Single-Phase Transformers (part 4)

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Testing the Transformer

Several tests can be made to determine the condition of the transformer. A simple test for grounds, shorts, or opens can be made with an ohmmeter. Ohmmeter A is connected to one lead of the primary and one lead of the secondary. This test checks for shorted windings between the primary and secondary. The ohmmeter should indicate infinity. If there is more than one primary or secondary winding, all isolated windings should be tested for shorts. Ohmmeter B illustrates testing the windings for grounds. One lead of the ohmmeter is connected to the case of the transformer, and the other is connected to the winding. All windings should be tested for grounds, and the ohmmeter should indicate infinity for each winding. Ohmmeter C illustrates testing the windings for continuity. The wire resistance of the winding should be indicated by the ohmmeter.

If the transformer appears to be in good condition after the ohmmeter test, it should then be tested for shorts and grounds with a megohmmeter. A MEGGER will reveal problems of insulation breakdown that an ohmmeter will not. Large oil-filled transformers should have the condition of the dielectric oil tested at periodic intervals. This test involves taking a sample of the oil and performing certain tests for dielectric strength and contamination.

Transformer Nameplates

Most transformers contain a nameplate that lists information concerning the transformer. NEC 450.11 requires the following information:

1. Name of manufacturer

2. Rated kilovolt-ampere

3. Frequency

4. Primary and secondary voltage

5. Impedance of transformers rated 25 kilovolt-ampere and larger

6. Required clearances of transformers with ventilating openings

7. Amount and kind of insulating liquid where used

8. The temperature class for the insulating system of dry-type transformers

Notice that the transformer is rated in kilovolt-amperes, not kilowatts, because the true power output of the transformer is determined by the power factor of the load. Other information that may be listed is temperature rise in 8C, model number, and whether the transformer is single phase or three phase.

Many nameplates also contain a schematic diagram of the windings to aid in connection.

Determining Maximum Current

The nameplate does not list the current rating of the windings. Because power input must equal power output, the current rating for a winding can be determined by dividing the kilovolt-ampere rating by the winding voltage.

For example, assume a transformer has a kilovolt-ampere rating of 0.5 kilovolt- ampere, a primary voltage of 480 volts, and a secondary voltage of 120 volts.

To determine the maximum current that can be supplied by the secondary, divide the kVA rating by the secondary voltage:

The primary current can be calculated in the same way:

Transformers with multiple secondary windings will generally have the current rating listed with the voltage rating.


Assume that the transformer shown is a 2400/480-V, 15-kVA transformer. To determine the impedance of the transformer, first calculate the full-load current rating of the secondary winding:

Next, increase the source voltage connected to the high-voltage winding until a current of 31.25 A flows in the low-voltage winding. For the purpose of this example, assume that the voltage value is 138 V. Finally, determine the percent age of applied voltage as compared to the rated voltage:

The impedance of this transformer is 5.75%.

Transformer impedance is a major factor in determining the amount of voltage drop a transformer will exhibit between no load and full load and in determining the amount of current flow in a short-circuit condition. Short-circuit current can be calculated using the formula ... because one of the formulas for determining current in a single-phase circuit is ...and one of the formulas for determining current in a three-phase circuit is ...

The preceding formulas for determining short-circuit current can be modified to show that the short-circuit current can be calculated by dividing the rated secondary current by the Ohm_sZ:


A single-phase transformer is rated at 50 kVA and has a secondary voltage of 240V. The nameplate reveals that the transformer has an internal impedance (Ohm Z) of 2.5 Ohm. What is the short-circuit current for this transformer?

It’s sometimes necessary to calculate the amount of short-circuit current when determining the correct fuse rating for a circuit. The fuse must have a high enough "interrupt" rating to clear the fault in the event of a short circuit.


Transformer Impedance

Transformer impedance is determined by the physical construction of the transformer. Factors such as the amount and type of core material, wire size used to construct the windings, the number of turns, and the degree of magnetic coupling between the windings greatly affect the transformer's impedance. Impedance is expressed as a percent (%Z or %IZ) and is measured by connecting a short circuit across the low-voltage winding of the transformer and then connecting a variable voltage source to the high-voltage winding. The variable voltage is then increased until rated current flows in the low- voltage winding. The transformer impedance is determined by calculating the percent age of variable voltage as compared to the rated voltage of the high-voltage winding.

+++++58 Determining transformer impedance.

Variable-voltage source; Voltmeter; High-voltage winding; Low-voltage winding; Ammeter; Short circuit

Constant-Current Transformers:

A very special type of isolation transformer is the constant-current transformer, often referred to as a current regulator. Constant-current transformers are designed to deliver a constant output current, generally 6.6 amperes, under varying load conditions. They are most often employed to provide power to series-connected street lamps. Street lamps are often connected in series instead of parallel because of the savings in wire. Series-connected lamps require a single conductor to be connected from lamp to lamp instead of two conductors.

