Testing Technique

Tests for open circuits, short circuits, turns ratio, and d-c resistance are made on pulse transformers in the same manner as in other transformers. The instruments used must be suitable for the low values of inductance encountered, but otherwise no special precautions are necessary. Usually the d-c resistance is somewhat lower than the winding resistance during most of the pulse, but even the latter value is so low that it causes no significant part of the transformer loss. Losses are measured as described in Efficiency.

Various methods have been used to check effective pulse OCL. These may involve substitution of known inductances, or current build-up, or decay, depending on the time constant of the transformer

inductance and an external known resistance. When such measurements are attempted under pulse conditions, there is usually a certain amount of error due to reflections, incidental capacitance, and the like. A method involving the measurement of pulse permeability and calculation by the OCL formula is given here.

If the air gap and pulse permeability are known, the OCL for a given core area and number of turns can be calculated. If the gap used is purposely made large to reduce saturation, proper allowance for it can be made in equation 38. If the gap is the minimum obtainable, it is necessarily included in the permeability measurement, but this is often done in taking pulse permeability data, as it was in the data of Fig. 241. With this definition of permeability equation 38 reduces to

OCL = (3.2μN2A10-8)/lc

[141]

Equation 141 is valid only when l_c/μ >> l_g.

B-H data for a pulse transformer are taken by means of a circuit similar to that of Fig. 247.

Fig. 247. B-H test for pulse transformers.

Primary current flowing through small resistor R₁ gives a horizontal deflection on the oscilloscope proportional to I and therefore H for a given core. R₁ should be low enough in ohmic value not to influence the magnetizing current wave form appreciably. If the voltage drop across a high-resistance load R₂ (≈ 50 times normal pulse load) is almost the entire secondary voltage e₂, then voltage e_c applied to the vertical plates is the time integral of e₂ and is therefore proportional to flux density at any instant. (See equation 138.)

Short leads and the reduction of incidental capacitance are essential to obtain accurate measurements. Distributed capacitance of the winding, shown dotted, should be minimized, as it introduces extraneous current into the measurement of H. One way to minimize this capacitance is to omit the high-voltage winding, and make all measurements from the low-voltage low-capacitance coil only.

The pulse source should be the kind for which the transformer is to be used. If it must be loaded to obtain proper pulse shape, a diode may be used to prevent backswing discharge through this load and therefore a reset core, unless reset core data are desired. Difficulty may be experienced in seeing the B-H loops of pulses having a low ratio of time on to time off because of the poor spot brilliance, unless an intensifier is used to brighten the trace.

With a calibrated oscilloscope it is possible to determine the slope of the dotted line ob in Fig. 247, drawn between the origin and the end of the pulse, and representing effective permeability μ at instant b, Fig. 226. This value of μ can be inserted in equation 141 to find OCL. Cores failing to meet the OCL should first be examined for air gap.

Effective values of leakage inductance and capacitance are difficult to measure. The calculations of capacitance and leakage inductance are based on the assumption of "lumped" values, the validity of which can be checked by observing the oscillations in an unloaded transformer when pulse voltage is applied. The frequency and amplitude of these oscillations should agree with those calculated from the leakage inductance and effective capacitance. The pulse source should be chosen for the squareness of its output pulse. Because of the light load, the transformer usually will be oscillatory, and produce a secondary pulse shape of the kind shown in Fig. 248.

Fig. 248. Transformer constants may be found from pulse shape.

In this figure, the dot-dash line is that of the impressed pulse and the solid curve is the resulting transformer output voltage. This curve is observed by connecting the vertical plates of a synchroscope (oscilloscope with synchronized sweep) across the transformer output winding.

The first check of leakage L_s and C₂ is made by finding the time constant T from

This time constant can be related to the time interval t_o-t_r in Fig. 248 by consulting Fig. 230. Formulas in this figure can be used for finding values of parameter k₁ using L_s, C₂, source resistance R₁ and load resistance R₂. This load resistance will be that corresponding to transformer losses only; hence R₂ R₁ for a pulse source with plenty of power, and

With this value of k₁, the increase or overshoot of the first voltage oscillation over the flat top value E may be found from Fig. 230, and may be compared with that observed in the test. When the load is resistive, or when the voltage pulse is the criterion of pulse shape, these are the only checks that need to be made on leakage inductance and distributed capacitance.

When the load is a magnetron, triode, blocking oscillator, grid circuit, or other non-linear load, the shape of the current pulse is important. Ordinarily the current will not build up appreciably before time t_r in Fig. 248. The shape of this current pulse and sometimes the operation of the load are determined to a large extent by slope AB of the no-load voltage at time t_r. This time is the instant when the first oscillation crosses the horizontal line E in Fig. 248. As indicated in Fig. 244, there is a relationship between this slope and the parameter fci. If the slope AB is confirmed, correct current pulse shape is also assured.

Insulation can be tested in one of two ways, depending on whether the insulation and margins are the same throughout the winding or whether the insulation is graded to suit the voltage. In the former case an equivalent 60-cycle peak voltage, applied from winding to ground at the regular 60-cycle insulation level, is sufficient. But, if the winding is graded, this cannot be done because the voltage must be applied across the winding and there is not sufficient OCL to support low-frequency induced voltage; hence a pulse voltage of greater than normal magnitude must be applied across the winding. Adequate margins support a voltage of the order of twice normal without insulation failure.

Such pulse testing also stresses the windings as in regular operation, including the non-uniform distribution of voltage gradient throughout the winding. The higher-voltage test ought to be done at a shorter pulse width so that saturation does not set in. In cases of saturation, the voltage backswing is likely to exceed the pulse voltage of normal polarity and thus subject the insulation to an excessive test. This backswing may be purposely used to obtain higher voltage than the equipment can provide, but it must be carefully controlled. Corona tests are sometimes used in place of insulation tests, and this can be done, where the insulation is not graded, by using a 60-cycle voltage and a sensitive receiver to pick up the corona noise. With graded insulation a high frequency must be used. The method becomes too difficult to use because the receiver may pick up part of the high-frequency power emitted from the transmitter, or the transmitter parts may generate a certain amount of corona which is more troublesome than at 60 cycles.

In pulse amplifiers, the mode of operation of the tubes and circuit elements is important. A round irregular pulse may be changed by grid saturation, or by non-linear loading of some other sort, into a practically square wave pulse. It may take several stages of amplification to do this in certain instances, and a transformer may be used at each stage. Often the function of the transformer is to invert the pulse for each stage; that is, the transformer changes it from a negative pulse at the plate of one stage to a positive pulse on the grid of the next. Polarity is therefore important and should be checked during the turns ratio test. If the transformer fails to deliver the proper shape of pulse, it may he deficient in one of the properties for which tests are mentioned above.

Fig. 249. Pulse amplifier with oscilloscope connections.

Figure 249 shows a pulse amplifier with normal pulse shapes for each stage. Checking each stage at the points indicated, without spoiling the pulse shape by the measuring apparatus, requires attention to circuit impedance, stray capacitance, cable termination, and lead length.