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Important considerations when transitioning to Higher Efficiency Motors

20 January 2009

Changing motors to reduce energy cost is good for the bottom line and will be good for the environment, so what could be a problem?

Baldor IPM motor
Baldor IPM motor

In any industrial manufacturing operation the machinery has often been modified to suit productivity, quality improvements, and production process changes. For these reasons the current motors may not reflect the best selection for the lowest energy cost.

Moving to motor efficiency IE2 and higher (IE3) achieves the highest savings in terms of energy and attracts carbon tax credits in some countries. This results in much shorter payback periods that are more readily acceptable financially. This type of motor will, however, often require some further engineering and logistical considerations to optimise the installation.

An understanding of the mechanical and performance differences of these motors is essential in order to realise maximum financial benefits and a trouble free installation.

MECHANICAL DIFFERENCES

The first difference is the use of grain oriented electrical steel. Even though this type of steel may be better magnetically, it cannot be 'worked' electrically as hard as standard steel. By introducing more higher performance steel, we add cost and size, but it works better, cooler and at a higher efficiency. The impact of this is explained later.

Most motor losses are dissipated in the form of heat. The next highest loss is through the cooling fan (friction and windage component). As the premium efficiency motor produces less heat, this allows the use of a much smaller fan thereby reducing the losses of the fan. This also results in reduced acoustic noise.

Tighter manufacturing processes make sure the rotor is in the centre of the magnetic field. The rotor will inherently move axially to the magnetic centre, or try to achieve that position. If it is not manufactured accurately enough the rotor will continuously exert axial force, which is energy that is lost as heat and bearing stress. Tighter manufacturing tolerances result in greater product performance accuracy (repeatability) and will typically last longer due to less mechanical stresses and cooler operating temperatures throughout the machine.

The rotor will be balanced to tighter tolerances which will ensure that energy is not wasted inside the motor but converted to torque. This again means improved reliability through less mechanical stresses.

A change in motor frame size is possible with premium efficiency motors. The motor manufacturer wants the motor to be in the smallest frame size (barrel diameter) possible, to achieve the lowest cost. To achieve a higher efficiency the length of laminations inside the motor (the 'stack') will be increased until the magnetic steel becomes 'saturated' (or over 'worked') and the losses in the steel begin to rise. Once saturation begins, the stack length cannot be increased any further and therefore the motor frame size will have to increase in order to increase the power rating. Typically, higher efficiency motors are longer in length and occasionally one frame size larger at lower power ratings which can create problems when retrofitting.

Overall a premium efficient motor will operate cooler, quieter and with less mechanical stresses. This will result in providing a more reliable, longer life-cycle motor installation. Improved productivity from less down time is an additional benefit.

ELECTRICAL POWER

Using a premium efficient motor will reduce the full load current drawn from the power supply. Therefore the electrical protection of the motor must be reviewed with the overload protection adjusted as necessary.

Conversely, the starting current (or inrush current) which is normally 4-5 times full load for a standard efficiency motor, will usually increase to 6-8 times full load for a premium efficient motor. Again, the motor overload protection system will need to be reviewed to accommodate this factor.

INSTALLATION AND APPLICATION

The primary goal should be to assess and optimise the system as a whole from the motor through the power transmission to the load itself. The motor should always be optimised to the driven load.

The process may have been modified since the original installation and there is a very good chance the motor is not the best motor for the current operation. Measure the amps and confirm it is not overloaded or under loaded. The best gauge for verifying the amount of loading is the actual running RPM.

Note that measuring the amps against the nameplate does not tell you how much it is under-loaded, as current consumption is not linear. A motor nameplated at 10 A will not be half loaded at 5 A. The actual load at 5 A is typically almost no load on the shaft. No-load amps is the amperage with nothing connected to the shaft. It is also called the magnetising current. The smaller the motor, the higher the percentage of the nameplate amps corresponds to the no load or magnetising current. (CAUTION: no-load amps will typically increase each time the motor is rewound.)

RPM will tell more about under loading. If a motor's synchronous speed is 1500 RPM it will achieve that speed at no load. A tachometer will cycle between 1499 -1500 RPM typically at no load.

