Read any trade magazine and it is rare these days that you see drives manufacturers promoting the energy efficiency benefits of variable speed drives. Their focus tends to be Y2K, EMC, serial communications and small size, but little is reported about one of the basic reasons a variable speed drive exists - to save energy.
The Climate Change Levy has put a spotlight beam on industry’s consumption of electricity. 65% of electricity in industry is used by electric motors, many of them running far less efficiently than they could. Many industrial companies could easily alleviate or even eliminate the effects of the levy, by addressing inefficiencies in motor drive systems.
There are about 10 million motors installed in the UK industry alone. Of all these motors, it is estimated that only some 3% are controlled by variable speed drives. This makes the efficiency and savings potential phenomenal, as energy savings in excess of 50% are not unusual when using drives in pump and fan applications.
In fixed speed operations, high efficiency motors can contribute to considerable savings. For example, a new high efficiency EFF1 motor rated at 90kW with 95.2% efficiency, will cost around £5,900 and will use electricity costing around £37,250 per year, but will save nearly £9,000 compared to a standard efficiency EFF3 motor with 93% efficiency, over a 10-year service life. For companies operating large industrial complexes with many motor driven machines, such savings can mean tens of thousands of pounds annually.
To optimise operational costs and energy consumption, it is important to be able to calculate the overall efficiency of the combined system when choosing an AC motor and drive system.
The nature of efficiency
Efficiency in terms of electrical and mechanical systems has a simple definition:
Efficiency = output power divided by input power.
However, in the real world of industrial processes, this simple definition has to be tempered by actual performance and the imperfect nature of actual machines. The total efficiency of a drive system depends on the losses in the motor and its control system. Drive and motor losses are thermal in nature, so they are dissipated as heat.
Input power to the drive system is electrical, while output is mechanical. Knowledge of both mechanical and electrical engineering is required when calculating the overall efficiency of any motor and associated drive system.
Electrical input power depends on voltage, current and the power factor (Figure 1).
The power factor determines what proportion of the total electrical power is 'active' power and how much is 'reactive' power. Active power provides the required mechanical power while reactive power is needed to produce magnetisation in the motor itself.
Figure 1: The efficiency of the drive system.
Mechanical power, P out, is dependent on the required torque,T, and the rotating speed, n. The faster the speed or the greater the torque required, the more power needs to be delivered. This has a direct effect on the amount of power the drive systems draws from the electrical supply. The frequency converter regulates the voltage that is fed to the motor so directly controlling the power used in the motor as well as the process being controlled.
Motor efficiency
Motor efficiency is typically between 0.70 and 0.97 depending on the motor size and rated speed. There are four different factors that impact motor efficiency. As outlined as follows.
• Motor size
• Motor rated speed
• Motor load
• Type of control
Motor size
In general, smaller motors with rated power of 1 kW or below are less efficient than larger ones, typically efficiencies are 70-80 per cent. Larger motors rated 100 kW and above have efficiencies higher than 95 per cent (Fig.2)
Figure 2: Typical efficiencies of Different Motor Sizes
Motor speed
Commercially available AC motors are rated according to speed, which is determined by the number of poles. Overall, four pole motors with speeds up to 1500 rpm tend to be the most efficient due to the fact that the four pole motor tends to use the motor geometry and material better than other types of AC motors. In Figure 3 the differences in efficiencies for 250 kW motors for different rated speeds shows that the 4 pole motor has an overall efficiency of 96.5 compared to 95.7 for a slower motor with the same output.
Figure 3: Efficiencies of four 250 kW motors with different rated speed
Motor load
Just as for motor car engines, AC motors work at their peak efficiency over a limited range of their power output. Modern EFF1 electric motors, are usually working at the peak efficiency at around 75 per cent of rated load (Fig.2), whereas older designs often have peak efficiency in a very narrow band around full load .
Type of control
There are some additional losses induced by speed control. To understand the nature of these losses, it is important to look more closely at the internal design of an AC motor. There are four types of losses - frictional losses, resistive rotor losses, iron losses and resistive stator losses. Stator and rotor winding losses are the most significant followed by iron losses with frictional losses accounting for less than 10 per cent of the total.
Frequency converters with non-sinusoidal current can cause harmonic losses in the motor and increase motor losses by up to 10 per cent, which translates into an overall reduction in motor efficiency of up to 1 per cent.
Frequency converter efficiency
Electrical switching with transistors is very efficient. So the efficiency of a frequency converter is also high - from 0.97 to 0.99. Figure 4 breaks down the typical inverter losses at rated load. ‘No load’ dependent losses are about 10 per cent and the ‘load dependent’ losses about 90 per cent. The main part of the ‘load dependent’ losses are generated in the inverter and the rest in the rectifier.
Figure 4: Typical Inverter Losses at Rated Load
Simplifying calculations
ABB has assembled this knowledge of motors and drives to create a tool for the selection of the optimum system for a particular application. The company has looked at the impact of constant torque and variable torque on the losses in its own AC drive designs and then charted the motor and drive losses combined (see Fig.5) which provides a clear picture of the overall performance of a motor and associated drive under all load conditions.
Figure 5: Typical efficiency for a 75 kW drive system
Its application engineers can use this chart as the basis of a calculation programme. By inputting the motor and drive data, it is possible to establish the various efficiencies that will be expected at various selected load and frequency levels to select the optimum performance.
The system provides not only useful data about future installations but can be used to estimate the total energy costs of existing systems and evaluate potential energy savings.
ABB have also developed a 6 step plan for users to determine the best ways of achieving savings in their own plants..
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