How to Specify Vacuum Contactors for High Altitude Environments

Motor control applications in high altitude environments pose a unique set of obstacles. As altitude increases, the density of the air decreases. Thinner air leads to fewer molecules to dissipate heat, causing electrical equipment to run hotter. More importantly, higher altitudes also decrease the dielectric strength through air. Hotter operating temperatures derate current-carrying components, while a weaker dielectric strength derates the electrical basic insulation level (BIL) and in some cases rated operating voltage.

Vacuum Contactor's Altitude Derating Guideline

It is best to separate the concept of altitude derating in vacuum contactors into two sections—first, power components, including terminals and vacuum interrupters. Second, the control which consists of the magnetic coils utilized to actuate the vacuum interrupters.

1. Power Components Derating

BIL requires derating at higher altitudes because weaker dielectric strength in the air increases the likelihood of flashover. A lower thermal transfer rate of thinner air at higher altitudes requires operating current to derate, albeit less dramatically than BIL. Below, Table A illustrates the recommended derating by altitude for Joslyn Clark's 7.2kV, 3.6kV, and 1.5kV Vacuum Contactors.

Note: All vacuum contactors are fully qualified at 2000 meters, and current derating starts when altitude exceeds approximately 2000 meters. For further detail, please refer to our datasheets.

Table A. Vacuum Contactor Power Section De-Rating By Altitude

2. Control Module Derating

Coils in the control utilize a generated magnetic field to pull on an adequately sized ferromagnetic contact material. The motion produced by the coils actuates the contactor. The coil circuit has two states. The first state draws the maximum current when energized as the ferromagnetic material is at its largest gap (open state). Once the ferromagnetic material is pulled together (closed state), it activates a switch which changes the coil circuit to its economizing state. This prevents the coils from overheating during continuous operation.

Thinner air at high altitudes causes coils to have less natural convective heat dissipation. In this case, premature failures from frequent cycling can occur. To avoid these premature failures, you will want to reduce the switching frequency of your contactor. For the recommended derating for Joslyn Clark's Vacuum Contactors, see Table B below.

Table B. Cycles/Hr Derating

Vacuum contactors rely on the vacuum in the interrupter to provide some of the force required to close the contacts. As the atmosphere thins external to the interrupter, the force created by the vacuum inside the interrupter which assists closing forces is also reduced. Lower atmospheric pressure requires operational derating such as pick- up and drop- out voltages. Lower air pressure at higher altitudes will reduce the force of the closing contacts. Consequently, contact opening time is faster. However, such benefit is marginal. The data in Table C demonstrates the impact of altitude on pick-up and drop-out voltages.

Table C. Pickup and Drop Out Derating

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