Part 2. Three-phase motors
By Ed Butts, PE, CPI
We began a four-part series on causes of electric motor failure and troubleshooting techniques last month with an overview on single-phase motor failure. We continue the series with common causes of three-phase motor failures with this column. Next month will be an overview on single-phase electric motor troubleshooting and we will wrap things up with three-phase electric motor troubleshooting in April.
Causes of Three-Phase Motor Failures
Historically, the most common causes and percentages of three-phase electric motor failure are shown in Table 1.
When overload and overheat are combined with single phasing failures, it becomes apparent that almost half (44%) of all motor failures is the direct or indirect result of heat.
This discussion has been oriented around the most probable causes of motor failure, and what as designers and operators, we can do to help prevent them.
First, it is well known that heat is the most common cause of failure before a motor reaches its normal service life. It is important to remember that for every increase of 10°C or 50°F of a motor’s winding temperature over its design operating temperature, the life of the motor’s windings insulation is reduced by up to 50% (the half-life rule) even if the overheating was or is only temporary.
Motor overheating can result due to several pertinent factors. For example, if a designer selects a motor that is undersized for a specific application or selects a motor with the inadequate starting current and torque characteristics, it will start or operate at a higher temperature than its design temperature. Thus, beyond voltage and phase, motors should always be matched to their connected loads for horsepower,
speed, and torque requirements.
While undersizing a motor often leads to overheating, oversizing a motor costs more in capital investment and often lowers the application’s overall energy efficiency.
Excessive motor cycling is another frequent cause of motor failure and pump failure from overheat to the motor and wear to the pump and its components. Generally, water systems that operate pumps with on-off pressure systems must ensure that motor starting cycles do not exceed the specified daily values.
Pumping systems with capacities in excess of the required demand may need to incorporate a variable frequency drive, control valve, or other method to lengthen operation to reduce motor cycling. Various factors evolve when you deal with motor operating temperatures regardless of the application. These include:
- The electrical efficiency and power factor of the motor
- The rated versus actual load in horsepower or kilowatts the motor is driving
- If the motor is exposed to cycling, the number of daily cycles and off-time between cycles
- Instantaneous or daily variances or imbalance in operating power factor, voltage, and frequency
- The ambient temperature for which the motor is rated and the temperature it will operate at
- The increase in temperature the motor will incur when it is loaded to its rated load
- The manufacturer’s NEMA or IEC class of winding insulation
- The motor’s service factor.
Environmental and Operating Factors
Another common cause of overheating is operating the motor in an environment with a high ambient temperature or dirty operating environment that reduces the rate at which heat can be conducted from the motor into the adjacent environment.
This condition results in greater winding temperatures and a resultant shortened service life. Locating submersible motors in a dead area where water flowing to the pump cannot adequately flow past the motor first, or applying air-cooled motors in insufficiently ventilated areas or next to heat-producing equipment such as furnaces, can easily result in ambient or operating temperatures high enough to cause motor damage.
Designers and technicians should always verify ambient temperatures and paths for cooling air in enclosed areas where motors are to be installed and use ambient-compensated thermal overloads that are designed for higher ambient temperature environments. If necessary, adding a shroud to submersible units to direct adequate water flow past the motor or using forced or supplemental ventilation to the area if ambient temperatures exceed the rating for the intended motor.
Although the ambient temperature may be within the manufacturer’s recommended temperature guidelines—blocked, plugged, or restricted air passages; inadequate or dirty cooling fans; or vanes may result in elevated motor operating temperatures. Even grease or oil lubricated motors must receive adequate airflow around the motor to remove operating heat and maintain the winding temperatures below maximum values.
All motor environments should be inspected at least annually to verify all paths to cooling air movement in and out of the motor are kept clear, including air inlet or outlet openings and screens. Motors that are floor mounted or drive a water pump should be checked to ensure that dirt, water spray, or misting is not being drawn into the motor from leaking packing, mechanical seal, or floor debris/moisture.
This situation can cause moistened or dirty air to be sent across the windings by the fan during operation, often leading to deposition on the windings. This can result in hot spots and eventual failure.
