Mechanics and Manufacturing Methods -- Process Control in Commutator Fusing

Fusing has been found to be the most critical process in electric motor manufacturing. This is particularly true for the new generation of asbestos-free, high-speed motors.

This section describes how to evaluate fusing results, the fusing process, the effects of commutator design and construction on the fusing process, and improvements to fusing-machine technology for improved process control.

Historically, the fusing process has been the single greatest source of rejects in armature manufacture. Thus, it’s one essential area on which efforts to improve technology must be focused.

The process of fusing tang commutators is the focus for analysis here, although many of the observations, methods, and conclusions are also valid for fusing slot commutators. The electrical, mechanical, and thermal characteristics of fusing the two types of commutators remain the same, but the observations on the geometry and the thermal and electrical phenomena inside the tang are different for the slot type of commutator.

To begin an analysis of the tang-fusing process, the objective of commutator fusing can be defined as establishing an electrical and mechanical connection between the wire and the commutator bar, with characteristics that guarantee the correct working of the motor in operating conditions for the expected life of the motor.

The process by which commutator fusing is done and solutions for optimizing the fusing process are presented. With this result in mind, we now examine the typical battery of tests that are performed either on the manufacturing floor or in the laboratory to qualify the fused commutator.

Generally speaking, fusing is accomplished by contacting the bar with a ground electrode, then compressing the tang and heating the tang with the fusing electrode, as in Ill. 78.This is discussed in detail later.

Evaluation of Commutator Fuse Testing

The first step is to define a list that represents the tests for qualifying fusing results.

The following subsections present a brief description of each test and the relevance it has for analyzing the fusing process. The tests must be divided into two categories, qualifying checks and measurements.

Qualifying Checks or Observations. All of these tests are done randomly by visual inspection through a magnifying glass or a microscope. These tests require a low investment and can be performed on the manufacturing floor by the technician.

They can be done quickly, but they don’t offer any quantitative results as per formed. Generally, it’s very useful to test all the commutator tangs in order to verify the fusing uniformity.

1. Insulation peeling or "burn-back" is observed, as in Ill. 79, where the wire extends from the sides of the fused tang. This test is carried out on the entire commutator to check the consistency of power applied during fusing.

ILL. 78 Evaluation of commutator fuse testing.

Fusing electrode Tang Wire core Wire insulation Ground electrode; Commutator bar

ILL. 79 Well-closed tang with visible insulation peeling.

2. Bar oxidation gives an indication of the heat stress caused to the commutator during fusing. This test may also be used to gauge the consistency of energy application during fusing, although it’s heavily affected by external conditions such as the presence of cooling airflow.

3. Tang roughness gives an indication of the wear status of the electrode. The technician observes the top of the tang for pitting or scars. This occurs because the tungsten fusing-electrode surface breaks down due to the heat and pressure applied.

4. Observation of the tang closing checks for air gaps between the tang and the bar. A perfect closure, as in Ill. 83, prevents accumulation of extraneous material under the tang that would otherwise insulate the wire from the bar.

5. Subjective test of resistance to mechanical stress-stress the tang transversely or vertically, as shown in Ill. 80, to check the mechanical connection to the bar.

When the joining force is reached the two parts separate, emitting an easily audible snap. When the tang is not connected, no sound is heard.

6. Visual test of the tang impression on the bar-the degree of mechanical cohesion can be observed by lifting the tang in order to observe its impression on the bar. A well marked impression does not always correspond to a good mechanical cohesion, while the presence of oxidation under the tang is a clear indication of non-cohesion.

7. Visual test of the wire impression on the tang and bar-normally, with a good fuse, you can clearly see the impression of the wire under the tang and on the bar.

The copper of the tang and bar surrounding the wire is a good indication that the wire has made good surface contact with the tang and the bar.

Measurements. These tests are generally carried out either in the laboratory or directly on the production line. The benefit of testing on the line is that it gives the manufacturer the ability to create a database for quality control and for tracking serial parts, and it guarantees that each part has been measured against a set of tolerances.

