Motors
This guide provides an overview of the operation of high, or premium, efficiency motors in the context of the Accelerated Capital Allowances (ACA) and illustrates their relative merits for inclusion in new and retrofit installations.
The ACA covers asynchronous electric motors, or three-phase induction motors, with a power rating of 1.1kW or greater that meet a defined energy efficiency standard. A qualifying motor may be either a standalone unit or a part of other equipment.
Far too often motors are poorly sized and selected, resulting in significant energy waste. Energy-efficient motors typically pay for themselves in a few years or sometimes even a few months. After this period, they will continue to accrue savings worth many times their purchase cost for as long as they remain in service.
In order to size and select an efficient motor for a task, a number of key criteria should be considered:
An electric motor is a machine that converts electrical energy into mechanical energy to perform work. Motor driven systems account for 65% of industrial electricity consumption in Europe. While several different types of motors exist, the AC induction motor is by far the most widely used in manufacturing industry, accounting for around 90% of electricity consumed by all small to medium-sized motors in industrial applications. An induction motor has two main parts, a fixed external stator and an internal rotor. The stator has windings through which AC current flows and it produces a rotating magnetic field. The rotor is attached to the output shaft and gets a torque by the rotating magnetic field. The speed of the motor is dependent on the frequency of the electrical source and the number of windings, or poles, in the stator.
Figure 1: Induction Motor (courtesy CMG Motors)
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The efficiency of a motor is the ratio of mechanical power output to the electrical power input and is typically expressed as a percentage. An energy-efficient motor is simply a motor that uses less energy than a conventional one. It will deliver the same mechanical power output but requires less electrical input power. Energy-efficient motors are manufactured with higher quality materials and techniques, and therefore usually have longer bearing lives, less waste heat output, and less vibration, all of which can increase their reliability and longevity.
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In general, you should consider investing in an energy-efficient motor whenever you are purchasing a new or a replacement motor or thinking about repairing an existing motor. Choosing an energy-efficient motor makes sense in the following circumstances:
When purchasing new equipment packages or machines that have integrated electric motors;
When purchasing spare motors or replacing faulty motors;
As an alternative to rewinding old standard-efficiency motors;
To replace oversized and under loaded motors;
As part of an energy management program, such as Energy MAP <hyperlink?>.
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Normally yes. AC induction motors are a commodity product and most are produced in standardised frame sizes, making them interchangeable between manufacturers and between efficiency classes. However, European and US standard sizes will differ. The nameplate of your existing motor will list all important design characteristics that need to be taken into account when replacing it. A description of a typical nameplate can be found below <hyperlink>. You should consult with your supplier of energy-efficient motors to help identify the correct replacement for an existing standard-efficiency motor.
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Yes. The capital cost of an energy-efficient motor can be 20-30% higher than that of a standard-efficiency motor. However, the running costs of a motor are a multiple of the capital cost, and an energy-efficient motor saves money during its entire lifetime. The payback period for an energy-efficient motor will depend on how much it’s used, on the size of the motor load and on the price of electricity. An example of a payback calculation is given below <hyperlink>. The simple payback of a continuously operated energy-efficient motor can be recovered through energy savings typically in less than 2 years. Of course, the introduction of the ACA scheme makes the payback on motors included in the scheme even quicker.
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There are several standards and classifications which are used by motor manufacturers to categorise their products as “energy-efficient”. The one most commonly referenced in Ireland is the European CEMEP classification. CEMEP (European Committee of Manufacturers of Electrical Machines and Power Electronics) defines three efficiency classes: EFF1, EFF2 and EFF3, with EFF1 being the highest efficiency classification. Minimum efficiency values required to qualify for EFF1 classification have been used as the threshold for qualification for ACA for all motors with a rated output up to 90kW. For motors with rated output greater than 90kW, minimum efficiency values for IE3 premium efficiency motors, defined by the IEC 60034-30 draft standard, have been used as a prerequisite for qualification for the scheme.
From the user’s perspective, a purchased motor is considered energy-efficient and qualifies for the scheme if it conforms to the qualification criteria defined by SEI, (given here <hyperlink>) and is included in the list of qualifying products.
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Motor size: Motors should be sized to operate with a load factor between 65% and 100%. If a motor is oversized for its application, it results in less efficient operation. Motors are at their most efficient when operating with in excess of 60% of full load. Low load conditions for the selected motor will result in lower efficiencies.
Motor speed: The average speed of an energy-efficient motor can be slightly higher than the average speed of an equivalent-sized standard-efficiency motor. Installing a new motor with a higher speed can result in reduced energy savings. For centrifugal pump or fan applications in particular, it is important to select a replacement motor that has a comparable full-load speed.