Single Phasing of Three-Phase Motors
The effects of single-phasing (Figures 1 and 2) on three-phase motors varies with the service and motor winding configurations, location of circuit opening, and motor thermal capacities. When single phasing occurs, the motor temperature rise may not vary or rise directly with the motor current but may increase at a rate greater than the increase in current.
In some cases, protective devices that sense only current in two legs may not provide complete single-phasing protection. If a circuit opens on the line or primary side of a transformer, such as for a wye-delta system, two of the load legs increase to 115% of full load value while the remaining leg increases to 230% of full load value.
When one lead or phase on the load side of a motor opens, as shown in Figure 2, the motor circuit becomes secondary single-phased and the motor current in the remaining two phases increases to 173% (√3) of the normal running current although the increase can be as much as two times (200%) due to power factor changes.
Where the motor has a high inertia load or the motor tries to start on single-phase power, the current can reach locked rotor values. Normally, if three-leg, quick-trip overloads are used and properly adjusted, the overload relays will safely clear the motor from the power supply within a few minutes.
However, in some cases using an oversized motor in which the overloads are sized to the horsepower and not the actual load, the increase in load current may not be adequate to generate enough current to trip the overloads.
In these instances, the motor will continue to run on single-phase power to destruction or until the fuses or circuit breaker clears the circuit. For example, given a lightly loaded three-phase motor that operates at 70% of normal full load amperes, during a secondary single-phasing condition, the phase current will increase by the square root of three (√3, or 1.732).
This will result in a current draw of approximately 20% more than the nameplate full-load current, which if the overloads are sized at 125% of the motor nameplate, circulating currents can still damage the motor. This is the reason it is recommended that motor overload protection should always be based upon the actual running current of the motor under its actual loading, rather than the nameplate current rating.
Likely, the greatest risk from single phasing is when a condition of primary (line) single-phasing occurs, as the motor current in one secondary phase will increase to 230% while the remaining two phases increase to 115% of normal current.
Normally, as before, the overload relays will protect the motor. However, real-world experience has confirmed that properly sized and adjusted three-leg motor running overload devices can greatly reduce the problems of single-phasing for the majority of motor installations.
In some instances, additional protective means may be necessary when a higher degree of single-phasing protection is required. This can be the case when other loads are present on the circuit and the motor in question is a minor load, or circumstances using an oversized motor with HP rated overload devices.
In these instances, as well as with all highly valued or large (greater than 50 HP) motor installations, the use of a three-phase phase-voltage monitor (PVM) is highly recommended. A PVM can automatically monitor line phase conditions, high or low voltages, phase reversal, and even phase unbalance with some devices and open the control circuit to quickly shut down the motor if necessary.
Typical Causes of Three-Phase Motor Bearing Failures
In addition to the possible damage to bearings caused from environmental factors and contamination of and inadequate lubricant, potential damage or failure of bearings on three-phase motors is a distinct possibility when driven by a variable frequency drive (VFD).
The high frequencies in the switching of voltages and transient spikes are induced into the motor rotor and build up a voltage potential between the rotor and stator. This high voltage is often dissipated by internal arcing or fluting through or across the motor’s ball bearings. Fluting damage is related to the characteristics of the PWM waveform, VFD programming and characteristics, and the installation.
Shaft grounding is recommended (NEMA MG1 31.4.4.3) as an effective means of bearing protection for motors operated from VFD supplied power. Shaft voltage is likely to occur in motors powered by VFDs, which induce shaft voltages onto the shaft of the driven motor because of the extremely high-speed switching of the insulated gate bipolar transistors (IGBTs) producing the pulse width modulation (PWM) used to control AC motors.
The presence of these high frequency ground currents can cause sparking, arcing, and electrical shocks as well as potentially damage bearings. One grounding device is generally adequate to bleed down inverter-sourced shaft voltages, thereby protecting both bearings for motors as large as 6000 size frames. Part 4 of this series will outline other damage prevention methods to bearings when driven by a VFD.
Voltage/Current/Frequency Unbalance and Motor Derating Protocol
Voltage and frequency unbalance degrades the performance and shortens the life of a three-phase motor. Table 2 illustrates the impact that unbalanced voltage or frequency can play on various motor performance characteristics.
