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Sometimes, budget constraints keep you from buying a new transformer for a specific application. You already have a boneyard of transformers; can you make do with what you have? In many cases, the answer is yes.
Designers and installers sometimes configure transformer banks from single-phase devices and apply non-standard connections. You can use these unconventional methods to adapt distribution-system voltage levels to equipment of dissimilar voltage ratings; typically for small loads near transformer sets.
We'll examine field situations where you could solve a problem with off-the-shelf dry-type or machine-tool transformers. These transformers typically have dual 240V (rated 2402480V) primaries, and single or dual 120V (rated 120/240V) secondaries. You can use fewer or smaller-sized transformers than those you might see with more-familiar symmetrical hook-ups. In one case, a single-phase unit matches a 3-phase, 4-wire delta system to wye-voltage utilization equipment. In another case, a 3-phase source serves as a convenient cure for single-phase overvoltage difficulties in a machine-tool control console. The interconnections in the 10 cases for this article (four of which appear here in Part 1) take advantage of the inherent 180 degrees phase shift, ratio change, and electrical isolation available with single-phase devices.
Case One: open-wye/open-delta connection (OY-OD). Many light-commercial neighborhoods have 208Y/120V, 4-wire distribution. A small building "saved" money by foregoing the expense of a 4-wire electrical service. Instead, occupants had an uncommon 3-wire lateral network service, consisting of two ungrounded conductors and one grounded conductor served from a 4-wire wye system.
The tenant shop owner needed to operate his prime electrical appliance: an ancient, but sturdy, cord-connected 3-hp 230V dough mixer. Unfortunately, he had only 208Y/120V 3-wire, single-phase network service available within a reasonable distance. This simple transformer set (OY-OD) was a low-cost solution. Using this connection was cheaper than using a single- to 3-phase upgrade of the electrical service entrance, 125-ft feeder, and subpanel.
To apply this configuration, nipple two 3-kVA transformers to a suitable panelboard, and run a three-quarter-inch raceway to a NEMA L15-20 receptacle. Wire as shown in Fig. 1, on page 66 (in the original article). The NEC Sec. 250-5(b) allows you to leave the separately derived 3-phase, 3-wire circuit ungrounded. A 3-pole motor-overload relay in a manual starter will provide secondary-side overcurrent protection for the transformer bank. A 20A 2-pole molded-case breaker furnishes primary short-circuit and ground-fault protection.
While running the motor, check current in the grounded "neutral" lead of the 208Y/120V single-phase, 3-wire circuit that supplies the transformer pair. To do so, use a clothespin-type current transformer and a digital multimeter. Your reading on this conductor will be similar to the two primary-phase currents with balanced 3-phase load on the secondary. (This particular neutral current has nothing to do with harmonics. You'll find a description of it in the NEC Art. 310 Notes to Ampacity Tables, 10(b).) You cannot apply this configuration on a 120/240V single-phase, 3-wire system. The 120 degrees phase difference between the two phase-to-neutral voltages is absent in this type of circuit.
Motor phase currents showed reasonable balance under load. No currents exceeded nameplate rating during routine operation. Stator overheating from inordinate voltage imbalance was no problem for this pre-letter-frame induction motor.
System voltage doesn't matter for this and most other configurations we're looking at. A utility used an OY-OD connection on a remote 115Y/66-kV transmission circuit to provide local corner-grounded 7.2-kV 3-phase distribution service. The rural electric co-op constructed a wood-pole H-frame platform to berth the two single-phase transformers. In each case, transformer ratings must match the application. Case 2 also demonstrates this.
Case Two: open-delta/lambda connection (OD-LM). An explosives-research site applied this unusual connection while under time constraints. They needed limited-power 208Y/120V 3-phase, 4-wire service from the usual 3-phase, 3-wire source. They had, at their disposal, only two single-phase transformers.
The Greek letter lambda (L) best represents the shape of this secondary interconnection. It is an odd but handy adaptation of an already peculiar configuration you'll find in ANSI C57.105, IEEE Guide for Application of Transformer Connections in Three-Phase Distribution Systems.