As the motor is loaded, the speed will decrease to create the required torque. Older motor nameplate information for RPM is generally not accurate. For a low efficiency motor, slip (where slip is the difference between synchronous speed and actual operating speed) of less than 3% (1455 RPM shaft speed) will show the motor is under loaded. Higher efficiency motors typically run at full load with no more than 3% slip (1455 RPM or higher shaft speed). Larger motors may be fully loaded at 2% slip or 1470 RPM shaft speed. By looking at the actual motor RPM and the amperage with some knowledge of the motor history, an estimate can be made of the motor's loading.

If a motor is oversized there may be a reason. Motors can be oversized due to starting requirements or possible overload conditions to prevent stalling. Knowledge of the process and history is also very important.

Regardless of these issues, replacing a low efficiency motor with a high efficiency motor will save money over the life of the motor, many times.


Higher efficiency motors operate at higher RPM for the same load. On fans and pumps this increase in impeller speed may mean an increase in load and potentially a higher current draw. Pump impellers may need to be 'trimmed' and fans and blower systems rebalanced to prevent overload. Increased speed may actually improve the process or volume of product delivered, which can be a side benefit to higher efficiency. This issue may need to be reviewed with the driven equipment OEM and presents an opportunity to update and perform the maintenance to ensure the overall system performance.

Starting torque on higher efficiency motors can be lower. This may also be the reason for an apparent over sizing of the motor. Hard to start loads may need a different solution. A high efficiency motor will meet the specified IEC values for a given kW power rating. However, most lower efficiency motors inherently exceed this rating. Establishing the worse case for starting is a very important question before resizing. Extreme loads may need assistance from the motor manufacturer or others to ensure operation in all conditions.

Under loading the motor DOES NOT improve efficiency. This may have been true with older, less efficient designs, but higher efficient motors will typically have the best power factor and efficiency at near full load. This is another change in practice for some motor manufacturers. It may also have been an installation strategy used to keep motors in service longer by upsizing/under loading. The root cause of the reason for upsizing (prior failures in service), needs to be reviewed and solved so the motor can be properly sized for long service and lowest cost.

PRIMARY RETROFIT TARGETS

Locate motors that operate for the longest period of time per day and per week. Examples are water supply pumps, recirculation fans, air compressors, conveyor motors, and exhaust fans. Typically, these motors would have a relatively short payback if they run 24/7. In general, the longer the motor runs per day, the shorter the payback period. It is also fair to assume that in most instances, the larger the motor, the quicker the payback period.

On any variable torque load such as a fan or pump, the addition of a variable speed drive (VSD) should also be considered in addition to the motor retrofit to achieve maximum efficiency. A variable torque load where demand is varying, such as water from a feed pump, will benefit greatly from the addition of a VSD that will slow the motor speed to match the demand for delivery, whereby reducing motor current (power consumption) at the same time . This will result in the best and fastest payback for the retrofit.

There are many software tools available, including the Baldor BE$T software, that will help to identify, quantify and justify the benefits for a motor alone retrofit, a VSD retrofit, or both on any given application.

HAVE A MOTOR PLAN

Once the retrofit process begins, be sure that logistics of motor replacement and sparing of such replacements is covered. Selection of similar power motors for retrofit will help in this matter. It is important to avoid the future replacement of an optimised motor with one that is not! A carefully planned schedule of replacement over time is essential for successful management of the overall programme. Guides for such replacement programmes are also available from manufacturers and government agencies where needed.

High efficiency motors will improve productivity with more reliability, and they will certainly reduce consumption of electricity. What are not as obvious are the subtle changes inside the motor that may present challenges.

When considering a retrofit:

• Consider the system approach first;
• Optimise the motor in terms of power rating to help reduce operating costs further;
• Choose retrofit targets based on best returns first;
• Review fan and pump applications that may benefit from the addition of a VSD;
• Where many motors are to be replaced, ensure an actively managed plan is in place; and
• Review causes of any chronic motor failures.

