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History and different types of power meters |
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- THERMAL WATTMETER
This instrument uses a thermocouple to convert an AC-current into a DC-equivalent current (its so called RMS-value). When a current flows through the hot-wire, its temperature goes up.
A thermocouple is connected to this hot-wire, so thermocouple is also heated up.
In turn, the thermocouple generates a DC-voltage proportionally to the developed heat at the junction.
A moving coil instrument is used to indicate the result. For measuring power, The rising temperature of the thermocouple is proportional to square value of the current and equals the active power.
In this case, this instrument uses two thermocouples with a differential circuit. This instrument can be used to measure both DC and AC.
Frequency bandwidth Current: up to approx. 10 MHz
Voltage: up to approx. 100 kHz
Heat-wire is very thin, so it is easy to burn out by the influence of over current.
- Electro-dynamic instrument
When a current flows into the fixed coil, the fixed coil generates a flux field. The mobile coil rotates in this flux field. This is the principle of this instrument. The amount of torque to rotate of the mobile coil is proportional to multiplication of the current flowing through the fixed coil and the mobile coil.
To flow the difference current into the fixed coil and the mobile coil, this instrument indicates averaging value which is integrated multiplication value of the each instantaneous current by time. For measuring power, it use this characteristic.
Measuring voltage to attach resistance to the mobile coil and measuring current to flow current into the fixed coil, this instrument indicates multiplication value between voltage and current. It is power.
This instrument can use to measure DC and AC.
Frequency bandwidth current: up to approx. 1 kHz
voltage: up to approx. 150 Hz
*YOKOGAWA 2041,2042 ( DC, 25 Hz to 1000 Hz)
- Induction instrument
Putting a conductor into a moving or rotating flux field generates a so called eddy current in the conductor. The force between this eddy current and the flux field, moves or rotates the conductor. This instrument uses this driving force.
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An electrodynamics wattmeter is composed of a pair of fixed coils and one mobile coil. The mobile coil moves in a spinning direction in the magnetic field generated by the fixed coils. The torque applied to the mobile coil is proportional to the product of the currents flowing in the fixed and mobile coils. When a load current flows in the fixed coils and a small current proportional to the load current flows in the mobile coil, the torque on the mobile coil is proportional to the product of the load current and voltage. For an alternating current, the phase-angles of the currents in both coils
(i.e., phase angles of the currents against the voltages) affects the torque in the same relationship as the power of the load power. This means that the torque is proportional to the electric power whether the current is alternating or direct, and the power can be indicated by this meter mechanism.
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An analog computation wattmeter computes the instantaneous electric power with the full use of an analog switching circuit. It generates a constant-period pulse-width modulation waveform proportional to the input level of the voltage or current (such that the duty of each cycle of the pulse is proportional to the input level), and superimposes the input current (or voltage) waveform on that pulse waveform to obtain a multiplication of the voltage and current waveforms. This method is referred to as time-division multiplication. At the last stage, the effective power value is obtained from the time-division multiplication waveform through averaging by an analog filter circuit.
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In general, the following equation is used to express the effective power, where the instantaneous power values, products of the instantaneous voltage and current values, are integrated and averaged by cycle T.
- u(t): instantaneous voltage value at time t
- i (t) instantaneous current value at time t
- u (k): instantaneous voltage value at k-th sample
- i (k) instantaneous current value at k-th sample
- T : cycle
From the approximation on the right above, we can tell that the effective power can be obtained by averaging the number of k (=T/ _ t) segments of the width of _ t by cycle T. A digital sampling wattmeter executes this computation almost as is. In an actual wattmeter, the waveform measurement time period is often set longer than one cycle. _ t is generally around tens of microseconds and the sampling frequency is the inverse number of the _ t.
Click here for an example of a Full-Digital Type: Yokogawa WT 2030
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