Lab Information
EXPERIMENT NO. 2
EXCITING CURRENT (OPEN CIRCUIT TEST)
PURPOSE:
To measure the Exciting current and determine the core losses in a transformer.
DISCUSSION:
The current input to the primary winding, without a load connected to the secondary winding is
usually from 1 to 5 percent of the full load current rating. This no load primary current is called
the exciting current and consist of the following two components:
1. The magnetizing current that supplies the alternating flux in the core which produces the
primary and secondary induced voltages. This current component lags the applied voltage by
900.
2. Due to eddy currents and hysteresis, power is lost in the core. The changing flux induces a
voltage and current will flow in the iron core causing an I2R loss. This eddy current "loss" is
minimized by laminating the core and insulating each lamination with a varnish or oxide coating.
The inability of the magnetic domains of the core material to instantly follow the changing flux
(due to inter-domain friction) incurs a power loss as heat. This hystersis loop loss is minimized
by the use of special steel and various core configurations. The core current is in phase with the
voltage supplied to the primary.
The vectorial sum of the in-phase core loss and lagging magnetizing currents produces the
exciting current of a transformer.
APPARATUS REQUIRED:
1. One Hampden transformer (Model T-100-3A)
2. One Hampden AC Wattmeter
3. One Hampden variable AC power supply
4. One Hampden AC voltmeter
5. One portable AC ammeter (rated at least 50 milliamps)
PROCEDURE:
1. Make the connections shown in Figure 3. Apply 240 volts to the primary side, as shown.
Measure the input current, voltage, and power and record the data.
2. Make the connections shown in Figure 4. Apply 120 volts to the secondary side as shown.
Measure the input current, voltage, and power and record the data.
3. Calculate the inphase and lagging components of the exciting current for each step. The
transformer is rated at 240/120 volts, 120VA for the connections made. Calculate the exciting
currents' percentage of the full load current rating. Calculate the magnetizing branch impedances.
REPORT:
Prepare a written report to include:
1. Calculations.
2. A discussion comparing the relative values obtained in steps 1 and 2.
3. An error analysis.
4. Vector diagrams of the exciting current and its components for steps 1 and 2.
OPEN CIRCUIT TESTS