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There is an inherent capacitance in cables. The cable capacitance indicates how much charge the cable can store within itself. If a voltage is being applied the insulation on the individual wires becomes charged by the voltage. This cable capacitance is defined in pF/km. With higher lengths of cable the ability to store charge also increases.
With advent of technology and focus on energy saving, contactors are also becoming energy efficient and compact. However there is one issue linked with low VA consumption of AC coil of contactors. When the control cable length is high, the cable capacitance is more and it may be sufficient to store energy for providing hold on VA to contactor coil. This may lead to delayed drop off of the contactor. This applies to contactors operating with AC control supply only.
The effect of cable capacitance depends on the design of the control circuit as shown below,
In this case, when the coil is de-energized the net cable capacitance is disconnected from the supply and discharges through the coil. Hence there may be a slight delay in contactor drop off due to CL
In this case, the net cable capacitance continues to remain connected to the supply even after the coil supply is switched off. This capacitance will feed the coil and if the energy stored in the capacitance is more than coil Hold-on VA, the coil will continue to remain energized and the contactor will fail to drop.
The problem of cable capacitances is mainly encountered in large scale installations such a crane systems in container terminals or where control is from field devices or automation system situated away from the control panels.
Where,
L - Length of control cable in meter
m - ratio of minimum drop-off voltage to rated coil voltage
PH - Rated Hold-On power of the contactor in VA
CC - Cable capacitance per unit length in pF/km
UC - Rated control supply voltage in V
The following table gives a ready reckoner of maximum control cable length for MX contactor (AC) to ensure a clear drop off when control supply is cut off:
| Family | Contactors | Hold-On Power PH (VA) | Min Drop-off Voltage/Rated voltage | L (m) < | ||
| Ud / Uc | 110 V | 240 V | 415 V | |||
| MX mini | MX 0/6/9/12 AC | 4.5 | 0.3 | 1365.74 | 286.63 | 95.95 |
| MNX | MNX 9 - 40 | 11 | 0.35 | 3751.0 | 788.0 | 263.5 |
| MNX 50 - 80 | 21 | 0.35 | 7161.0 | 1504.3 | 503.1 | |
| MNX 95 - 140 | 36 | 0.35 | 12276.0 | 2578.8 | 862.5 | |
| MNX 185 - 225 | 56 | 0.35 | 19096.1 | 4011.5 | 1341.6 | |
| MNX 300 - 400 | 95 | 0.35 | 32395.1 | 6805.2 | 2276.0 | |
| MNX 550 - 650 | 25 | 0.35 | 8525.0 | 1790.8 | 598.9 | |
| MO | MO 9 - 45 | 9 | 0.35 | 3069.0 | 644.7 | 215.6 |
| MO 50 - 70 | 15 | 0.35 | 5115.0 | 1074.5 | 359.4 | |
| MO 80 - 110 | 25 | 0.35 | 8525.0 | 1790.8 | 598.9 | |
| MO 140 - 225 | 50 | 0.35 | 17050.0 | 3581.7 | 1197.9 | |
| MO 140 - 225 * | 17 | 0.25 | 4472.0 | 939.4 | 314.2 | |
| MO 250 - 300 | 65 | 0.35 | 22165.0 | 4656.2 | 1557.2 | |
| MO 250 - 300 * | 17 | 0.25 | 4472.0 | 939.4 | 314.2 | |
| MO0 | MO0 AC | 9 | 0.4 | 1365.74 | 710.42 | 237.6 |
*Electronic coil version
The following graphs give the trend of permissible control must be noted that the trends are given at most common cable lengths with Hold-On VA at different voltage ranges. It values of cable capacitances Cc.
Cc= 0.2 and Cc = 0.3 pf/km
Sandeep Pawar,
General Manager - Product ManagementElectrical Engineer with over 20 years of experience in Product Management and Application Engineering for Protection Relays, Power Quality Solutions, and Intelligent Motor Control Centres (MCCs). Possesses strong expertise in LV and MV power system protection schemes, motor protection, monitoring, and control applications. Well-versed in both conventional and intelligent motor management systems, with extensive knowledge of protection, control, and condition monitoring solutions.
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