Improving motor efficiency

July 23rd, 2014, Published in Articles: EE Publishers, Articles: Vector, Featured: EE Publishers

 

Some 20% of the estimated 65% of industrial energy used by electric motors is lost by wasteful throttling mechanisms, making it essential to reduce the energy appetite of motors.

Energy consumption by electric motors can be reduced in two main ways – efficient control of the speed at which they run, and making the motors themselves more efficient.

Optimum motor speed

By far the most effective method of controlling a motor’s speed is through the use of variable speed drives. Much control is, however, still performed with throttling valves in pump systems or vanes in fan applications while the demands for rotating machinery are solved by gears or belt drives  (see Fig. 1).

Fig. 1: Variable-speed drives offer the most effective method of controlling a motor’s speed, thereby contributing significantly to energy saving.

Fig. 1: Variable-speed drives offer the most effective method of controlling a motor’s speed, thereby contributing significantly to energy saving.

Speed control with belt drives, gearboxes and hydraulic couplings all add to the inefficiency of the system to varying degrees and require the motor to run at full speed all of the time. In addition, mechanical drives can be noisy as well as difficult to service, situated, as they are, between the motor and the driven machinery.

These arrangements often seem cost-effective at first sight, but they are energy wasters.

Imagine trying to regulate the speed of your car by keeping one foot on the accelerator and the other on the brake. Running a motor at full speed while throttling the output has the same effect; a part of the produced output immediately goes to waste.

In fact, so much energy is wasted by inefficient constant speed and mechanical control mechanisms that every industrialised nation around the world could make several power stations redundant simply by using variable speed drives (VSDs) instead. In the right applications, VSDs can make a huge difference.

In pump and fan applications, using variable-speed drives can cut the energy bill by as much as 60% (see Fig. 2). A pump or fan running at half speed uses only one eighth of the energy compared to one running at full speed. Or, put differently, the power required to run a pump or a fan is proportional to the cube of the speed. This means that if 100% flow requires full power, 75% requires (0,75)³ = 42% of full power, and 50% flow requires (0,5)³ = 12,5% of the power.

Fig. 2: In pump and fan applications, VSDs contribute to large energy savings, which in turn impacts the environment.

Fig. 2: In pump and fan applications, VSDs contribute to large energy savings, which in turn impacts the environment.

As a small reduction in speed can make a big difference in energy consumption, and as many fan and pump systems run at less than full capacity much of the time, a variable speed drive can produce huge savings. This is particularly so when compared to a motor that is continuously running at full speed.

The efficiency of motors and drives has improved considerably over the years. Motors have improved in efficiency by an average of 3% over the last decade, while ABB AC drives delivered in the past ten years for the speed control of pumps and fans are estimated to reduce electricity consumption by about  81 000 GWh per year worldwide.

This means that the company’s AC drives now in use reduce global CO2 emissions by over 68-million tonnes every year, equivalent to the emissions of a country the size of Finland, with a population of over 5-million people.

If we replace an average 1980s motor and frequency converter with an ABB high-efficiency motor and an ABB drive, the payback time due to lower energy consumption is a few years, depending on annual operating hours and energy price. This points to a great potential replacement market as users seek to improve their energy consumption.

Regulating the motor speed has the added benefit that it accommodates production rises easily without extra investment, as speed increases of  5 – 20% are not a problem with an AC variable speed drive. By matching the performance of the motor to the needs of the process, variable speed drives can give major savings compared to the wasteful practice of running the motor at full speed against a restriction to modulate output. In an ideal world, we would be approaching the point where energy was applied with pinpoint accuracy when and where needed, and never wasted.

Despite these obvious energy saving advantages, 97% of all motors in applications under 2,2 kW have no form of speed control at all, equating to some 37-million industrial motors sold annually, worldwide.

In the past, this might have been understandable, as a small drive cost in the region of $500 per kW. Over the past few years, however, drives across the range have become smaller and cheaper and now start at around
$150 per kW.

This can make investment in a VSD a viable proposition on energy grounds alone. The new generation drives is smaller and so installation might be possible in places where a space constraint was an issue in the past. They are also more energy efficient than their predecessors.

An example of these smaller, cheaper drives is the range of ABB component drives; these are used in new, small-scale operations where no-one would have thought of employing a VSD in the past, such as potters’ wheels, spa baths and oven hobs. It is estimated that 40% of the value (and 90% in units) of all drives shipped are rated at less than 40 kW.

The company is developing drive technology with radical new control techniques such as direct torque control (DTC). A feature of DTC which contributes directly to energy efficiency is motor flux optimisation which greatly improves the efficiency of the total drive, the controller and the motor in pump and fan applications.

The drives themselves are becoming leaner too, not only smaller in size but more energy efficient to manufacture, with smaller circuit boards and enclosures made of recyclable plastic.

