PUMPS STANDARD ELECTRICAL


Voltage. When a motor is operating on voltage lower than its nameplate rating, it will draw more current to create the power needed to operate. Since power equals current multiplied by voltage (P=I*V), if the voltage is low the current must increase to supply the power required. When excessive current is used by the motor, it dissipates in the form of heat. The rise in temperature of the motor winding will start to deteriorate the insulation until it finally fails and the winding shorts out. In most cases the damage from increased heat takes time to cause failure. This process could take months or even years depending on the severity and frequency of the undervoltage events.


PROTECTIONS INCLUDING FUSES, CIRCUIT BREAKERS AND ADJUSTABLE OVERLOADS SHOULD BE INCLUDED IN YOUR PUMPING SYSTEM DESIGN TO SAFEGUARD THE MOTOR.


Based on the effects of low voltage and the formula for power calculation, one may expect that if the motor is operating on voltage higher than its nameplate rating, the current would be lower. This, however, is not the case. Overvoltage, beyond


the manufacturer’s tolerance, can cause the magnetic portion of the motor to be pushed into saturation where it will draw more current. As with low voltage, the increased current in an overvoltage situation will create increased heat in the motor and ultimately premature failure.


Things to consider: Standard electrical protections including fuses, circuit breakers and adjustable overloads should be included in your pumping system design to safeguard the motor. It is also important to confirm that the transformer supplying the location can support all loads that are connected to it. Contact the local utility and have professionals check the voltage at the transformer. In many cases the transformer feeding the site can be adjusted up or down if needed. It is also important to ensure that the correct wire size is used to connect the service panel to the motor. Undersized wire will cause voltage to drop through the cable just like pressure drop in an undersized pipe.


Asymmetry (voltage and current). Asymmetry is the lack of balance of voltage or current in


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3-phase electrical loads. Voltage asymmetry (or imbalance) occurs when the electrical load is unequally distributed across the three legs of the power supply. This can occur because of the varying demands of each leg’s connections.


Even a relatively small voltage asymmetry can have a significant effect on a pump motor. As the voltage imbalance increases in a system, the temperature in the motor winding increases. At a 2% voltage imbalance, the motor winding temperature will increase by 8%. This 8% increase in winding temperature can reduce the motor life by 50% compared to a motor running at a normal temperature.


When voltage supplied to a motor is not balanced, the individual windings will consume current in an unequal manner. This is referred to as current asymmetry or imbalance. A voltage imbalance of 1% can affect the current imbalance by up to 10%. When a 3-phase motor consumes current in an unbalanced manner, one winding will “pull” harder on the rotor. This can cause the motor to have increased vibration, uneven wear on the shaft and bearings and increase heat rise in the windings. The excessive heat and vibration will lead to motor failure.


Things to consider: Voltage and current asymmetry should be calculated at the time of installation to determine the best connection to the incoming power. The National Electrical Manufacturers Association recommends that 3-phase motors should not operate with a voltage imbalance greater than 1%. If the voltage imbalance is greater than 1% the installer or owner should have a dialogue with the local utility to find a solution. For current imbalance, the target should be at or below 5% for best operation. If the voltage asymmetry is within acceptable limits and the current asymmetry remains above 5% for all connections, it is important to determine if the largest difference in current consumption is consistently drawn from the same incoming power leg (L1-L2-L3) or, if it follows the same motor winding with each connection. If the largest difference is consistently from the same incoming leg, the issue may be from the utility side of the system. If the higher current is consistently on the same motor winding, the motor should be evaluated for a potential issue.


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