Recognizing motor winding problems
It’s always important to identify the real cause of burned windings and not just to replace the electric motor. Motor windings have a different appearance in all these failure situations: single-phase burnout, overload, unbalanced voltage, and voltage spikes. Voltage spike damage occurs more often in motors controlled by variable frequency drives.
These problems are all caused by in-plant faults that require correction. A replacement motor can fail sometimes immediately if the in-plant problem isn’t corrected.
It’s very important to accurately identify problems that require a motor’s removal and replacement. Winding problems that are identified should be documented. A history of the plant’s motor problems (on computer software) will point out problem areas that ran be improved, or even eliminated.
- Shorted turns
- Ground (winding shorted to frame)
- Phase-to-phase short
- Open winding
- Burned windings from operating on single phase
- Submerged motor
- Assorted rotor problems:
These problems require replacing or rewinding the motor.
A short is a common winding breakdown, and it requires rewinding or replacing the motor. Shorted turns are caused by nicked coil wire, high-voltage spikes, conductive contaminants, overheated winding, aged insulation, and loose, vibrating coil wires.
The most of the resistance to current flow in an AC motor is furnished by inductive reactance. The resistance of the wire in a complete phase is a very small percentage of the motor’s total impedance (resistance plus inductive reactance). Inductive reactance makes each turn very significant in the motor’s ampere demand. Each turn supplies much more inductive reactance than resistance.
A high circulating current is transformed into the loop (Figure 1a).
Power consumed by the circulating current increases the amperes of the faulty phase, making it easy to identify the problem. Circulating current in the closed loop often melts the circuit open. When this happens, the circulating current and the turns within the closed loop are eliminated.
See Figure 1(b).
Only the resistance of the wire (turns) within the closed loop is now eliminated from the phase winding. Without the ampere demand of the circulating current, the difference lessens between the amperes of the faulty phase and those of the normal phases.
A very small difference in resistance is all that is needed to identify the faulty phase. Please note that the rotor should be turned during this test to eliminate its effect. Shorted turns in any AC winding are usually visible. They become charred quickly from the high circulating current that is transformed into them (Figure 2).
When a motor is “grounded“, the winding is shorted either to the laminated core or to the motor’s frame. The problem is usually found in a slot, where the slot insulation has broken down. Water is the most common cause of a grounded winding. A solid ground requires rewinding or replacing the motor.
Some causes of slot insulation breakdowns are overheating, conducting contaminants, lightning, age, pressure of a tight coil fit, hot spots caused by lamination damage (from a previous winding failure), and excessive coil movement.
Excessive coil movement is often caused by thermal growth and/ or coil twisting torque, brought on by reversing (plugging) or a momentary power interruption.
A phase-to-phase short is caused by insulation breakdown at the coil ends or in the slots. This type of fault requires rewinding or replacing the motor. Voltage between phases can be very high. When a short occurs, a large amount of winding is bypassed. Both phase windings are usually melted open, so the problem is easily detected.
Among the causes of interphase breakdown are contaminants, tight fit (in the slot), age, mechanical damage, and high-voltage spikes. Coils that form the poles for each phase are placed on top of each other in all three-phase motors.
Figure 3 is a concentric-type winding. The coils don’t share the slots with other poles in some concentric-type windings.
Figure 4 is the lap-winding type. The ends of the coils are nested within each other and have phase insulation between the poles. The coils usually share the slots with other poles. Insulation also separates the coils of each phase in the slots. Some motors (up to 5 horsepower) are wound with no insulation separating the phases.
Phase-to-phase insulation is important because there is a line voltage potential between phases regardless of a motor’s horsepower.
Figure 5 is an end coil phase-to-phase short. A phase-to-phase short occurs in the slot more often than at the coil ends. When a breakdown occurs in the slot, copper usually melts and fuses to the slot laminations.
This copper has to be ground out and removed before the motor is rewound, or it becomes a hot spot and deteriorates the new insulation.
A common cause of an open winding is undersized lead lugs. Charred connections in the motor’s connection (terminal) box are a sure indication of this problem. Open windings are also caused by shorted turns, phase-to-phase shorts, ground-to-frame shorts, faulty internal coil-to-coil connections, severe overloads, and physically damaged coils.
These faults require rewinding or replacing the motor. An open winding will show several different symptoms (depending on the motor’s internal connection).
A microhmmeter is used to identify this problem. A motor with a high number of parallel circuits, that is, four and eight wye, will show less power loss when one circuit is open. Multiparallel circuit connections are used in motors above 5 horsepower. The windings of a severely overloaded motor (operating on 250 volts) usually become completely charred before an open winding occurs.
An overloaded motor operating on 490 volts, however, often will have no sign of burned wires before its windings melt open. In either case, the overload protection isn’t working, and the motor should be rewound or replaced.
Motor lead connecting lugs should be thick enough (throughout the connection) to represent the circular mil area (size) of the motor’s lead wire. If any part of the lug is too small, it becomes a resistor in series with the motor, and current will be restricted when the motor needs it the most – to start the load.
Figure 7 shows lugs that aren’t made for electric motors. Lug (a) is a piece of copper tubing, which has been partially flattened and has a hole
punched in it for the connecting bolt. Its ferrule will hold wire that has a much greater circular mil area than that of the bolted part of the lug.
Lug (b) is clearly not a motor duty lug.
When one line of a three-phase power supply opens, the power becomes single phase. If this happens while the motor is running, its power output is cut approximately in half. It will continue to run, but it can no longer start by itself. Like a single-phase motor without its start winding energized, it has no rotating magnetic field to get it started.
