Is power quality killing your motors?
By Alan Bixler and Brendan Watson
Understanding electrical power and how it affects electric motors is critical for successful pump installations. Power quality can directly affect both the operation and the life of a motor.
Since many individuals in the water well pump industry are not electricians or electrical engineers, let’s start by reviewing power and how it is used in the electric motor. In this case, power is a measurement of how much energy is being consumed by the motor.
For electric motors, power is measured in watts, which are calculated by multiplying the supply voltage (V) by the current being consumed in amps (I).
P (watts) = I (current) × V (volts)
Since we cannot see electricity, let’s visualize it as water in a garden hose. Voltage represents the pressure of the water in the hose and current (amps) represents the flow of the water.
When a motor is running, it is consuming energy in the form of current at a particular voltage. If the voltage and current are within the design parameters of the motor, it will operate as designed with normal heat rise. If power quality issues occur, motor operation can be affected.
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 times 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.
Based on the effects of low voltage and the formula for power calculation, one may expect 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.
If overvoltage or undervoltage is an issue, there are a few things that can be done to correct the problem. First, make sure that the transformer supplying the location can support all loads connected to it. Contact the local utility and have them 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 the correct wire size is used from the service panel to the motor. Undersized wire will cause voltage drop through the cable just like pressure drop in an undersized pipe.
Asymmetry is the lack of balance of voltage or current in 3-phase electrical loads. Voltage and current asymmetry are critical with 3-phase motor installations as the imbalance in voltage, and ultimately, current can cause severe motor issues.
The illustration on the previous page is an example of a local grid supplying power to both residential and commercial real estate. Within this grid we also have a pump station. The residential and commercial properties are connected to single phase, 240V supply using either L1-L2 or L2-L3 connections.
The L1-L2 connection is supporting more of the load than the L2-L3 connection and there are no connections to the L1-L3 connection. Even though the grid has unbalanced load connections, the voltage asymmetry at the pump station (0.60%) is acceptable at less than 1%.
Now consider the illustration above of the same grid example on a hot summer afternoon (95°F). The residential and commercial load on the grid will increase drastically due to demands from air conditioning (A/C) units operating at peak levels.
As is shown, the grid is more heavily loaded on the L1-L2 connection and the increased power consumption from the A/C units will cause the voltage to drop on those two legs. Since the L3 leg has a minimal load, the voltage would not be reduced in the same way as the L1 and L2 legs. This results in a voltage asymmetry or imbalance at the pump station. There is now a 2% voltage imbalance which is significant and will have a negative effect on the pump motor.
Even a relatively small voltage asymmetry can have a significant effect on a motor. As the voltage imbalance increases in a system, the temperature in the motor winding increases. Heat kills motors. 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.
Voltage asymmetry should be calculated when the motor is installed to determine the best connection of the motor leads to the incoming power. The National Electrical Manufacturers Association (NEMA) recommends that 3-phase motors should not operate with a voltage imbalance greater than 1%. If the imbalance is greater than 1%, the installer or owner should have a dialogue with the local utility to find a solution.
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 current 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.
Current asymmetry should be calculated at the time of installation to determine the best connection to the incoming power. 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.
Detection and Protection
Both voltage asymmetry and current asymmetry can be detrimental to electric motor operation and life expectancy. Typical electrical protections are made using fuses, circuit breakers, and adjustable overloads.
As pump and motor installers, it is critical to understand the basics of electrical power and the effect it can have on motors.
While these devices are good safety measures to protect the motor from significant overload and provide safety from a short circuit, they do not provide protection from voltage and current asymmetry. For detection of and protection from these power quality issues, the use of an advanced motor protection device is recommended.
These types of devices are available from several suppliers, and in most cases will not only monitor the system for power issues, but also stop the motor when significant problems are detected.
An advanced motor protection device may also record voltage and current in real time and either store this data or send it to a cloud server via telemetry or direct internet connection. Additional advanced motor protection device functions can include monitoring for loss of phase, change in phase sequence, motor temperature, power factor, and more.
As pump and motor installers, it is critical to understand the basics of electrical power and the effect it can have on motors. The ability to measure and calculate the voltage and current balance for 3-phase motors is imperative to ensure a safe operating range for the equipment.
It is also important to note these values will never be perfect and can sometimes fluctuate based on other loads on the grid. If the asymmetry exceeds safe limits, a dialogue with the power supplier may be required to determine if they can regulate the incoming power to assure safe operating limits.
Lastly, make sure a good quality advanced motor protection device is installed to monitor for and protect the equipment from power quality issues that kill motors.
Alan Bixler is a lead sales engineer, groundwater, for Grundfos Americas Corp. He is located in Lenexa, Kansas, and can be reached at firstname.lastname@example.org.
Brendan Watson is a senior product manager, water utility for Grundfos Americas Corp. His office is in Lenexa, Kansas, and can be reached at email@example.com.