Electric motors transfer electrical power to mechanical rotational forces that are the muscle of the industrial world. Keeping them running means keeping them healthy and working properly. Regularly measuring voltage and current unbalance as well as power factor can help predict failure and avert downtime. They can also help determine whether additional inspections, such as vibration testing, shaft alignment analysis or insulation testing, are needed.
Voltage Unbalance
Voltage unbalance is a measure of voltage differences among the phases of a three-phase system. For optimum motor performance, the phase voltages should be equal or close to equal. Besides causing poor motor performance, voltage unbalance shortens a motor’s life.
Voltage unbalance is 100 times the maximum voltage variation from average divided by the average voltage of the three phases. This calculation produces the unbalance as a percentage. The U.S. Department of Energy (DOE) presents the following example: If measured line voltages are 462 volts (V), 463 V and 455 V, the average is 460 V. So, the voltage unbalance is shown in Equation 1.
Overall, voltage unbalance should be less than 1% and never more than 5%. Standard EN50160 requires less than 2% voltage unbalance at the point of common coupling. National Electrical Manufacturers Association (NEMA) specifications call for less than 5% for motor loads but recommend that voltage unbalances at motor terminals not exceed 1% and that the motors be derated for higher percentages. Measuring for voltage unbalance should be performed regularly at motor terminals using a power quality analyzer to verify that voltage unbalance is below 5%. In addition, regular thermal inspections might reveal high-resistance connections in the switchgear, disconnects or motor connection boxes, which can cause voltage unbalance. Other possible sources of voltage unbalance include faulty power-factor correction devices, unbalanced or inconsistent supply voltages, unbalanced transformer banks, unevenly distributed single-phase loads, single-phase to ground faults or an open circuit on a distribution system primary.
Corrections should be done by an experienced electrician or power specialist. Begin by checking supply voltages from the adjustable speed drive (if one is used in the system). Also, check utility inputs to the plant and transformer outputs to the system. If balanced phases are found at these “sources,” then the best approach is to begin at the motor and systematically work back to the initial source: the utility’s electric supply.
Potential Savings & ROI
The best way to calculate overall savings is to use a software tool.
Here is how the basic calculation works if the following is known (sample values appear in parentheses):
- loading on the motor (100%)
- horsepower (100 hp)
- runtime (8,000 hr/yr)
- efficiency at nominal unbalance for the loading (94.4%)
- efficiency at actual unbalance and loading (93%)
- one horsepower converts to 0.746 kilowatts
Using the sample values provided, the annual energy savings following corrective action is shown in Equation 2.
If electricity costs $0.05 per kWh, the annual savings in dollars is shown in Equation 3.
In industrial settings, many motors may be powered from the same unbalanced power supply. Therefore, potential savings will be more than for a single motor, with the actual savings dependent on loading, runtimes, horsepower, etc. Remember that motors run hotter when their power supplies are unbalanced—roughly twice the square of the voltage unbalance: 2 x % voltage unbalance. For example, at 2% voltage unbalance, a motor will experience an 8 C (46 F) temperature rise. Every operating temperature increase of 10 C (50 F) halves the life of the motor-winding insulation.
Current Unbalance
Current unbalance is a measure of difference in current drawn by a motor on each leg of a three-phase system. Correcting current unbalance helps prevent overheating and the deterioration of motor-winding insulation. The draw on each leg should be equal or close to equal. One cause of current unbalance is voltage unbalance, which can cause current unbalance disproportionate to the voltage unbalance itself. When current unbalance occurs in the absence of voltage unbalance, look for another cause of the current unbalance, like faulty insulation or a phase shorted to ground.
Current unbalance is calculated the same way as voltage unbalance. It is 100 times the maximum current variation from average, divided by the average current of the three phases. So, if the measured current is 30 amps, 35 amps and 30 amps, the average is 31.7 amps. The current unbalance is shown in Equation 4.
The current unbalance for three-phase motors should not exceed 10%. Measuring current unbalance should involve an experienced electrician or power specialist. As with voltage unbalance, it should be performed regularly at motor terminals using a power quality analyzer. The two measurements for unbalance—voltage and current—can be made and saved simultaneously with the same power quality analyzer.
