Motor Contactor
Captive Power Plant and Motor Problems
Captive Power Plant and Motor Problems
Squirrel cage induction motors in general are sturdy and robust in construction and are supposed to have sufficient overload capacity. But even then failure experiences due to winding burn-out is not uncommon. Such results from impropriety of application, deviations in supply system, incapacity of motor accessories, inadequacy of protective devices, aggressive environmental factors, pollutions etc. Here below are considered few of these in relation to few peculiar abnormalities experienced in an industry having captive power station.
General Captive Power House Consideration
A general power house practice is to maintain generated voltage with plus tolerance limits so that at consumer end it is at the right level. The consumer also takes precautions wherever necessary using tap changing transformers so that available voltage at the equipment terminal does not cause starvation. Relevant Indian Standard Specification [ISS] specifies +/- 10% tolerance on 11 kV H.V. and +/- 5% tolerance on 415 V L.V. systems. Thus there is the criticality of ensuring right voltage levels [tolerance being of a wider band for H.V. system] at utility points. In study of a certain case it was seen that relevant Plant design Specification [PDS] specifies generation voltage as 11500 V +/- 10% volts.
Over Voltage Condition in System
A system designed with these paramount consideration with interposing transformer ratio as 11000/433 volts the receiving end voltage at utility points on L.V. system becomes 440 V disregarding transformers regulations while generator are operated at 11.2KV. Such is a marked deviation from the ascribed 415V of the L.V. system as +5% gives a maximum of 435.75V as the permissible limit.
Low System Voltage and Motor Starting
Again if a motor designed at 415V is required to maintain developed torque at a higher value throughout the acceleration period than the resistive torque even when the Terminal Voltage is 340V as is envisaged in relevant PDS [the motor is to give required torque for acceleration even when the system voltage is 80% of the rated] such voltage may become injurious. At 80% terminal voltage during starting a cage induction motor develops 64% of the rated torque. In case the torque is to be maintained higher than this level special winding and slot configurations will be necessary like multiple cages, stepped slots, formation of end turns otherwise the motor will be overloaded.
Incapability of Conventional Bi – Metallic Thermal Over Load Relay
Under above conditions a bi-metallic Thermal Overload relay, which replicates the winding temperature rise, caused by the current flow remains overset to protect the system from overloads. A conventional thermal O/L relay with rated current set as Relay current [Ir] will have its trip operated only after the motor current rises above 135% of the load current.
Effect of System Unbalance
Under considerations of system-unbalance-abnormalities in the rotating system excepting only the winding heating due only to current flow remains unaccounted till a breakdown occurs. Such leaves the motor unprotected also against voltage unbalances creeping into the system due to resistive contacts/joints in the current path since 3.5% unbalance will cause about 25% increase in the winding temperature rise. In cooling tower installations such effect is very much pronounced in cooling tower fans since cooling tower is a wooden structure and have problem of high vibration due to which the cable terminal contacts to the motor winding gets loosened.
Such situation becomes unavoidably grave when motor absorbed power remains in much lower range. To exemplify we can consider an 11 KW 415 Volts 3 phase squirrel cage induction motor rated at 21 Amps, over load thermal trip set at 100% and drawing a load current of 11-to-13 Amps during normal loaded run. This motor will evidently be not overloaded even when the rotating system abnormalities demand an additional 4 KW due to bearing failure, cooling air fan failure, friction between stator and rotor due to loosening of slot insulation etc. And this is a huge amount of energy capable of causing physical damages even to the motor body by breaking it into pieces. Again under severest condition of unbalance when one phase-supply to the motor totally fails the motor current will reach a maximum of 23 Amps. A thermal overload relay set at 21 Amps having threshold operating current at 23.1 Amps remains inadequate to protect the motor. Such an overload protection will be found incompatible if further conditions of motor de-ratings due to environmental and pollution conditions are included.
An Approach for better protection.
With the development of technology and microelectronics establishing inroads into electrical protections more comprehensive protection systems are made possible. International Electro technical Commission had brought about major changes on standards covering low voltage switchgears and control gears in 1988. Indian standard institution with a view to adopt these changes introduced a new standard in 1993 having a dual number IS 13947/IEC 947. Further introduction of Intelligent Electronic Devices (IED) have made possible evaluating starting, operating, stalling conditions separately, measuring residual heat in the motor winding/laminations/cores; qualifying supply source etc. — the current related phenomenon of which in earlier times were necessarily to be co-ordinate in a single characteristics of thermal overload relay. These facilities afforded, an IED become lucrative but its cost is also high. One such type is the Motor Vision Relay of M/s L&T. As a cost effective measure different manufacturers have developed protective gears, which can be coordinated with current limiting MCCB / Contactor after thorough consideration and analytical study of the drive and the driven machine.
Various designs of motors and their protection gears are being developed to suit specific applications. All these changes are not globally done and manufacturers are competing hard for economic solution for the already installed and operating industries. Indigenous manufacturers like L&T, Siemens, Minilec are making available in the market low watt loss, high voltage fuses, faster and slower Overload Relays incorporating current sensing on single phasing, current limiting MCCB and Contactors, PTC Thermistor protections etc. These properly selected and coordinated are certain to afford better motor safety.
About the Author
P. Malakar, author of this article practiced electrical engineering for 30years after graduation and he, during his service period, headed electrical department and captive power plat in a fertilizer industry. He was, hence, required to consider the requirements of both plants and generators. The article is based on his experience.
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