Voltage unbalance at the motor terminals can cause current unbalance that is far out of proportion to the voltage unbalance. Unbalanced currents lead to torque pulsations, increased vibrations and mechanical stresses, increased losses resulting in lower efficiency, and motor overheating, which reduces winding insulation life.
The percent of voltage unbalance is defined by the National Electrical Manufacturers Association (NEMA) as 100 times the absolute value of the maximum deviation of the line voltage from the average voltage on a three-phase system divided by the average voltage.
For example, if the measured line voltages are 462, 463, and 455 volts, the average is: (462 + 463 + 455)/3 = 460 volts and the maximum deviation from the average is: 460 volts (average) – 455 volts = 5 volts. The voltage unbalance percentage is:
Maximum Deviation from Avg. 5 volts
Average of Three Voltage Values = 460 volts
= 0.01087 × 100 = 1.087%
It is recommended that voltage unbalances at the motor terminals do not exceed 2%. Voltage unbalances that exceed 1% will require derating of the motor per NEMA MG-1-2011 and will void most manufacturers’ warranties.
Common causes of voltage unbalance include:
- Faulty operation or inadequate capacity of power factor correction equipment
- Unbalanced or unstable utility supply (primary power)
- Unbalanced transformer bank supplying a three-phase load that is too large for the bank’s capacity
- Unevenly distributed or excessive single-phase loads on a common power system
- Open-delta (two transformer) three-phase power system on dual-primary distribution system
- Unidentified or bleeding single-phase to ground faults
- An open circuit on the distribution system primary.
The general trend of efficiency reduction with increased voltage unbalance is observed for motors at all load conditions. When the line voltages applied to a three-phase induction motor are not equal, unbalanced currents in the stator windings will result. This small percentage of voltage unbalance will result in a much larger percentage of current unbalance.
Consequently, the temperature rise of the motor operating at a particular load and related percentage of voltage unbalance will be greater than for the same motor operating under the identical conditions with balanced voltages. Maintaining the proper balance of voltage between phases on a three-phase system is crucial to ensure proper performance and optimum life of the motor.
Most motor manufacturers and designers recognize the potential impact that unbalanced currents can have on a motor’s performance and life. However, many do not readily understand that voltage unbalance plays the primary role in unbalanced current.
The decrease in motor efficiency associated with voltage unbalance causes the motor to run hotter with a resultant decrease in performance and a proportional reduction in winding insulation life is to be expected. Winding insulation life is reduced by one-half for each 10°C (50°F) increase in its operating temperature. In addition, significant amounts of voltage unbalance will increase the operating cost of a motor.
For example, assume that a 100 HP motor is fully loaded and operated for 8000 hours per year with an unbalanced voltage of 2%. If the motor exhibits a full load efficiency of 95% and a 2% unbalanced voltage corrected efficiency of 93%, and with energy priced at $0.10/kilowatt-hour (kWh), the annual energy and resulting cost penalties are:
Annual Energy Penalty =
100 HP × 0.746 kW/HP × 8000 hours/year × (100/93 –
100/95) = 13,509.9 kWh
Annual Cost Increase =
13,509.9 kWh × $0.10/kWh = $1350.99
Voltage unbalance leads to current unbalance, which increases the resistance losses in the stator windings and rotor bars, and more supplied power is converted into heat and the motor runs hotter. Increased rotor losses result in an increase in slip, so the motor rotates more slowly and provides less work over a given time.
For example, as the voltage unbalance increases from none (0%) to 1%, motor winding temperatures will increase from 120ºC to 130ºC, efficiency decreases by 1/2%, and the projected winding life drops in half, from 20 years to 10 years. The impact with a voltage unbalance between 0% to 5% is even more pronounced: Motor winding temperatures increase to 180°C, efficiency decreases by 5%, and projected winding life falls to less than one year.
As a rule of thumb, three-phase voltage unbalance should be limited to 2% or less and current unbalance to 5% or less. In addition to the motor, three-phase distribution systems often serve other diversified single-phase loads.
An imbalance caused from system impedance, harmonics, or load distribution across the three phases can contribute to imbalance across all three phases. In addition, potential faults may occur in the branch circuit cable to the motor, terminations at the starter or motor, and potentially within the windings themselves. This imbalance can lead to stresses in each of the phase circuits in a three-phase power system.