1. Electrical continuity testing checks for electrical continuity between the commutator bars. It requires a simple and low-cost equipment setup; however, it does not distinguish poor from good electrical connections.

2. Fuse resistance testing is carried out on a commercial machine in the production line to measure each tang's fuse resistance. The test permits accurate evaluation of the electrical connection across the wire and bar.

ILL. 80 Resistance to mechanical stress.

3. Measurement of final tang thickness indicates whether the force and energy used during fusing have been maintained within required tolerances.

4. Measured resistance to mechanical stress testing, a more accurate measurement of tang-to-bar cohesion, can be obtained by using a dynamometer to measure the breaking load. In high-rpm applications, the armature is rotated to very high speed in order to accomplish a further check for bar lift.

5. Wire-squeezing testing evaluates the result of the motion of the tang during fusing. This test is destructive, because it requires that the tang be opened to measure the thickness of the wire. Normally, the optimal decrease in the wire diameter (minor diameter of ellipse-shaped wire) is 15 to 40 percent. Reduction of the wire diameter within this range ensures good wire stability while also ensuring that the wire is not excessively weakened.

6. Measurement of the commutator temperature, carried out immediately after fusing, indicates the thermal stress on the commutator. Generally, the commutator manufacturer indicates a maximum allowable temperature. This test has become very important with asbestos-free commutators.

Fusing Process

Basically, the fusing process requires compressing and heating by applying current flow through the parts that need to be fused.

The following is a description of the most important stages that occur during fusing:

ILL. 81 Stages 1 to 3 of fusing.

ILL. 82 Stage 4 of fusing.

Stage 1. A properly dimensioned ground electrode is brought into contact with the commutator bar without damaging the commutator, by means of a pneumatic cylinder.

Stage 2. The fusing electrode is brought into contact with the commutator tang ( Ill. 81) without excessively bending the tang. In physical terms, this requires that a well-proportioned ratio between the kinetic energy of the fusing head and the mechanical strength of the tang and the commutator be achieved. Since the mass of the fusing head can be reduced only within certain limits, the speed of the electrode becomes an important control variable in efforts to minimize the kinetic energy. In order to accomplish a low-speed approach in first contacting the tang, low electrode force must initially be applied. Then the force should be gradually increased to reach the preset values as required during the current-flow phase.

Stage 3. The electrode applies a resultant force F on the tang ( Ill. 81), which, due to the increasing surface contact of the electrode, moves until it becomes balanced by the opposing deformation of the tang. When this balance is reached, the electrode stops moving and current should be applied. In particular, the geometry of the tang, the size of the wire, the wire position beneath the tang, and the sharpening angle of the electrode determine this balance position.

Stage 4. At the beginning of the power application, the current flows through the tang in a lengthwise direction ( Ill. 82).The tang begins to heat at the elbow, the tang portion enveloping the wire, where the current density is higher. In this first stage, the wire insulating material is eliminated. The insulation evaporates easily because the tang is still open. When the temperature rises, the mechanical characteristics of the copper change; therefore, the tang becomes more plastic in its elbow (this is the point with the highest temperature), and the tang continues to close until it contacts the bar ( Ill. 83).

Stage 5. Once the tip of the tang contacts the bar, the current starts passing through the tang in a transverse direction. As the contact area with the bar increases, the current through this area also increases, thereby reducing the cur rent density in the tang elbow. At this point the heat is mostly applied to the tip of the tang ( Ill. 84).

Stage 6. Once the current supply is terminated, the electrode continues to apply force to the tang for a preset time. During this time the temperature decreases and mechanical cohesion is completed ( Ill. 85). Variables which influence cohesion are typically the electrode force, the time for applying the cohesion force, and the presence of forced cooling of the tang (e.g., by using low temperature airflow).

Importance of the Wire and the Commutator in the Fusing Process. The geometric characteristics and the composition of the commutator and the winding wire are very important in achieving good results in the fusing process. The most important characteristics are the following:

1. Tang cross section and length. Together, these determine the minimum amount of force applied for "cold" fusing (see Stage 3 of the fusing process). The cross-sectional area and copper alloy determine the electrical resistance and thus the upper limit of the current to be applied during Stage 4 of the fusing process. If the current exceeds this limit, the tang will melt at the elbow.

