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Microwave ovens can kill. Please recognize what you are doing prior to taking one apart, and be really careful throughout.

## The microwave oven transformer (MOT)

If you have removed a microwave oven transformer (MOT) native an old microwave oven, the is feasible to reuse it as a short voltage power source for an electronics project request hefty power. The an approach described below keeps the mains major winding intact, and also removes the high voltage (HV) an additional and the filament windings without having actually to reduced the transformer laminations. A brand-new secondary winding can then it is in wound roughly the gaps in the laminations, giving a stepped under voltage however high present AC source.

removing the magnetic shunts

The magnetic shunts an initial require removed to assist the removed of the an additional winding. The shunts space removable laminations that room fitted in between the main winding and also the filament winding. Usage a 4mm punch and also a hammer to push out the shunts, one in ~ a time.

Magnetic shunts gotten rid of

The shunts are used in a microwave cooktop to limit the flux density in the second and so limit the maximum an additional current. Unless existing limiting is required for your project, then there is no additional use the the shunts.

removed the second winding

The HV secondary used to it is provided the 2kV for the microwave range magnetron, calls for removal. Following using a hack saw carefully from the top cut through the secondary coil. The an additional coil deserve to be further established as it provides thinner wire. Be mindful not to nick or damages the major coil below.

an additional winding nearly removed

once the second has been cut through, use a 10mm punch and hammer out the copper windings. Alternative between both sides to attract out the coil from the various other side the the transformer. You might decide to save the document former depending upon how fine it has survived or it deserve to just be removed if no longer required. Making use of wire cutters, eliminate the filament windings (the orange wire presented in the photos).

an additional winding eliminated

using a brush clean away any kind of copper filings and also debris, and you have to be left v something prefer the photo shows. Be afflicted with in psychic an MOT will certainly get very hot as soon as run repetitively at the nominal primary voltage, even with no load as result of core losses. They space after all cheaply manufactured and also designed to just operate for brief periods together in a microwave oven. However these main point losses deserve to be lessened by running the transformer major at a lower voltage than the in the name voltage, or even by using 2 MOTs with their primaries associated in series.

brand-new secondary winding added

To conveniently test the MOT a new temporary secondary winding to be wound making use of multi-core cable which happened to be laying approximately in the junk box. ~ winding 40 turns and also attaching two crimp terminals the mains was linked to the major winding. The RMS voltage on the an additional was measure up to be 35.9V, offering 0.9 second volts every turn. When complete wave rectified and also smoothed with a perfect capacitor, the maximum DC voltage is about given by: maximum DC voltage, ( eginaligned V_DC = sqrt2.V_RMS endaligned )

## Measurements and calculations

By testing the transformer under open circuit and also short circuit conditions it is feasible to discover the transformer losses, including iron-core, magnetising, and eddy-current / hysteresis losses. The test equipment required to carry out the test encompass a voltmeter, ammeter and also wattmeter. The voltmeter and ammeter procedures "apparent power" in VA when the wattmeter procedures "true power" in watts. The brief circuit test additionally requires a variac or part other method to minimize the voltage throughout the main winding.

open circuit check (iron-core and also magnetising losses)

A transformer v an open circuit an additional can be modelled as shown in the circuit below. Because that the open circuit test case, R1, R2, X1 and also X2 have the right to be ignored leaving simply Rw (iron-core loss resistance) and also Xm (magnetising lose reactance). The open circuit check is carried out by leaving an additional winding open, when the complete rated main voltage is applied to the main winding. At this allude Eo, Io and Po readings room taken.

Iron-core loss as measured by the wattmeter, ( eginaligned P_o = E_o . I_o . Cos phi_o endaligned ) No load power factor, ( eginaligned cos phi_o = frac P_o E_o . I_o endaligned ) Iron-core loss current component, ( eginaligned I_w = I_o . Cos phi_o endaligned ) Magnetising loss current component, ( eginaligned I_m = I_o . Sin phi_o endaligned ) Iron-core ns resistance, ( eginaligned R_w = frac E_o I_w endaligned ) Magnetising lose reactance, ( eginaligned X_m = frac E_o I_m endaligned ) No fill impedance, ( eginaligned Z_o = sqrt R_w^2 + X_m^2 = R_w + jX_m endaligned )

