Introduction to Industrial Automation

2.1.5   Power  Relays

Power relays are made in order to feed the various kinds of electric motors with the required 

electric power. In proportion to the typical motors’ powers, these power relays are made in sev-

eral sizes, from the smallest of nominal power 5.5 KW, to the largest of 500 KW under 660 V, 

while it is obvious that the power relays and particularly their electric contacts must withstand a 

“switching under load”. This means simply that an electric contact of a power relay should have 

the mechanical strength required in order to open while the nominal current is passing through 

it. The reason that causes damage to the contact material is that an electric arc is created during 

the opening or closing of the electric contact. Thus, it is obvious that the electric contact consist-

ing of two thin metallic plates, as shown in Figure 2.6, does not have the strength to break a high 

electric current. Therefore, the construction of a power relay leads to the increase of the contact 

surface and subsequently to larger dimensions of the metallic plates. This, in turn, leads to the 

need for stronger attraction of the movable arm and hence to larger dimensions in the coil and 

the corresponding coil’s core. In conclusion, a power relay has the same principle of operation as 

the one described in Section 2.1.1, but the higher its nominal power, the larger its size is.

Power relays are characterized from both the nominal current of its electric contacts and the 

nominal power of the electric motor that supplies it. These magnitudes are different for the vari-

ous categories of power relay use, such as for example AC1, AC2, etc. (IEC 158-1, BS 5424, and 

VDE 0660). Figure 2.7 shows the side cross section of a large size power relay, while the internal 

structure of the power relay includes all the components described in the “principle of operation” 

(Figure 2.6), which are the coil, the fixed iron core, the movable iron core or arm, the electric 

contacts, and the arc chamber for protection. It should be noted that the rotary motion of the 

movable contact found on small relays, shown in Figure 2.6, is replaced in the case of power relays 

with parallel motion, resulting in the realization of the electric contact at two different points and 

the so-called double-break contact. To reduce the effects of contact arcing, modern contact tips 

are made of, or coated with, a variety of silver-based alloys (silver-copper, silver-cadmium-oxide, 

silver-nickel, etc.) to extend their life span. Since power relays switch their rated loads, their electric 

contacts are characterized by an electrical and mechanical life, expressed in thousands or millions 

of operations, while the usual values of electrical or mechanical life expectancy may be one mil-

lion, 10 million, or even 100 million operations. The electrical life is usually lower in comparison