Engineering Expert Witness Blog

     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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