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Are your motors out of control?

04 January 2012

Steven Brambley asks: why don’t we apply those simple energy efficiency measures we all use at home to that much larger energy consumer, the industrial electric motor?

It sounds like a very simple and obvious principle that most people would agree with: to save energy, switch it off when not needed; turn it down when full power is not required. It is a method commonly applied in our homes, but it is somehow not widely applied in industrial motor driven systems, the single largest consumer of electrical energy in industry in the UK and worldwide.

Why not? A mixture of tradition, lack of investment and the indirect supply chain are all barriers, but many businesses are now starting to see the value in getting their motors under control and saving significant amounts of energy and money. There are three reasons why system efficiency should be given a high profile when considering the purchase of any motor driven system, whether a machine, a ventilation system, a swimming pool pump or a baggage handling system. These are:

- The control of a system can often give greater energy savings than simply installing high efficiency components.
- The lifetime energy costs of a typical motor are more than 40 times greater than the purchase price, making system control vital to energy reduction.
- Many end users buy systems, not components, and need to understand how much the system will cost to run as well as to buy.

In any given system, the efficiency of the components can usually be found in the data sheets and documentation. This doesn’t, however, necessarily give an indication of the system efficiency once the components are assembled together. Motors are commonly oversized to cope with rare extremes and then further oversized just in case. Combinations of components, gearboxes, alignment and configuration can all contribute to system efficiency being very different from the sum of the components within.

Motor control is one part of a system that can often give the greatest efficiency gain, simply by applying the principle: switch it off, turn it down.

As hard as it is to believe, the majority of motors have no form of control, other than an on/off switch, which in many cases is never in the off position. Imagine if your car had no accelerator pedal, just a hand brake. Aside from the uncomfortable stop/start process for the passengers and the increased wear and tear on the vehicle, the fuel consumption would be dramatically increased as the engine maintains a steady 4,000rpm at each traffic light. It sounds ridiculous, but this is how we commonly use electric motors in industry.


Lifetime energy cost dwarfs purchase cost
There is a very important reason why the energy efficiency of a motor driven system should be highlighted - cost. Electric motors consume about 65% of the electricity used in industry; it is the largest cost in the energy bill by a significant margin.

It might surprise you to learn that your average 11kW motor may cost about £500 to buy but will consume £2,000 of electricity per year running only 2,000 hours. If the motor runs for 15 years, your £500 motor will cost you £30,000 to run over its life (not including energy price increases). A motor running 8000 hours per year is going to cost £120,000 to run over its lifetime. It’s worth considering the payback on any investment in motor control that will reduce this significant running cost. Payback times can often be less than one year and of course continue to give a saving over the lifetime of the system, particularly as energy costs increase.

Continuing the car analogy, it is a minority of car drivers who purchase engines and fit them into their cars. People don’t buy a component, they buy a system - in this analogy, the car. Similarly, in many instances, electric motors are often purchased not individually, but as part of a system. But just as you would usually consider the running cost of a car when you buy it, do you also consider the running cost of your machine?

Every car is sold with a fuel consumption declaration, so you can work out that your £20,000 car will cost £2,000 per year in fuel. But does your machine come with a similar declaration? Unlikely, but it doesn’t stop you asking for one. A capital purchase will often require three quotes for comparison, but what if those three quotes also included a projection of the running cost per year? It would be worth considering the quote that costs less over its lifetime rather than the one that costs less to purchase.

Motor loads and how they are controlled
Systems will fall into three types of load: variable torque (20% reduction in speed gives 50% reduction in energy used); constant torque (20% reduction in speed gives 20% reduction in energy used) and constant power (no energy reduction by reducing speed).

When it comes to controlling motors, there are three broad methods you can apply: manual, fixed speed and variable speed. To decide the most appropriate control method for an application requires an individual assessment of the load, the usage profile and the current motor installation. However, to give some broad brush guidance, variable torque loads such as pumps and fans often gain significant energy benefits from variable speed control, whereas constant power loads would not. Constant torque loads may benefit from either fixed speed or variable speed controls, depending on the application. Consider the following control strategies:

- When the output is too high the load is mechanically damped, braked or diverted, which is wasted energy. This is often the case because the motor has been oversized to meet a peak demand or for added safety, but may never be required to operate at this level.
- When the output requirement is variable the motor speed can be varied to match the requirement, rather than running the motor at constant speed, constant output. This is often the case with refrigeration and ventilation, for example, where temperature varies through the process or over time.

If the speed of the system cannot be varied, it can still be automatically switched off when not needed to save energy. This can be for long or short periods of time, at specific times or as detected by sensors. Examples include process waiting times, planned downtime (breaks, maintenance, shift handovers), non-production time (nights, weekends), and unplanned downtime (breakdowns, unscheduled stoppages).

For pump and fan type loads, GAMBICA’s VSD energy saving calculator, which gives an indication of the saving potential and payback for the various load types, is available from the GAMBICA website.

This article is based on a paper presented by Steven Brambley at the Motor Driven Systems conference, held in November 2011. Steven Brambley is deputy director, industrial automation, GAMBICA
 


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