Inrush current: Overloading of circuits should be avoided. Energy-efficient motors will typically have a lower electrical resistance than a standard-efficiency motor, with the result that inrush currents will be higher. The duration of the inrush current is too short to trip thermal protection devices but could result in nuisance trips of in-built magnetic circuit protectors in some energy-efficient motors on start-up.
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The simplest and often-overlooked method of saving energy with motors is to switch them off when they are not being used.
For motors that operate with variable loads, the use of a Variable Speed Drive (VSD) can yield significant energy savings. VSDs included in SEI’s list of energy-efficient products qualify for ACA <hyperlink>.
The mechanical efficiency of the driven equipment (i.e. pumps, fans, belts, etc.) directly influences the efficiency of the overall motor system. It’s important that this equipment is regularly maintained and lubricated and that wear-and-tear of mechanical parts is monitored. The casing of the motor should be kept clean to ensure that its operating temperature does not rise unnecessarily. An overheated motor can lead to premature failure.
Electrical power consists of “active” and “reactive” power. Active power is metered with a normal electricity meter. For large industrial installations, reactive power is also metered and a charge is applied if a threshold value is exceeded. The relationship between active and reactive power is known as the “power factor”. To avoid additional cost for reactive power consumption, or “wattless charges”, the power factor should be maintained as high as possible, i.e. as close as possible to 1. Running inductive loads such as induction motors will reduce the power factor, although energy-efficient motors tend to have a less adverse effect than standard-efficiency motors. The power factor at your site can be improved by installing power factor correction equipment. You should contact electrical contractor to check your power factor.
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Under IEC standards, all motors are described in terms of their operating and design characteristics using a motor nameplate attached to body of the motor’s housing. Nameplates which can display the following specific pieces of information:
1. Manufacturer name and manufacturer-specific ordering code. This code will typically contain information about frame size and mounting arrangement.
2. Rated volts - The voltage at which the motor is designed to operates and give optimum performance.
3. Full load amps –.used to facilitate cable sizing, starter selection and motor protection.
4. Rated frequency in Hertz, and rated full load speed in revs/minute at that frequency.
5. Rated temp rise or insulation class – Industry standard specification of the thermal tolerance of the motor insulation.
6. Rated power (kW).
7. Electrical connection and associated operating parameters – i.e. three phase motors can be connected in a ‘star’ (Υ) or ‘delta’ (Δ) configuration. In a star configuration the current flowing from the supply is reduced as is the torque.
8. Power factor for the motor.
9. Efficiency class – IEC nameplates quote EFF ratings at full load. It is a measure of how well the motor converts electrical energy to mechanical power.
Figure 2 illustrates a typical motor nameplate with showing the items of information listed above. The information displayed on a motor nameplate may vary depending on manufacturer and motor size.
Figure 2: Typical motor nameplate (Courtesy of ABB Ltd.)
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In this example, the payback for using an energy-efficient EFF1 classified 5.5kW motor instead of an equivalent EFF3 motor is calculated. First, the running costs of each is calculated using an average motor loading value of 80%, assuming permanent use and a cost of electricity of €0.14 per kWh. The input power for each motor is calculated by dividing the rated output power by the efficiency value.
Running costs for a 5.5kW EFF1 motor with an efficiency of 88.6% at full load:
Annual running cost = Input power * Load Factor * Run hours*Electrical cost
= (5.5kW/0.886)*(0.8)*(24*7*52)*(€0.14)
= €6,073.79 per annum
For a 5.5kW EFF3 motor with an efficiency of 84.7% at full load:
Annual running cost = Input power * Load Factor * Run hours*Electrical cost
= (5.5kW/0.847)*(0.8)*(24*7*52)*(€0.14)
= €6,353.45 per annum
The application of the energy-efficient motor in this example results in a saving of € 6,353.45 - € 6,073.79 = € 279.66 per annum. To calculate the payback period, difference in the price of the two motors is divided by the annual savings. In the example the price of the EFF1 motor is assumed to be 50% greater than the less efficient motor. The payback is calculated both with and without the ACA scheme being taken into account.
Payback period without ACA:
EFF3 motor cost = € 1,000.00
EFF1 motor cost = € 1,500.00
Additional Cost = € 500.00
Payback = € 500 / € 279.66 = 1.78 Years
Payback period with ACA:
EFF3 motor cost = € 1,000.00
EFF1 motor cost = € 1,500.00 – (0.109 * 1,500) = € 1,336
Additional Cost = € 336.00
Payback = € 336.00 / € 279.66 = 1.20 Years
The simple payback period for the energy efficient motor is 1.78 years. This means that the running costs saved in that period cancel out the additional capital cost of the EFF1 motor. Under the ACA scheme the payback period is reduced to 1.20 years in this example.
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