Problems with Over and Under Electric Motor Voltage

        Failure to properly take into consideration over and under voltage considerations.

Motors must be selected to serve in the service conditions that will be found at the site.  All too often motor efficiencies and service life suffer drastically because a motor selected looks good on paper but has its performance degraded because of the on site conditions.

You can help defray this problem by making proper selections for motors at the initial design stage.  When choosing motors for a 208-volt electrical system, select a motor that is manufactured for that voltage in lieu of utilizing a combination type motor such as one rated at 208-230/460. 

These types of combination voltage motors compromise the motor design for convenience and flexibility.  They automatically run hotter than a specifically selected voltage motor and are also less efficient.   It is better to select a 200-volt motor to operate at 200 or 208 volts.

Over Voltage Conditions:

Whenever the voltage applied to a motor is higher than its rated voltage, it will operate with an increase in the magnetizing current.  This magnetizing current increases as an exponential function.  At some point, depending on the design of the motor, saturation of the motor's core will occur and will cause overheating. 

For standard efficiency motors, when the over voltage condition reaches about 10 to 15% above the rated voltage, both the efficiency and the power factor significantly decreases.  The starting current, starting torque and breakdown torque, all significantly increase with over voltage conditions.  Experience shows that over voltages of 10% or greater significantly reduces the service life of a motor.

Under Voltage Conditions:

Whenever a motor operates in an under voltage condition it will operate at increased amperage in order to be able to develop the torque requirements imposed by the load.  This very fact will cause an increase in operating losses.  The losses occur in the stator and rotor as I²R losses.

Low voltage can also prevent a motor from developing adequate starting torque.  Figure 1-5 describes the effect of voltage variation on Power Factor, Full Load Amperage and Efficiency. 

Fig. 1-5