MOTORS AND DRIVES

Calculating Overall Motor/Drive System Efficiency Key to Understanding Impact on Operational Costs, Energy Consumption

Motor and drive manufacturers have been facing increased legislative pressure in recent years to meet higher standards of efficiency, in an effort to induce overall energy consumption by industry.

Clear Benefits
Higher efficiency motors can contribute to considerable energy savings and reduce their own payback time. For example, a three per cent difference in efficiency of a 90 kW motor -- from 92 to 95 percent -- can add up to savings in excess of $20,000 over the lifetime of the motor. For companies operating large industrial complexes with many motor driven machines, such savings can add up to millions of dollars.

ABB Industry has invested considerable effort in not only increasing the efficiency of its own range of motors and associated drives but also in understanding the nature of factors that affect efficiency.

Besides energy efficiency ratings, equipment manufacturers are aware that industrial users will look increasingly at the environmental benefits when choosing motor models. In the long run the best performing designs will offer energy savings as well as environmental ones.

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. (See 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.

Mechanical power, Pout, 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 draw from the electrical supply. The frequency converter regulates the voltage that is fed to the motor thereby 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, however, as outlined below.
- 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)

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 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.

Motor load
Just as for car engines, AC motors work at their peak efficiency over a limited range of their power output. For electric motors, they are usually working at peak efficiency at around 75 per cent of rated load (Figure 2).

Typical catalog data show that the efficiency of a 0.75kW motor -- which at its peak achieves 70 per cent efficiency -- will be reduced to only 55 per cent when operating 25 per cent of rated load. Larger motors also become less efficient at low loads, thus a 160 kW will experience a 5 per cent drop in efficiency at 25 per cent of rated 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. Figure 4 shows the typical losses in a 37 kW 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 about 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 5 breaks down the typical inverter losses at rated load.

No-load losses are about 10 per cent and the load losses about 90 per cent. Main part of the load dependent losses are generated in the inverter and the rest in the rectifier.

Simplifying calculations
ABB Industry's Drives Division has assembled its 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 (Figures 6 and 7) and then charted the motor and drive losses combined (Figure 8) which provides a clear picture of the overall performance of a motor and associated drive under all load conditions.

Application engineers can use this chart as the basis of a calculation program. 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 not only provides useful data about future installations, but can be used to estimate the total energy costs of existing systems and evaluate potential energy savings.