NJIT
Electrical and Computer Engineering Department
Undergraduate Laboratory

ECE 494 - Electrical Engineering Laboratory III

Lab 2: Separation of Eddy Current and Hysteresis Losses

Objectives

To separate the eddy-current and hysteresis losses at various frequencies and flux densities using the Epstein Core Loss Testing equipment.

Equipment

References

Background

Designers of electrical machines must know the magnetic characteristics of the material they use in order to predict the performance of their finished products. In this experiment core losses resulting from eddy currents and hysteresis in steel sheets will be measured. The Epstein test frame is a special one-to-one transformer having provisions for inserting the sample where it serves as a core. The testing procedure is specified by the American Society for Testing Materials (ASTM).

Description of the Apparatus

The windings are on four sections hollow square tube, each 1.57 × 1.57 inches square and about 17.25 inches long. Each section is wound with 150 turns of No. 18 wire. These turns are wound parallel to each other and in the same plane so that the primary and secondary turns lie alternately adjacent to each other in order to improve their magnetic coupling. The core material used is Armco 6M (USS Transformer 66).

The four sections are arranged to form a square. Primary turns on all sections are connected in series. The secondary turns are also connected in series.

The sample to be tested consists of 10 kg (22 lbs) of strips 3 cm wide and 59 cm long. One half of the sample is cut with the grain and the other half is cut across the grain. Four equal bundles are made of the specimen and each bundle is tightly taped and placed in a section of the winding. The four ends are then butted together in the form of a square with a piece of 0.004 inch thick paper in each joint, and all joints made as tight as possible.

Because of the tight coupling between the primary and secondary coils, the voltage induced in them by the AC magnetic flux is the same. Since the primary winding carries the current which establishes the magnetic flux in the core, the voltage applied to the primary winding includes the ohmic voltage drop due to the resistance of that winding. The secondary winding, on the other hand, is open-circuited; hence, its terminal voltage is equal to the induced voltage. The latter is given by


VSrms = 4.44 f NS (BmA)
(2.1)

A = m /( l ⋅ p)
(2.2)

NS = number of secondary turns = 600
(2.3)

Bm = maximum flux density in W b/m2
(2.4)

m = weight of bundle of strips = 10 kg
(2.5)

l = total length of strips = 2 m
(2.6)

P = density of steel in kg/m3 = 7700
(2.7)

A = cross-sectional area of bundle strips
(2.8)

f = frequency of AC supply.
(2.9)

Substituting the values for Ns , m, l and p, we get.


| V̅Srms | = 1.73 × f × Bm
(2.10)

In order to separate the eddy-current loss (Pe ) and hysteresis losses (Ph ) when only total power loss (W ) is measured, the following calculations must be performed.


Formula 2.11
(2.11)

Formula 2.12
(2.12)

Formula 2.13
(2.13)

where Kh and Ke are constants related to the material of the transformer core and its volume.
In (2.13) we see that if (W/f ) is plotted against  f  for fixed Bm, a straight line is obtained whose slope is k 001 and y-axis intercept k 002  The hysteresis power loss for that value of Bm  is then obtained by multiplying the y-intercept by the frequency. The corresponding eddy-current loss is the slope multiplied by the frequency squared. The procedure is repeated for each value of Bm.

To obtain the value of Kh, the logarithmic values of k 002 obtained above are plotted against log Bm.The slope of the resulting straight line is n and its y-intercept is log Kh. Thus Kh and n can be obtained. Similarly, by plotting log k 001 against log Bm  as a straight line of slope 2, log Ke can be obtained and, hence, Ke.

An alternator-dc motor set is used as a variable frequency AC voltage supply.
The frequency can be changed by varying the motor speed. The magnitude of voltage can be altered by varying the alternator field current.

Note: Only the instructor can change the frequency and the maximum AC voltage. The students can then obtain fractions of the supplied voltage by turning the three-phase variac.

Prelab

  1. Complete table 2.1 using formula 2.10 Remember that the Vs in 2.10 refers to the root mean square (RMS) of the secondary voltage. The DMM reads the average value of the secondary voltage and converts it to an approximate RMS value by multiplying by 1.11 which assumes a pure sinusoidal voltage. (VRMS  =  VAV(s) * 1.11)

  2. Connect the meters into the circuit of figure 2.1 to measure the total power lost (W) of the Epstein test frame and primary current (IP). Show connections with a Low Power Factor (LPF) Wattmeter and with Power Quality Meter (Fluke 43B).

  3. Read the reference sections 10.1 through 10.5. Explain which of the precautions at the end of section 10.5 you think we might have the most difficulty with.

Eddy Current and Hysteresis Losses

  1. Connect the circuit as shown in figure 2.1. Note: Use only the primary and secondary connections outside the coil, ignore the connections inside.
  2. Figure  2.1: Circuit for Epstein core loss test set-up.
    Figure 2.1: Circuit for Epstein core loss test set-up.
  3. Connect the power quality meter to monitor only core losses by connecting to the primary current and to the secondary voltage (one side of the DPDT switch ).
  4. Connect the power supply from the bench panel to the variac and connect the OUTPUT of the variac to the circuit.
  5. Wait for the instructor to adjust the frequency and maximum output voltage available for your panel.
  6. Adjust the variac to obtain voltages Es as calculated in table 2.1. For each applied voltage, measure and record Vs and W in table 2.2.

  7. T able 2.1: Es = 1 .73 f Bm

    B m

    f  = 30 Hz

    f  = 40 Hz

    f  = 50 Hz

    f  = 60 Hz

    0.4
    0.6
    0.8
    1.0
    1.2

     

     

     

     


  8. Perform the previous steps for frequencies of 30, 40, 50 and 60 hertz.

Report

  1. Plot a graph of kg core loss (W/10), against the frequency f at different flux densities Bm on the same graph.
  2. Separate the Eddy-Current Pe and hysteresis Ph losses at different flux densities Bm and frequencies f . Complete table 2.3.
  3. Plot graphs for Pe and Ph against the frequencies for different flux densities on the same graph.

Discussion Questions

  1. Discuss how eddy-current losses and hysteresis losses can be reduced in a transformer core.
  2. Plot the log of the k 002   values against the log of Bm. Determine Kn and n.
  3. Plot the log of the k 001 values against the log of Bm (slope should be approximately 2) Determine the values of Ke from the intercept (log Ke)
  4. Explain why the wattmeter voltage coil must be connected across the secondary winding terminals.

  5. Table 2.2: Core Loss Data.

     

    f = 30 Hz

    f = 40 Hz

    f = 50 Hz

    f = 60 Hz

    Bm

    Es
    Volts

    W
    Watts

    Es
    Volts

    W
    Watts

    Es
    Volts

    W
    Watts

    Es
    Volts

    W
    Watts

    0.4                
    0.6                
    0.8                
    1.0                
    1.2                


    Table 2.3: Data Sheet for Eddy-Current and Hysteresis Losses.

     

    f = 30 Hz

    f = 40 Hz

    f = 50 Hz

    f = 60 Hz

    Bm

    Pe
    Watts

    Ph
    Watts

    Pe
    Watts

    Ph
    Watts

    Pe
    Watts

    Ph
    Watts

    Pe
    Watts

    Ph
    Watts

    0.4
    0.6
    0.8
    1.0
    1.2