Standard Coil Voltage Ratings used in India
240V
Coils with rated voltage of 240V are the most widely used coils in Industrial and commercial applications. 240V single phase-neutral supply can be easily derived from a 415V Three Phase Four Wire system by connecting across one phase and neutral point (415/sqrt(3)=240). Since this distribution system is prevalent across many industrial applications, 240V coils find their application in majority of contactor applications. Common applications are industrial motor feeder systems. Also, in most of the industrial installations voltage values are quite stable and variations are limited. Hence in such systems with very less voltage fluctuations, it is viable to go for 240V coil with a standard coil band of 80% to 110% of rated coil voltage.
220V
220V coils are generally preferred in applications where the available supply is slightly less than the rated voltage of 240V. In such applications it is advisable to go for a 220V coil because one gets a lower value of pick up voltage as compared to 240V. For example for a 240V coil the coil band would be 156 – 288V. If one goes for a 220V coil then the available coil band is 143 – 264V. This takes care of the slight fluctuation in voltage which is below the band specified for 240V or a consistent low voltage.
415V
415V coils are used when there is a possibility of neutral floating condition affecting contactor operation. Neutral floating arises when the neutral is not properly grounded or ground connection is completely broken. Conventional distribution systems are three phase four wire systems in which individual single phase systems are derived from a three phase supply. In such cases the neutral is grounded and ideally must be at zero potential. In a perfectly balanced three phase four wire systems, loss of neutral conductor will not cause any abnormal voltage variation on connected single phase loads. However this condition is extremely rare and there is always some current flowing through the neutral owing to imbalances in the single phase loads. In such a scenario a loss of neutral will lead to abnormal voltage variations across the connected single phase loads. The extent of voltage variation will depend on the extent of unbalance in the single phase loads. However the imbalance in voltages will not affect the line voltages and they will continue to be at 415V.
In such a scenario if one used 240V coils then they may get damaged due to overvoltage condition arising out of neutral floating. This problem can be efficiently eliminated by going for 415V coils as neutral floating condition does not affect the line voltages. Hence the issue of coil burning due to neutral floating is completely eliminated. Improper neutral grounding can lead to voltage rise and hence going for 415V coils is advisable.
Hence for all changeover application involving four Pole contactors, it is recommended to go for 415V coils. But, it should be noted that the allowable control cable length due to cable capacitance is lowest at 415V.
360 or 380V
These coil voltages are mainly used in agricultural applications. In agriculture applications even though the rated secondary of transformer is 415V, because of simultaneous running of loads leading to sustained voltage drop and absence of voltage stablilizers, many of the users get voltages in the range of 360-380V. Since this voltage levels are much lower than 415V special coils of 360 or 380 volts have to be designed specifically for agricultural applications. These coils are restricted to applications where it is known that reduced voltage is available. These coils don’t find their applications in industrial applications where voltage supply is as per rated and stable. The choice of 360V and 380V coils can be based on how low the supply voltage can dip to in that particular installation. It is also to be noted
that in such installation Phase to neutral voltage connection is not preferred for coil voltages, due to the possible problem of neutral floating.
440V
These coil voltages are mainly used in Industrial applications, and there are chances of failure of coils due to sustained high voltages These coils are restricted to applications where it is known that higher voltage is available. These coils don’t find their applications in industrial applications where voltage supply is rated and stable.
110V
110V coils are generally used in applications where one wants to prevent any unauthorized start of the contactor. For example in many applications, operating personnel tend to override the contactor drop command given by a Distributed control system (DCS). This is mainly done by using easily available 240V single phase supply to on the contactor. However if one uses 110V coils, it acts as an efficient deterrent against overriding DCS commands as 240V supply to an 110V coil will damage the coil beyond repair. This acts as an efficient safety feature in the system. It also efficiently isolates the coil supply from the main supply through a control transformer. 110V 60Hz supply is also used mainly in western countries as 110V is much safer to operating personnel as compared to 240V. Also it should be noted that the allowable control cable length due to cable capacitance is highest at 110V.
24V DC
24V DC coils are mainly used in automation applications and in contactors which are used along with backup supplies. In many process industries having the entire control through PLC one finds applications of 24V DC coil contactors as 24V DC is predominantly required for PLC. Some of the contactors have low coil consumption coils and can be directly actuated by the PLC without the use of an interface relay. PLC output, generally being 24VDC, DC coil voltage is required. 24V DC Coils are also largely used in battery backed up systems and UPS applications. For example, in power plant a lot of critical equipment is kept on backup supply where actuation is done through a DC coil contactor, 24VDC being the most widely used.
To Summarize
240 VAC: Most commonly used coil voltage, Limitation where pickup at low voltage is required
220 VAC: Used where voltage fluctuation on lower side. Can pick up at lower voltage, Overvoltage withstand will be limited as band shifts to lower side
415 VAC: Ideal for DG applications, there is a chance of neutral floating, Allowable control cable length reduces
360 or 380 VAC: To be used in agricultural applications , where undervoltage is prevalent Overvoltage withstand will be limited as band shifts to lower side
440 VAC:
Used where voltage fluctuation on higher side. Better withstand at sustained high voltage Pick up at lower side gets limited as band shifts to higher side
110 VAC
Provides separation between control voltage and common available single phase supply. Separate control transformer is needed which makes it expensive
24 VDC
Used in PLC applications or Automation systems, Eliminates need for interposing relay
Expensive due to high cost of DC Coils and limitations of NC contacts