This invention relates to a simple, electronic, detection circuit for detecting variations in the mechanical motor load of an induction motor and more particularly for detecting the extent to which the outdoor heat exchanger of a heat pump system is blocked by ice.
Heat pump systems for heating and cooling homes and other structures are becoming popular. Such heat pump systems ordinarily have an indoor heat exchanger, an outdoor heat exchanger and a compressor. In operation one heat exchanger functions as a part of the condensor and therefore provides a heat output and the other functions as part of the evaporator and therefore absorbs heat. Refrigerant is circulated through the closed loop. These functions are alternatively switched or interchanged between the indoor and the outdoor heat exchangers depending upon whether it is desired to cool or heat the indoors.
In the winter season, the outdoor heat exchanger operates as an evaporator for collecting heat from the atmosphere. In this mode of operation, the outdoor heat exchanger is cooled to temperatures substantially below the outdoor temperature. Consequently, under appropriate temperature and humidity conditions atmospheric water will condense on the outdoor heat exchanger and freeze.
Most outdoor heat exchangers are forced air systems having an induction motor connected to an impeller for effecting a flow of outdoor air across the heat exchanger. This air flow prevents the buildup of a thick thermal boundary layer and therefore improves the heat transfer from the outdoor air to the heat exchanger.
Unfortunately, over a period of time of from 20 minutes to several hours, the continuous accumulation of ice upon the outdoor heat exchanger not only provides an insulative thermal barrier which retards the heat flow between the exchanger and the outdoor air but also restricts and eventually entirely blocks the flow of air. Such ice blockage so significantly reduces the efficiency of the heat exchanger that the heat pump system must periodically be stopped and steps must be taken to remove the ice. This is ordinarily done by reversing the operation of the heat pump for a brief time interval so that the outdoor heat exchanger becomes a condenser and therefore is warmed by the compressed refrigerant.
A major problem with the heat pump system has been the detection of the excessively iced condition in order to initiate the de-icing procedures.
One prior art system for de-icing the outdoor heat exchanger uses a timer which, at selected time intervals and at selected times of the day, causes the heat pump to be reversed. This system, however, has caused problems because it is often operated when excessive icing has not occurred and under conditions of extreme icing may not permit enough time to remove all of the formed ice.
The timer system described above was improved by use of a temperature sensor which can effect the removal of all of the ice from the heat exchanger. When the exchanger rises to a temperature significantly above 32.degree. F. the de-icing cycle is terminated. Such a temperature sensor, however, only serves to eliminate an excessively long de-icing cycle. It does not solve the problem of an inadequate de-icing interval.
Still another attempt to solve the problem of the prior art is the differential temperature control in which the temperature of the heat exchanger coil as well as the temperature of the outside ambient air is sensed. When the difference between these temperatures is more than a selected trigger level, the de-icing procedure is initiated. The magnitude of the trigger level is automatically varied in accordance with the outdoor temperature variations. However, the differential temperature control requires a different control circuit parameter for each heat exchanger unit because the temperature differentials and icing characteristics vary radically with small design variations in the heat exchanger.
Still another system for sensing the formation of ice on the heat exchanger is the air switch which detects the pressure differential between the air input side and the air exhaust side of the heat exchanger. Such a system initiates the de-ice procedures when the pressure differential exceeds a selected value. These air switches, however, suffer from the disadvantages that they are sensitive and respond to changes in wind velocity and are affected by any foreign matter that enters the sensing element.
Still a further attempt to detect the formation of ice was the temperature permissive, time initiated, temperature terminated, time override system. This system uses both a timer and a transducer which senses the temperature of the outside heat exchanger. Such a system, under control of the timer, automatically initiates the de-icing procedures unless the temperature sensor senses that the outdoor heat exchanger is too warm to have ice formation. Then, after a selected period de-icing is halted if the thermal sensor did not earlier terminate the de-icing function. However, this system can permit excessive icing to occur between successive time cycles. Conversely, the timer also permits initiation of defrost when the outdoor heat exchanger is cold enough, even though no icing may have occurred.