Direct Current Power Switching
Vacuum and hydrogen gas filled relays and contactors are being selected more and more often for use in direct current electrical systems. Switching a direct current load is one of the toughest tasks a relay can be asked to perform. Many times the relay or contactor will also be called upon to clear the maximum fault current, often several times higher than the normal load. Unlike AC power, where both voltage and current regularly pass through zero allowing the arc formed during switching to naturally extinguish, a DC load can only be interrupted by forcing the arc voltage higher than the source voltage. A number of schemes are commonly used to increase the arc voltage, ranging from arc chutes or multiple contacts to magnetic blowout, which lengthens the arc path.
Kilovac's power switching relays are designed to interrupt rated power as supplied. However, reactive inductive loads can result in significant voltage overshoot in some applica-tions. Voltage overshoot can be suppressed by a variety of measures.
Figure 27a, b, and c show three methods used to limit the voltage transient developed across the relay contacts when interrupting inductive loads. The same circuits impressed across the load will protect it from voltage overshoot:
- Figure 27a shows a Metal Oxide Varistor (MOV) across the power contacts. Kilovac recommends this circuit for most general purpose applications and suggests the user consult MOV manufacturers application notes since the proper MOV selection depends on transient energy, etc. MOV's are compact in size and low in cost.
- Figure 27b indicates an MOV in series with an SVP (Surge Voltage Protector - spark gap). The MOV absorbs transient overshoot energy, the SOV provides excellent dielectric once the circuit is open. This solution is also compact in size and low cost.
- Figure 27c illustrates the traditional RC Snubber. The snubber will suffice for low power and energy situations, but suffers a size and cost penalty at high power.
Kilovac vacuum and hydrogen gas-filled relays have some significant inherent advantages in switching high current DC loads. These include:
- Longer load life due to ability to use high temperature contact materials
- Low contact resistance due to the elimination of contact oxidation/contamination
- Lighter weight and smaller size due to smaller contacts and shorter contact gaps
- Low coil power due to optimized magnetic circuits and small size
- High integrity, durable ceramic to metal hermetic seals
Today, Kilovac's products have been rated at 320 Vdc since this is the most common voltage being adopted by new applications. However, the new products are versatile, encompassing a wide range of power, voltage, and current applications.
For detailed application notes on EV relays, contact the factory to request a copy of Kilovac's EVS-13 paper entitled, "Use of High Current Relays in Electric and Hybrid Electric Vehicles".
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