Industrial Control Basics – Thermal Expansion Effect on Overload Relays

     Imagine driving on steel tires, not rubber.  Don’t think it would work too well?  On asphalt highways maybe not, but on the steel rails that steam locomotives travel upon, steel wheels work surprisingly well and it’s due in large part to the principles of thermal expansion and the different rates at which metal alloys expand and contract.  Allow me to explain by analyzing how a locomotive“tire” is changed.

     As you can imagine changing locomotive tires isn’t easy.  Firstly, locomotive shop mechanics have to actually build a fire around the steel tire to heat it up.  The intense heat causes its steel tire to thermally expand, meaning its steel molecules become energized by the heat and begin to vibrate.  This causes the molecules to move away from each other, and this results in the tire actually growing slightly in size.  This enlargement is just enough to enable mechanics to slip the tire back onto the locomotive’s wheel.  Now in place, the tire is allowed to cool back down to ambient air temperatures.  Cooling results in the tire’s steel molecules relaxing and moving closer to each other.  The tire shrinks back to its original preheated size and tightly wraps itself around the wheel. 

     Thermal expansion properties of metals comes into play in many other instances, including the workings of motor overload relays.  Please refer to Figure 1.

motor overload relay

Figure 1

 

     Here overload relay components are shown in the foreground box.  We see that the relay includes an electric heater and a set of two peculiar looking curved objects.  These are bimetal switch contacts, so named because each is made of two, that’s the “bi” part, metal strips with different thermal properties.  These strips are positioned back to back, then bonded together and curved into a shape resembling a question mark.

     Each of the two metals has different properties, namely, one expands at a faster rate and to a greater extent than the other when heated.  This differing rate of expansion is indicative of the two metals’ diverse thermal properties.  When the bimetal contact is exposed to heat, one metal strip wants to expand a lot, but it is bonded to the other metal strip which only wants to expand a little.  The end result is that their point of contact distorts and changes shape.  When allowed to cool back down, the metal strips contract and the contact point returns to its original shape.  In our next blog we’ll see how the contact shape changes and why this shape change is important.

     In Figure 1 the motor is running normally and there is no overload situation.  Under these conditions the motor draws electric current within the normal limits of its design.  That current also flows through the heater in the overload relay causing it to generate heat, but in this situation the heat change is small enough that it doesn’t affect the bimetal switch contacts and cause them to change shape.  The temperature at which the switch contacts will warp depends on the overall design of the overload relay as well as its application.

     Next time we’ll see what part a motor overload plays in conjunction with the overload relay’s heater and bimetal contacts.

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