Mike Howell
EASA Technical Support Specialist
Most AC stator windings installed by EASA service centers are balanced, three-phase, two-layer, lap windings. But, what does it mean for such a winding to be balanced? If balanced, the voltages generated in each phase are equal in magnitude and displaced from each other by the same angle (See Figure 1 Balanced). If there is any difference in magnitude or angle displacement, the winding is unbalanced (See Figure 1 Unbalanced). It is well established that unbalanced windings can cause undesirable vibration, electromagnetic noise, and additional conductor heating due to circulating currents.
With very few exceptions, duplicating a factory winding, balanced or unbalanced, is a reasonable approach to a rewind. Generally, when a manufacturer utilizes an unbalanced winding the consequences of doing so have been considered and deemed acceptable. However, service centers should avoid redesigns that require unbalanced windings and where an unbalanced winding is unavoidable, parallel circuits and delta connections should be avoided; that is, use a 1Y if possible.
This article will not go into detail to discuss coil grouping and layout of windings, but there is a need for some general information required to discuss conditions of balance. First, it is assumed for this article that the number of slots is divisible by the number of phases; if this isn’t the case, one or more coils will likely need to be removed from the winding, and you should contact EASA technical support for help with that layout. Next, it is assumed that if the phases are divided into parallel circuits, then the number of circuits being used is valid and that each circuit has the same number of total coil turns.
Integral slot windings
he integral slot winding is a winding where the number of slots per pole per phase (SPP) is an integer or whole number. Integral slot windings are balanced windings. We can write the calculation for SPP as follows.
SPP = Q / (M·P)
where:
SPP is the number of slots per pole per phase
Q is the number of stator slots
P is the number of poles
M is the number of phases
Examples of common integral slot windings are 36 slot 2-pole with SPP = 6, 48 slot 4-pole with SPP = 4, and 54 slot 6-pole with SPP = 3.
Fractional slot windings
If the slots per pole per phase (SPP) is not an integer or whole number, the winding is referred to as a fractional slot winding. These windings may be balanced or unbalanced and the SPP can be expressed as a mixed fraction as follows.
SPP = Q / (M·P) = I + n / d
where:
I is the integer part of the SPP
n is the numerator of the remainder
d is the denominator of the remainder
Examples of fractional slot windings are 36 slot 8-pole with SPP = 1+1/2, 48 slot 6-pole with SPP = 2+2/3, and 72 slot 10-pole with SPP = 2+2/5.
The two conditions for balance of a fractional slot winding are easy to check. A fractional slot winding is balanced if the two following conditions are true.
P / d = integer
d / M = not an integer
For a 72 slot 10-pole, P/d = 2 and d/M = 1.67, so the winding is balanced. For a 48 slot 6-pole, P/d = 2 but d/M = 1, so the winding is unbalanced.
For windings where the number of slots is divisible by three (same number of coils per phase) but the winding is unbalanced, the number of poles is generally divisible by six (e.g., 6,12,18,24). Table 1 provides some combinations of slot and pole counts where unbalanced windings occur. If the unbalance of voltage magnitude is less than 3% and the error in angle between phases is less than 3°, the combination is listed under “1Y”; that is, the winding should be ok to use but should be designed for a 1Y connection if possible. For all other cases, the combination is listed under “Do not use.”
As previously mentioned, manufacturers may sometimes use the combinations shown in Table 1 and this usually is for economic reasons. For the service center, the take-away is to avoid using these combinations when redesigning machines and where one must be used, avoid delta connections and parallel circuit connections.
For other unbalanced windings where the number of slots is not divisible by the number of phases, we recommend you contact EASA technical support before proceeding. This is often encountered when a single-phase or two-phase machine is redesigned for three-phase. Other times, if a manufacturer has selected an unbalanced winding, duplicating the OEM winding layout may be important for controlling the effects of the unbalance.
AVAILABLE IN SPANISH
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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