The motor system - introduction
Understanding the complete motor system is the key to unlocking large energy savings. Optimising the motor system, when coupled with best practice motor management can generally deliver savings of between 30 and 60 per cent.
What determines motor system efficiency?
Motor system efficiency is determined by the motor efficiency, control system (such as a variable speed drive or damper regulation), driven system (such as a pump, fan or conveyor), power transmission, and the size and configuration of any pipe work or ducting. It is also determined by how well the system has been designed, installed and maintained.
Control systems
The control system varies the output of the motor system as required by the end use. Sophisticated control methods, such as variable speed drives, can often efficiently match the load on the motor to the demand of the end use (where the end use demand is variable). Common control methods, such as damper regulation and throttling to reduce flow rates in pump and fan systems, in some cases waste significant amounts of energy because the motor continues working at fixed speed against the restriction. These inefficient control devices can often be eliminated by simple adjustments to change the driven speed or by trimming impellors to reduce flow.
Driven systems
Driven systems vary widely in efficiency and are generally much less efficient than electric motors. Selecting and maintaining an efficient driven system will generate large energy savings. Often suppliers can assist in selecting the most appropriate driven system for your application. As with motors, purchase price is a poor indicator of the most cost effective system.
Power transmission
The efficiency of the motor system depends on the efficiency of power transmission between the motor and the driven system. Common transmission systems include direct coupled, belt driven and geared.
Pipe work, ducting, etc
Industrial sites have large amounts of pipe work, plumbing and ducting. The shorter, straighter and larger diameter of the piping, the greater the energy efficiency, as friction and turbulent flow are reduced. For more information, see fine tuning pump and fan applications.
Important system considerations influencing motor selection
In general, premium efficiency motors offer significantly greater energy performance than standard efficiency motors. However, care is required in selecting the most appropriate motor for your purpose, because motor performance depends on the system in which it is used.
In some pump and fan applications, premium efficiency motors can consume more power than standard efficiency motors. This happens when the premium efficiency motor spins faster than the standard efficiency motor causing the pump or fan to also spin faster. The subsequent increase in energy use by the pump or fan is proportional to the operating speed cubed. Motor Selector can assist you in taking into account operating speed changes in these situations.
Due to over design, many motors are underloaded, and this significantly impacts on efficiency and power factor. Downsizing is often a cost-effective way to improve efficiency. However, downsizing a motor will affect its operating speed. This has important implications for pump and fan applications.
Therefore, to achieve optimum efficiency you need to check each component in the system at regular intervals to confirm it is operating at its maximum economic efficiency. Wasted energy increases running costs, as well as often reducing equipment life and product quality.
Investigating the difference in efficiency between two pump systems at an oil refinery
This case study investigated the difference in energy efficiency between two pump systems performing the same duty at an oil refinery. The investigation found that one pump system was costing twice as much to run as the other. This surprising result is not uncommon in industry and it demonstrates that well considered changes to the motor and motor system can result in significant cost savings.
This case study also demonstrates that:
- Significant financial and greenhouse savings can be made through improved motor system performance, often at minimal or no capital cost.
- It is important to investigate the complete motor system in order to make the most cost-effective energy efficiency savings.
- The energy used by a motor system is not always what might be expected. In this case, there were significant differences in energy consumption, which were most likely due to differences in the pump systems (which look identical), not in the motors (which are visibly different).
The characteristics of the systems
The investigation considered two pump systems connected to the same circuit, which are alternately required to pump fresh water 24 hours a day all year round. Each pump system operates approximately half the time, so that when one pump system is operating, the other is on standby. The water flow rate and load on both systems is relatively constant.
System 1 consists of a new 30 kW 415 V, two-pole standard efficiency, direct drive 2 940 RPM (full load speed) spark proof motor connected to a fresh-water pump of unknown characteristics.
System 2 consists of a 45-year-old 30 kW 415V, direct drive 2 960 RPM (full load speed) spark proof motor connected to another fresh water pump of unknown characteristics.
What was investigated?
The power consumption, current and power factor of the motors were logged over a six-day period. The old motor (in System 2) was logged for two and a half days prior to a shutdown. During the shutdown the systems were switched over and then the new motor (in System 1) was logged for three days.
The logging exercise produced the following results:
- The new motor and its pump (System 1) drew 18.7 kW on average to meet the pumping requirements, with a measured power factor of 0.6.
