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Energy saving tips for motors

09 July 2012

Industry is flooded with advice on how to save energy with electric motors. Most of this advice only tackles one issue or, even worse, only provides a single biased viewpoint. Mark Shepherd offers, in his words, an honest, unbiased and reliable set of tips for efficient motor use.

Measure - The phrase, ‘If you can’t measure it, you can’t manage it’ remains true for electric motors. To make the biggest impact you must have a clear understanding of which motors and processes are consuming the most energy in your plant. This will allow you to target your efforts and gain the quickest return on investment.

Understand energy use - The energy consumption of a motor is related both to speed and load. A slow motor with a full load will consume more energy than a fast motor with no load. All motor driven loads are a combination of one or more forces (friction, gravity, inertia, drag) and the type of load will determine the best energy saving strategy to be employed. For example, the energy saving strategy for friction loads (such as power tools) is to reduce operating time, whereas for inertia loads (such as flywheels and centrifuges) the strategy is to adopt intelligent motor control.

Fixed speed versus variable speed - Consider which applications are already variable speed, those that must remain fixed speed and those where the speed could be reduced. Be careful though, reducing speed on some applications will not reduce energy consumption.

Turn it off - It sounds simple, but the most effective way to save energy is to switch the motor off when it’s not needed. Often the reason for not doing this is the perceived risk of additional wear and tear at motor start up. This is especially true for motors started direct on line or with star delta starters. Using a soft starter will reduce the wear and tear at every start, and can reduce the strain on mechanical and electrical systems by as much as 70 percent.

Efficient system design - There is little point installing the latest high efficiency motors and equipment, if the entire system is fundamentally inefficient. Study how the system works and identify when and where the motor is doing work unnecessarily. Sensors, PLCs and soft starters can be combined to create a system that starts and stops automatically to meet demand.

Slow down - In simple terms, at the same load, a slow motor does less work than a fast motor, so you can only save energy in applications where you need less work done. Variable speed drives save energy by allowing the motor to do less work. They are very effective in reducing speed and saving energy in applications where the main opposing force is drag; however, reducing motor speed in applications where the main opposing force is gravity or inertia will not save any energy.

Use energy saving motor controls - All motors, even IE3/NEMA Premium Efficiency motors, are most efficient at near full loads. As motor load falls below 50 percent, efficiency begins to reduce. In applications where the motor load is variable or the motor runs at light loads for long periods, intelligent energy saving motor controllers will constantly match the amount of energy supplied to the motor to the load. The motor still always runs at full speed, but during the lightly load periods, energy savings of 20% or more can be achieved.

Size motors correctly - It is good engineering practice to slightly oversize a motor for a particular application; this will extend motor life and provide some extra capacity when it is required. However, if a motor is oversized larger than required the installation should be reviewed. Energy saving motor controllers are very effective at saving energy in oversized motors.

Use high efficiency motors - The latest IE3/NEMA Premium Efficiency motors are more efficient, but the efficiency gains are marginal. Only in a very few cases where the motor is very old and running 24/7, will it make financial sense to replace a perfectly functioning motor with a new motor. However upgrading the motor as it reaches the end of its service life, or when the motor fails, should be considered as best practice. Motor rewinds should only be considered when the motor is, for any reason, irreplaceable.

Be aware that the design and higher copper content of IE3/NEMA Premium Efficiency motors means that the inrush currents and peak torque on start can be considerably higher, and in some cases it may be necessary to upgrade the motor starting control gear.

Reduce wear & tear - A large amount of wear occurs when an electric motor is started. To reduce the damaging effects, soft starters should be used in all fixed speed applications to extend motor life. Soft starting can also be achieved by using variable speed drives, but this is less efficient and far more costly.

Mark Shepherd is managing director of Fairford Electronics

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