Tom Bishop, P.E.
EASA Senior Technical Support Specialist

We will begin this article by clarify­ing the terms “alternator” and “gen­erator.” Both terms refer to a machine that converts mechanical to electrical power. An alternator is a synchronous machine that converts mechanical to AC electrical power. A generator is a more general term and is a machine that converts mechanical to electrical power, either AC or DC. An alternator is always a generator, but not vice-versa. Our focus in this article will be on 3-phase alternators. However, much of the information provided also applies to single-phase alternators and single- or three-phase generators.

Identify the leads
The initial step in the rewind data taking process is to determine the number of leads and their identifi­cation. Consider this example: The main stator of an alternator has 10 leads, with numbers 0 through 9. See Figure 1. The “0” lead is probably the neutral; however, the lead identifica­tion should be verified. If that is the case, the alternator has an internal wye or multiple internal wyes and the lead coming from it is labeled “0.” To confirm this, we need a low-resistance ohmmeter. There will be continuity between the four leads 7-8-9-0, and between each pair 1-4, 2-5, 3-6. The resistance from leads 7, 8 or 9 to lead 0 should all be the same and should match that of leads 1-4, 2-5, and 3-6. There will be continuity between leads 7, 8, 9 and 0 and the resistance between leads 7-8, 8-9 and 9-7 will be twice the values of the previous test.

 Many large alternators have 12 or 24 external leads. For the purposes of using EASA’s internal con­nection diagrams, these can be considered wye-delta 12-lead connections. The 24 leads consist of leads that will be paired to result in 12 leads. Al­though most alternators utilize external wye con­nections (to provide a neu­tral), there are cases where the alternator output is delta connected. These possibilities reinforce the need to carefully record the complete as-received connections. See Figure 2. If there are leads in addi­tion to the main winding output, they may be taps used to provide voltage for control circuits such as for the voltage regulator. See the section on determining the inter­nal connections for further guidance.

Note: Numerical lead labeling is common with North American alternators, and alphabetical (letter) or alphanumeric lead identifications are commonly found on alternators manufactured to IEC standards.  

Determining lead wire size
The lead wire size used on an al­ternator is frequently smaller than the size that would be determined from a lead wire ampacity table. This is particularly the case with alternators rated for less than continuous, e.g., standby duty. Record the as-received lead wire size if it is marked on the leads. If the leads are not marked, slip a terminal lug that fits snugly onto the leads, thereby using the lug as sizing guide to determine the correct lead wire size. Note: Lugs are not sized for lead cable, so the correct lug will be snug to very tight.  

Determine the internal connections
Prior to the burnout process, it is a good practice to lift and completely identify the connections. If there are leads other than output power leads, such as main winding taps for a control circuit or leads for an auxiliary wind­ing, these need to be identified, and their position in the winding located. See Figures 3 and 4. Label every lead: main winding, taps, auxiliary, etc. with identification that will withstand the burnout process.  

Draw the winding connection be­fore the burnout process and verify that it is correct. Most but not all al­ternators are synchronous machines having a rotor with wound poles supplied by direct current (DC). A straightforward but important check is to confirm that the winding poles match the number of wound poles on the synchronous rotor. 

Connection data for alternators is frequently not available from the origi­nal manufacturer. In some cases the manufacturer name on the alternator nameplate is not the actual alternator manufacturer, making the task of ob­taining original information more dif­ficult and frequently not possible. It is much better to carefully take connection data rather than have to purchase a replacement stator or complete alterna­tor if the correct and complete connection information was not obtained during the data taking process.

After identifying and labeling all of the main external leads from the winding, pro­ceed to do the same with any taps or aux­iliary winding leads. If there are taps, count the number of coils between the tap and the associated main lead. Likewise, count the number of coils from the tap to the nearest non-output lead, most often a lead that is used to form a wye connec­tion. In many cases the tap is within a coil. Therefore, count the number of turns to the tap. See the July 2004 Currents article titled “Powering Up: Deter­mining Where To Tap Stator Windings” for detailed instructions to identify and locate tap locations. Note: If the winding is tapped, it may be prudent to burn out the stator without first cut­ting off the coil end.

If there is an auxiliary winding, permanently label a slot as “slot #1.” A good place to do this is the slot where the main winding lead #1 is located. Draw the connection diagrams of the main and auxiliary windings referenc­ing coil and lead positions relative to slot #1. Relative to the main winding, and to slot #1, the auxiliary winding should be inserted in the same slots. For example, if the auxiliary winding begins in the same slot as coil #1 of the main winding, it should be reinserted in the same slot as coil #1 of the main winding. The auxiliary winding is the secondary of a transformer with the main winding being the primary. If the auxiliary winding is not cor­rectly placed back in the stator it may develop an incorrect output voltage and the alternator will not perform properly.