When lamps are connected in series, some device must be used to continue the circuit in the event that one or more lamps should fail. Some lights use a reactor coil connected in parallel with the lamp. Another method uses a film cut-out device, consisting of two pieces of metal separated by an insulator designed to puncture at a predetermined volt age. As long as the lamp is in operation, the voltage drop across the cut-out device is not sufficient to cause the film to puncture. If the lamp should burn out, producing an open circuit, the entire circuit voltage will appear across the cut-out device and cause it to short circuit.

+++++59 Street lamps are often connected in series. Constant current transformer

+++++60 An inductor maintains the circuit if the lamp should fail.

+++++61 A film cut-out device shorts and maintains the circuit if the lamp should fail. Film cut-out device

+++++62 Magnetic flux of the two windings repel each other.

Constant-current transformers contain primary and secondary winding that are movable with respect to each other. Either winding can be made movable.

Both windings are wound on the same core material.

The constant-current regulator operates by producing a magnetic field in the movable winding that is the same polarity as the magnetic field produced in the stationary winding. Because the two magnetic fields have the same polarity, they oppose each other. If the load current increases, the magnetic field strength of the two windings increases, causing the two coils to move further apart. Moving coils farther apart increases the amount of magnetic flux leakage, resulting in a reduction in output voltage. If the load current decreases, the magnetic field strength of the two windings decreases, causing the movable coil to move closer to the stationary coil, producing an increase in output voltage.

Many constant-current transformers employ a counterweight and dashpot mechanism to help reduce sudden changes in the spacing between the two coils. The counterweight helps balance the weight of the movable coil, and the dashpot device helps reduce the "hunting" action between the two coils.

+++++63 A counterweight and dashpot device help reduce sudden changes in the output current. 4800 Vac 60 Hz Counter Weight Dashpot Device Laminated Core Movable Coil Stationary Coil Series Street Lamps

+++++64 Transformer secondary windings connected in series. Primary windings are connected in parallel 120 Volts -- Connection becomes transformer center tap 240 Volt output

Series Connection of Transformer Secondaries:

As a general rule, connecting the secondary windings of transformers in series does not present a problem. Because the current is the same in a series circuit, the problem of high circulating current does not exist in a series connection.

The secondary windings can be connected in series to produce a higher output voltage. Assume that two transformers have a secondary voltage of 120 volts.

The primary windings can be connected in parallel without a problem.

When making this connection, the polarity of the two secondary windings must be connected additive of boost. The series connection of the two secondary windings will produce an output of 240 volts center tapped. If the polarity is not correct, the output voltage will be zero (0).

Parallel Transformer Connections:

It’s sometimes necessary to connect the secondary windings of transformers in parallel to increase the current capacity, but generally it’s not done unless there is no other alternative. Connecting transformer primary windings in parallel is not a problem, but connecting the secondary windings in parallel can cause high circulating currents or extremely unbalance currents that can lead to transformer failure. Transformers that are to be connected in parallel must have:

  • the same kVA rating.
  • the same turns ratio,
  • the same impedance, and
  • the same secondary voltage;
  • and the secondary windings must have the same polarity.

If any of these factors are different, it can cause failure of one or both transformers. Assume for example, that two transformers have the same kVA rating, same secondary voltage, and same turns ratio, but the impedance is not the same. When load is added to the parallel connection, the transformer with the higher internal impedance will exhibit a greater voltage drop, causing the other transformer to supply more of the load current. This unbalance can lead to the failure of the transformer that is supplying the majority of the load current.

In another example, assume that two transformers have the same kVA rating, same secondary voltage, and same impedance, but the turns ratio is different. When power is applied to the connection, the difference in turns ratio can cause a very high no-load circulating current. This circulating current can cause burn-out of both transformers. When load is added, the secondary cur rent of each transformer will be a combination of both the load current and circulating current.

Connecting Parallel Transformers:

Care should be exercised when paralleling transformers to ensure that the polarity is correct. If the polarity is incorrect, it produces a short circuit. In this example, assume that one transformer is already connected to the line. Also assumed that the primary voltage is 4160 volts and the secondary voltage is 120 volts. Connect one of the secondary leads to the line and then energize the primary winding.

Connect a voltmeter between the secondary lead that has not been connected and the line of the intended connection. If the polarity of the two transformers is correct, the voltmeter should indicate zero (0) volts.

If the polarity is not correct, the voltmeter will indicate double the amount of secondary voltage. In this example, the voltmeter would indicate 240 volts if the polarity were not correct.

+++++65 Transformers are sometimes connected in parallel to increase the current capacity of the circuit.

+++++66 Checking the polarity of a parallel transformer connection.

+++++67 The secondary winding should be disconnected before disconnecting the primary winding.

Precautions When Servicing Parallel Transformers:

If it should become necessary to remove one of the transformers for service, the secondary should be disconnected first. This can generally be accomplished by removing the secondary fuse.

In this example, the primary is connected to 4160 volts. If the secondary winding is not disconnected first, the transformer becomes a step-up transformer. The primary winding will still have a voltage of 4160 even if it’s disconnected from the power line. Anytime parallel transformers are employed, a sign reading CAUTION FEEDBACK VOLTAGE should be located at each primary fuse.

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