In other situations, the use of an open-delta or two transformer system for a three-phase power supply can also cause severe voltage unbalance between the phases, resulting in current unbalance.
Open-delta systems were popular in rural areas where only two primary phases were available. Although this system has increasingly fallen out of favor with utilities and users alike, due to the impact on primary feeders and shortened motor life, the system is still used in many regions of the country.
This can present a problem with any three-phase motor, especially submersible types. In a system with unbalanced power, the current unbalance between phases results in a negative sequence voltage within the motor windings.
This negative voltage has the effect of causing a counter voltage in the motor that resists the normal current and can lead to current unbalance over 10%.
At the simplest and ideal level, all three phases of voltage should always have the same magnitude of voltage, but there are cases in which this is just not possible, particularly where true or full three-phase power (three primaries) are not available, so an open-delta power supply may be the only viable option to provide three-phase power.
Alternatively, in areas with adequate single-phase primary power, the use of a rotary single to three-phase converter or variable frequency drive can offer a viable and cost-effective option to the use of an open-delta electrical system.
If either of these options is not viable due to inadequate single-phase primary power, the application and use of an open-delta transformer bank using two single phases may be the only choice. In these circumstances, derating a load to counteract the negative sequence voltage of the open bank will often reduce the voltage unbalance to an acceptable level.
The values shown in Table 3 can be used for motor load derating.
The procedure for determining voltage unbalance and motor selection on an open-delta transformer bank is:
Example 1: Determine (a) the voltage unbalance and (b)
required motor HP for a submersible motor to a 29.5 BHP,
460VAC, 3ϕ load with the following combination of voltages:
Phase 1 to Phase 2: 457 volts, Phase 1 to Phase 3: 461 volts,
Phase 2 to Phase 3: 483 volts
Solution (a):
Step 1: Find average voltage = 457V (1-2) + 461V (1-3) + 483V (2-3) = 1402/3 = 467.3V
Step 2: Subtract the greatest variation of readings from the average: 483V – 467.3V = 15.7V
Step 3: Divide the difference by the average voltage: 15.7V/467.3V = 0.0336 × 100 = 3.36%.
Since this exceeds a 2% voltage unbalance, refer to Table 3 and interpolate. Thus, a 3.36% voltage unbalance will necessitate a motor derate of about 0.86 (86%) of full load. Required motor HP: 29.5 BHP ÷ 0.86 = 34.30 HP > 30 HP.
Solution (b): Therefore, use a 40 HP motor for the 29.5 HP load.
The same procedure can be used to determine the current unbalance and best lead combination:
Example 2: Determine the best lead combination for the following observed currents:
After rolling motor leads 3 times:
(1) Phase 1: 65 amps, Phase 2: 73 amps, Phase 3: 66 amps. Average = 68 amps
(2) Phase 1: 64 amps, Phase 2: 75 amps, Phase 3: 65 amps. Average = 68 amps
(3) Phase 1: 62 amps, Phase 2: 76 amps, Phase 3: 59 amps. Average = 65.6 amps
Solution:
Current unbalance for combination (1): 73A – 68A = 5A/68A = 0.0735 × 100 = 7.35%
Current unbalance for combination (2): 75A – 68A = 7A/68A = 0.1029 × 100 = 10.29%
Current unbalance for combination (3): 76A – 65.6A = 10.4A/65.6A = 0.1585 × 100 = 15.85%
Referring to Example 2, using combination (1) as 7.35% is the lowest value of current unbalance and less than the maximum value of 10%, although it is still considerably higher than the recommended limit of 5%.
In this case, note that the highest current value remains on the same leg (Phase 2) each time the motor legs are rotated. This tends to indicate that the power supply may be the culprit, requiring a motor HP derating or working with the utility to improve the power quality and supply to the site.
If the high current followed the leg or moved with the same leg as they were rotated, this would tend to indicate the problem is within the motor or drop cable (possible leakage).
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This concludes this month’s column and discussion on the causes of electric motor failures. Next month, we will concentrate on troubleshooting single-phase motors.
Until next month, work safe and smart.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.