This week, guest writer, Robin Cowley gives us the low-down on high efficiency motors – their construction, why we need to install them and what needs to be considered when embarking on a retrofit programme
With continual increases in energy prices and the carbon tax credits and incentives that are currently available, it is likely that most companies will consider making a change in production equipment to higher efficiency motors. From the point of view of the boardroom, 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?
In any manufacturing or industrial operation machinery will often have been modified to suit production process changes, and meet productivity and quality targets. For these reasons and many others the motors currently being used may not reflect the best selection to give the lowest energy cost for the current operation.
Moving from lowest efficiency ‘Eff3’ to mid range ‘Eff2’ generally creates few issues. These motors are very similar in construction and performance so migration is usually trouble free, unless a change in physical frame sizing can be made. However, moving from lowest efficiency motors to Eff2 level, will only yield relatively small benefits in terms of energy saving and no benefit in terms of carbon tax savings. This would result in relatively long payback periods for such projects.
Moving to motors of the very highest efficiency – ‘Eff1’ and higher - will yield the highest savings in terms of energy as well as attracting carbon tax credits. This in turn yields much shorter payback periods that are more readily financially acceptable. However, this type of motor will often require some further engineering and there may be logistical considerations in terms of installation optimisation. To ensure a smooth transition, an understanding of the mechanical and performance differences of these motors is essential if you want to achieve maximum financial benefits and a trouble-free installation.
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 adding higher performance steel, we add cost and size, but it works better, runs cooler and at a higher efficiency. The impact of this will be explained later.
Most motor losses are dissipated in the form of heat; there are also windage losses via the cooling fan. As a premium efficiency motor will produce less heat, the fan is smaller, quieter and its losses are reduced. Tighter manufacturing tolerances ensure that the Eff1 motor rotor is in the centre of the magnetic field. A rotor will inherently move axially to the magnetic centre, or try to achieve that position. If high manufacturing tolerances are not achieved, the rotor will exert an axial force, resulting in energy loss and bearing stress.
The motor manufacturer always seeks the smallest frame size (barrel diameter) possible for a given power rating, to ensure 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’ and the losses in the steel begin to rise. Once saturation begins, the stack length cannot be increased and therefore the motor frame size must be greater in order to increase the power rating. Typically, higher efficiency motors will be longer and occasionally one frame size larger, which can sometimes create a problem when replacing existing equipment.
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 relay settings adjusted or replaced if necessary. Conversely, the starting current (or inrush current) will usually increase from 4-5 times full load for a normal efficiency motor, to 6-8 times full load for premium efficiency motors. Therefore the Motor overload protection system will also need to be reviewed and adjusted where necessary to accommodate this factor.
Never assume the motor currently installed is the correct size! The process may have been modified since the original installation, or a failed motor was replaced with what was readily to hand at two-o-clock in the morning. Measure the current and confirm that the motor is not overloaded or under-loaded. The best gauge for verifying the amount of loading is the actual running rpm.
It is worth mentioning at this point that measuring the amps against the nameplate does not give a true indication of under-loading as current consumption is not linear. A motor with a 10A nameplate will not be half-loaded at 5A. Indeed, the actual load at 5A is typically almost no load on the shaft. The no-load or magnetising current will be measured when nothing is connected to the shaft. The smaller the motor, the higher will be the percentage of the nameplate current corresponding to no-load or magnetising current.
A note of caution here: no-load current will typically increase each time the motor is rewound. Motor speed will tell more about under-loading. If a motor’s synchronous speed is 1,500rpm it will achieve that speed at no load. A tachometer will cycle between 1,499 – 1,500 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% (1,455 rpm shaft speed) will indicate that the motor is under-loaded. Higher Efficiency motors typically run at full load with no more than 3% slip (1,455rpm or higher shaft speed). Larger motors may be fully loaded at 2% slip or 1,470rpm shaft speed. Motors are often oversized for a reason – to prevent stalling on starting a high-torque load, for example. A knowledge of the process and its history is therefore important before you consider re-sizing an installation.
Higher efficiency motors operate at higher rpm for the same load. On fan and pump duties 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 re-balanced to prevent overload. Increased speed may actually improve the process or volume of product delivered, which can be a side benefit to higher efficiency.
Starting torque on higher efficiency motors will typically be less than lower efficiency motors. It may also be the reason for an apparently oversized 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 will exceed this rating. Establishing the worse case for starting is a very important question before resizing. Extreme loads may need assistance from the motor manufacture or other parties to ensure operation in all conditions.
Under-loading a motor does NOT improve its efficiency. This may have been true with older, less efficient designs, but higher-efficiency motors will typically have the best power factor and efficiency at near full load. Remember, in any given installation the strategy to upsize/under-load may have been instituted simply to keep motors in service longer. The underlying decision to upsize (prior failures in service, for example), needs to be reviewed and solved so the motor can be properly sized for long service and lowest cost.
So, what are the primary targets for retrofit? Identify 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 pay back 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 cases the larger the motor, the quicker the payback period.
On any variable torque load such as a fan or pump, the addition of an Variable Speed Drive (VSD) should also be considered in addition to the motor retrofit to achieve maximum efficiency. Variable torque loads will benefit greatly from the addition of a VSD, which will slow the motor speed to match the demand, reducing motor current (and therefore power consumption) at the same time. This will result in the best and fastest payback for the retrofit.
Have a ‘motor plan’ to hand. Once the retrofit process begins, be sure that the logistics of motor replacement and associated spare parts requirements are 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 programme management. Guides for such replacement programmes are available from manufacturers and government agencies.
High efficiency motors will improve productivity with more reliability, and they will certainly reduce consumption of electricity. What is not as obvious is the subtle changes inside the motor that may present challenges. When considering a retrofit, use it as an opportunity to 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 an VSD. Where many motors are to be replaced, ensure an actively managed plan is in place. Review causes of any chronic motor failures.
Robin Cowley
Baldor UK
www.baldor.co.uk
Do you have any comments to make on this or any other subject covered in these newsletters? We are always pleased to receive feedback from readers; simply email les.hunt@imlgroup.co.uk. Bob Dobson responded to my comment posted last week on the prevailing economic climate. I reproduce it in full here:
Mr Hunt - I've just read the leader in your latest newsletter. It may be the best summary of the current financial situation I've seen – not because of its searing economic insight, but because it concludes with an honest 'I don’t know', rather than the more egotistical 'It's all because of....' or 'And the answer is....'
The truth of the matter is nobody knows where the economy is going – if they did, we’d have avoided this mess in the first place.
It is truly alarming to watch the world’s most powerful leaders floundering and to realise that the best banking brains got it so wrong. But looking for someone to blame is not going to solve the problem.
What we need – on a global scale – is as much economic stability as we can muster. This will allow businesses to continue to operate, and thus keep the economy going.
It’s a bit unpalatable, but we need the banks like never before. So we must support and protect them, although this does not mean we have to maintain the current banking structure or condone the actions of its people.
Once we’ve got the world economy onto firmer foundations, then we can – and should – look at ways to restructure world finances.
But for now we need to keep as many of the wheels of industry as possible turning. And perhaps the first step is to stop worrying about things we personally can’t fix and focus on maintaining to our own sectors. Bob Dobson
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