open up circuit measurements / calculations

Mains voltage is used to the primary, v the an additional open circuit. Readings the Eo, Io, and also Po space taken. ( eginaligned E_o = 240V, ext I_o = 2.72A, ext P_o = 53W. endaligned ) ( eginaligned cos phi_o = frac 53 240 imes 2.72 = 0.081 endaligned ) ( eginaligned I_w = 2.72 imes 0.081 = 0.221A endaligned ) ( eginaligned I_m = 2.72 imes 0.997 = 2.711A endaligned ) ( eginaligned R_w = frac 240 0.221 = 1087 Omega endaligned ) ( eginaligned X_m = frac 240 2.711 = 89 Omega endaligned ) ( eginaligned Z_o = sqrt 1087^2 + 89^2 = 1090 Omega endaligned )

quick circuit check (copper and eddy-current / hysteresis losses)

A transformer with a short circuit second can be modelled as displayed in the circuit below. Because that the short circuit test case, Rw and Xm can be ignored. R1 and R2 are linked together as Rc (copper loss resistance), and also X1 and also X2 are an unified together as Xi (eddy-current / hysteresis ns reactance). The brief circuit check is conducted by shorting the secondary winding, when the main voltage is slowly increased making use of the variac until the maximum rated primary current is reached. In ~ this point Es, Is and also Ps readings room taken.

Copper loss as measured through the wattmeter, ( eginaligned P_s = I_s^2 . R_c endaligned ) Copper lose resistance, ( eginaligned R_c = frac P_s I_s^2 endaligned ) complete load impedance, ( eginaligned Z_s = frac E_s I_s endaligned ) Eddy-current / hysteresis ns reactance, ( eginaligned X_i = sqrt Z_s^2 - R_c^2 endaligned ) full load strength factor, ( eginaligned cos phi_s = frac R_c Z_s endaligned )

brief circuit measurements / calculations

The variac is enhanced until the preferably primary present is reached, with the an additional short circuit. Readings of Es, Is, and also Ps are taken. ( eginaligned E_s = 88.7V, ext I_s = 3.33A, ext P_s = 186W. endaligned ) ( eginaligned R_c = frac 186 3.33^2 = 16.8 Omega endaligned ) ( eginaligned Z_s = frac 88.7 3.33 = 26.6 Omega endaligned ) ( eginaligned X_i = sqrt 26.6^2 - 16.8^2 = 20.7 Omega endaligned ) ( eginaligned cos phi_s = frac 16.8 26.6 = 0.63 endaligned )

performance

Transformer performance is provided by,

Efficiency, ( eginaligned eta = frac output extpower input extpower = frac output extpower output extpower + losses endaligned ) The worth of transformer efficiency will be maximum once the copper losses, computer are equal to the iron-core losses, Pw in the transformer. This will certainly be somewhere in between no load and also full load conditions. Efficiency, ( eginaligned eta = frac E.I.cos phi E.I.cos phi + P_c + P_w endaligned ) Assuming that the copper losses are lumped in simply one winding we can approximate copper losses. Efficiency, ( eginaligned eta approx frac E.I.cos phi E.I.cos phi + I^2.R_c + I_w^2.R_w endaligned )

complete load effectiveness calculations

( eginaligned E = 240V, ext i = 3.33A, ext cos phi = 0.63. endaligned ) ( eginaligned eta approx frac (240 imes 3.33 imes 0.63) (240 imes 3.33 imes 0.63) + (3.33^2 imes 16.8) + (0.221^2 imes 1087) endaligned ) ( eginaligned eta approx 0.68 ext (68 ext%) endaligned )

## Conclusion

In the MOT defined here, the an additional winding might of food be wound an ext efficiently by using single core copper wire v thinner insulation and a certain gauge to minimise wait gaps in the core. These changes would assist to enhance the transformer efficiency, calculated to be around 68%. If there is a need for consistent operation, climate it is recipient to use two MOTs with the primaries connected in series. This method reduces warmth generated and noise led to by the MOT"s high magnetising current. fan to the microwave range being a mass sector appliance, and so the requirement for minimal production costs, the MOT will certainly not be the most effective of transformer designs. But the MOT have the right to still be properly used as a way of producing a cheap high power mains action down strength supply.

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