ABSTRACT
In practical service conditions, relays are affected by negative environmental factors, which can considerably change their characteristics. Changes in the temperature of the environment may cause changes in the linear dimension of a core, an armature, a case, and other significant elements of a relay. As a result, distortion of movable parts and even jamming may occur. Changes in the resistance of relay windings and the modulus of elasticity of a restorable spring can lead to considerable changes in relay pickup (operate) and dropout (release). As the temperature increases from þ20 to þ1008, the insulation resistance of a relay
decreases practically by ten times. Strange as it might seem, even increased air humidity can lead to changes in pickup and release currents of a relay. Oxide films and corrosion in joints of the movable parts of a relay may cause pickup current multiplication of 10 to 15%. Temperature fluctuations varying from 0 to 20-608may result in malfunctions of a relay caused by freezing of contacts. As air-pressure goes down, its electric strength considerably decreases, in accordance with Paschen’s curve (Figure 4.1). As it can be seen from the curve, minimum electric air strength is about 320V/mm at a pressure of 4 to 5mm Hg, corresponding to 42 km height. This fact must be taken into account for relays designed for aircraft and rockets. When relays are used in movable units or stationary equipment affected by vibration,
they are prone to external mechanical vibration loads of different frequencies and amplitude. Under vibration the pickup current of a relay usually decreases by 5 to 25% because of recurrent reduction of the magnetic circuit gap, easier relay operation at the moment, and also because of a decrease in the constant of friction between movable and stationary elements. In addition, the pressing strength of the closed contacts may also change occasionally. A weakening of this effort may lead to contact welding. If this frequency of external oscillations is in accordance with the frequency of natural oscillations, resonance may occur, causing a sharp increase in the amplitude of oscillations, bringing about opening of closed contacts or closing of open contacts, breaking winding outlets, and eventually causing mechanical collapse of the relay. Relay specifications usually indicate frequency band and amplitude (acceleration) range, at which no spontaneous closing or opening of the contacts will occur, and at which the pressing effort (strength) remains great enough for a continuation of reliable work. Apart from vibration, a relay installed on movable units is also prone to linear accel-
eration. Most relays are affected by strong acceleration during aircraft take-off, in-flight maneuvers of military aircraft, and missile take-off. In these cases if special precautions are not taken, relays may be picked-up spontaneously. Relay with so-called ‘‘balanced
armature’’ (see above) have the highest resistance to linear accelerations. This type of relay is the most widespread today. Some years ago in some cases relays with an attracted-armature magnetic system (see Section 2.3.1) were used in operations with linear accelerations. Such relays were designed in such a way that spontaneous energizing in the direction of acceleration could be avoided. Relays are also very sensitive to current-conducting dust or gases causing metallic
corrosion. For example, unprotected relays installed in automation systems of big poultry-yards or stock farms will go out of action quickly due to the corrosive impact of ammonia.
