Chuck Yung
EASA Senior Technical Support Specialist
My purpose in writing this article is to explain in layman’s terms what electromechanical professionals refer to as circulating currents, why they exist in three-phase electric motors and to offer practical solutions.
When a winding has parallel circuits, it’s easy to assume that the winding equally divides the current such that every parallel path draws the same current. The reality is very different. More current is required to drive magnetic flux through air than through a permeable steel core. That is one reason motor designers control the physical distance between the rotor and stator cores, which is referred to as the airgap.
The flux must cross the airgap twice per pole for each revolution. The more poles a winding has, the more times the flux must cross the airgap between stator and rotor. This is illustrated in Figure 1. You have probably noticed that slower speed machines (more poles) have smaller airgaps than two-pole machines of comparable size, and that motors with more poles have progressively smaller airgaps.
This is not a new phenomenon. American mathematician and engineer Charles Steinmetz observed in 1897 that “the magnetic field must be of constant intensity in all directions,” which we recognize as the reason our airgap tolerance is +/- 10% of the average. You may have heard an electric motor with so much electrical noise that you suspected the rotor was dragging the stator bore, only to find that the actual cause of the noise was an eccentric airgap. If it was practical to use a clamp-on ammeter to compare current between parallel circuits of such a motor, you would find a considerable difference. (See Figure 2.)
Small variations in the airgap between stator and rotor cause coil groups in positions where the airgap is larger to draw higher magnetizing current than identical groups in positions where the airgap distance is smaller. That can affect the current drawn by different paths within the same phase or between different phases.
In simple terms, currents will circulate between the parallel paths as they try to balance the uneven magnetizing current. The circulating current creates additional heat within the windings. Let’s look at some common examples that our industry encounters.
When a motor is machine-wound in layers by the OEM, the first layer installed is in the slot bottoms with subsequent layers stacked above the first layer. The mean length of turn (MLT) differs, as does the resistance of each phase. The difference in resistance contributes to circulating currents. Most winders have seen windings with equalized delta connections that are easily mistaken for a wye connection (e.g., an equalized 2-delta closely resembles a 4-wye) as shown in Figure 3. The purpose of those equalizers is to balance, or equalize, the current in parallel paths. This is done to reduce I2R losses and to improve efficiency.
Another area where we see equalized connections used is when a winding has half as many circuits as poles. There are two factors to consider in those cases. First, the more poles, the smaller the physical airgap the manufacturer works toward. The second is that the airgap tolerance of +/- 10% of the average must be met. The more imperfect the radial airgap distance, the greater the likelihood of circulating currents. A frequent issue EASA engineering helps callers with is a rewound motor that is noisy, runs hot and/or has unbalanced current. When the problem motor is a six-pole machine with a 3-circuit connection, our go-to solution is to reconnect the winding using 1-10 jumpers. Table I provides the circuits, poles and recommended jumpers for commonly encountered windings in this category.
When a winding has half as many circuits as poles, the extra-long jumpers connect groups in series that are directly across the stator from each other. Forcing the coil groups to carry the same current reduces the circulating current when the radial airgap is not perfect.
In Figure 4, it is easy to see how awkward an equalized connection can be with the risk of misidentification when taking data, or accidental misconnection when rewinding the motor. Rather than six group ends brought together for each phase, the extra-long 1-10 jumpers simplify the connection and reduce the risk of errors.
Compared to the equalized 3-delta in Figure 4, this connection (Figure 5) is much simpler and less likely to result in errors during the connection process.
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