Current density  
When checking the lead wire size, also count and measure the winding coil wires and calculate the wire area per amp. The winding wire area per amp (CMA) will often be less than typical for motors, especially if the alternator is rated for standby duty. If the current density is found to be less than about 275 CMA, check the nameplate to be certain the machine is rated for standby duty. Continu­ous duty alternators typically have current densities above 300 CMA. If slot space allows, wire area per turn can be increased. The winding will operate cooler and be more efficient with the larger wire area.

Winding changes
In general, it is best to maintain the original winding data. This may not be possible when converting metric to AWG wire sizes or half to full size wire gauges. However, this type of conversion is satisfactory if wire area per turn is maintained or increased, provided the slot fit is not made too tight. The coils should fit snugly in the slots; if too tight, a short could result.  

Concentric to lap changes can be performed, but with caution. It may not be enough to have the lap wind­ing match the magnetic flux densities of the original concentric winding. The lap span must be equivalent to the concentric. For example, some alternator windings use 2/3, 4/5, 6/7 or 5/6 pitch. The 2/3 pitch is used to eliminate third harmonic content in the output; the 4/5 eliminates the fifth harmonic; the 6/7 eliminates the seventh harmonic: and the 5/6 pitch reduces the 5th and 7th harmonics. Table 1 illustrates the combination of slots and pitch that can be used with a 4-pole alternator. Note that while the 2/3 pitch is common to all slot combinations in the table, the other useable fractional pitches occur much less frequently.

Another consideration is that par­alleled alternators often need to have the same pitch to provide matching output waveforms. Changing the pitch could cause sensitive load or protective circuits to reject the alterna­tor output and not allow the alterna­tor to operate. The same caution ap­plies to an original lap winding; that is, the span should not be changed.

How is the lap span made the equivalent of the concentric span? Here is an example: 4-pole wind­ing, 48 slots, spans 1-6-8-10-12. The teeth spanned are 5-7-9-11, with an aver­age value of 8 (5+7+9+11 = 32/4 =8). With 48 slots and 4 poles, full pitch is 12 (48/12). The winding pitch is 8/12, or 2/3. The chord factor of the 2/3 pitch winding is 0.866, lower than normal for a 4-pole winding. Our training with winding design inclines us to increase the span to a chord factor greater than 0.900. Doing so may also allow a reduction in turns and an increase in wire area. That would reduce winding losses and result in cooler operation, except that the span change may not allow the alternator to connect to the line or parallel with other alternators. As tempting as an increase in span may seem, we need to maintain the original equivalent span so as not to create difficulties.

Occasionally the wires in multiple are much more than may be on hand or that the winding machine can pull. If the number of circuits can be doubled, that will reduce the area of wires in multiple by 50%. However, check that the volts per coil do not exceed about 80 volts. Also, maintain the original type of connection, which is usually wye for the 3-phase line-to-line output. And check that the desired number of new circuits is possible. For example, if the original connec­tion of a 6-pole generator was 2 wye, a doubling to 4 wye would require an impossible connection. 

It is a good practice to use the EASA AC Motor Verification and Redesign Program when performing changes such as doubling the circuits to reduce wires in multiple. The pro­gram has built-in checks to prevent selection of circuits that can’t be used. It also confirms that the magnetic flux and current densities are maintained. One practical alternative solution is to wind two sets of coils, each using half the needed wires in hand, lay them in exactly the same slots, then parallel the two sets.

Voltage changes
Proceed cautiously with requests to change the voltage rating of an alterna­tor. If the machine is rated 600 volts or less, a change to a higher voltage such as 4160 volts will probably not be feasible even if the original winding had formed coils. The reason is that the higher volt­age rating requires much more coil to ground insulation, thus reducing the CMA to a value that may be too low even for standby duty. Voltage changes from a higher voltage such as 4160 volts to a lower voltage such as 480 volts may also be problematic. The theoreti­cal conversion often requires fractional turns such as a combination of 1 and 2 turn coils that are not practical to use. A coil with 2 turns would have only about 50% of the CMA of the 1 turn coil and would probably overheat. In some cases the conversion results in a fractional turn much less than 1, making it impossible to convert.  

Categories: Winding connections, Alternators/generators

Tags: Winding connections Taking data Alternators

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