| 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.
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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Posts Tagged ‘electrical contacts’
Industrial Control Basics – Latching Circuit
Sunday, January 29th, 2012| When I think of latches the first thing that comes to mind is my Uncle Jake’s outhouse and how I got stuck in it as a kid. Its door was outfitted with a rusty old latch that had a nasty habit of locking up when someone entered, and it would take a tricky set of raps and bangs to loosen. One day it was being particularly unresponsive to my repeated attempts to open it, and the scene became like something out of a horror movie. There was a lot of screaming.
When latches operate well, they’re indispensable. Let’s take our example circuit from last time a bit further by adding more components and wires. We’ll see how a latch can be applied to take the place of pressure exerted by an index finger. See Figure 1.
Figure 1
Our relay now contains an additional pivoting steel armature connected by a mechanical link to the original steel armature and spring. The relay still has one N.C. contact, but it now has two N.O. contacts. When the relay is in its normal state the spring holds both armatures away from the N.O. contacts so that no electric current will flow through them. One armature touches the N.C. contact, and this is the point at which current will flow between hot and neutral sides, lighting the red bulb. The parts of the circuit diagram with electric current flowing through them are denoted by red lines. Figure 1 reveals that there are now two pushbuttons instead of one. Now let’s go to Figure 2 to see what happens when someone presses on Button 1.
Figure 2
Again, the parts of the circuit diagram with current flowing through them are denoted by red lines. From this diagram you can see that when Button 1 is depressed, current flows through the wire coil, making it and its steel core magnetic. This electromagnet in turn attracts both steel armatures in our relay, causing them to pivot and touch their respective N.O. contacts. Electric current now flows between hot and neutral sides, lighting up the green bulb. Current no longer flows through the N.C. contact and the red bulb, making it go dark. If you look closely at Figure 2, you’ll notice that current can flow to the wire coil along two paths, either that of Button 1 or Button 2. It will also flow through both N.O. contact points, as well as the additional armature. So how is this scenario different from last week’s blog discussion? That becomes evident in Figure 3, when Button 1 is no longer depressed.
Figure 3
In Figure 3 Button 1 is not depressed, and electric current does not flow through it. The red bulb remains dark, and the green bulb lit. How can this state exist without the human intervention of a finger depressing the button? Because although one path for current flow was broken by releasing Button 1, the other path through Button 2 remains intact, allowing current to continue to flow through the wire coil. This situation exists because Button 2’s path is “latched.” Latching results in the relay’s wire coil keeping itself energized by maintaining armature contact at the N.O. contact points, even after Button 1 is released. When in the latched state, the magnetic attraction maintained by the wire coil and steel core won’t allow the armature to release from the N.O. contacts. This keeps current flowing through the wire coil and on to the green bulb. Under these conditions the relay will remain latched. But, just like my Uncle’s outhouse door, the relay can be unlatched if you know the trick to it. Relays may be latched or unlatched, and next week we’ll see how Button 2 comes into play to create an unlatched condition in which the green bulb is dark and the red bulb lit. We’ll also see how it is all represented in a ladder diagram. ____________________________________________ |
