Induction Motors and Adjustable-Speed Drive Systems (Electric Motor)

7.1

ENERGY CONSERVATION

The potential energy conservation in any system can best be determined by examining each element of the system and its contribution to the losses and inefficiency of the system. Every device that does any work or causes a change in the state of a material has energy losses. Thus, typical losses include the following:
1. Electrical transmission losses from the metering point to the system. (This is where the electric power consumption is measured and the power bill determined.)
2. Conversion losses in any power conditioning equipment. (This includes variable-frequency inverters and the effect of the inverter output on the motor efficiency.)
3. Electric motor losses to convert electric power to mechanical power.
4. Mechanical losses in devices such as gears, belts, and clutches to change the output speed of the motor.
5. Losses in the driven unit, such as a pump or fan or any other device that performs work on material.
6. Transmission losses, such as friction losses to move material from one location to another.
7. Losses caused by throttling or other means to control material flow by absorbing or bypassing excess output.
Each element in a particular system has an efficiency that can be defined as

or
Losses — input power — output power
The overall efficiency of the system is the product of the efficiencies of all elements of the system; thus,

Therefore, the proper selection of each element can contribute to electric energy conservation.
This can be illustrated by an example of a constant-speed pumping system. The pumping system is to move water from one location to another at 1000 gpm with a static head of 100 ft. The friction head is 30 ft with a 4-in.-diameter supply pipe. What is the energy saving using a 5-in.-diameter supply pipe and an energy-efficient motor? Table 7.1 shows a summary of the calculations for the two systems. The net result is an annual saving of 28,520 kWh, or 19.7% of the input. Note that the savings were achieved by improved performance in several elements: lower motor losses due to improved efficiency and lower horsepower required, lower pump losses due to increased efficiency and lower horsepower required, and lower pipe friction

TABLE 7.1 Summary Calculations for Example of Pump Installation

losses with improved hydraulic efficiency. The overall system efficiencies are as follows:

The conclusion is that the complete system needs to be considered to obtain the most energy-efficient installation.

One aspect not to be overlooked is that the losses in the system are dissipated as heat at each device, such as the motors, pumps, and compressors.
Therefore, if the devices are in a conditioned environment, the effect of the losses or change in the losses on the conditioning system must also be considered.
In many installations, additional energy savings can be achieved by combining the fixed-speed induction motor with some method of varying the output speed of the unit. This is particularly true in any application in which output is a fluid flow that must vary in response to some other variable. A similar opportunity exists if output pressure must be controlled with a varying flow or varying input pressure.
Many fluid processes (including air processes) involve pumping the fluid to a high pressure and controlling flow and pressure to the required levels by throttling or bypassing. These throttling and bypass methods of control are inherently inefficient.
Centrifugal pumps, fans, and blowers have characteristics in accordance with the laws of fan performance, which state the following:
Flow varies directly with speed. Pressure varies as the square of the speed. Power varies as the cube of the speed.
These types of applications lend themselves to conversion from throttled constant-speed systems to adjustable-speed systems and offer a large potential for energy savings.