to introduce cost-saving measures wherever possible.

Understanding electric motors

An electric motor is a device that converts electrical energy into rotary kinetic energy that is used to power a particular process.

There are several different types of motor, all of which have different characteristics, advantages and uses. The most commonly used motor is the induction motor. This uses conventional alternating electric current to induce a force on its rotor, causing it to rotate.

Ordinary induction motors usually run at a single fixed speed. But a variable speed drive can be used that controls the electrical supply to an induction motor, allowing it to run at different speeds. Other types of motor, such as multiple speed motors, can run at between two and four different speeds.

Cost savings on electric motors

Electric motors are used to drive many items of machinery such as pumps, fans, conveyor belts and compressors. To make cost savings you need to look at the whole process, not just the motor itself, otherwise you could overlook ways to make significant and often inexpensive cost reductions.

By understanding how an electric motor works you can get a better idea of how energy wastage in your business may occur.

Reducing wear when starting an electric motor

A motor connected directly to an electrical power supply accelerates to a fixed speed. When it starts up, the motor draws a very high current as it accelerates - this is known as the 'motor starting current' and it generates a great deal of heat. This excess heat considerably increases motor wear and reduces its life expectancy. For this reason, motor manufacturers normally give a maximum number of 'starts' per hour.

However, by connecting a motor to a 'soft starter' or a variable-speed drive (VSD) the starting current can be limited. This gives a much smoother start and results in less wear on the motor. To find out more about soft starters and VSDs, see upgrading your system equipment.

Electric motor loading

Only around 75 to 80 per cent of the energy put into an electric motor is used by the load being moved. This includes:

• heat wastage of around five per cent in the motor's drive unit
• internal losses in the motor of around eight per cent
• additional losses through friction if the motor is attached to a transmission system like a gearbox or pulley

The 'loading' of a motor is the actual amount of work it does compared with its maximum rated power output. For example, a motor rated at 90 kilowatts driving an 81 kilowatt load is described as 90 per cent loaded. Modern motors typically operate most efficiently at above 75 per cent loading, peaking at around 90 per cent efficiency.

It is very important that you consider the type of load that is being put on each electric motor, as this can affect the opportunity for energy saving.

The three main electric motor load types are:

• Variable-torque loads - for example fans and pumps where the speed varies. With this type of load, reducing the speed of the motor by even a small amount can save a lot of energy.
• Constant-torque loads - for example conveyors, screw and reciprocating compressors, and crushers. With these loads, the amount of power consumed is in direct proportion to the useful work done. So halving the speed will halve the energy consumed.
• Constant-power loads - for example machine tools and centre winders. With these loads, the power is constant so there will rarely be energy savings when the speed is reduced.

Using a variable-speed drive (VSD)

For variable torque loads in particular, using a VSD in conjunction with a suitable motor allows you to get a better match between the speed of the motor, the speed of the machine it is driving, and the requirements of the process. A VSD converts the incoming electrical supply from a fixed frequency to a variable frequency output. This frequency variation allows the VSD to control the speed of the motor.

You can achieve significant energy and cost savings by using a VSD in certain applications, particularly those that use variable torque such as fans and pumps. For example, you could save up to 50 per cent of the energy used by a fan or pump if you reduce the speed by 20 per cent.

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When you use electric motors in your business you can save energy through:

• good 'housekeeping' and correct usage
• proper maintenance
• correct motor sizing and system optimisation

Electric motor usage and housekeeping

Leaving electric motors running when they are not needed, for example, during evenings or at the weekend, is expensive and wasteful. Depending on motor size, it can cost up to 拢2,000 a year per motor and it also shortens the motor's lifespan. Overheating and dirt can also reduce the lifespan of a motor.

There are various steps you can take to make sure that the day-to-day operation of your electric motors is carried out as efficiently as possible. These include:

• switching off motors wherever possible, perhaps including the introduction of an automated switch-off system
• locating motors and drives in areas that don't get too hot
• keeping components clean

Electric motor maintenance

Motor maintenance is one of the most important ways of ensuring that your motors continue to work efficiently.

There are two types of maintenance - planned preventative maintenance (PPM) and breakdown maintenance. PPM is essential for the long-term reliability and energy efficiency of your critical motor systems.

Electric motor sizing and optimising

Lightly-loaded motors are less efficient than fully-loaded ones, so it is much better that they are loaded as near to their full capacity as possible. There are different ways that you can achieve this, including:

• replacing larger, partially-loaded motors with smaller, fully-loaded ones - see replacing motors to save energy
• optimising a system or process so that the motor is running at full capacity for shorter time periods instead of running continually with a partial load

If it's not practical to change your existing motors or optimise existing systems you can still make energy savings on motors running with very light loads by:

• fitting motor optimisers that reduce the average voltage and current
• running the motor continually using a different connection mode - for example, star/delta, which reduces starting current and starting torque

Buying new and more efficient equipment is an excellent way to improve energy performance. When you look at the whole-life costs, replacing a motor is often more cost-effective than rewinding an existing motor because rewound motors are up to five per cent less energy-efficient.