— Robin C H Cowley MCIM MIET, Baldor


Useful links
•Baldor Energy savings site: http://www.baldor.com/support/energy_savings.asp
•BE$T Energy saving software tool: http://www.baldor.com/support/software_BEST.asp
•Premium efficiency motors: http://www.baldor.com/products/product.asp?product=AC+Motors&family=Premium+Efficiency|vw_ACMotors_PremiumEfficiency
•Baldor VSD drives overview: http://www.baldor.com/products/accontrols/vs_overview.asp
•Baldor UK/Europe homepage: http://www.baldoreurope.com/default_eu.asp



********** UPDATE ON MOTOR ENERGY EFFICIENCY STANDARDS **********

The largest world body dealing with electrotechnical standards and specifications is the International Electrotechnical Commission (IEC). The IEC has a main standard that covers asynchronous AC induction motors, IEC 60034.

There are two sub-sections to this standard that relate to efficiency: 60034-2 defines the methods to actually measure the efficiency and 60034-30 defines the efficiency classifications and the associated efficiency value. Both have been recently set or updated with respect to Low Voltage (LV) motors.

• IEC 60034-2 revised in 2007. This introduces a new method that significantly increases the accuracy of motor efficiency measurement. As a result, the new motor efficiency levels are as much as a few percentage points below those using the old method. The old method (60034-2 1996) is not void until November 2010, so it is vitally important for us to know which standard is being used when a motor efficiency is stated.

• IEC 60034-30. This is a new standard and has four levels. IE1 = Standard Efficiency, IE2 = High Efficiency, IE3 = Premium Efficiency and IE4 has yet to be named. IE1, 2 and 3 have defined efficiency values for motors from 0.75 to 375 kW in 2, 4 and 6 pole speed ratings. This standard stipulates that IEC 60034-2 2007 is to be used to measure these levels.

CEMEP STANDARDS

Prior to IEC 60034-30, the European motor sector had a standardised level of efficiency in three classes through the CEMEP (Comité Européen de Constructeurs de Machines Electriques et d'Electronique de Puissance) manufacturers association voluntary agreement. Broadly these three classes equate to the new IEC levels as follows, CEMEP Eff3 has no IE equivalent as it is too low or inefficient to consider in the current market. CEMEP Eff2 = IEC IE1; CEMEP Eff1= IEC IE2. There was no CEMEP equivalent to IE3 and the levels were established using the older measuring standard IEC 60034-2 1996. The CEMEP standards are now superseded by the IEC levels.

MANDATORY MINIMUM EFFICIENCY LEVELS IN EUROPE

The EU is currently developing legislation regarding ‘Eco-design requirements for Energy using Products’ — EuP (2005/32/EC). Under this legislation, low voltage motors will be subject to a minimum efficiency level across the EU. This legislation is expected to be published in early 2009 and proposes that LV motors from 0.75 to 375 kW, of 2, 4 and 6 pole construction will be mandated at efficiency level IE2 from January 2011. It further proposes to increase this mandatory efficiency level to IE3 by 2015.

It is worth noting that most of the industrialised world either already has or is currently setting up mandatory minimum efficiency and performance standards (MEPS). In fact North America has had such a MEPS of an equivalent level to IE2 since the mid 1990s. This legislation called EPAct is currently moving to mandatory level of IE3. In this respect Europe has been slow to embrace higher levels of motor efficiency and even slower to adopt respective legislation.

FUTURE HIGH EFFICIENCY MOTORS

As noted above, the level IE4 is yet to have a title or values. With the efficiency levels of IE3, it is unlikely induction motors will reach significantly higher levels under current technology.

The use of electronic drives with motors is seen as the most likely future to higher efficiency levels. For example, integrated or internal permanent magnet (IPM) motors driven by electronic adjustable speed drives may be a solution. This latest motor technology has the magnets embedded within the rotor, and achieve efficiency levels well in excess of IE3. The added benefits of IPM machines includes cooler operation, higher speed ratings, and lower noise and vibration, and up to five times smaller frame size for the same power ratings. The down side is the cost, but it is expected that as demand (volume) increases, then costs should decrease proportionally.

Useful web links (note: these are not hot linked; you will have to copy and paste into your browser.)
IEC main site http://www.iec.ch/
CEMEP main site http://www.cemep.org/index.php?id=4
EU Site for EuP http://ec.europa.eu/enterprise/eco_design/index_en.htm


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