2. Top surface of the tang. This surface determines the current value to be used in Stage 5 of the fusing process. An excessive current density causes surface melting of the tang and can result in a blown tang, while a low current density produces insufficient cohesion.

3. Type of copper alloy. The yield point and conductivity of the copper commutator varies with the silver content. This means the tang's mechanical resistance and temperature will vary as the copper alloy varies. Therefore, it’s necessary to adjust the electrical and mechanical parameters accordingly.

4. Wire diameter and position. The wire hooked around the tang behaves as a support for the tang; thus, the wire affects the tang closing and therefore the forces necessary for the tang elbow. This effect often occurs as a result of wire hooking with insufficient wire tension or of double wire hooking and is more evident the shorter the tang.

5. Wire insulation. Wire insulation with a high thermal resistance requires a considerable increase in the current and fusing time to vaporize the insulation. In addition, using a lower force slows tang closure, thereby increasing the heating of the tang elbow.

6. Commutator. The inherent eccentricities in the commutator as supplied by the manufacturer can be eliminated as a negative influencing factor in the fusing process by measuring the electrode displacement relative to the bar rather than to the center of the armature shaft.

Commutators without asbestos are less resistant to thermal stress. To avoid damage, it’s necessary to reduce the applied energy. The best results have been obtained by applying high power for a short time, in order to concentrate heating as much as possible to the tang area. Reducing the fusing time avoids excessive heat diffusion in the commutator bar and thus reduces defects caused by thermal stress. Less heat stress also results in lower bar-to-bar drop development after turning.

ILL. 83 Stage 4 of fusing.

ILL. 84 Stage 5 of fusing.

ILL. 85 Stage 6 of fusing.

7. Bar and tang geometry. The relationship between tang mass mt and bar mass mb are correlated to the temperature T of the commutator bar at the end of fusing, as follows:

T = K (3.12)

where K = temperature dependent coefficient If the tang is small compared to the bar, fusing can be done with lower thermal stress on the commutator.

Improvements to Fusing-Machine Technology

1. During the fusing process, current distribution in various portions of the tang varies considerably, as has been described previously. This variation results in significant variations in the associated electrical resistance rt of the tang. The heat Q applied to the tang by means of current i can be expressed as follows:

Q = _ rt (t) * i 2 (t)dt (3.13)

If heat application occurs by regulating only the RMS value of the current i, the variations in resistance rt will cause unwanted variations in heat Q. To avoid this, for obtaining more precise heat application, it’s necessary to regulate the power P applied during fusing.

Q = _P(t)dt = _v(t) * i(t)dt (3.14)

For this power regulation, and to obtain fusing in a sufficiently short time, medium frequency dc inverters are the most adequate electrical power supply equipment that can be used. Normally, fusing cycle times are completed in 70 to 150 ms. DC medium frequency inverters (1 to 1.2 kHz) regulate using closed-loop feedback every 415 to 500 µs. Line-frequency equipment regulates every 8,000 µs. Considering, for example, a fusing done in 72 ms, which corresponds to a little more than 4 cycles in American line frequency, line-frequency regulation is likely to be inadequate most of the time.

2. During the fusing process, preset electrode force profiles need to be applied to precisely control tang closure and to distribute the heat correctly in the various portions of the tang. To achieve force application in this way with necessary control accuracy, the fusing system should use closed-loop force control.

3. Quality fusing requires that the machine be designed and built to guarantee high repeatability and reliability. In particular, the fusing machine needs to have adequate cooling of the electrode to maintain consistent working temperature, design construction that guarantees accurate alignment of the electrode with the tang, and provisions for quick electrode replacement by the operator without affecting the precision and repeatability of the machine's performance.

Conclusions

Today, fusing-machine technology allows reliable and consistent fusing of commutators using low energy. In particular, this is required to process modern high-speed motors having asbestos-free commutators. Since the fusing process is also significantly affected by the commutator and wire-insulation characteristics, it should be carefully considered in new motor design.