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Enery saving through Variable Speed Drives


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I wanted to know whether an energy saving can be obtained by using a variable seep drive. The motor details are as follows.


10hp, 3-phase Induction motor

720 rpm, 50hz, rating AOM

volts 380/440, 16.1 FLAmps


This is a trough motor in a tea factory (ventilation). Continuous operation for 10-14 hours. Currently running at the full speed. about 5 hours can be run at half the rpm. What will be the percentage energy saving possible.


Is there a chaper way of acheiving speed variation in steps. In this case 4 levels between Full spped and half the spped will be more than enough. Continuous speed variation based on sensors is not needed.


Are 3-speed motors in the market?





[Edited on 10/10/02 by Madhura]

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Hello Madhura


Yes, you can certainly save energy, if it is being wasted. If you have a fan applications as you describe, and at full speed it is circulating more air than is necessary, then slowing it down will reduce the work done, and therefore the energy consumed.

One point to remember however, is that while the fan is running at full speed, it is consuming more energy when used with a drive, than connected directly to the supply. The drive itself consumes energy, typical efficiencies are in the order of 95%, so there is an energy loss of up to 5% in the drive, plus in most drives, there is a very distorted current drawn from the supply resulting in a poor power factor, higher KVA demand than a corrected motor and higher supply losses due to the harmonic currents.

If the fan system is currently controlled by shutters or similar mechanical means to control the air flow, then the energy wasted is already reduced, reducing the potential savings from speed control.

This means of energy optimisation is being heavily promoted by drive companies and definitely will reduce wasted energy, but I believe that it is sometimes oversold.


Best regards,

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  • 3 weeks later...


I was recently asked this question. Anybody got any comments?


Can you inform me as to what will be the chances of Energy saving with 3 phase Induction motor coupled to Compressor and is driven by AC variable frequency drive.Pl note this is constant torque application.The frequency (speed) reduction could be to about 30 Hz against rated Frequency of 50 Hz.


Anil Deshpande


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What type of compressor is it? Not all compressors are constant torque. If it is a piston type, it is (more than likely) a variable torque load. Oil flooded and rotary compressors are generally constant torque. Piston compressors are usually rare above 15kW.


The VSD can be modelled as a power source ie. VA in = VA out. The VSD not only changes the frequency of the motor, but also the voltage (have you heard of the VVVF?). Therefore, as the voltage and frequency decrease at the motor, the input current will decrease proportionally (assuming the motor current remains the same~constant torque). This of course does not take into account the relatively constant magnetising current required by the motor, which requires an "offset" in the V/Hz ratio.


As marke states in an earlier post, don't rule out other losses associated with the VSD.

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