The payback from energy-efficiency
This article was originally written in the period 1995-2000
Energy-efficient motors cost more than the standard, so can they always be usually justified on a payback basis? This advice from Allen-Bradley.
All major manufacturers produce both standard motors and high efficiency motors. Although energy-efficient motors cost more than the standard line, they are usually justified on a payback basis, since they consume less energy per horsepower.
The energy-efficient motor differs from a standard motor in both design and manufacturing techniques. It is not, as some suppose, a return to the old U frame motors of the past. First of all, the energy-efficient motor employs larger stator conductors with higher conductivity. The rotor bars are larger as well, to reduce the overall copper (or aluminium) loss, which comprises 55-60% of total motor losses. The motor flux density and air gap are reduced to minimise the magnetising current required. Stator and rotor laminations are thinner to increase resistance to the flow of eddy currents. More laminations are added to the core stacks as well, producing a longer stator and rotor for increased torque.
Hysteresis losses, which are normally 20-25% of the total motor losses, are reduced by utilising silicon steel instead of low carbon steel for the laminations. As a result, the motor uses less current, has better power factor and runs cooler.
Since the energy-efficient motor runs cooler, ventilation requirements are reduced, allowing a smaller fan to be installed. Windage loss (typically 5-9% of total losses) decreases and the smaller fan runs quieter. These motors are less susceptible to damage from impaired ventilation and also operate well at higher altitudes. Cooler operation also increases motor life.
Insulation life is up to four times longer – an important fact, since insulation breakdown is the primary cause of motor failures. Bearing lubrication lasts longer too, doubling the interval between required lubrication.
Cooler operation means that less burden is placed on air conditioning equipment. Energy-efficient motors can also operate in higher ambient temperatures without requiring extra cooling, which in the case of a standard motor, could add substantially to the purchase price of the motor.
Energy-efficient motors are also more rugged than standard motors, tolerating greater fluctuations in applied voltage, voltage imbalance and overload.
The lines of energy-efficient motors from some manufacturers possess 1.15 service factors. Some can even tolerate 30-40% overloads for prolonged periods. They are also capable of starting higher inertia loads than standard motors, because of their increased thermal capacity.
And energy-efficient motors tolerate non-sinusoidal waveforms better than standard motors, which is important when the motor is powered by an ac drive, such as the six-step ac drive of Fig 1. Here, the standard motor has been derated to 85%, due to the heating caused by the waveform shown in the right-hand corner. To provide the same performance as the energy-efficient motor, a larger standard motor is required. No derating is required on the energy-efficient motor, since it can tolerate the disturbances caused by the six-step waveform. Other types of ac drives produce smoother waveforms that would not require derating of the standard motor. One such type is the PWM (pulse width modulated) ac drive.
This feature is an extract from an Allen-Bradley white paper: “A Comprehensive Guide to Understanding Motor Fundamentals”. The full text is available free in printed format and covers the basic construction of squirrel cage motors, motor acceleration, torque characteristics, reduced voltage starting and the effects of frequency variations. Energy-efficient motors and special motors (wound motor rotor and reluctance synchronous motors are also covered. By analysing the relationship between motors and adjustable frequency drives, the paper considers the performance advantages of using this combination.
- Allen-Bradley
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