US2850698A - Wattmeter - Google Patents

Description

G. E. PIHL sepnz, 195s WATTMETER Filed Feb. 8, 1955 Mw 523mm Patented Sept. 2, 1958 2,850,698 WATMTER George E. Phl, Abington, Mass., assgnor to Acton Laboratories, Inc., Acton, Mass.

Application February 8, 1955, Serial No. 486,868 4 Claims. (Cl. 324-142) This invention relates to wattmeters, and more particularly to Wattmeters usable at higher alternating current frequencies.

It is an object of this invention to provide a new and improved wattmeter.

Another object of this invention is to provide a wattmeter which is rugged, dependable and does not require any rotating parts in the wattage sensing portion of the device.

A still further object of this invention is to provide an improved multiplying device in combination with the wattage determining portion of the device.

Other and further objects of the invention will be apparent by reference to the following description taken in conjunction with the accompanying drawing, wherein:

Figure l is a functional block diagram of the invention; and

Figure 2 'is a cross-sectional view of the electrostatic multiplying device used in the wattmeter.

Fig. 3 is a functional diagram showing how the voltage and current components of the power transmitted from au electrical source to a load are derived for application to the wattmeter of Fig. l.

Referring now more particularly to Figure l, there is shown a wattmeter of this invention for measuring the power transmitted from an electrical source to a load, having two input voltages el and e2. The input voltage el is developed across a resistor 1l) and is proportional to the current transmitted to the load whose power consumption is to be determined. The input voltage e2 is proportional to the voltage applied to the load whose power consumption is to be measured,

The heart of the wattmeter is an electrostatic multiplying device 11. Referring momentarily to Figure 2 for a detailed showing of this device, it is seen that a deflectable electrically conducting element or diaphragm 12, which may be made of a thin, metallic foil with vent holes 13 piercing its surface, has a pair of spaced insulating spacers 14 on one side adjacent its periphery and a pair of spaced insulating spacers 15 on its other side adjacent its periphery. These spacers 1Q- and 15 may be made of any well-known insulating material, such as fibre-board. An electrically conducting member or metallic end plate 16 contacts the insulating spacers 14 on the side remote from the diaphragm 12. A second electrically conducting member or metallic end plate 17 contacts the insulating spacers 15 on the side remote from the diaphragm 12. A pair of bolts 13 and nuts 19 rmly hold together the layered structure consisting of the end or charge plates 16 and 17, the spacers 14 and 15 and the diaphragm 12. The bolts 18 and nuts 19 are insulated from the end plates by washers 2l). Provision is made for applying input voltages to the diaphragm 12 and each of the end plates 16' and 17.

In order to clearly illustrate the manner in which this structure performs the process of multiplication, let it be assumed that a time-varying input signal 221 is applied between the end plates 16 and 17 having instantaneous polarities as shown by the arrow heads in Figure 2. Let there further be applied a second time-varying input signal e2 between the diaphragm 12 and the neutral of the rst signal voltage 2e1. The potential difference between the diaphragm and the plate e predates a force on diaphragm 12 in an upward direction, i. e. toward the plate 16, duc to the difference between e, and e2. This can be expressed as f uP=k1 e1e2l2 (l) The potential difference between the diaphrgam 12 and the lower plate 17 produces a downward force f d0W11=k2(e1+e2)2 (2) The net downward force on the diaphragm is then f--kzi-hezlz-kilreala (3) where k1 and k2 are constants determined by the geometry of the physical structure. lf, by reason of the geometry of the system, k1 is equal to k2, then f= 4kele2 (4) Equation 4 shows that the force deliecting the diaphragm 12 is a function of the product of the voltages el and e2. Stating this differently, an instantaneous force is produced acting on the dellectable element or diaphragm 12 which is proportional to the product of the instantaneous signal voltages el and e2. Due to the mass compliance and damping of the system, this force is integrated and produces an average deflection, depending upon the average value of the product of the two signal voltages.

Since the deflection of the diaphragm 12 is a function of the force applied to the diaphragm, either a mechanical or electrical means for measuring or sensing the deflection of the diaphragm could be used. If a mechanical sensing means is used, the deflection must of necessity be relatively large. This would modify the geometry of the system to the extent that k1 no longer equals k2 and errors are accordingly, introduced.

Therefore, in the preferred embodiment, an electrical method of sensing is used in which the average force due to el and e2 is opposed by a force 4kE1E2 (where E, is the value of the D. C. signal produced by the phase sensitive detector and applied to the diaphragm 12 and E2 is the value of each of the balanced D. C. voltages applied to the summing circuits 22 and This force is very nearly equal to the average value ^f the force 412182, because of the feedback loop to be described hereafter. As a result the deflection of the diaphragm 12 is very small, being only that necessary to produce the required error voltage to excite the feedback loop.

In particular, a carrier source having two outputs of equal amplitude but opposite polarities, both outputs being balanced to ground, has one of its output voltages appliedv to the plate 17. The diaphragm 12 together with the plates 16 and 17 forms a capacitive voltage divider so that, if the diaphragm 12 is deflected from its center position, the capacitive arms of the divider are unbalanced and the diaphragm will pick up one of the'two carrier output voltages. 1f the deflection of the diaphragm 12 is toward the plate 16, then the diaphragm will piel; up the carrier voltage applied to that plate, the other carrier voltage being picked up if the deflection is in the direction of the plate 17.

From the above discussion it is clear that the actual physical deflection of the diaphragm is extremely small (approaching zero), but that the tendency to be deflected as counteracted by the high-gain feedback loop can be accurately observed from the magnitude of the feedback loop voltage providing this counteracting force.

VReferring now again to Figure l, the output of the summing amplifier 22 is applied through a lead 23 to the plate 16 of the multiplying device 11. A summing circuit 2li has its output connected through a lead 2S to the plate 17 of the multiplying device 11. Each of the summing networks 22 and 24 has three inputs which may .have Yaltases applied to 'te be summed by

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