Lab Information
EXPERIMENT NO. 11
OPEN DELTA CONNECTION
PURPOSE:
To Show the operating characteristics of the open delta connection.
DISCUSSION:
One advantage of the delta-delta connection is that it can be used to deliver three-phase power,
but at a reduced level, should any one of the transformers in the three-phase bank be removed.
The resulting configuration is called open delta. It can be used for an initial transformer
installation in an area where growth is anticipated. The third phase of the bank is added when the
area's demand increases beyond the capacity of the open delta configuration.
The maximum open delta power is 57.7% of the maximum closed delta power. In addition the
maximum open delta power is 8.6% of the rating of the two transformers. That is, if each
transformer is rated at Vs Is, then the two transformers of the open delta will have a rated
capacity of 2 Vs Is. But the maximum open delta power is 86.6% of this or 3 Vs Is, where Vs and
Is are both phase values. The above power expression applies to resistive loads. For general
loads, one transformer of the open delta will provide Vsis cos (30 + 0) and the other provides Vs
Is cos (30 - 0), where 0 is the phase angle of the load.
APPARATUS REQUIRED:
1. One Hampden T-100-3 Transformer
2. One Hampden RLC-100 resistance-reactance load
3. Two Hampden AC ammeters
4. One Hampden AC voltmeter
5. One portable voltmeter
6. Two Hampden wattmeters
7. One 208 volt three-phase power source.
PROCEDURE:
1. Make the connections shown in Figure 24 apply a balanced load of 333 (each leg) to the
open delta and record the meter readings.
2. With the power off, place one of the voltmeters across either secondary winding. Turn the
power on and record this voltage.
3. Assume that the secondary current measured in Step #1 above, is the rated secondary current
of the transformer (the rated current is actually 1A but do not use this value). Calculate the
output power for Figure 25 if rated secondary current flows, provided that the input voltage is the
same as in Step #1 above.
4. Verify that the open delta maximum power is 57.7% of the closed delta maximum power
using the data from Steps 1 and 3, above.
REPORT:
Prepare a written report in include the following:
1. Data, calculations, and circuit schematics.
2. An answer to this question:
What will be the output power for Figure 25 if the input voltage and the load are the same as in
Step #1 of the procedure section.