EP1296102B1 - Method for operating an electric multistage heating having several heating elements - Google Patents

Description

The invention relates to a method for operating a multi-stage electric heater consisting of a plurality of heating elements, wherein the power converted in each heating element is maintained at a preset value by controlling the electrical power consumption.

In the published on 26.03.2003 WO 99/07185 Such a method is described in which power semiconductors are regulated via control circuits in order to set the heating power continuously. The regulation of the heating power is made possible by the fact that the power semiconductors can be controlled individually by means of a jointly predefinable setpoint value and by means of actual values derived from the power semiconductors in the output power.

Furthermore, a method of the type mentioned in the EP 1 091 621 A1 known. There, a method for controlling an electric heater is described consisting of at least two heating resistors and a control unit. In this case, a setpoint temperature in a cell to be heated is compared with an actual temperature, at regulators of the heating resistors are given by means of a control circuit pulse-shaped time-shifted currents, and the duration of the pulses is adjusted depending on the difference of the temperatures.

A multi-stage electric heater, for example a heater with positive resistance temperature coefficient PTC, consists of several individual elements, which are often referred to as heating elements and are electrically connected in parallel. Each heating element in turn may consist of several sub-elements, that is, individual PTC bricks.

Each individual heating element can be switched on or off via an example electronically formed switch. The power P _H converted in a heating element, that is to say the supplied electrical power, which is equal to the thermal power output, is dependent on the electrical resistance R _{H of} the heating element at the operating point for a given operating voltage U _B : $$\mathrm{PH}\mathrm{=}{\left({\mathrm{U}}_{\mathrm{B}}\right)}^{\mathrm{2}}\mathrm{/}{\mathrm{R}}_{\mathrm{H}}$$

However, the electrical resistance R _{H of} the heating elements is subject to production due to large variations. As a result, the power dissipated by the heating elements differs to the same extent. To meet the demand, for example, the air conditioning manufacturer for a defined performance at a given operating point, therefore, complex measures, such as matching or sorting, required to comply with the required for the operating point electrical resistance R _{H of} the individual heating elements.

Although a heater composed of a plurality of heating elements can satisfy the requirement for a defined total power, the heating power of the individual heating elements may be different. This leads due to the usually delivered over a larger area heat output to a temperature stratification of the exiting the heating air flow. The heated air has noticeable temperature differences across the exit surface. This is undesirable, for example, in heating or air conditioning systems, since it leads to irregularities in the temperature of the heated room, for example, a vehicle interior. It follows that all individual heating elements of a heater should implement or deliver the same power.

The object underlying the invention is to provide a method of the type mentioned above, is ensured by the fact that even if the individual heating elements have scattering resistance values, all the heating elements implement the same power or deliver.

This object is achieved according to the invention by claim 1. Based on a maximum electrical resistance of the individual heating elements, in which the full applied operating voltage results in the required performance of the individual heating elements for the operation, the voltage at the individual heating elements is separately controlled down to the required rated power.

In the following, the inventive method will be described in detail with reference to a particularly preferred embodiment.

The power converted in an electric multistage heater and thus in each heating element is kept at a preset value P _HSoll by controlling the electrical power consumption. With an n-stage heating, each heating element is thus kept at a predetermined value P _Hsetpoint / n. This default value can be variable, so that the power output of the heater is adjustable.

Each individual heating element is regulated individually, so that the dimensioning of the individual heating elements is thereby considerably simplified. It only has to be ensured that a value R _{Hmax is} not exceeded by the scattering of the electrical resistance R _{H of} the individual heating elements. With this assumed maximum value of the resistance, a heating element just reaches its rated power required at the operating point when the full operating voltage U _B is present: $${\mathrm{P}}_{\mathrm{H}}\mathrm{=}{\left({\mathrm{U}}_{\mathrm{B}}\right)}^{\mathrm{2}}\mathrm{/}{\mathrm{R}}_{\mathrm{Hmax}}$$

If the resistance R _{H of} a single heating element below the value R _Hmax , the voltage at the heating element, for example, by clocking the operating voltage, in particular by pulse width modulation and thereby the power consumption of Heating element to the required value P _H reduced. The power consumption of the heating element is determined by measuring the applied voltage and the absorbed current.

Another advantage of this regulation of the individual heating elements to the required rated power is that not only the variations of the resistance R _{H of} the heating elements can be compensated, but that fluctuations in the operating voltage U _B can be compensated, as long as this voltage U _{B is} not below a minimum value U _Bmin decreases. For this applies: $${\mathrm{U}}_{\mathrm{Bmin}}\mathrm{=}{\left({\mathrm{P}}_{\mathrm{H}}\mathrm{*}{\mathrm{R}}_{\mathrm{H}}\right)}^{\mathrm{1}\mathrm{/}\mathrm{2}}\mathrm{,}$$

The inventive method has the advantage that despite the variations in the resistance of the heating elements required at the operating point rated power can be met, the exiting air flow everywhere has the same temperature, that is, no temperature stratification occur, and significantly reduces the sorting cost of the heating elements in terms of their electrical resistance becomes what greatly reduces or even avoids the committee. Depending on the scattering range, sorting can even be completely eliminated. Separated individual heating elements can also be used in heaters with other rated power.

Claims (3)

1. A method for operating a multi-stage electrical heater comprising a plurality of heating elements, in which method a set value of the power consumption in each of the heating elements is maintained by controlling the electrical input power,
   characterized in that separately from the full operating voltage the operating voltage applied to the individual heating elements is so down-regulated as a function of the power consumption, which has been determined for the individual heating elements, that the power consumption is reduced to the set value,
   the full operating voltage being that voltage at which the heating elements would have a power consumption at the set value, provided their resistance would have a maximum value which is not exceeded by the resistance of any of the heating elements which may deviate from a nominal value.
2. The method according to claim 1,
   characterized in that the set value of the power consumption is selected to be the same for all heating elements.
3. The method according to any of the preceding claims,
   characterized in that the set value of the power consumption of the heating elements is adjustable.