Originally, the crew installed a 1300-ft run of direct-buried 8AWG/3-conductor jacketed MC cable to operate a half-hp 200V 3-phase gearmotor and a 115V intercom at a newly relocated electric gate. Unfortunately, the intercom got its power from one insulated phase conductor and the bare equipment-grounding conductor in the MC cable. Two weeks later, carpenters built a small security kiosk near the gate. This structure had external and inside lighting, a 2-way radio and a one-and-a-half-hp 115V window air conditioner.
The owner powered the new load from this fairly long circuit intended only for the gate motor, notwithstanding the missing neutral conductor. While still connected phase-to-ground, the lights and radio worked, but an attempt to start the air conditioner severely depressed feeder voltage at the load end. The cooling unit's motor would not budge from a locked-rotor condition (The starting current in a 115V phase-to-neutral-connected one-and-a-half-hp load is about nine times greater than in a 3-phase 200V half-hp load).
To fix this, the crew mounted a transformer bank close to the kiosk. This bank consisted of two 5-kVA dry-type units (left after a change order). A 30A 3-pole molded-case circuit breaker protected the secondary windings. At its source (quarter mile away), an electrician moved the MC-cable end from the serving 208V panelboard to a nearby 480V circuit. The voltage-drop problem ceased when they energized the small bank. You can see the hookup in Fig. 2, on page 68 (in the original article).
This application could use almost any pair of transformers having dual 120V secondaries. For example, nothing prevents you from using devices with 240- or 600V primaries. However, make sure the primary-winding voltage matches the source voltage.
The lambda-secondary connection was an easy solution for a 208V 3-phase station service needing transformer cooling-fan motors in a substation. Space permitted swapping out the existing 25kVA single-phase unit with two 15-kVA (cast-coil 95-kVBIL) units. These had 12,470V primaries and 120/240V two-winding secondaries. For initial 1-phase station service, the manufacturer furnished metal-clad switchgear with a "stock" 3-pole, 14.4kV fuse drawer and 225A-frame, 3-pole interchangeable-trip breaker. This configuration simplified retrofit tasks. The equipment included a mechanical interlock between these two components. Accommodating the usual single-unit core-type 30kVA delta-wye bank or three single-phase units would have required significant structural and sheet-metal modifications, greatly escalating job costs and downtime.
Case Three: open-wye/lambda connection (OY-LM). You can assemble a combined variation of the previous two configurations (Fig. 3, on page 69 in the original article). The distribution substation in Case Two is one example. You may be able to upgrade a phase-to-phase-fed single-phase station-power transformer with its two-pole fuseholder and still have 208V 3-phase, 4-wire station service. For a 12,000V system, the two replacement transformers need 6900V primaries.
Short-circuit and ground-fault protection require only two primary fuses, if you have a solidly grounded wye-system neutral connection. You cannot use ungrounded or resistance-grounded neutrals to serve open-wye-primary station-power transformer sets. Doing so risks having a bus or line phase-to-ground fault induce a phase-to-neutral overvoltage on two of three phases of the local medium-voltage bus. Besides introducing poor voltage regulation, this scenario may drive the transformer primaries into saturation. In that condition, the bank will be prone to operating fuses frequently.
As in the Case Two installation (with a lambda-connected secondary), each of the two transformers must have dual (split) secondary windings.
Case Four: lambda quasi-autotransformer connection (LQA). Case One presented a solution for serving 240V or 480V 3-phase load where only 208Y/120V 3-wire network service is available. Here's another scenario: a 200V 3-phase motor in an auto-repair shop with network service needs power. The next largest loads are a puny buzzbox welder and half-hp bench grinder.
The owner says, "That's exactly the motor you told me I needed, right?" Well, you have to get that extra conductor from the utility transformer and through the service lateral. Then you have to get it past an extra set of jaws in the watthour-meter socket and a third pole on the service-entrance disconnect/overcurrent protection.
Don't forget the 150A feeder, extra busbar in the panelboard, and one more branch-circuit pole. Doesn't this seem a bit much for a 5-hp air compressor? You could get a (typically higher-priced) replacement single-phase motor, but then you'd have to eat the one the owner provided. After that, you get a bill from the industrial-supply installer for an extra $175 for moving the machine away from a side wall to fit the new motor: with its almost cubic-foot-sized start-capacitor box.
For this particular application, you need a pair of 1202240-to-120/240V dry-type transformers. Fig 4, on page 70 (in the original article), shows a fairly straightforward hookup. It's similar to the secondary-winding configurations of Fig. 2 and Fig. 3.