Alternator winding pitch and power system design

Overview

The paper defines pitch factor as Kp = cos(N x 180 x (1 - pitch)/2), with N the harmonic order. Table 1 gives Kp for 2/3 pitch as 0.87 at the fundamental, 0.00 at the 3rd and 0.87 at the 5th and 7th; for 5/6 pitch the values are 0.97, 0.71, 0.26 and 0.26. A 2/3 pitch winding therefore generates no triplen harmonics (3rd, 9th, 15th, 21st).

The conclusion is a selection rule: 2/3 pitch for all four-wire distribution systems, and 5/6 pitch where no neutral is distributed, typical of MV and HV plants. The trade-off is capacity: a 5/6 pitch winding yields about 97% of full-pitch fundamental voltage against about 87% for 2/3, so it extracts more kVA from the same frame.

The final part covers paralleling alternators of different pitch. It is allowed provided circulating neutral current is mitigated, with reactors or resistors in the neutral leg or neutral switching; Figures 8-11 show these arrangements.

Key points

1. Pitch factor: Kp = cos(N x 180 x (1 - pitch)/2).
2. 2/3 pitch Kp: fundamental 0.87, 3rd 0.00, 5th and 7th 0.87.
3. 5/6 pitch Kp: 0.97, 0.71, 0.26, 0.26.
4. 2/3 pitch produces no triplen harmonics (3rd, 9th, 15th, 21st).
5. Recommendation: 2/3 pitch on all 4-wire systems; 5/6 acceptable without a distributed neutral (most MV/HV).
6. 5/6 pitch gives about 97% fundamental voltage versus about 87% for 2/3, hence more kVA per frame.
7. Different-pitch alternators can be paralleled if circulating neutral current is handled by neutral reactors or neutral switching.

Using it in design

• Add a windingPitch field to the alternator catalog and require 2/3 pitch (or flag) for LV 4-wire generator systems.
• When sets of different pitch with bonded neutrals are paralleled, raise a warning and suggest a neutral reactor or 3-pole isolation.
• Accept 5/6 pitch for MV plants with delta transformer primaries.

Who should read it

Electrical engineers specifying generators and alternators, especially for four-wire LV standby systems.

Related standards

• IEC 60034-1
• NEMA MG 1

Publication details

Authors

Rich Scroggins; David Matuseski

Publisher

Cummins

Year

2014

Document number

Power Topic #5981

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation