GB1605034A

GB1605034A - Tumbler dryers

Info

Publication number: GB1605034A
Application number: GB21691/77A
Authority: GB
Original assignee: TI Domestic Appliances Ltd
Current assignee: TI Domestic Appliances Ltd
Priority date: 1978-05-31
Filing date: 1978-05-31
Publication date: 1981-12-16

Description

(54) IMPROVEMENTS IN AND RELATING TO TUMBLER DRYERS (71) We, T.I. DOMESTIC AP PLIANCES LIMITED, a British Company, of Radiation House, North Circular Road, London, NW10 0JP, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to tumbler dryers and is moreover a modification of or an improvement in the tumbler dryer described in Specification No. 1 470 163.

Specification No. 1 470 163 describes a tumbler dryer in which articles are dried by tumbling them in a rotating drum through which hot air is circulated. The Specification also refers to the difficulty of ensuring that the articles reach a desired state of dryness irrespective of ambient temperature. Many conventional tumbler dryers rely on the temperature of air leaving the drum to provide an indication of the state of dryness of articles in the drum with the result that underdrying tends to occur in conditions of high ambient temperature whilst overdrying tends to occur in conditions of low ambient temperature.

The Specification referred to above refers to a tumbler dryer incorporating an electronic temperature control system in which two temperature sensing elements are employed to detect the difference in temperature between air entering the drum and air leaving the drum after passing over articles therein. In particular, the Specification describes in detail a tumbler dryer in which the temperature sensing element employed to detect the temperature of air entering the drum is located at a position where it responds to the temperature of air that has been heated. That temperature depends upon ambient temperature and the amount of heat imparted to the air during heating.

According to the present invention, a tumbler dryer comprises a heater for heating air prior to entry of the air into the drum of the dryer, a thermal cut-out associated with the heater and operable to disconnect the heater on detection of thermal overload conditions, and means for controlling the heater after the commencement of a drying cycle, the means comprising a first electronic sensing device responsive to ambient air temperature (as herein defined), a second electronic sensing device responsive to the temperature of air that has passed through the drum during a drying cycle, and an electronic control circuit responsive to the first and second devices and adapted, at the commencement of a drying cycle, to produce a control signal to cause energisation of the heater and to maintain the control signal in an unchanged condition until a predetermined difference is detected between the temperatures sensed by the two sensing devices when the control signal is modified to cause de-energisation of the heater.

By the expression "ambient air temperature" is meant the temperature of air before being heated by the heater. The first electronic temperature sensing device may thus be located at any position where it is exposed to ambient air temperature and not affected by air exiting from the drum. Such a location is to be found behind the front panel of the dryer casing adjacent the lower edge thereof, i.e. a location in which the device is affected by changes in ambient temperature only, and is not influenced to any substantial extent by air that has passed through the drum.

The device may be located at a point where air being drawn in by the impeller that circulates air through the drum can flow over the device. Alternatively, the device may be located in the impeller air flow path at a position upstream of the heater.

By way of example only a tumbler dryer embodying the invention will now be described in greater detail with reference to the accompanying drawings of which: Figure 1 is a sectional elevation of the dryer showing the mechanical components only, and, Figure 2 is a circuit diagram.

Figure 1 shows a tumbler dryer that is described in detail in Specification No. 1 226 951 and exemplifies a tumbler dryer to which the present invention is applicable.

The mechanical construction of the tumbler dryer will not be described in detail as it is fully dealt with in the Specification just mentioned.

Specification No. 1496656 describes an alternative construction of tumbler dryer to which the present invention is also applicable.

Figure 2 is a diagram of the electrical circuit of the dryer and shows, within rectangle 1 the details of the electronic temperature sensing control.

The circuit employs two thermistors X1 and X2 as temperature sensing devices and of these X1 is responsive to the temperature of air exiting from the drum during a drying cycle and X2 is responsive to ambient air temperature. The precise location of the thermistors in the dryer will be described below.

The control is powered through ac input terminals L' and N', the input being rectified by diode D1, whose dc output is smoothed by capacitor C1 connected to diode D1 via a potential dropping resistor R1. Voltage stabilisation is provided by a Zener diode D2 at, in the control shown, 11 volts.

The stabilized voltage appears across resistor R2 in series connection with thermistor X1. The junction between resistor R2 and thermistor X1 is joined via resistor R3 to input terminal 3 of an operational amplifier MA which in the embodiment is of Type 741.

