3 Phase Autotransformer Connections

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Wiring Diagram Book A1 15 B1 B2 16 18 B3 A2. Class 8606 Autotransformer Type 60-61. 4-Wire 3-Phase Line Markings L1, L2 L1, L3: Phase 1. The autotransformer connection is not available with certain three-phase connections. Higher (and possibly more damaging) short-circuit currents can result from a lower series impedance. Short circuits can impress voltages significantly higher than operating voltages across the windings of an autotransformer. Example: 1 phase autotransformer / Input voltage 440 V / Output voltage 400 V / „Rated power“ needed for application 5 kVA Identical equation works with 3 phase transformers. Regarding input/ output “upper“ or „lower voltage“ is independant, voltage related only for 3 phase autotransformers value of phase voltage is important. Three-phase Transformers. A transformer bank composed of three single-phase units may be replaced by one 3-phase transformer – the magnetic core has three flux-carrying legs. two yokes, bottom and top, couple the flux of the three legs. the sum of the three fluxes equals zero – each leg contains the primary and secondary windings. The AC supply is connected to the fixed terminals (A & B) of the autotransformer, while the load is connected between terminals C & B. The number of turns in the primary winding N1 between point A & B exceeds the number of turns in secondary winding N2. Hence the turn ratio becomes less than 1, which is the condition for step down transformer.

Using ordinary single-phase transformers in unusual applications can have extraordinary benefits. Jmsn velvet album. We'll examine three more configurations here.

Case Eight: Zig-Zag Autotransformer (ZZA). A steel building (under construction) has a temporary 480V, 3-phase, 3-wire feeder for welders and general power. Unfortunately, the contractor encloses the facility without windows, before the busduct feeding the 277V lighting is complete. Non-stock fittings delay energizing the duct. Table 1, (in original article) shows the electrical parameters.

3 phase transformer wiring

To serve phase-to-neutral loads from an ungrounded 3-phase 3-wire source, use the ZZA connection for a neutral conductor for grounded-wye service. Here, we nippled together three 11/2-kVA transformers connected in this fashion. We added a 480V, 20A enclosed circuit breaker, and connected a 4-conductor No. 6 AWG, Type W flexible cord with a 60A, 480V pin-and-sleeve plug to the breaker line terminals. We wired a 5-conductor No. 10 AWG, Type SO cord between the breaker load side and the lighting-control enclosure. This assembly provided a temporary 480Y/277V, 3-phase, 4-wire branch circuit for fixed low-bay lighting. We bonded the neutral to nearby building steel as required by NEC Sec. 250-5(b). Sec. 450-5(a) recognizes autotransformer use for grounded-neutral derivations. When serving 277V loads, you must solidly ground the ZZA wye point.

Per Sec. 450-5(a)(1), you can't use independent overcurrent protection or disconnecting means between the ZZA set and its connected load. This configuration has phase-to-ground fault currents anywhere on the system divided among the phase connections of this transformer set. Yes, a ground fault of sufficient magnitude and duration downstream of the overcurrent device protecting the autotransformer will cause an outage. However, an equal or greater ground fault upstream will also cause local tripping, and possibly a second simultaneous branch-circuit or feeder overcurrent-device operation somewhere else on the 480V system. You sacrifice some selective coordination with ZZA grounding.

Case Nine: Autotransformer Connection (AT). A maintenance shop needs an industrial-grade 0.60 duty-cycle transformer / rectifier-type arc welder. The facility rents one having single-phase nameplate listed full-load currents of 104A at 200V, 90A at 230V, and 45A at 460V. A supply house delivers the welder strapped for highest-voltage operation: Fitted with a 4-conductor No. 6 AWG, Type SO flexible cord and a 60A, 3-pole, 4-wire pin-and-sleeve plug. For the listed duty cycle, NEC Sec. 630-11(a) permits a multiplier of 0.78 for conductor ampacity in a welder branch circuit. A surplus equipment dealer has coils of 2-conductor No. 8 AWG, Type ACL cable and a suitable transformer in good condition available at minimal cost. The 10kVA four-winding transformer (configured for step up service:parallel 240V windings and series 120V windings) allows the device to handle 20kVA of one phase of a 480V load. This short-term application (using surplus materials) may not be cost-effective in every situation.


The welder is 90 ft from the shop's service switchboard, with the transformer placed at the switchboard. At the welder location, we installed a 60A 600V fused disconnect switch and mating 3-pole, 4-wire pin-and-sleeve receptacle. Table 2 (in original article) shows the electrical parameters.

NEC Sec. 450-4(b) recognizes field-connected autotransformers. Sec. 210-9 mandates the system-neutral connection shown for case nine. Sec. 450-4(a) describes the basic 125% overcurrent-rating requirement for autotransformer applications. We provided a 100A molded-case breaker in the service switchboard for the welder circuit.

