Deck 17: Single-Phase Transformers Connected in Delta and Open Delta

Question 1

Check to see that the voltage output of each of the three transformers is 240 volts.

Answer 1

To check that the voltage output of each of the three transformers is 240 volts, you will need to follow these steps:

1. **Safety Precautions**: Before you begin, ensure that you have taken all necessary safety precautions. Wear appropriate personal protective equipment (PPE) such as gloves and safety glasses. Make sure that you are qualified to perform electrical measurements, and that you are familiar with the equipment and the risks involved.

2. **Prepare the Multimeter**: Set up a digital multimeter (DMM) to measure AC voltage. Ensure that the multimeter is rated to handle the voltage levels you are testing. Select the appropriate voltage range if your multimeter is not auto-ranging.

3. **Access Transformer Terminals**: Locate the output terminals of the first transformer. Ensure that the transformer is operational and that it is safe to access the terminals.

4. **Measure Voltage**: Connect the multimeter probes to the output terminals of the transformer. The red probe should connect to the phase (hot) terminal, and the black probe should connect to the neutral or ground terminal. Observe the reading on the multimeter display.

5. **Verify Voltage**: Check that the multimeter reading is 240 volts. A small variation is normal due to fluctuations in the supply and load conditions, but the voltage should be close to 240 volts.

6. **Repeat for Other Transformers**: Disconnect the multimeter probes from the first transformer and repeat the measurement process for the second and third transformers, ensuring that each one is outputting approximately 240 volts.

7. **Document Results**: Record the voltage readings for each transformer. If any of the transformers are not outputting the correct voltage, it may indicate a problem that needs to be addressed.

8. **Troubleshoot if Necessary**: If any transformer is not providing the correct voltage, further investigation is required. Check for issues such as loose connections, overloading, transformer defects, or incorrect tap settings.

9. **Ensure Safety**: Once the measurements are complete, ensure that all covers and safety barriers are replaced, and that the area is safe for normal operations.

10. **Report Findings**: Report your findings to the appropriate personnel. If any transformers are not functioning correctly, recommend further action to repair or replace the faulty equipment.

Remember, working with electrical equipment can be dangerous, and should only be performed by qualified individuals. If you are not trained or certified to work with electrical systems, it is important to consult with or hire a professional electrician to perform these checks for you.

Question 2

In any closed-delta transformer connection, the line voltage and the voltage across the transformer windings are the same.

Answer 2

True

Question 3

What is the relationship between line current and coil current in a delta-connected transformer bank?

Answer 3

The line current is greater than the coil current in a delta-connected transformer bank. The line current is equal to 1.73 x coil current. A study of a closed-delta transformer connection shows that each line lead is fed by two transformer coil currents that are out of phase and thus cannot be added directly by simple arithmetic.

Question 4

Although it appears that the total kVA of the open-delta bank should be two-thirds that of a closed-delta bank, the actual kVA rating of an open-delta bank is only ____________________% of the capacity of a closed-delta bank.

Question 5

What is a common use of the open-delta connection?

Question 6

The high-phase situation can be a serious hazard to human life as well as to any 120-volt equipment connected improperly between the high phase and neutral.

Question 7

A delta-delta connected transformer bank, with one transformer secondary center tapped, may be used to feed what two types of load?

Question 8

What procedure must be followed when connecting the low-voltage output or secondary windings in the closed-delta pattern?

Question 9

When three transformers are connected with their primary windings in delta and their secondary windings in delta, the total connection is called a(n) ____________________ connection.

Question 10

When three single-phase coils are connected so that each coil is connected to the beginning of another coil, a simple open-delta system is formed.

Question 11

What problems may result if the taps are not connected properly on a three-phase transformer or bank of transformers?

Question 12

When two or three single-phase transformers are used to step down or step up voltage on a three-phase system, the group is called a(n) ____________________.

Question 13

Article 110.15 of the NEC requires that the high phase be marked ____________________ in color.

Question 14

Never complete the last connection if there is a voltage difference greater than zero.

Question 15

A three-phase transformation of energy is not possible using only two transformers.

Question 16

With primary power off, connect the end of one secondary winding with the beginning of another secondary winding.

Question 17

If three single-phase transformers are to be used to step down 2400 volts, three phase to 240 volts, three phase, a(n) ____________________ connection is used.