Engineering Expert Witness Blog

    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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