By Tom Bishop, P.E.
EASA Technical Support Specialist
Being accustomed to rewinding AC stators, we may not realize that there is an equivalent repair service that can be performed on some rotors…namely, rebarring. Our focus in this article will be the rebarring of fabricated copper squirrel cage rotors. Redesign of rotors is outside the scope of this article.
After it has been determined that a rotor needs to be rebarred, the first step should be to obtain samples of the bars and the end rings and send them to a laboratory for analysis. The laboratory analysis should determine the composition of the bars and end rings by metal and exact percentage quantity of that metal. Another critical factor that should be identified by the laboratory is the conductivity of the bars and end rings. The conductivity of a rotor bar or end ring that is made of a copper alloy can vary greatly with a change in composition of less than a percentage point of any one of the metal elements.
Effects of resistance
In general, a rotor with higher resistance will have greater slip, more losses, lower locked rotor current and higher starting torque than a comparable rotor with lower resistance. The converse also applies; a rotor with lower resistance will have less slip, less losses, higher locked rotor current and lower starting torque than a comparable rotor with higher resistance. Note: Resistance/resistivity is the opposite of conductance/conductivity.
The rotor design determines the shape of the motor speed torque starting curve, and therefore the NEMA design characteristic. That is, the designation and conformance to a design letter code of A, B, C or D is determined by the rotor design. It is critical that the resistance of the rotor bars and end rings does not change. Otherwise the starting, and running, performance of the motor will be affected.
For example, the higher locked rotor current of a lower resistance rebarred rotor may fuse the motor starter contacts or cause fuses to blow or circuit breakers to trip.
We have elaborated on the importance of matching rotor material resistance because that is an issue that frequently arises when a rebarred rotor does not perform properly. Another important reason for determining the bar and end ring material conductivity and composition is to identify the copper alloy number associated with it. If that copper alloy or its equivalent is not available in reasonable quantities, it may not be possible to proceed with the rebar. Better to know that prior to applying labor and material to remove the rotor bars and end rings.
Take measurements
After determining that the needed alloy(s) are available, the next step is to measure the dimensions of the rotor core, bars and end rings. A good practice is to take digital photos of the overall rotor, and close-up photos of the bar extensions and connections to the end rings. Measure the distance from the core to the end ring at each end of the rotor, and note if the bars go through or over the end rings. Next place the rotor in a lathe and use a parting tool to cut through the bar extensions as close to the end rings as possible without cutting into the brazed connections. This will remove the end rings and make it possible to access the bar extensions.
It is often possible to reuse the end rings; therefore, do not discard them. A good practice is to save the end rings (unless they are reused) and a sample bar until the repaired motor has been returned to the customer and operated successfully. Having samples of the original bar and end rings can be invaluable if there is a question as to whether they were dimensioned properly, and of the correct conductivity.
After parting off the end rings, the bars are removed. The level of difficulty in removing copper bars depends on several factors including tightness of the bars in the slots, whether or not the bars have been tightened by swaging (or dipping the rotor), and slot configuration. Note: The bar removal process described here is taken from the “Rotor Bar Removal Tips” subsection of the EASA “Mechanical Repair Fundamentals” seminar text.
Apply pulling force parallel to bar
Begin the removal process by trying to pull bars out by hand. Some rotors are designed with loose bars, and hand pulling may be all that is required. If bars are not easily removed by hand, continue by using one of the methods given below. No matter what method is used, always apply the pulling force parallel to the bar.
Use a pinch clamp (like those used to lift steel plate on edge) and grip a bar. Use a come-along or puller to remove the bar. Skip bars that are too tight to readily remove. Soak the tight bars with penetrating oil. Let the bars soak for approximately 1 hour. Try again with the pinch clamp and remove any bars that have been freed up.
If the pinch clamp slips, drill a hole through the bar and use the pulling jig shown in Figure 1. Build the pulling jig so that its opening is a slip fit over the bar. When the bolt is tightened, the clamping force helps grip the bar. A loose fit puts all the pulling force on the bolt hole through the soft copper bar.
For tighter bars, there are a couple of methods that help, depending on the rotor design. If the bars have been swaged, use a thin blade in a die grinder to relieve the swaged areas. In extreme cases, it is necessary to slit the bars full length.
When rotor bars can’t be pulled
There may be instances where the rotor bars cannot be pulled. In these cases, there are two options. First, if the rotor has vent ducts, it is sometimes possible to cut or drill through the bar. By splitting the bar at the midpoint, two shorter sections can be pulled towards their respective ends. Since the bar is half as long, there is only half as much friction holding it. For a difficult-to-remove rotor bar, locate the bar within an air duct and cut the bar in half using a saw or drill. Once the bar has been cut, pull the two shorter sections toward their respective ends.
