Power absorbed and B vs H plot for microwave oven transformer

[Russ Edmonds]

TL;DR Summary

Net power absorbed and a tentative B vs H plot for an unloaded microwave oven transformer (MOT)

A DI-149 data acquisition module was used to record a microwave oven transformer (MOT) primary voltage and current time histories. The secondary of the MOT was open. The DI-149 analog input specs are: +- 10 volts full scale, 5000 samples per second for each channel (assuming two channels being recorded) and a ten bit ADC. One second of data was was recorded for 90v, 100v, 110v and 120v rms.The setup.

Note: The MOT HV cap is not in the circuit. One terminal of this cap is being used as a connection point for the blade/push on connectors.

Schematic of the setup.

The power absorbed by the MOT at time t is given by V(t)I(t) where V(t) and I(t) are the real voltage and current at time t. The current lags the voltage by nearly 90 degrees so this term shows that the MOT alternatively absorbs and emits power. The time integral of V(t)I(t) over one period of the AC cycle (T) gives the net energy absorbed by the MOT in one T. Dividing the net energy by T then gives the net power absorbed.

Magnetic flux density in the core is given by the time integral of voltage divided by the number of primary turns and cross sectional area of the core at the primary winding. There can also be a constant flux and flux density term in the core but this term is generally set to zero in making B vs H plots. In getting the value of the magnetizing field H, I assumed a constant H around a intermediate closed path in the core. This is probably not very accurate, also the effects of the magnetic shunts were ignored.

A tentative B vs H plot.

 

[Future Bruno]

The data was plotted in a B vs H plot and the results are shown in the figure below. The plot suggests that the MOT saturates at around 110V rms. The hysteresis loop indicates that the core is non-linear, which is expected since the MOT core is made of a ferromagnetic material. The shape of the hysteresis loop shows the typical behavior of a ferromagnetic material, with a shallow slope at low field and a steep slope at high field. This suggests that the MOT core is made of a soft ferromagnetic material, which is typical for MOT cores.

 

[Future Bruno]

The 90v, 100v, 110v and 120v rms data was used to determine the B vs H plot. This was done by taking the time integral of the voltage divided by the number of primary turns and cross sectional area of the core at the primary winding. This gave the flux density B. The magnetizing field H was assumed to be constant around a intermediate closed path in the core. This is probably not very accurate, also the effects of the magnetic shunts were ignored. The plot below shows the estimated B vs H curve for the MOT.

 

[Future Bruno]

The B vs H plot is difficult to obtain because of the limited resolution of the data. The DI-149 can only measure up to 10 volts, so it is unable to accurately measure the MOT primary voltage at higher applied voltages (120v). Also, the 5000 samples per second for each channel is probably not enough to get accurate readings of the MOT primary current. The B vs H plot was constructed by combining the 90v and 110v data points, and then interpolating the 100v and 120v points.The B vs H plot is not very accurate due to the limited resolution of the data. The DI-149 can only measure up to 10 volts, so it is unable to accurately measure the MOT primary voltage at higher applied voltages (120v). Also, the 5000 samples per second for each channel is probably not enough to get accurate readings of the MOT primary current. If a better data acquisition system with higher resolution is used, a more accurate B vs H plot can be obtained.