Kent Henry
EASA Technical Support Specialist
When taking winding data, equalizer connections can be mistaken for wye points. You may wonder what purpose equalizer connections serve and whether they can just be eliminated to simplify the repair process. Before discussing equalizers, we will explore the factors that lead to a need for equalizers.
A magnetic unbalance within a motor or generator can be a very serious problem. The magnetic balance Stator relies on a marriage of electrical and mechanical elements. When either of these electro-mechanical elements changes, it may create a magnetic unbalance.
By looking at the magnetic forces within a new motor we can see a relationship between the electrical and mechanical partnership built on symmetry:
- The mechanical alignment of the rotor within the stator bore is concentric and has acceptable symmetry in every aspect with the stator bore (see Figure 1).
- The electrical alignment of the stator windings and poles within the stator is symmetrical in every aspect with the stator bore as well as the rotor.
- When we energize the stator windings, the rotor experiences a symmetrical magnetic .ux induced through the air gap and there is acceptable magnetic balance.
- While lap windings are symmetrical, some concentric designs are not. We are therefore more likely to see equalized connections used on concentric windings to counter circulating currents.
The machine-wound concentric windings can have different slot leakage reactance in each layer as they lay in by phases, so the top layer (phase) could have quite a different reactance from the bottom layer (phase). Leakage reactance is the inductive resistance to current .ow within the slot. The effect of leakage reactance is to increase current as the winding becomes further from the top of the slot. This is the reason Stator we put the low-speed winding of a 2-speed motor on the top layer; to reduce current, and to increase torque.
Mechanical degradation
The manufacturing tolerances result in small imperfections that, in some cases, lead to an eccentric air gap. These small imperfections of symmetry cause a difference in the magnetic .ux of paralleled paths within the winding.
The unequal air gap illustrated in Figure 2 leads to magnetic and electrical dissymmetry. The motor may experience noise, vibration, heating, elevated current draw and an increase in losses.
Countering circulating currents
To counteract circulating currents that result from air gap dissymmetry, equalizers may be utilized. The easiest way to visualize the equalized connection is to consider one phase at a time.
The equalizers in a 3-phase motor need to connect the parallel paths at points of equal potential (see Figure 3). Circulating currents are only possible when a winding has two or more circuits.
They are equally likely to occur in adjacent pole (1-4 jumpers) or skip pole (1-7 jumpers) connections. The equalizer jumper carries the equalizing current required to balance the paralleled paths and should be at least 1/3 of the circular mils of the wire area of the coils.
Maximum correction
Technicians should duplicate the equalized connection found in a winding. That is, do not reduce the number of equalizers or eliminate equalizing.
Note: In most modern designs, the exact midpoint is equalized; more than one equalizer is used only when the exact midpoint is inaccessible, in which case you might see two equalizers at 1/3 and 2/3 through the paths; for example, if there was an odd number of groups in each path (see Figure 4).
The greater the number of poles, the smaller the air gap becomes. It requires more current to drive flux through air than through iron, so designers reduce the physical air gap size to compensate. Practical considerations limit how small they can go, but a 0.040” (1 mm) air gap is a typical lower limit. The air gap should be equal, within 10% of the average; for this example, that tolerance is 0.004” (0.1 mm).
The greater number of poles increases the potential for air gap dissymmetry. Windings with a large number of poles are more likely to use equalizers. In form coil windings with a large number of poles, we often .nd that the equalizers are connected via a ring-connection.
The resulting connections can confuse the technician during data acquisition, as they resemble individual wye points. When we examine the 6-pole, 6-circuit wye connection (see Figure 5 top), we find 3 wye rings each connects 6 group ends to form a ring wye. A 6-pole, 3-circuit delta diagram (Figure 5 bottom) can confuse the technician. In this diagram we have 3 ring jumpers, each equalizing 6 poles. The points at which coil groups connect to the ring connection for equalization could be mistaken for wye points.
It is important to inspect the number of coil ends connected to the input leads to identify the correct number of parallel circuits. If there appear to be too many wyes, or if the connecto be delta and what appears to be wyes are found, check for equalizers. A good practice methodology is to make a connection diagram. Tip for determining whether wye or delta: If the group ends coming to a lead are from the same phase, you have a wye connection. Conversely, if the group ends coming to a lead are from different phases, you have a delta connection.
Long jumpers and circulating currents
We often get calls from members who are experiencing noise, vibration, and higher than expected current draw. These problems are often found to be due to an unbalanced magnetic circuit; circulating currents are one result of that unbalance.
The circulating currents are caused by a disparity within the air gap between the rotor and stator. Ideally, the air gap would be corrected to eliminate magnetic unbalance. Often that is not practical, particularly in motors without provisions to measure the air gap. The alternative is to use equalized connections or extra-long jumpers.
One thing about circulating currents is that they may appear to happen at random. The customer may have several identical motors and just one motor that experiences problems.
A no-load run test is noisy, the motor vibrates, and may draw higher than normal current.
The problems can intensify as you increase the voltage to full nameplate voltage. When a load is applied to a motor, the circulating currents may be reduced as the load increases and it masks the effect of the circulating currents.
The reason for this comes back to the air gap variation, and variation in manufacturing tolerances. If the air gap was perfectly uniform, no circulating currents would occur. If a motor exhibits symptoms of circulating currents, look for these potential causes:
- A bearing housing machined off-center
- A bent shaft
- An eccentric rotor
- A warped stator or frame
- Abnormally loose fit between mating parts, such as the end bracket and stator frame
Any of these conditions could cause an eccentric air gap.
Be proactive
The worst-case scenario for circulating currents due to air gap dissymmetry is when a winding has half as many circuits as poles. To neutralize these circulating currents we can use the coil jumpers so that they connect two pole groups 180 mechanical degrees apart. For example, a 3-circuit on a 6-pole machine would use a 1 to 10 jumper (see Figure 6).
Similarly, a 4-circuit connection on an 8-pole machine would use a 1 to 13 jumper for coil connections. These jumpers will connect groups 180 mechanical degrees apart and neutralize the circulating currents.
Key considerations
To ensure the correct connection, we must diligently examine the existing circuitry while taking data. Be sure to compare the jumpers on the leads to the suspected wye points in order to determine the proper connection. To avoid problems, the best practice is to use equalizers in our connections if the winding originally utilized them. If the number of circuits is half the number of poles, we need to be proactive and use extra-long jumpers. If we encounter or suspect circulating current symptoms, inspect the mechanical conditions that can in.uence air gap symmetry. The special long jumpers connect groups that are 180 mechanical degrees apart, as shown for the Pole-Circuit combinations shown in Table 1.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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