The stabilized voltage also appears across a circuit consisting of resistor R4 is series connection with resistor R5 that is parallel connected to potentiometer resistor R6, and with preset potentiometer R7 and thermistor X2.

The tapping point of potentiometer R6 is connected to input 2 of the amplifier MA, and input 2 is coupled to input 3 by capacitor C2.

The output of amplifier MA appears at terminal 6 thereof and is fed via potential divider resistors R8 and R9 to the trigger of thyristor To 1. The anode of the thyristor is joined via diode D3 to power input terminal L' and the cathode is connected to neutral terminal N'. The purpose of the fuse FS1 is to safeguard components in the event that diode D3 fails and thyristor TH1 is destroyed.

The amplifier MA has a feedback loop including resistor R10.

Figure 2 also shows the control circuits of the heater H of the tumbler dryer and its driving motor M. Power input leads L nd N are connected to a terminal block B as shown and lead L is joined to a microswitch S1, which is a door switch and is opencircuited when the door of the tumbler dryer is open and is closed when the door is closed. From switch S1 lead L is extended to contact L1 and L2 of relay RL1 and to "start" microswitch S2 and "cool-run" thermostat T1.

The start microswitch S2 is normally open-circuited and is operated by a user at the commencement of a drying cycle via a suitably marked button (not shown). Microswitch S2 remains closed only as long as the user maintains pressure on the button.

On release of the closing pressure, the microswitch returns to its open circuit condition.

Thermostat T1 is employed to terminate energisation of the motor M via the contacts S3 as shown. In series with contacts S3 is a signal lamp SL3.

The heater H of the tumbler dryer includes parallel connected heaters HC1 and HC2 and energisation of these is controlled by relay RL1 over its contacts L1 and L3.

Overheating of the heaters HC1 and HC2 is prevented by a resettable thermal cut-out TC1.

Relay RL1 has a second set of contacts L2 and L4 that are series connected with the motor M as shown.

Motor M is a two-directional motor having two sets F1 and F2 of field windings.

Reversal of the direction of rotation of the motor is affected by a reversing microswitch S4 operated during a drying cycle by a mechanism driven from a rotating member of the tumbler dryer. Details of suitable mechanisms are found in Specification No.

1495936.

Before the commencement of a drying cycle, thermistors X1 and X2 are at substantially the same temperature. As the cycle proceeds, the temperature to which thermistor X1 is exposed gradually increases as the articles in the drum gradually dry with a consequent increase in the temperature of air exiting from the drum. The thermistors are negative temperature coefficient devices and as the resistance of thermistor X1 falls as the temperature to which it is exposed rises, the potential at the junction with resistor R2 will increase and this increasing potential is fed to input 3 of the amplifier MA where it is compared with the potential at terminal 2. The potential at terminal 2 depends upon the resistance of thermistor X2, i.e. on ambient temperature, and upon the position of the tapping point of potentiometer R6. The adjustment of the tapping point is under the control of the user who sets the position in accordance with whether the articles are to be fully dried or merely "damp" dried.

Having loaded the machine with the articles to be dried, and with the machine connected to a source of electricity, the user closes the door of the dryer and this action closes microswitch S1.

The user then operates microswitch S2 by means of its control button. Closure of microswitch S2 completes a circuit through the winding of relay RL1 and input R of the electronic control 1 to the anode of thyristor TH1. At the same time the electronic control circuit is also energised and an output appears at terminal 6 of amplifier MA and a potential of about 9 volts is applied to the trigger of thyristor TH1.

The thyristor anode is supplied with ac via the circuit just described and during the positive half cycles of the ac supply the thyristor will conduct and the winding of relay RL1 will be energised and contacts L1, L2 and L3, L4 will close.

During the negative half cycles of the ac supply, the winding of relay RL1 will remain energised by current flow through D3 induced by the decay of the magnetic field associated with the relay winding. This avoids "chatter" of the contacts of relay RL1.

Closure of contacts L1, L3; L2, L4 energises the heater coils HC1 and HC2 and field windings of the motor M which rotates the drum of the dryer and the impeller so that heated air commences to flow through the drum and over articles therein.

Closure of microswitch energises lamp SL2 and this remains alight even when the user has released the closing pressure on the push button of microswitch S2.

Lamp SL3 also lights up at the commencement of a tumble drying cycle.

