Electricity: power supply or regulation systems – External or operator controlled – Phase controlled switching using electronic tube or a three...
Patent
1986-03-14
1988-10-18
Wong, Peter S.
Electricity: power supply or regulation systems
External or operator controlled
Phase controlled switching using electronic tube or a three...
323241, 323243, G05F 1455
Patent
active
047790385
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an electric continuous-flow water heater with controllable outlet temperature, comprising
(a) a through-flow path with a plurality of heating passages connected in series in the flow path,
(b) three-pole switching means arranged to be connected to three phases of a three phase current and responding to flow rate in the through-flow path,
(c) a temperature sensor arranged on the outlet side in the through-flow path,
(d) first heating resistors, which are arranged in some of these heating passages and are arranged to be switched on by the switching means when a predetermined flow rate has been reached,
(e) second heating resistors, which are arranged in other ones of these heating passages, and
(f) a control system, which is connected to the temperature sensor and by which the second heating resistors are controllable through electronic power output stages in order to maintain a desired value.
The invention relates also to an electronic power output stage, particularly for controlling the outlet temperature of electric continuous-flow water heaters, comprising
(a) a rectifier bridge connected to a mains-frequency a.c. voltage and supplying a d.c. voltage pulsating with twice the mains-frequency,
(b) a Schmitt-trigger, to which the pulsating d.c. voltage is applied for generating a square wave pulse sequence with twice the mains frequency,
(c) a frequency divider, to which the square wave pulse sequence is applied,
(d) a semiconductor relay with zero voltage switch performance connected to the mains a.c. voltage and arranged to switch the power, and
(e) means for controlling the semiconductor relay as a function of output signals of the frequency divider.
The control of the outlet temperature with electric continuous-flow water heaters presents special problems. Electric continuous-flow water heaters have a very high installed heating capacity of, for example, 22 kW. The outlet temperature must be maintained very exactly and is only allowed to hunt slightly around the desired temperature. It is difficult to control the heating capacity with corresponding sensitivity.
The electric heating capacity can be controlled continuously by a phase control. But a phase control of the high power present here is not permissible because of the higher harmonic waves generated and the disturbances caused thereby. It is only permissible to switch the alternating current fully on or off, whereby a zero voltage switch takes care that the switching on and the switching off occur always at moments at which the a.c. voltage passes through zero anyway, such that no phase control occurs.
But a control of the heating power by switching on or off requires a relatively high switching frequency, if the requirement is to be met, that a desired outlet temperature is maintained with high accuracy. If the heating capacity would be switched on and off with low switching frequency, too cold or too hot water would alternately flow out of a continuous-flow water heater, even if the average heating power corresponds to the desired temperature. But a high switching frequency of the switching on and off of a high heating power results again in other problems: The high heating power of the electric continuous-flow water heater affects the mains-frequency due to the mains impedance. Therefore a high switching frequency would be accompanied by an unpleasant flicker of electric filament lamps. Therefore the switched heating power permitted by the public utility company is the smaller the higher the switching frequency is. These requirements counteract the efforts to control the outlet temperature of a continuous-flow water heater of high power with high accuracy.
For this reason it is known to subdivide the installed heating power in an electric continuous-flow water heater into at least two stages. Thereby a control system with a temperature sensor arranged in the outlet switches only a respective one of the stages for the temperature control (DE-OS No. 28 37 934).
From DE-OS No. 28 37 934 a device is known for controlling th
REFERENCES:
patent: 4090107 (1978-05-01), Seib
patent: 4317975 (1982-03-01), Mizukawa et al.
patent: 4463269 (1984-07-01), Yashima
Eckerfeld Erika
Wong Peter S.
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