Tom Bishop, P.E.
EASA Technical Support Specialist 

This article describes machine balancing of the rotating components of motors and generators, primarily rotors and armatures. The methods described here, in general, can be applied to on-site balancing if the rotating component is accessible. The intent is to describe the methods of attaching balance weights, not determining acceptable balance level or the location and amount of correction weight. 

The advent of computerized balancing machines has made the latter steps rather straightforward. However, the challenge of how to attach a weight in such a way that it will remain secure and not negatively affect machine operation remains at times a vexing problem. 

What is the purpose of dynamic balancing a rotating part? It is to reduce unbalance and consequently to bring vibration to acceptable levels to allow for normal bearing and other component life. The acceptable levels of vibration are described in EASA Tech Note 32, “Standards For Dynamic Balancing,” thus we won’t explain them here.

Heavy or light spot 
Balancing consists of identifying a physically heavy or light spot in a rotating component and placing an equal weight in opposition to the heavy or light spot. To simplify the explanation we will consider the case of a die cast squirrel cage rotor. If the unbalance is due to a heavy spot, that means there is too much weight on one side of the rotor. By placing an equal weight on the opposite side of the rotor, at the same effective radius, the heavy spot is cancelled out. For example, let’s assume the heavy spot is 1.2 ounces at a radius of 2.5 inches. The terminology for the units of unbalance in this case is ounce-inches, and the unbalance would be 3.0 ounce-inches (1.2 ounces times 2.5 inches). 

Note that the units of unbalance are a force at a distance (much like Table 1 Keyseat Dimensions (in inches) torque), whereas vibration is typically expressed in units of velocity (e.g., inches or millimeters per second) or displacement (e.g., mils – thousandths of an inch or millimeters). Also note that the distance measurement is a radius, not a diameter. A weight of 1 unit at radius r would have the same effect as a weight of 1/2 unit at a radius of 2r. The further the weight can be safely located from the center of the shaft, the less weight is required. 

A general caution: balance correction weights must not extend above the outer diameter of the rotating component. Otherwise, the balance weight could come in contact with a stationary component such as the stator winding, or could prevent insertion of the rotating component into the frame. Bolt and nut threads should be locked with a thread-locking compound and the ends of the bolts “staked,” if possible, to prevent the nut from loosening. 

Weight of the shaft key 
Balance quality can be affected if the weight of the shaft key is not determined correctly. The half-key for most shafts should be the length and width of the actual key used in that keyseat (keyway), but half of the key height. Avoid using a full-size key of half the length. That raises the center of gravity of the key, making it have the effect of being too heavy. If the machine was received without coupling or key, the length of the key being used is usually evident with a visual inspection of the keyway. If you are unsure of the correct length for the key, use the “ES” value for the applicable frame size from NEMA MG 1 standard. (See Table 1.) The ES dimension is given in the EASA Technical Manual, Electrical Engi­neering Pocket Handbook and Mechanical

Reference Handbook. For a metric motor, the IEC stipulates that the shaft and rotor be balanced using a full key. The pulley or coupling for an IEC motor is balanced without a key, to compensate. 

For 2-pole machines, after the rotor has been balanced to the appropriate tolerance, add the fans, coupling, etc., and balance them. Use the rotor as a balance mandrel for those parts, making weight correc­tions directly to the fan/coupling. Weight corrections should not be made to the rotor at this stage. Doing so will result in unbalance in multiple planes, and may cause the shaft to deflect at operating speed. 

Die cast rotors 
The most prevalent locations for balance correction on a die cast rotor are the projections between the end ring cooling fan extensions, some­times referred to as “nibs.” Typically, flat washers are added to correct for unbalance. After installing washers on these nibs, the nib ends are peened over to secure the washers. (See Figure 1.) The washers should then be checked for tightness before the next balance run, i.e., there should not be any clearance between the balance washers and the nibs. If the nibs are deformed so as to facilitate washer removal, the affected nibs should not be reused for attaching balance weights. The probability is great that the nib structure may no longer be capable of retaining a balance washer. 

The die cast end rings should not be used to correct for unbalance by removing material from them, such as by drilling or grinding. Fan blades on the end rings may be drilled to accommodate a bolt, nut and washers for balance correction if there are no nibs on the end rings or the existing nibs cannot be used. Make certain that the balancing machine is protected from particles and debris whenever drilling or grinding or other such machining operations are used to adjust the part being dynamically balanced. 

As a guide, the bolt hole diameter in the fan extension should not be more than about one-fifth of either the extension height or length from the end ring; the bolt length should not be greater than about three times its diameter. Preferably, the bolt hole should be in the middle one-third of the fan extension, as measured from any edge. 

