Energy Saving Tip – September

Energy Saving in Induction Motors -III

In the last two energy tips, we reviewed the efficiency perspective while buying new motor or replacing an existing motor & how to improve efficiency during less load conditions .

In this energy tip, we will discuss about how to save energy by reducing speed of the motor , which is dependant upon type of load on the motor .There are two types of loads

  • 01. Constant torque
  • 02. Variable torque

In constant torque type of loads, the torque requirement does not vary with the speed. It is constant , e.g. Plastic extruder . So, even if one changes the speed , power requirement will remain as it is . So, in constant load applications , energy saving can not be achieved by reducing speed of the motor.

Variable loads are the ones in which torque requirement varies as speed varies . Typical examples are centrifugal pumps & blowers . These loads follow affinity laws ,when it comes to power consumed to speed . This can be explained best as follows :

The basic principle is that the faster you try to move a liquid the more force it requires. This is the same principle as walking versus running through water, a slow walk requires little effort, but try to run through water and you will find it takes a considerable more amount of work. This is why speed reduction provides significant energy savings . e,g, Running a pump at 60% of its operating volume requires only 22% input power.

Volume Pressure / Head Power required
100% 100% 100%
80% 64% 51%
60% 36% 22%
40% 16% 6%

Generally , whenever there is a need of reduction of flow , a valve is used in case of pumps and dampers are used in case of blowers .

The valve restricts the output of the pump system and therefore the flow volume is reduced. Controlling the flow with a throttling valve is like changing the speed of a car by only using the brake pedal. The accelerator is at a fixed point and applying the brake restricts the output by increasing resistance. The car consumes a fixed amount of fuel and the engine must work to overcome friction losses in the system. This would be an inefficient way to drive a car, and yet most pump systems with valves operate in much the same fashion. A better approach is to simply change the operating speed using the gas pedal; or in the case of our pump by using a variable speed drive (VFD) or a pully system.

By Nilesh Salgaonkar , Founder Director , Teknocrat’s Control Systems (I) Pvt, Ltd.

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