Energy Saving in Induction Motors -II
In the last month energy audit tip, we reviewed the efficiency perspective while buying new motor or replacing an existing motor.
In this energy tip, we will check how loading patterns cause energy losses.
We all are aware that the power consumption of any loaded motor is decided by the load, which is being driven by the motor. We have also seen in the last energy tip that the efficiency of the motor depends upon the loading % of the motor . There are many applications & situations , wherein the load drops below 50% of the rated full load of the motor . e.g .
- 01. Compressor during unload period
- 02. Grinder during idle running
- 03. Mechanical press machine during idle period
- 04. Hydraulic pump of the injection moulding machine during cooling cycle
- 03. Lightly loaded conveyors
I have also observed many blower motors running with less than 50% loading because the motors were deliberately selected on higher side to take care of the higher starting current .
Now, when these motors run with less than 50% load , the iron losses remain same as the one during full load condition . This results in reduction of rated efficiency & power factor also. So, if the supply voltage is optimised to suit the torque requirement during this unload condition , the iron loss reduces & PF increases . Increase in PF also results in slight reduction in current which again reduces Copper losses . Both of these effects result into energy saving .It can be achieved by two ways :
01 . Delta to Star connection: The motor is hardwired into star connection. Star connection results into less voltage appied to the motor windings .This can be employed only if the motor loading is continuously less than 40% . I will not recommend the automatic start delta changeover due to frequent failures of contactors .
02. Motor Power Optimiser: This is an intelligent thyristor based device. It senses the low loading situation based on the PF & instantaneously reduces output voltage proportionally . It can be used with variable load conditions . Please note that this is not a VFD as it does not change the speed . With VFD, if one wants to reduce the voltage, frequency needs to proportionally reduced.
We will discuss more other aspects of energy saving in motors in the next energy tip ...
By Nilesh Salgaonkar , Founder Director , Teknocrat’s Control Systems (I) Pvt, Ltd.www.greenmagic.co.in