Correcting current unbalance might include any or all of the following strategies:
- If the unbalance is the result of the supplied power, a power-factor correction device can solve the problem.
- If the problem is the motor itself due to, for example, faulty insulation or a phase shorted to ground, carefully weigh the options. The decision to repair (rewind) a motor versus replacing it with a new one is difficult. According to the DOE, rewinding almost always reduces a motor’s efficiency and reliability. Consider such variables as rewind cost, expected rewind losses, new motor purchase price (for both standard and energy-efficient models), motor size and original efficiency, load factor, annual operating hours, electricity price, availability of a utility rebate and simple payback criteria.
- In most cases, buy a new motor if the faulty motor is less than 40 hp and more than 15 years old, especially if it has been previously rewound, the motor is a nonspecialty motor of less than 15 hp or the rewind cost is more than 50% of the cost of a new motor.
In the latter case, increased efficiency and reliability should provide a fast ROI.
Potential Savings & ROI
ROI takes two forms: energy savings and long-term production savings (preventing motor failure and downtime). Possible utility rebates can also come into play. Energy savings can be difficult to determine, especially when rewinding is the chosen solution. Final rewind losses are unknown until after rewinding.
If the decision is made to buy a new motor, use a software tool to calculate the annual energy savings (expected from the replacement). The following information is needed:
- motor rated hp
- load factor (L = percentage of full load ÷ 100)
- annual operating hours (hr)
- average energy costs (C = $/kWh)
- existing motor efficiency (Estd, as a percentage)
- efficiency rating of the new motor (Eee, as a percentage)
- conversion factor from hp to kW (0.746)
This information is shown in Equation 5.
In general, premium efficiency motors are about 1% more efficient than standard efficiency motors, and the energy savings will typically result in a payback period of less than 18 months. Compared to an existing rewound unit, a new premium efficiency motor will be considerably more than 1% more efficient.
Power Factor
Bad power factor is generated by some types of equipment operation and results in penalty fees from the utility. Evaluate power factor on all major circuits and loads, including motors. The closer the power factor is to 100% or “1,” the better (utilities usually charge a penalty for power factor less than 95%). Increasing power factor will:
- reduce the electric bill
- increase electrical system capacity
- decrease voltage drop
Power factor is caused by inductive loads (loads with coils) such as motors and transformers. It is expressed as a percentage or a number, with 100% or 1 being ideal. Power factor is the ratio of real (working) power (kW) to apparent (total) power (kilovolt-ampere [kVA]). Apparent power is a combination of real power and reactive power (kVAR).
An increase in reactive power causes the apparent power to increase and, consequently, power factor to decrease. So, decreasing reactive power will increase power factor, and that is generally a good thing. Measuring power factor is best done with a power-quality analyzer. Before starting, find out:
- how the utility charges for low power factor or VARs
- what the utility says the power factor averages per month
- what the demand charge is
- how the utility measures power factor or VARs—peak intervals or averages
The goal is to identify loads that are causing lagging reactive power and develop a strategy for improving power factor. Start at the service entrance, where the utility monitors its data, and check individual loads. The power-quality analyzer will allow users to find the average power factor over a specific recording period.
Correct power factor using the following strategies:
- curtail or decrease the use of idling or lightly loaded motors
- avoid operating motors above their rated voltage
- replace failed standard motors with energy-efficient models
- install capacitors in the affected circuit(s) to decrease reactive power
Potential Savings & ROI
Use the information from the utility and from the investigation to calculate savings. Assume the utility adds a 1% demand charge for each 1% the power factor is below 0.97%. If the power factor averages 86% each month, then the operation is 11% (97% minus 86%) below the 97% threshold. If the demand charge is $7,000 per month, then the avoidable annual cost via power factor correction is shown in Equation 6.
Next Steps
When wrapping up the immediate motor efficiency investigation, evaluate long-term maintenance practices and start making changes there, too. Include these same voltage and current unbalance checks in regular inspections. Also, consider regularly inspecting connections and grounds, off-design voltage and insulation resistance for additional long-term performance improvements.