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Last week we explored the topic of thermal expansion, and we learned how the bimetal contacts in a motor overload relay distort when heated. We also discussed how the overload relay comes into play to prevent overheating in electric motor circuits. Now let’s see what happens when an overload situation occurs.
Figure 1
Figure 1 shows a motor becoming overloaded, as it draws in abnormally high amounts of electric current. Since this current also flows through the electric heater in the overload relay, the heater starts producing more heat than it would if the motor were running normally. This abnormally high heat is directed towards the bimetal switch contacts, causing them to curl up tightly until they no longer touch each other and open up. They will only close again when the overload condition is cleared up and the heater cools back down to normal operating temperature. Let’s now take a look at Figure 2 to see how the motor overload relay fits into our example of a conveyor belt motor control circuit. Once again, the path of electric current flow is denoted by red lines.
Figure 2
The circuit in Figure 2 represents what happens after Button 1 is depressed. That is, the electric relay has become latched and current flows between hot and neutral sides through one of the N.O. contacts along the path of the green indicator bulb, the motor overload relay heater, and the conveyor belt motor. The current also flows through the other N.O. contact, the Emergency Stop button, Button 2, the electric relay’s wire coil, and the motor overload relay bimetal contacts. The motor becomes overloaded, causing the overload relay heater to produce abnormally high heat. This heat is directed towards the bimetal contacts, also causing them to heat up.
Figure 3
In Figure 3 the bimetal contacts have heated to the point that they have curled away from each other until they no longer touch. With the bimetal contacts open, electric current is unable to flow through to the electric relay’s wire coil. This in turn ends the magnetic attraction which formerly held the relay armatures against the N.O. contacts. The spring in the electric relay has pulled the armatures up, causing the N.O. contacts to open, simultaneously closing the N.C. contact. These actions have resulted in a loss of current to the green indicator bulb and electric motor. The red indicator bulb is now activated, and the conveyor motor is caused to automatically shut down to prevent damage and possible fire due to overheating. This means that even if the conveyor operator were to immediately press Button 1 in an attempt to restart the line, he would be prevented from doing so. Under these conditions the electric relay is prevented from latching, and the motor remains shut down because the bimetal contacts have been separated, preventing current from flowing through to the wire coil. The bimetal contacts will remain open until they once again cool to normal operating temperature. Once cooled, they will once again close, and the motor can be restarted. If the cause of the motor overload is not diagnosed and its ability to recur eliminated, the automatic shutdown process will repeat this cycle. Next time we’ll see how the overload relay is represented in a ladder diagram. We’ll also see how switches can be added to the circuit to allow maintenance staff to safely work. ____________________________________________
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Posts Tagged ‘motor damage’
Industrial Control Basics – Motor Overload
Sunday, March 4th, 2012|
Last summer my wife and I did a lot of work in the garden. Many holes were dug, bags of garden soil lifted, and plants planted. It’s a new garden, and my wife has very big plans for it, so needless to say there was a lot of work to be done. On more than one occasion we would end the day moaning about our body aches and how we had overdone it. The next day we would hurt even worse, and we’d end up taking time off to recuperate. Pain is your body’s way of telling you that it needs attention, and you’d better listen to it or you may have an even heavier price to pay down the road. Electric motors can get overworked, just like our bodies. Motors are often placed into situations where they are expected to perform tasks beyond their capability. Sometimes this happens through poor planning, sometimes due to wishful thinking on the user’s part. Motors can sustain damage when stressed in this way, but they don’t have a pain system to tell them to stop. Instead, motors benefit by a specific type of electric relay known as an overload relay. But before we get into how an overload relay works, let’s get a better understanding of how overloads happen. Suppose we’re back in the telephone factory discussed in previous blogs, watching a conveyor belt move phones through the manufacturing process. An electric motor drives the conveyor belt by converting electrical energy into mechanical energy. Everything is moving along normally when all of a sudden a machine malfunctions. Telephones start piling up on a belt, and the pile up gets so bad the belt eventually gets jammed and its motor overloaded. If the electricity flow to the motor isn’t shut down promptly by means of a nearby emergency stop button or an astute operator sitting in central control, then an even bigger problem is in the making, that of a potential fire. When electricity is applied to motors they begin to operate, and their natural tendency is to want to keep operating. They do so by continuously drawing energy from the electric current being supplied to them. The greater the workload demand on the motor, the more current it requires to operate. When motors become overloaded as in the scenario presented above, they continue to draw energy unless forced to a stop. The result is an overabundance of current flowing through the motor and no outlet for its task of converting electrical energy into mechanical energy. And where is all that pent up energy to go? It becomes heat energy trapped inside the motor itself, and this heat can build up to the point where the motor becomes damaged or even bursts into flames. Next time we’ll look at how overload relays work to keep electric motors from overheating, just as our body’s pain sensors protect us from overdoing it.
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