Jim Bryan
EASA Technical Support Specialist (retired)
The connection of a three-phase motor is one of the many variables a motor designer can use to optimize the performance and life of the machine. The designer determines whether to use a wye or delta connection and how many parallel circuits to maximize current density (circular mils per amp or cm/A) while optimizing flux densities and manufacturability.
In three-phase motors, the square root of three is an important number. Because of the phase relationships of the three windings shown in Figure 1, the voltage and current are intertwined with this factor. In the delta winding, the phase voltage is applied to each phase winding but the current has two possible paths. Due to the phase relationship of the winding, the current is not split in two but by the square root of three (1.73). The opposite is true for the wye connection; the phase voltage impressed on each phase is the line voltage divided by 1.73, and the phase current equals the current in each coil. This is the reason that wye wound motors have fewer turns of heavier wire than do delta-connected motors.
Choosing wye or delta
The reason for using wye or delta connections is fairly arbitrary. Either will be able to provide similar performance as the various parameters of the design are adjusted. In a large motor with high-load current, it may be an advantage to use the greater number of turns with lower total circular mils in the delta connection. In smaller motors, fewer turns of heavier wire may be an advantage. For the same number of circuits, the turns per coil are inversely proportional to the horsepower. So it follows that machines with higher horsepower ratings are more likely to use a delta connection.
The number of parallel circuits (see Figure 2) is also important. The parallel path will never be more than the number of poles except for some 2-pole motors with interspersed windings (see EASA Tech Note 37 for more information) or special, but rare, interleaved designs (see “Interleaved Windings Provide Useful Alternative” in January 2003 Currents). The number of parallel paths may be further restricted by the number of slots in the stator core. For instance, a 6 pole motor can have 1, 2, 3 or 6 circuits (6 is evenly divisible by each). If the motor has 48 slots, it will have 6 groups of 2 coils and 12 groups of 3 coils. No balanced arrangement of this coil grouping is possible with 3 or 6 parallel circuits, so we are restricted to using 1 or 2 circuits.
Number of parallel circuits
The voltage applied can be influenced by the number of parallel circuits. For instance, a motor designed with one circuit for 460v operation can be applied to 230v if the connection is changed to 2 parallel circuits. In the same machine, if the number of wires in hand becomes a problem (not enough buckets of wire to meet the requirement), the number of circuits can be doubled. This will double the number of turns and reduce the total circular mils by one-half to maintain the same performance. The down side is more coil ends in the connections make that process a little more difficult. Also, it will increase the volts per coil; if this exceeds 80v, additional insulation should be placed between the coils to reduce potential damage.
It is critical to properly determine the number of parallel circuits and the connection when taking data in preparation for a rewind or redesign. It does not matter if the motor is concentric or lap wound; the connection details are the same. Figure 3 on Page 4 is an excerpt from EASA’s How to Wind Three-Phase Stators interactive training program showing how to determine the connection for three-, six-, nine- or twelve-lead motors. This represents a very large portion of the motor connections but is not all-inclusive. Some motor connections are special, such as tri-voltage or multi-speed ratings. These special connections are a topic for another discussion.
Importance of lead numbering
The lead numbering is important in determining the connection for motors with more than three leads. If the motor has six leads and the leads are labeled 1,1,2,2,3,3, the line leads are in parallel and this is the same as a three lead motor. If they are labeled 1,2,3,4,5,6, it is a wye start, delta run motor. For design considerations, the delta connection should be used, as that is the way the motor will operate under load. All six leads will go to the motor starter and three separate contactors will transition the connection from wye for reduced current starting to delta for full torque running. This motor may also be a dual voltage motor where the high voltage is 1.73 times the low voltage (e.g. 2300v / 4160v). If they are labeled 1,2,3,7,8,9, it is a part winding start, single voltage. The motor is designed to run on the higher number of parallel circuits in this case.
If the motor has nine or twelve leads, the lead numbering may also indicate parallel line leads such as three or four number 1s, 2s and 3s. A nine-lead motor numbered one through nine is a dual voltage motor where the high voltage is twice the low voltage. The leads are configured in the outlet box by the end user for the appropriate number of parallel circuits for the voltage being utilized. With twelve leads numbered one through twelve, the motor is capable of wye start/delta run application on two voltages, the high voltage being two times the low voltage. In addition to choosing the number of parallel paths for the applied voltage, the motor will be able to transition as the six-lead wye start/delta run motor in the previous paragraph. If the motor is rated single voltage and has 12 leads, it can be connected part winding, wye start/delta run, or across the line.
A two-circuit wye connection will have performance close to that of a one-circuit delta connection so that both can fall in the acceptable range of performance criteria. The difference in the optimum design here is proportional to the difference between two and the square root of three. If a design with a one-delta connection has a moderate airgap flux density of 50,000 lines of flux per square inch, a two-wye misconnection would result in 57,735 lines of flux or a 33% increase in horsepower (horsepower varies as the square of flux) with a corresponding increase in inrush current, torque and operating temperature. Going the other way, if the motor should be two-wye and is connected one delta, it would not have sufficient horsepower to drive the load.
Conclusion
In conclusion, when taking data for a rewind or to redesign a three-phase motor, it is important to correctly determine the connection and number of parallel circuits used in the original design. An error could result in the new winding being inadequate for the application and severely reduce its performance and life.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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