When a desired degree of dryness has been effected, the potential difference across inputs 2 and 3 of the amplifier MA reverses and the latter rapidly switches to a state in which the output on terminal 6 becomes zero. During the next negative half cycle of the ac supply on the anode of thyristor TH1 the latter will cease to conduct and will remain non-conducting during succeeding positive half-cycles because of the absence of a trigger signal on the trigger electrode.The magnetic field of the winding of relay RL1 rapidly dies away, the induced current flowing via diode D3 as described above.

Upon de-energisation of the winding of the relay RL1, contacts L1, L3 and L2, L4 open and the heaters HC1 and HC2 are de-energised.

However, the motor M continues to be energised via a circuit including the contacts of the switch S3 controlled by the cool-run thermostat T1. When air exiting from the drum reaches a temperature substantially equal to ambient temperature, thermostat T1 responds and opens the contacts of switch S3 and the motor stops. At the same time, signal light SL3 becomes extinguished thus indicating to the user that the machine has now reached the end of its drying cycle.

The user can then open the door of the dryer to remove the articles from the drum.

Opening the door opens microswitch S1 which disconnects lead L and power from the dryer and the circuit shown in Figure 2.

Conveniently, thermistor X1 is located adjacent to the loading port in the front panel at a point where the thermistor is exposed to the flow of air exiting from the drum. Thermistor X2 is located on the front panel below the loading port at a point where it cannot be influenced to any substantial extent by air exiting from the drum.

Thermistor X2 is thus responsive to changes in ambient temperature only. It will be apparent that it is not necessary that air entering the drum should actually flow over thermistor X2. Alternatively, thermistor X2 may be positioned in the impeller air flow path upstream of the heaters HC1 and HC2 ut spaced from the latter by a distance sufficient to ensure that the thermistor is unaffected by the heat from the heaters.

The thermistors X1 and X2 are, respectively, types 2322/640/90004 and VA 1039.

It will be desirable, in most cases, to lag thermistor X1 thermally in order to delay slightly its response to changes in the temperature of air exiting from the drum. Such lagging avoids the premature deenergisation of the heater that otherwise occurs when the dryer is operating with a small load of articles. In such cases, it is found that the temperature of air exiting from the drum rapidly increases in temperature and de-energisation of the heater occurs before the articles have reached a desired state of dryness. The amount of lagging will depend upon the minimum load to be catered for.

It will be appreciated that by suitable choice of components, the energisation of the heaters HC1 and HC2 and the field windings of the motor could be controlled directly without the use of relay RL1, in this case thyristor TH1 would be replaced by a triac of suitable current carrying capacity and necessary circuit modifications effected.

WHAT WE CLAIM IS: 1. A tumbler dryer comprising a heater for heating air prior to entry of the air into the drum of the dryer, a thermal cut-out associated with the heater and operable to disconnect the heater on detection of ther