5.1 Single-Phase Damage to a Wye-Connected Nine-Lead Motor
Figure 8 shows the current path through the wye connection. Two phases of the windings are energized; the third phase has no current flow. If the motor’s protection device doesn’t function, the two phases that carry current will overheat and char The phase without current flow will look normal.
Figure 9 is a picture of a single-phase-mused burnout in a four-pole winding.
Single-Phase Damage to a Delta-Connected Nine-Lead Motor
Figure 10 shows the current path through the delta connection, with an open phase. The A phase has extremely high current flowing through it. The other two phases have about half as much. The phase with high current will overheat and char if the motor’s protection device doesn’t disconnect it.
The phases that carry less current will look normal.
Figure 11 is a picture of a single-phase-mused burnout in a four-pole winding.
If a three-phase motor has been submerged in water but not energized, there’s a good chance it won’t need rewinding or replacing. Cleaning and baking the windings may be all that’s needed.
What should you do?
The motor should be disassembled as soon as possible. If the motor has ball bearings, they should be replaced. If it has sleeve bearings, the oil wicking material will pit or rust the shaft area located in the bearing window. Replace the oil wick material immediately. If the motor has an oil reservoir and oil ring, the reservoir should be thoroughly cleaned. The windings should be first tested with an ohmmeter.
The baking temperature shouldn’t exceed 93°C. The ohmmeter test should read infinity after baking. After the windings have been cleaned, dried, and tested, they will need a coat of air-drying varnish. When water soaks the slot insulation, the copper windings and the core become a form of battery. A small voltage can be read (with a millivoltmeter) between the winding and the frame when the slot insulation is wet.
A zero reading indicates the motor has been baked long enough. A megohmmeter, hi-pot, or surge tester can be used when an ohmmeter test shows infinity.
Further reading: Recommended maintenance practice for electric motors and generators
This is a review of the rotor problems found in Chapter 3, with more detailed information:
- Open rotor bars
- Open end rings
- Misaligned rotor/stator iron
- Rotor dragging on the stator
- Rotor loose on shaft
Open rotor bars or end rings usually necessitate replacing the motor. They can be repaired, recast, or rebarred (if it’s economical). It’s important that any replaced metal be the same as the original. See Figure 12.
Open rotor bars are usually caused by:
- Overload burnout,
- Arcing in the slot from a shorted winding,
- Loose bar vibration,
- Thermal growth stress (from starting),
- Flaws in bar material (casting flaw), and
- Poor connections with end rings.
Open rotor bars cause loss of power. If too many rotor bars are open, a loaded motor will draw amperes high enough to open its protection device. With no load, the amperes will be very low. Slow starting and lower-than-rated RPM are a sign of broken rotor bars.
Open end rings muse uneven torque and some power loss. A ring with one open spot will soon develop more open spots. Each time the open spot crosses a 90° spot between poles, the current will double in the ring area between the next two poles. See Figure 13 below.
Causes of open end rings and/or cracked end rings include the following:
- Flawed casting;
- Motor burned out from overload;
- Motor redesigned for a higher speed (without increased size of end ring);
- Ring material drilled away for balancing; thermal growth stress; and
- Mechanical damage.
A bubble or void in an end ring can cause an electrical vibration. This type of vibration can’t be corrected by balancing. It can be detected by cutting the power and allowing the motor to coast. Electrically mused vibrations will always cease as soon as the power is shut off.
A motor with a misaligned rotor will draw high amperes and will lose power. The magnetic path becomes distorted, causing the magnetizing amperes to increase. The stator windings will char and resemble an overload burnout.
Possible causes of a misaligned rotor include:
- Wrong bearing shim placement
- Bearings not installed correctly on the shaft (extended race on wrong side)
- Wrong bearing width
- Captive bearing not held as originally placed
- End bells interchanged
- Stator core shifted on its shell
- A rotor shifted on its shaft
- A rotor replaced with a shorter rotor. A rotor with the same diameter but longer than the original will work, but some efficiency is lost.
If the rotor drags on the stator and the bearings aren’t worn, it’s common practice to “skim” the rotor on a lathe. The process increases the air gap, which increases the no-load amperes. The increased amperes are similar to a misaligned rotor and stator iron. The magnetic circuit is degraded, so it takes more amperes to magnetize the motor’s iron.
The motor will run hotter than normal, because the motor is drawing more magnetizing amperes. If the load is at maximum or there are any adverse conditions (such as low voltage or frequent starts), the motor should be replaced. There will be some permanent power and efficiency loss.
A loose rotor on a shaft makes a rumbling or vibrating sound. The sound will cease after the power is turned off (while the motor is coasting). If the motor has operated this way for very long, a red dust will form between the shaft and rotor iron. This dust is oxidized iron, caused by the rubbing action between the shaft and the rotor iron. The same thing happens when a pulley or bearing is loose on a shaft.
The decision to repair a loose rotor depends on the price of a replacement motor and the importance of the motor in the plant operation.
There are options for this problem:
- The rotor (and shaft) can be replaced;
- The rotor can be bored out and a new shaft fitted to it; or
- A thin metal wedge can be driven between the shaft and rotor to secure it.
Wedging the rotor may offset it enough to make it drag on the stator. It would then be necessary to skim the rotor-on a lathe-to keep it from dragging. The rotor should be bored and fitted with a new shaft. In most cases, it is more economical to replace the motor.
- Electric motor maintenance and troubleshooting by Augie Hand
- Detecting Broken Rotor Bars With Zero-Setting Protection by Carlos Pezzani, Pablo Donolo, and Guillermo Bossio (Universidad Nacional de Río Cuarto) and Marcos Donolo, Armando Guzmán, and Stanley E. Zocholl (Schweitzer Engineering Laboratories, Inc.)