Understanding Motor Overloads Caused by Low Voltage

Motor overloads are a common industrial problem when motors are not protected adequately.  This can include incorrectly sized circuit breakers or no circuit panel at all.  It can also be due to an undervoltage situation.

The definition of undervoltage is the condition when the applied voltage drops to 90% less than the motor's rated voltage for at least 1 minute.  Low-voltage situations occur when the voltage demand exceeds what the line can deliver.  This can be caused by a number of reasons; a transformer can act as a choke, restricting the amount of total power that gets through when demand is high.  Maybe the demand from newly installed equipment sharing the same line exceeds what the line can deliver.  In some cases, a motor with a long cable run can drop the voltage on a circuit, resulting in low voltage to that motor.  Brief low-voltage conditions can occur when a large load is started up unexpectedly, or when power is shorted to ground or another line.  Even loose cable connections can be the culprit.

To understand how low-voltage effects electric motors, one must understand the applied voltage to motor torque ratio.  Torque changes as the square to the voltage applied.  So a 10% increase in voltage will result in a 21% increase in torque.  Conversely, a reduction to 90% of the rated voltage will result in a 19% reduction in torque.  A more severe undervoltage condition - say 20% below the rated value, will reduce the motor's torque to 64% of its rating, resulting in 156% motor overload condition and catastrophic failure.

When does overload come into play in a low-voltage situation?  When the motor torque decreases below the torque required by the load, the motor will stall.  This is a bad situation if not caught immediately, as the motor now simply becomes a generator of heat.  Increased heat can damage the motors windings and insulation if the low-voltage event is long enough in duration.  This damage is permanent, and will result in a shorter life span or even a catastrophic failure.  In a prolonged low-voltage event, the only motor that is safe is one that is turned off.

Quick response techniques and computer monitoring and control equipment can reduce or even eliminate the damage of low-voltage events.  Thermal protectors or condition-monitoring devices can detect abnormally high winding temperatures and automatically shut down the motor. 

Being proactive is key in avoiding serious problems.  And thanks to computers, most everything can be monitored and controlled more accurately now.  This includes the electric motors that you rely and depend on.