Getting the rotor back up and running
For most motor users, the rotors of squirrel-cage motors are the simplest and most trouble-free rotating electrical elements. Yet, like any piece of machine equipment with moving parts, rotors can fail. However, rotors are repairable, and you have several options. For a large motor, a repair shop is often your best resource for determining if the rotor failed, how it failed, and for repairing it.
Rotor failure
Any change in operating conditions can upset the performance, reliability, and efficiency in today’s high-efficiency motors and lead to rotor failure. Rotors typically fail from two causes:
• Excessive number of starts. In some large squirrel-cage rotors, the length of rotor life is inversely proportional to the number of starts.
• Longer starting times than the motor is designed for. This situation is typically encountered when starting large inertia loads.
• Most, but not all, rotor failures are caused by broken rotor bars, typically: • Inside the slot portion of the rotor.
• Where the end ring (also known as the shorting ring) joins the bar
.• Where the bar enters the core.
These breaks are caused by combinations of mechanical and electrical stresses, including vibrational, pendulum, thermal, and centrifugal.
For example, one combination of these stresses might begin with thermal. Accelerating a load, especially one with large inertia, increases rotor temperature. Repeated starts increase the temperature further, pyramiding on top of the normal running temperature.
This heat expands the end rings and stresses the bar extensions much like a cantilever beam. The shorter the extension of the bars and rings, the greater the tensile and compressive stresses in the lower and upper edges of the bar — one area where breaks commonly occur. Long extensions reduce these compression and tensile stresses, but increase stresses, which can lead to cracks and breaks in the rotor bars.
Manufacturers can control the effects of some of these stresses during motor design. For example, varying certain design parameters can control the effects of centrifugal stresses. There are materials that are strong enough to resist breakdown forces. However, designers must balance a material’s fatigue strength and endurance with its electrical conductivity. High-strength materials generally possess high resistivity, making them poor choices for rotor design.
The shape of the bar can also be used to control the affects of some of these stresses. Modifying bar shape can distribute the stresses and extend rotor life. The most common bar shapes are rectangular, trapezoidal, inverted “T” shape, and combinations of these. If a change is made to the bar shape, engineers must maintain reasonable proportions between the slot and the tooth.
Finding the failure
There is no infallible method to determine if broken rotor bars exist in an assembled motor. When severe vibration or other motor problems occur, most likely you’ll need a repair shop to perform a full inspection of the motor components to determine the problem, especially if the motor is large. Not only can the repair shop diagnose the problem, many also have equipment for extensive repair, including fabricating new laminations.
A repair shop will need the following information when a squirrel cage motor fails:
• Length of time in service and type of service.
• Total number of starts and the interval between starts.
• Speed torque curve of load.
• Load inertia.
• Expected future service.
• Data sheet on stator.
• Nameplate data.
The repair staff will remove the rotor from the motor, and either visually inspect the bars or use a dye to detect cracks or breaks in the ends. Dye checking involves coating the rotor bars with a fluorescent dye that highlights any cracks or breaks.
Fractures in the slot section of the bar are often difficult to locate. In some cases, particularly where there has been a history of arcing, discoloration of the core may be the main indication of bar breakage.
Methods of repair
After finding the broken bars, you can choose from several repair methods. The method selected depends upon future use of the motor.
1. When a break is on the bar ends, brazing can repair it. Brazing provides a low-cost, temporary solution when you need to continue your operations while you consider other options. This method is fine for a few cracked bars, but other bars may be fatigued and will break after a short time.
2. Replace the regular copper bars with oxygen-free or silver- bearing copper. This material lends greater ductility and can increase bar life. With a change in bar material, however, the new bars will not have the same dimensions as the old bars. Thus, the use of a spring arrangement will keep the bars tight in the slot while permitting free longitudinal movement for thermal expansion.
In this method, replacing the end ring with a material that is the same or similar to the new bars is optional. If the end ring is not changed, though, there may be differences in conductivity between the two materials. These differences may require a further change in bar size as well as new laminations, which is option three.
3. Replace the copper bars with alloy bars, put in new laminations, and make the new bars an inverted T-shaped. Such bars can withstand centrifugal forces better than a trapezoidal shape because of the flat plane of the T section. The T keeps the bar in place as well as helps distribute the centrifugal force. A trapezoidal shape can move up and down in the slots.
Electrically, this design gives the same characteristics as a trapezoidal or semi-trapezoidal bar, while providing a strong tooth construction and free longitudinal bar movement.
The first repair method is probably sufficient if the motor is reaching the end of its expected life. If the motor is entering the peak period where the number of starts will increase or many more years of life are expected, then the second or third method may be more economical. Continual brazing of the bars as they break, added to the amount of time the motor is out of service can make method two or three the more cost-effective option.
After the rotor has been repaired and the motor reassembled, closely check rotor alignment. A change in the air gap in the squirrel cage rotors effects the efficiency of the motor. Too small of an air gap will increase the rotor resistance. Too large of an air gap will require derating the unit.
Beant Nindra is engineering manager for National Electric Coil, Columbus, Ohio.
When rotor bars aren’t the problemSometimes broken rotor bars are blamed for problems caused by other component failures. For example, a two-pole motor rotor experiencing severe vibration may indicate more elusive problems. In this example, engineers in a repair shop removed the rotor from the motor to inspect it. No broken bars were seen, so the rotor was balanced in an open set-up at full speed. This seemed to solve the vibration problem. Under load, however, the vibration returned indicating the problem was not purely mechanical. The engineers used other inspection methods to check the rotor, including ultrasound. These methods gave readings that indicated the problem was broken bars. The engineers removed the rotor again and checked for fractures. However, they confirmed that all the bars were intact and all connections to the end rings in good condition. This left the laminated core as a possible cause of trouble. The engineers inspected the condition of the laminations and shaft, and found that there was a gap between them. This gap could account for the vibration. The engineers could not determine whether this was a manufacturing flaw or the result of operating conditions, although they found evidence of fretting corrosion on the shaft surface. After this inspection, the engineers concluded that the problem involved the temperature of the motor. They reasoned that as heat penetrated the laminations, the laminated core expanded. The expansion, combined with centrifugal force, caused the laminations to loosen on the motor shaft. New laminations were fabricated, stacked in a mandrel, and heated. The laminations were then slipped onto the shaft, cooled, and contracted for a solid shrink fit. This procedure is similar to a process performed with large two-pole motors, where the core is honed to eliminate all burrs, heated, and finally shrunk on the shaft. |