Stopping a Costly Leak: The Effects of Unbalanced Voltage on the Life and Efficiency of Three-Phase Electric Motors

From the U.S. Department of Energy's Publication, Energy Matters

Electrical power quality problems cost U.S. industry $40-150 billion each year, according to some estimates. Most problems originate outside our plant and are therefore beyond our control-for example, outages, voltage interruptions, voltage sag, voltage reductions, and blackouts. Others may be traceable either to the utility grid, plant conditions, or some combination of the two.

The good news is that we can do something about one of the most pervasive (and insidious) power quality problems—voltage unbalance. The bad news is that this problem is easy (and costly) to overlook. At the very least, voltage unbalance reduces motor efficiency, potentially robbing you of the savings you expected to realize by upgrading to EPACT or National Electrical Manufacturers Association (NEMA) Premium^TM efficiency motors. More serious consequences include premature motor failure, costly shutdowns, and lost production.

What is unbalanced voltage, and where does it come from?

Voltage unbalance describes the condition when the voltages of all phases of a 3-phase power supply are not equal. You might expect the electricity used by your 3-phase electric motors to be balanced, but it rarely is. According to ANSI C84.1 -1995, Electrical Power Systems and Equipment — Voltage Ratings (60Hz), only 66% of the 3-phase power delivered to industrial plants is within 1% voltage unbalance. In addition, 98% of all voltage generated by electric utilities has 3% or less unbalance. Only 2% of the voltage produced by the electric utilities has a voltage unbalance greater than 3%.

The underlying causes of voltage unbalance are numerous, and may include:

• Lack of symmetry in transmission lines
• Large single-phase loads (for example, arc furnaces, welders, and so on)
• Faulty power factor correction capacitor banks
• Open delta or wye transformers.

Plant conditions that can cause or contribute to voltage unbalance include unbalanced or overloaded transformers, malfunctioning power factor correction devices, cyclical controls, and detuned reactors. Even what's happening at the plant next door or farther up the power line could affect the voltage unbalance at your facility. One plant reported 8% voltage unbalance; the cause was an aluminum plant next door, with predominantly single-phase furnace loads.

The bottom line: if your plant uses 3-phase power and you haven't taken corrective measures already, there's a fairly good chance you have unbalanced voltage.

How to tell

To find out if your plant has a problem with voltage unbalance, measure the line voltages of your 3-phase power supply where it enters the plant and then again at several critical locations within the plant under normal operating conditions. Use those measurements to solve the following equation:

For example, if measured line voltages were 455, 460, and 492, the average would be 469 volts (455 + 460 + 492 = 1407 / 3 = 469). The maximum deviation from that average is 23 volts (492 - 469 = 23). To find the voltage unbalance, solve the equation for the average voltage and the maximum voltage deviation:

Why you should care

The most apparent effects of voltage unbalance are decreased motor efficiency and performance—both of which affect your company's profitability.

Any given motor's efficiency will vary, depending upon such factors as the type of application, the load, and the supply voltage. In fact, even the efficiency ratings of the new NEMA Premium motors are possible only if they operate on balanced voltage.

That's because motors built to comply with the NEMA standard MG 1 are designed to operate on voltage balanced to within 1%. Operating on a power supply with a larger voltage unbalance will increase the I2R losses (that is, current squared times resistance) in the rotor and stator, meaning more of the supplied power will be converted to heat and less to work. The motor therefore will run hotter and, consequently, less efficiently. Increased rotor losses also will increase "slip," so the motor will turn a little more slowly and do less work in a given time.

The following table shows how unbalanced voltage affects the temperature rise, losses, efficiency, and life expectancy of a typical 3-phase motor operating at rated load.

% voltage unbalance	Winding temp. (°C)	I2R losses (% of total)	Efficiency reduction	Expected winding life (years)
0	120	30%	—	20 years
1	130	33%	Up to 1/2%	10
2	140	35%	1-2%	5
3	150	38%	2-3%	2.5
4	160	40%	3-4%	1.25
5	180	45%	5% or more	Less than 1

Operating on unbalanced voltage also causes the rotor's temperature (heat = losses) to rise. That adversely affects its performance causing it to rotate slower and leading to an increase in its "slip". Slip can be calculated by:

To get a better grasp of how unbalanced voltage affects motor performance, consider the most common industrial application for electric motors-pumps. Take a 4-pole, 60 Hertz (Hz) pump motor with a synchronous speed of 1,800 revolutions per minute (rpm) that operates at 1,764 rpm at the correct balanced voltage. The slip for this motor would be:

With 3% unbalanced voltage, slip would double, reducing the speed to about 1,728 rpm: (1.0 - .04) x 1800 = 1728

Since the volume of product being pumped varies in proportion to the speed, not only would the motor be 2% to 3% less efficient (referring to the table) when operating from unbalanced voltage, but it also would have to run 3% longer to do the same amount of work. The savings by correcting the unbalanced voltage will roughly equal the sum of the increased losses (reduced efficiency) and the longer run time required.

Unbalanced voltage shortens motor life

Another cost of operating on unbalanced voltage is reduced motor life. Recall that operating with a voltage unbalance greater than 1% will increase the I²R losses in the rotor and stator, causing the motor to run hotter. In fact, the temperature of the windings (in degrees Celsius) will increase by twice the percent of voltage unbalance squared.

Since every 10°C increase in temperature cuts the insulation life in half, a 3% voltage unbalance could reduce the life of the winding to about one-fourth its expected life. A 5% voltage unbalance could reduce winding life to less than the typical warranty period for a new motor (2 x (5) 2 = 50° C). The accompanying figure illustrates this point.

Factor in replacement or repair costs for premature motor failures, unscheduled downtime, and lost production, and the true cost of unbalanced voltage can be much higher than the cost of the wasted electricity—possibly as high as a factor of 10. The table below shows the cost of downtime for some representative industries.

Industry	Cost of downtime, $/hour
Pulp and Paper	15,000
Petro-chemical	150,000
Computer manufacturing	$4 million per incident
Cost of downtime for various industrial segments.

How can we correct it?

The first step in correcting the problem is to measure the voltage unbalance in your plant. Especially if the unbalance exceeds the utilities' standard of 3%, make your utility aware of it. Next, measure the line voltages at several key locations in your plant to identify conditions that cause or contribute to unbalanced voltage. A number of products are available to help you correct the problem. At one end of the spectrum are basic reactors with adjustable taps on each phase. With these units, you can regularly monitor the voltage balance and make any needed corrections by changing taps. At the other end of the spectrum are sophisticated, computer-controlled devices that monitor and automatically correct voltage and power factor problems.

Industries vary in their use of 3-phase motors. The higher your electric motor usage, the greater the impact unbalanced voltage will have on electric bills. Additionally, if a neighboring plant creates an unbalanced voltage situation, your plant also may experience considerably higher costs related to unbalanced voltage.

Summing up

The cost of unbalanced voltage to U.S. industry may be as much as $28 billion a year. The savings are even more substantial when you consider the value of "uptime" and extended equipment life. Like a leaky faucet, even a small drip can waste hundreds of gallons daily. With voltage unbalance, though, it's not just water but your money that is going down the drain! - U.S. Department of Energy