Higher-efficiency single-speed motors don't cost much more to buy than standard-efficiency models but they use up to five per cent less energy.

Multiple-speed motors are a little more expensive to buy than single-speed motors but they can improve energy efficiency. They have up to four fixed-speed settings, which means that you can save energy by not having to use a transmission system.

Although upgrading system components requires capital investment, in almost all cases this is paid back in a short space of time as a result of efficiency savings.

You can improve the energy efficiency of your processes by adding variable-speed drives (VSDs), 'soft starters' and 'smart' motors.

Variable speed drives

Where an electric motor serves a variety of load conditions or has a continuously-variable demand, the use of a VSD will improve energy efficiency by optimising motor speed. As well has having the ability to save significant amounts of energy, VSDs also have other benefits, including improved process control and the ability to control more than one motor.

Many VSDs also have a further energy-saving mode that's normally described as an 'energy optimising' feature. This reduces inherent losses and makes further energy savings.

Soft starters

A motor draws a high current when it starts up. To reduce the starting current you can fit a soft starter to your motor. These limit the current to the motor during start-up, to provide a smoother start. As a result, the life of the motor is extended because wear on the mechanical parts is reduced and electrical components are prevented from overheating. Soft starters can also increase the recommended number of 'starts' per hour, which is useful if your motors are subject to frequent stopping and starting.

Smart motors

Smart motors combine the functionality of an electric motor, a VSD and a control unit. They are able to analyse load conditions without having to feed back information to a central control system. This means faster response time and reduced cable losses. All electrical cables have electrical resistance, so a small amount of power is lost in the form of heat between one end of the cable and the other. Using smart motors to improve process automation can save up to 15 per cent of a motor's running costs.

Transmission systems

It's important to match the correct transmission system to your motor. The most commonly used transmission systems are:

• direct drives, where the load is coupled directly to the motor shaft. It is important that the drive is correctly aligned with the motor to prevent energy losses.
• belt-driven pulleys, where the output speed is adjusted by a pulley system of different-sized wheels connected by a belt, and the pump/fan speed can be reduced by changing the pulley ratio.
• gearboxes, where the output speed is adjusted using a series of gears

It's very important to introduce monitoring and management systems for your motors to maximise your energy saving initiatives.

Monitoring electric motors

Without accurate monitoring to detect wastage it's difficult to make long-term energy savings. When you introduce motor monitoring, it is a good idea to take the following steps:

• make an inventory of your motor systems - this will provide a clear framework for identifying and prioritising actions and also give a history of each motor, which is useful for deciding whether to repair or replace when problems occur
• measure the power consumed by each motor using an 'hours-run' meter, a clamp-on ammeter and a portable power logging device, as well as permanent kilowatt-hour metering
• analyse and monitor the condition of each component to predict the risk of failure by using vibration analysis, oil analysis and thermographic surveys

Modern monitoring systems can carry out the analysis automatically and flag up any problems by sending an email or text message to the relevant operative. More advanced systems will automatically shut down a process if a fault is detected.

Electric motor management policies

By following a well-designed motor management policy you can make cost savings on every motor in your business. The policy will provide a structured approach to repair and maintenance, resulting in reduced system down-time.

Your motor management policy will generally include a:

• schedule and procedure for motor maintenance
• plan for repairing, or replacing, failed motors
• plan for purchasing new and more efficient motors

It's a good idea to plan the measures your business will take to increase the energy efficiency of your electric motor systems. The following steps will help you to understand how you use motors in your business, identify opportunities for improving efficiency, and measure the impact of any changes you make:

1. Understand the system. Look at the system and identify where motors are situated, recording them on an inventory. Check the condition and operation of the key components. If possible, monitor the power consumption over a fixed period (for example a week) to gain a baseline against which any improvements in energy efficiency can be measured.
2. Understand how the system is used. Examine the way in which the motors and drives are used. Think about issues such as switching off, loading appropriately and reducing motor speed.
3. Plan for motor failure. Identify which motors should be repaired on failure and which should be replaced. In general, the associated costs will favour the replacement of smaller motors, and the repair of larger motors.
4. Identify and prioritise actions. Identify the areas where energy efficiency can be improved and decide how and when energy-saving actions will be implemented. Some measures could be simple to carry out in-house but others may require specialist assistance.
5. Seek specialist help if necessary. Contact a specialist contractor or consultant - or the Carbon Trust - to discuss the more complex energy saving options if you need to.
6. Make the changes and measure the savings. Make the necessary changes to your system to improve energy efficiency. By measuring the savings you will get useful information that will help you make future management decisions about your motors and drive systems.
7. Continue to manage your systems for energy efficiency. Put in place policies and procedures to ensure that your system continues to operate efficiently and that energy savings are maintained in the future.