The other method that can be used if the bars cannot be pulled is to unstack the rotor to remove the last stubborn bars. If this step is necessary, a partial restacking job may be sufficient. In the worst-case scenario, it may be necessary to unstack and restack the rotor to remove stubborn rotor bars.
Clean and bake rotor
Following bar removal the rotor should be cleaned and baked to dry. File or lightly grind any burrs or rough edges of laminations. Inspect the slots to confirm that they are all clean and clear of any obstructions. Next, micrometer measure at least three bars to determine the thickness and height for the purpose of ordering replacement bar material. If possible, have bars made to the size required. Otherwise, order oversize bar material and machine it to size. The new bars should be made about ¼” (6 mm) longer than the finish length, so that they can be machined to fit flush against the end rings. Likewise, measure and order material for the two end rings, if the original rings cannot be reused. Make certain that the new bar and end ring material matches the conductivity of the original.
If new end rings are made, make certain the raw material has the same conductivity as the original. End rings that have the bars passing over them can often be made by rolling a rectangular or square shape of the same thickness and width as the original. A significant difficulty with rolled rings is obtaining a flush abutment of the end faces, and consequently brazing them. It is much better and more reliable to have new end rings cast to size or machined from a solid plate. Rolled end ring construction is not suitable for use with 2-pole rotors or motors operating well above 60 Hz (e.g., spindle motors). End rings that do not have the bars passing over them, such as those that are slotted for the bars, should be made from casting or plate material.
Place new bars into rotor slots
Verify bar dimensions before inserting any new bars into the rotor slots. In general, the bars should be about 0.010” (0.25 mm) thinner than the slot. If new end rings were made, also verify their dimensions. If the original bars were hand pulled from the slots, the new bars can probably be tapped into the slots with light blows from a rawhide or plastic mallet. For tighter bar fits, shrink the bar size by cooling before insertion. This can be done by placing the bars in a bed of dry ice, or “soaking” them with liquid nitrogen. Use appropriate personal protective equipment as the very low temperatures of dry ice or liquid nitrogen can cause freezing burns. Note: Do not make the mistake of heating the rotor. That expands the iron, and actually reduces the slot width; the bars will be even tighter to install.
The cooled bars can also be tapped in using a rawhide or plastic mallet. It may be possible to push the cooled bars through the slots by hand insertion. If a bar can not be easily inserted into a slot, inspect the slot for the obstruction and lightly grind it away. Do not try to force the bar through the slot. It will probably distort the laminations and itself, and could become jammed in the slot. Check that the bar extension matches the original, plus the intentional additional 1/8” (3 mm) extension.
Swaging of bars
Next, swage the bars to make them tight in the slots. Note: The swaging of rotor bars process described here is taken from the “Swaging of Rotor Bars” subsection of the EASA “Mechanical Repair Fundamentals” seminar text. Swaging tightens rotor bars in the slots and minimizes the possibility of bar cracking. Figure 2 illustrates a rotor bar before and after swaging.
Rotor bars should be swaged to prevent bar vibration that results in chatter during starting as well as metal fatigue that can drastically reduce rotor cage life. The rotor bars should be swaged every 3” to 8” (8 to 20 cm) depending on accessibility and looseness. Each bar should be swaged in the same locations; each row of swages should be in line around the circumference of the rotor. For a consistent swage, use a blunt chisel in an air hammer. All swages should be of uniform depth and force.
Premature cage failure possible
If loose bars are not swaged, premature cage failure is likely due to repeated fatigue cycles. The more frequently the motor is started and the more severe the starting cycle (across-the-line starting as opposed to a soft-starting method) the more important this becomes.
After all the bars have been inserted and swaged, braze the bars to the end rings. For the procedure to do this, see the article titled “Follow These Tips When Brazing Induction Rotors” in the February 2000 issue of CURRENTS. If the bar ends are to be brazed to the inboard vertical faces of the end rings, machine the bar ends. That will minimize the gap between bars and end rings, and better assure a satisfactory brazed joint. For this type of connection the end ring vertical should be counterbored about 1/8” (3 mm) to create a trough to retain the brazing material.
After brazing and visual inspection of the brazed joints, test the rotor for open circuits or high-resistance joints using a growler or core test. The rotor is now ready for dynamic balancing and motor assembly.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100- Section 2: Mechanical repair
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