Effect of Voltage Variation on Motors
Abhijeet Limaaye
Certified Energy Auditor committed to make the world a Safer and Energy Efficient place
Three phase Induction motors form the heart of Industrial systems. Thus healthy motors contribute to a healthy industry and high industrial growth. It has been observed that one of the prominent reasons for motor failure is unregulated voltage at the user end. Both low and high voltages affect the motor performance and motor deterioration. Let us have a look at these effects to have a better understanding of motor performance.
The graph below shows the effect of voltage sag or swell on motor performance.
Low Voltage:
As seen from the above graph, as input voltage dips, it results in rise of Full Load Current as Shaft Power drawn will remain constant. This will result in increase of Copper/Winding Losses and overheating. Most of the time motors can withstand under voltages due to the tune of 10% but any sag below it can result in dangerously high losses. Thus it reduces efficiency of the motor.
Motor torque drops with drop in voltage as it is proportional to square of voltage. Thus for 90% voltage, torque will be reduced to 81%. In case of heavy loads, this may result in locked rotor and winding overheating. Thus, care must be taken in case of loads with high starting torque. In case of light loads like pumps, reduced voltage will reduce mechanical stresses and result in better starting. In fact, reduced voltage starting is one of the recommended methods for starting of light loads.
As magnetizing current is low in case of low voltage, motor will have improved PF .
High Voltage :
As input voltage rises, Full load current is expected to dip. However, higher voltages result in higher magnetizing currents and higher Iron/Core Losses. This in fact results in higher full load currents and insulation overheating. Most of the time motors can withstand overvoltage due to the tune of 10% but any rise above it can result in dangerously high losses. Thus it reduces efficiency of the motor.
Motor torque rises with rise in voltage as it is proportional to square of voltage. Thus for 110% voltage, torque will rise to 121% . It will also result in rise of starting current resulting in overheating and very high mechanical stresses . This might damage the motor and/or the load. Hence, overvoltage is very dangerous for a motor.
As magnetizing current is rise , motor has lower PF in case of overvoltage .
Thus we can conclude that both overvoltage and under voltage are dangerous for motor health. Hence, a regulated supply is necessary to achieve best performance. As we cannot rely on utility to regulate the supply, it is recommended to use a On Load Tap Changer with the Transformer to achieve Voltage regulation.
Founder & Director at SearchTeachers.in
3 年does this mean that by decrease of 10% Voltage during operation of motor, the power consumption will also decrease by around 10%
Electrical Engineer at Nicico Company
5 年can we use on load tap changer for reduce under voltage in starting of mv motors? and response time of on load tap changer is suitable for this work?
Ing. Eléctrico, Confiabilidad, Predictiva, Gestión, LSS Green Belt, Liderazgo y Desarrollo
8 年Great info, what happend when the motor is controlled by VDF at low hz?? In plant, the VDF controls the relation V/Hz, in 440VCA circuits if the VDF provide 30hz the V drops to 220, we had aplications with 10hz
Electrical Engineer at Nishat CCPP power Ltd
8 年I think more important topic is motor selection wrt temperature class as well as application area. there are many well known venders making B insulation class motors with writing it F-class .Temperature kills motors. Ideally, a motor should have a relatively high insulation temperature rating,and a relatively low operating temperature for both the insulation system and bearing system. This provides thermal margin in the event of motor overload, severe starting duty, safety margin for adjustable speed drive applications. unbalance voltages means your LV bus bar is unbalance which is not possible.
Chevron| (CPEng, NER, Mechanical Integrity & Reliability Engineering)| Monash University, Edith Cowan University and Madras University
8 年Good information on the effect of unregulated voltage on motor reliability. Thank you