Example

A case in point is the German company Stadtwerke Strausberg, which operates the district heating scheme in the town of Strausberg, 30 km east of Berlin. Its 86 MW power plant produces 190 000 MWh of heating energy, distributed through a 32 km distribution network with seven substations, to most official buildings and 50% of the households in the town. The company decided to upgrade its control system, which was using throttling valves, to one with VSDs.

Using the throttling valves to reduce flow increased the head, making the system less efficient as the pump worked harder to overcome the extra head. Temperature changes were too large and fast, and high pressure through the control valves caused loss and noise.

The system is now equipped with VSDs and works on the principle of keeping constant pressure in the network. When temperatures drop, the thermostat valves open, causing the pressure to fall and the pressure transmitter output signal to decrease.

This increases the pump speed and the higher flow rate increases the water pressure until a control loop balance is reached.

The annual pumping energy consumption was about 550 MWh using throttling valves, but was reduced to  230 MWh when variable speed-controlled pumps were used throughout the year. The payback period of the variable-speed control system was twelve months.

Motor rewinds: a false economy

Many motor users faced with failed motors will opt to have them rewound rather than purchase another one, believing this to be the cheaper option. Although this is the case in a straight comparison between rewind cost and new purchase cost, the resulting loss of efficiency wipes out any initial cost advantage.

This was illustrated in the Ontario Hydro experiment. Ontario Hydro purchased ten new 15 kW motors, which were then tested independently. The motors were purposefully damaged and sent to nine different repair companies.

They were retested after winding, and the results are shown in Table 1.

Results of tests carried out on 15 kW motors rewound at nine different repair companies 
Motor Efficiency change %
1 – 3,4
2 – 0,9
3 – 0,6
4 – 0,3
5 – 1,0
6 – 0,7
7 – 0,4
8 – 0,9
9 – 1,5
Average  – 1,1

Table 1: Results of tests carried out on 15 kW motors rewound at nine different repair companies.

Ontario Hydro concluded that failed standard efficiency motors should, in many cases, be scrapped and replaced by high-efficiency models. Efficiency is lost in rewinds for several reasons: core losses increase due to the high temperatures experienced during failure; stripping the motor for repair also damages the laminations; copper losses increase because of the practice of using smaller conductors, increasing I2R losses, and fitting of universal cooling fans, which may not be designed for the particular motor, leads to an increase in windage losses.

This decrease in efficiency and the consequent increased running cost make the rewinding of motors not such an attractive option as it might first appear (see Table 2).

Rewinding a motor versus purchasing a new one
Example: 75 kW 4-pole motor; continuous running; $0,063/kWh
Original motor rewind New motor
Cost of rewind: $2226 ABB high-efficiency motor typical
capital cost: $3585
Increased annual cost with 1,1% efficiency loss: $613 3% Annual energy saving with increase
in efficiency: $1435
Actual cost in first year: $2840 Actual cost in first year: $2150

Table 2: Rewinding a motor versus purchasing a new one.

As the figures show, purchasing a new motor results in a saving of $690 over the first year.

Improving motor efficiency

What can be done to improve motor efficiency? Designers can minimise losses by improving the design of features which give rise to the main losses in the motor. The greatest losses are the iron losses which occur in the rotor and stator, accounting for 50% of the total loss. This can be improved by using low-loss steel and thinner laminations. Copper losses account for 20%. Using an optimum slot fill design and larger conductors can reduce these.

Bearing friction and windage losses total 23% and can be reduced by using a smaller cooling fan. Stray losses, which account for 7% of the total, can be reduced by improving the slot geometry.

Manage your motors

Users can also do much to ensure they are getting the highest efficiency from their motors. A defined motor management policy must be in place. One policy decision should be to select high-efficiency motors when purchasing new plant equipment.

Users must specify minimum acceptable efficiency values. A replace or rewind decision can be made long before failure occurs – there must be clear guidelines for all responsible personnel. High efficiency also means improved reliability and less downtime and maintenance.

Lower losses provide:

  • Better tolerance to thermal stress resulting from stalls or frequent starting.
  • Increased ability to handle overload conditions. Motor efficiency can be increased by improving the laminations, slot geometry and slot fill design, and by using smaller cooling fans and larger conductors.
  • Better resistance to abnormal operating conditions such as undervoltage and overvoltage or phase unbalance.
  • Higher tolerance to poorer voltage and current wave shapes.

A motor management policy helps bring together capital, maintenance and revenue budgets, showing the effect they have on each other when different types of motor are selected.

Users benefit from such a policy through reduced energy costs, by upgrading to high-efficiency motors at the most cost-effective time. The forward planning inherent in the practice helps reduce downtime and inventory can also be reduced through a fast track delivery agreement.

Contact Mark Sheldon, ABB,  Tel 010 202 5868, mark.sheldon@za.abb.com

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