- The old motor and its pump (System 2) drew 34.4 kW on average to meet the pumping requirements, with a measured power factor of 0.89.
- The load for both motors remained relatively constant.
- The currents before and after the changeover suggested that the voltages were balanced for each system.
The old motor in System 2 was found to consume 84 per cent more energy than the new motor in System 1 - a difference of 15.7 kW. In other words, System 2 performs the same duty as System 1, but consumes almost twice as much power.
Based on the information available, it was assumed that the load on both pump systems was identical over the monitoring period. Further testing would confirm if this assumption was sound.
What were the causes of the differences in efficiency?
Motor efficiency
One possible cause of System 2's comparative inefficiency is the type of motor used. The motor in System 2 is very old and is therefore likely to be less efficient than the new motor in System 1. The old motor is also quite likely to have failed and have been rewound. The quality and practices of rewinders impact on operating efficiency and if, for example, the rewind was poor, then the level of efficiency may have been reduced by up to five per cent. Likely efficiency comparisons are shown in Table 1. Overall, differences in motor efficiency may account for up to 4.7 kW of the performance differential observed between System 1 and System 2.
Table 1 - Likely operating efficiencies of the two motors at the time of logging
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Motor commissioning and maintenance
Other motor problems, such as soft foot, can also reduce operating efficiency but tend to cause unbalanced phases. Measurements undertaken indicated that the currents were balanced, so this type of problem is unlikely to have affected the efficiency of either system.
Differences in the pump systems
It is likely that at least 11 kW of energy is being wasted because of inefficiencies in the broader pump system. These inefficiencies are probably caused by a combination of factors in System 2, such as:
- poor conditions at the outlet of the pump;
- damaged, poorly installed or unsuitably sized impellor in the pump;
- excessive throttling in the line;
- excessive friction in the line; and
- partially blocked suction.
In some cases pipe configuration and diameter can also contribute to system inefficiencies, particularly through unnecessary bends. However, in this case the systems appeared to be identical.
What are the opportunities for improvement in this case?
There are three options to improve the energy efficiency of this operation: running System 1 continuously, upgrading System 1 and investigating System 1 and System 2 more thoroughly.
Option one: Run System 1 continuously
The most straightforward improvement would be to run System 1 continuously. System 2 could be tested during shutdowns or run occasionally to ensure that it remains operational. This would save the company $6 600 each year at no capital cost and would therefore be a great investment.
This analysis was based on continuous operation with a two-week shutdown and an energy tariff of 10 c/kWh.
Option two: Upgrade System 1
The new motor in System 1 was only 56 per cent loaded. Low load factors impact on motor efficiency and power - in this case the operating efficiency was estimated to be 90 per cent and the power factor was measured to be 0.6.
By downsizing the new motor and replacing it with a smaller, premium efficiency model (as designated by the new Australian Standard AS/NZS 1359 - see MEPS), the efficiency could be improved to 93.51 per cent and the operating power factor increased to 0.87. Downsizing would require modifying the base plate and coupling because the smaller, premium efficiency motor has a smaller frame and shaft. The smaller motor would also run marginally slower than the existing one, so the throttling would need to be checked and reduced to maintain flow rates, if these are critical.
The replacement options were analysed using Motor Selector. It showed that additional annual savings of around $700 and 6 tonnes of CO2 could be achieved by replacing the new motor with a smaller premium efficiency motor - see Table 2. The rate of return of this investment would be in excess of 40 per cent. Motor Selector highlights the fact that the motor will run marginally slower.
The analysis in Table 2 was based on continuous operation (with a two-week shutdown) and an energy tariff of 10 c/kWh. It did not take into account other savings or costs, such as savings from an improved power factor, or the costs related to modifying the base plate and coupling.
Table 2 - Results from using the Motor Selector software
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Option three: Investigate System 1 and 2 thoroughly
Both pump systems and the end use could be investigated thoroughly to identify inefficiencies. While System 1 was demonstrated to be more efficient than System 2, it is quite likely that significant room for improvement in the efficiencies of both systems exists. When comparing the energy use of motor systems it is important not to assume that the comparatively more efficient system is truly efficient - both systems may in fact be inefficient.
In this case, recommissioning the pump in System 2 would be a good place to start. To do so would involve measuring and checking the pump characteristics against the manufacturer's performance data and then using the commissioning and troubling shooting guide in the pump manual to fix any problems. If information about the performance of the pump cannot be found, the data may be available from the pump supplier or manufacturer.