Step-up autotransformers (single- and open-delta 3-phase) can serve larger outdoor areas where circuit runs associated with exterior lighting are long, but have only lower-voltage service.

The usual grounding for separately-derived systems is irrelevant to autotransformers, because you have a conductor common to all transformer windings (Sec. 210-9 and 250-23).

Variac Transformer Wiring Diagram

Case Ten: Dual Delta/Wye Configuration. At an explosives test facility, an electrical contractor nearly completes installation of a metal clad 12kV tie-breaker switchgear and associated pole line (Table 3, in original article).

The site owns a pair of 3-phase transformers two miles apart; each rated 20 MVA @ 115kV delta-12kV wye. In 1981, the in-house utility operations group energizes one of the transformers (T525) with C-B-A high-side phasing (per the utility). During a 1985 upgrade, they heat up the other (T301) with primary phasing per a government-agency standard, which is A-B-C. Engineering considers this inconsequential during design and construction. (Surely the resident crew could easily fix any phasing problems at the commissioning stage:flopping or rolling 15kV, 750kcmil, MV-90 cables at their terminations would be a simple task!)

3 Phase Autotransformer Connections

The Operations Dept. proposes a working model of the unfinished project to help resolve the problem. A protective-relay electrician assembles six identical 75VA, 480V - 120V machine-tool control-power transformers on a metal panel. Their wiring (identical to that of their larger delta-wye counterparts) gets temporary power from a motor-workbench 480V circuit (Fig. 2, in original article). The electrician uses a magnitude-indicating phase-angle meter on this mock-up while he records and plots voltages and angles. Tables 4a, 4b, & 4c (in original article), show the measurements with out-of-phase primaries, and Table 5 (in original article) shows those with in-phase primaries.

After obtaining a clearance from the serving utility's switchyard, testing for verification of de-energization, and applying personal protective grounds, the line crew reverses phases A and C slack spans on one of the power-transformer primaries. But, before we can place the two transformers in parallel, we need to verify that the respective phase-to-phase 12kV bus voltages are equal at the same instant. (We're dealing with a pair of 3-phase alternating voltages.)

Two linemen securely block open the primary stationary-disconnect shutters in the empty tie-breaker cubicle at the T525 gear. With a racked-out vacuum circuit breaker, we have two sets of 3-phase stabs exposed at their normal medium-voltage potential.

In industrial settings, having an open breaker energized on both sides is unique to a tie circuit.

Six initial phase-to-ground measurements (Table 6, in original article) verify the neutral point is at ground potential for each 12kV bus of the power-transformer pair. It's important to test for the presence of normal 3-phase voltage at the bus and line sides of the racked-out tie breaker. Lastly, we take the crucial three bus-to-line readings (Table 3, in original article). Prior to paralleling, readings must fit the maximum 6 DEGR-delta angle/10%-delta voltage rule of thumb. In this case, bus-to-line voltage for each pair of phase conductors can be no more than 1.2 kV.

Two Phase Transformer

3 Phase Autotransformer Connections

The model shows we can't perform in-phase secondary-side paralleling of the two transformers with the existing 115kV bus configuration. The bolted 12kV fault level at tie-breaker closing will exceed 350MVA. ANSI-standard delta-wye transformers served in A-B-C phase sequence have an inherent 30 DEGR lagging phase shift in voltage from primary to secondary. If you find reversed high-side phase sequence, then the secondary voltage leads the primary by 30 DEGR. These two 30 DEGR angles add to produce a most-undesirable 60 DEGR mismatch.

Getting a clearance from the utility to flop incoming 4/0-gauge high-voltage spans between the utility's pole-mounted air switch and our oil breaker (on the T525 high side) is easier than medium-voltage-cable rework.


Article series summary. The 10 configurations in this series are most useful for loads under 15 kVA. With the hookups discussed in this article, you'll find single-phase transformers appropriate where conventional 3-phase single-enclosure transformers may not be suitable for a particular situation or time constraint. Discrete single-phase units in the uncommon arrangements introduced here can be desirable where you need to power a small load, or where lower-capacity, symmetrically connected banks (typically delta-wye, delta-delta, or wye autotransformer) may be too expensive or have unacceptable lead times.

3 Phase Autotransformer Connections Diagram

Remember, you must carefully consider these transformer-connection capabilities and limitations, the requisite overcurrent protection, load ratings, and grounding specifics. These connections provide economical alternatives to the more-conventional.

3 Phase Transformer Wiring Connections

Your ability to apply technical competence to electrical systems is a valuable asset to any commercial or industrial operation. Circuits detailed in this article can give your facility the money- and timesaving advantages of using limited quantities of ordinary, easily obtained single-phase transformers.