Balancing epoxy 
If there are no provisions for attaching washers, or bolts and washers, balancing epoxy may be the only alternative. Balancing epoxy is epoxy formulated for the purpose of correcting mechanical unbalance, and is “loaded” with a high-density filler material. Care must be taken in attaching the epoxy because there are few rotor surfaces that will retain it. Often the only acceptable location is under the end rings, where there will be a relatively flat surface that can restrain the weight against centrifugal force. Other considerations in locating the balancing epoxy are to be certain that air flow will not be restricted, such as by epoxy blocking a vent duct. 

Fabricated rotors 
Common methods of correcting balance on fabricated rotors are insertion of bolts into pre-tapped balance ring holes, and attachment to cooling fans or fins. Less frequently, weights are bolted or welded to fans or fins. Care must be exercised to be certain that air passages are not blocked or changed. 

The bars and end rings should never be used to correct for balance by removing material from them, such as by drilling or grinding. Drilling or grinding could jeopardize the mechanical integrity and possibly affect the rotor electrical characteris­tics. Care must be exercised to be certain that air passages are not blocked or changed by balance weights. Also, differential thermal expansion of the various rotor parts must be considered. A balance weight attached to a balance ring or internal fan should not be positioned where the end ring might contact it. The acciden­tal circuit to ground may cause arcing between the end ring or bars and the weight. 

If there is no place to attach a weight by use of threads or welds, balancing epoxy may be an alternative as long as the following conditions apply: airflow will not be restricted, the epoxy is applied to the inner diameter of a part such as an end ring, the part can support the weight without adverse effect on the part or weight, and the weight will not be prone to becoming loose or otherwise damaged. The surface to which the balance compound is to be affixed must be clean and dry. 

If the rotor has a spider, and a new shaft has been made, pre-balance the shaft and spider before mounting the core. That will reduce the amount of correction weight to be placed on the completed rotor assembly. 

Wound rotors and armatures 
The most common method for balancing wound rotors and armatures is to apply balancing epoxy to the bore side of the coil extensions or to the glass bands on the periphery of the coil extensions (See Figure 2.) Be sure to verify that correction weights on bands do not have a diameter greater than the core; otherwise, they may come in contact with stationary parts such as the stator windings or field coils. Going around the core outside diameter with a straight edge extending over the winding extensions on both ends can be used to check this clearance. As a guide, the minimum radial distance between straight edge and weight should be 1/16” (1.5 mm). 

Balancing ring 
If the rotor or armature has a balancing ring, that should be used for balance correction. Most rings consist of a narrow flange-like circular plate with drilled and tapped holes to accommodate bolts and washers as balance weights. Some commutators have dovetail shaped grooves with pre-installed balance weights. The weights can be redistributed within the dovetail groove to improve or correct unbalance. 

Balance weights should only be attached to fan blades if weights cannot be attached to the windings or balance ring as described above. Many internal fans are of light metal or plastic construction and are not intended for attachment of relatively heavy balance weights. 

If an armature is rewound, remove the old balance weights and pre­balance the bare core before inserting coils. 

Leave the balancing provisions (e.g., dovetail slots) clear for final balancing after the rewound armature has been varnish-treated and the commutator has been machined. 

Synchronous rotors 
There is no predominant method of correcting balance on these rotors. On smaller rotors it is a common practice to apply balancing epoxy to the underside of the coil ends. Larger rotors are usually corrected for balance by welding or bolting weights to the hub to which the bolts are attached, or by adding washers to the bolts holding fan blades in place. If the force of rotation (centrifugal force) is too great, the weld may fail and the weight could strike the winding or other components. Avoid using the amortisseur cage for attaching balance weights as the bars and end rings are relatively low in structural strength. 

If the rotor is rewound and the poles are removed, pre-balance the bare-rotor assembly before reinstall­ing the pole pieces and coils. That will reduce the amount of correction weight to be placed on the completed rotor assembly. 

Balancing of other rotating components 
Fans/Blowers 
For guidance on balancing fans and blowers, see the November 2004 Currents article “Fan Balancing Tips.” 

Impellers 
Impellers are often balanced by grinding or drilling to remove weight. The balance machine must be protected from debris particles generated during the drilling or grinding process. Balance weights should not be attached to the flow passages (nor the flow passages drilled) within the impeller or to the inlet faces. Likewise, the impeller wear rings should not be used for balance correction, such as by grinding them. 

Care must be exercised when drilling to be certain that the struc­tural integrity of the impeller is not compromised. For instance, too large a hole could cause the blade to crack. Holes should not pass completely through a wall or face, except for the specific purpose of providing for bolt attachment. Make certain that metal chips are completely removed from the impeller, and likewise protect the balance machine by capturing all the chips that are generated. 

Single vane impellers are a special case for balancing. The fluid that flows through them becomes part of the dynamic mass, thus they should be balanced in a fluid, not dry. Balancing single vane impellers dry can result in unbalance in actual operation with fluid in them. 

Categories: AC motors, Rotors, Balancing, DC motors, Armatures

Tags: Balancing

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