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the position of the tapping point of potentiometer R6. The adjustment of the tapping point is under the control of the user who sets the position in accordance with whether the articles are to be fully dried or merely "damp" dried. Having loaded the machine with the articles to be dried, and with the machine connected to a source of electricity, the user closes the door of the dryer and this action closes microswitch S1. The user then operates microswitch S2 by means of its control button. Closure of microswitch S2 completes a circuit through the winding of relay RL1 and input R of the electronic control 1 to the anode of thyristor TH1. At the same time the electronic control circuit is also energised and an output appears at terminal 6 of amplifier MA and a potential of about 9 volts is applied to the trigger of thyristor TH1. The thyristor anode is supplied with ac via the circuit just described and during the positive half cycles of the ac supply the thyristor will conduct and the winding of relay RL1 will be energised and contacts L1, L2 and L3, L4 will close. During the negative half cycles of the ac supply, the winding of relay RL1 will remain energised by current flow through D3 induced by the decay of the magnetic field associated with the relay winding. This avoids "chatter" of the contacts of relay RL1. Closure of contacts L1, L3; L2, L4 energises the heater coils HC1 and HC2 and field windings of the motor M which rotates the drum of the dryer and the impeller so that heated air commences to flow through the drum and over articles therein. Closure of microswitch energises lamp SL2 and this remains alight even when the user has released the closing pressure on the push button of microswitch S2. Lamp SL3 also lights up at the commencement of a tumble drying cycle. When a desired degree of dryness has been effected, the potential difference across inputs 2 and 3 of the amplifier MA reverses and the latter rapidly switches to a state in which the output on terminal 6 becomes zero. During the next negative half cycle of the ac supply on the anode of thyristor TH1 the latter will cease to conduct and will remain non-conducting during succeeding positive half-cycles because of the absence of a trigger signal on the trigger electrode.The magnetic field of the winding of relay RL1 rapidly dies away, the induced current flowing via diode D3 as described above. Upon de-energisation of the winding of the relay RL1, contacts L1, L3 and L2, L4 open and the heaters HC1 and HC2 are de-energised. However, the motor M continues to be energised via a circuit including the contacts of the switch S3 controlled by the cool-run thermostat T1. When air exiting from the drum reaches a temperature substantially equal to ambient temperature, thermostat T1 responds and opens the contacts of switch S3 and the motor stops. At the same time, signal light SL3 becomes extinguished thus indicating to the user that the machine has now reached the end of its drying cycle. The user can then open the door of the dryer to remove the articles from the drum. Opening the door opens microswitch S1 which disconnects lead L and power from the dryer and the circuit shown in Figure 2. Conveniently, thermistor X1 is located adjacent to the loading port in the front panel at a point where the thermistor is exposed to the flow of air exiting from the drum. Thermistor X2 is located on the front panel below the loading port at a point where it cannot be influenced to any substantial extent by air exiting from the drum. Thermistor X2 is thus responsive to changes in ambient temperature only. It will be apparent that it is not necessary that air entering the drum should actually flow over thermistor X2. Alternatively, thermistor X2 may be positioned in the impeller air flow path upstream of the heaters HC1 and HC2 ut spaced from the latter by a distance sufficient to ensure that the thermistor is unaffected by the heat from the heaters. The thermistors X1 and X2 are, respectively, types 2322/640/90004 and VA 1039. It will be desirable, in most cases, to lag thermistor X1 thermally in order to delay slightly its response to changes in the temperature of air exiting from the drum. Such lagging avoids the premature deenergisation of the heater that otherwise occurs when the dryer is operating with a small load of articles. In such cases, it is found that the temperature of air exiting from the drum rapidly increases in temperature and de-energisation of the heater occurs before the articles have reached a desired state of dryness. The amount of lagging will depend upon the minimum load to be catered for. It will be appreciated that by suitable choice of components, the energisation of the heaters HC1 and HC2 and the field windings of the motor could be controlled directly without the use of relay RL1, in this case thyristor TH1 would be replaced by a triac of suitable current carrying capacity and necessary circuit modifications effected. WHAT WE CLAIM IS:

1. A tumbler dryer comprising a heater for heating air prior to entry of the air into the drum of the dryer, a thermal cut-out associated with the heater and operable to disconnect the heater on detection of ther

mal overload conditions, and means for controlling the heater after the commencement of a drying cycle, the means comprising a first electronic sensing device responsive to ambient air temperature (as herein defined), a second electronic sensing device responsive to the temperature of air that has passed through the drum during a drying cycle, and an electronic control circuit responsive to the first and second devices and adapted, at the commencement of a drying cycle, to produce a control signal to cause energisation of the heater and to maintain the control signal in an unchanged condition until a predetermined difference is detected between the temperatures sensed by the two sensing devices when the control signal is modified to cause de-energisation of the heater.

2. A tumbler dryer as claimed in claim 1 in which the dryer includes an impeller for circulating air over the heater and into the drum, and in which the first device is located in the impeller air flow path and upstream of the heater.

3. A tumbler dryer as claimed in claim 1 or 2 in which the dryer includes a loading port giving access to the interior of the drum, and in which the first device is located adjacent the loading port in a Position such that the first device is not affected to any substantial extent by the air that has passed through the drum.

4. A tumbler dryer as claimed in claim 3 in which the second device is positioned adjacent the loading port at a location where it is exposed to the flow of air from the drum.

5. A tumbler dryer as claimed in claim 2 in which the dryer includes a casing having a front panel member with a loading port giving access to the interior of the drum, in which the dryer includes an impeller for circulating air over the heater and into the drum, and in which the first device is located in the impeller air flow path upstream of the heater and the second device is positioned adjacent the loading port at a location where it is exposed to the flow of air from the drum.

6. A tumbler dryer according to any one of the preceding claims and further comprising a user operated control operable, in use, to modify the predetermined difference.

7. A tumbler dryer as claimed in any one of the preceding claims in which the dryer includes a driving motor for rotating the drum and means for periodically reversing the direction of rotation of the drum during a drying cycle.

8. A tumbler dryer as claimed in any one of the preceding claims in which the second device is thermally lagged to an extent sufficient to prevent premature deenergisation of the heater when the dryer is operating with a small load of articles.

9. A tumbler dryer substantially as herein described with reference to and as illustrated by the accompanying drawings.