GB1594036A - Motor protection circuit - Google Patents

Description

(54) MOTOR PROTECTION CIRCUIT (71) I, FRIEDRICH KRIWAN, a citizen of the German Federal Republic, of 16 Wulfinger Strasse, 7119 Forchtenberg, Germany, do hereby declare the invention for which I pray that a patent may be granted to me and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a circuit arrangement for protecting motors against thermal overloading, of the kind comprising a switching relay, a control circuit for the switching relay and at least one PTC-resistor temperature sensor which is part of a voltage divider activating the control circuit, the control circuit being activated when a certain voltage divider ratio is reached, whereby the motor is switched off by the switching relay.

PTC-Resistor temperature sensors of the kind employed in this application undergo only a slight change in their resistance value in the range from -20"C to just below the nominal transition or cut-off temperature (for example 110 C). It is only just before the nominal cut-off temperature is reached that the resistance of the PTC-resistor temperature sensor increases sharply.

Accordingly, after cooling of the motor by only a few degrees (about 123 ), the resistance value of a PTC-resistor temperature sensor drops back below the value at which the motor is switched on again.

Although this readiness on the part of the motor to be rapidly switched on again is desirable for certain applications, the high switching frequency of the connected motor attributable to the small temperature difference between the cut-off point and the reconnect point is generally a disadvantage.

On the other hand, there are some installations in which the off-state must not be automatically locked in the event of a fault (as for example in the case of refrigeration compressors for refrigeration plants where a general switch-off would result in loss of the contents).

One possibility of, on the one hand, preventing a motor which has been switched off on account of thermal overloading from being switched on again too quickly whilst, on the other hand, ensuring that the motor is switched on again after a certain time, is to use additional time-lag relays which reduce the high switching frequency attributable to the PTC-resistor temp era- ture sensor. However, the disadvantage of this solution lies in the relatively high extra cost of providing additional time-lag relays such as these.

I have previously proposed a motor protection circuit in which the voltage divider ratio of the voltage divider which contains the PTC-resistor temperature sensor and which activates the control circuit of the switching relay is displaced by another resistor at the cut-off instant to reduce the reconnect value of the PTC-resistor temperature sensor. In this way, the reconnect value of the PTC-resistor temperature sensor is reduced by a certain temperature range with very simple switching means.

Further development of the motor protection circuit according to this earlier proposal has now shown that it is desirable to be able even further to reduce the reconnect temperature value as and when necessary or, in other words, to be able to lock the off-state for a longer, but nevertheless limited period.

Accordingly, the object of the present invention is to construct a motor protection circuit of the type described above in such a way that this requirement is satisfied.

According to the invention there is provided a circuit arrangement for protecting a motor against thermal overloading, comprising a switching relay, a control circuit for the switching relay, and a voltage divider including at least one PTC-resistor temperature sensor of a kind exhibiting a sharp increase in resistance above a predetermined transition temperature the divider activating the control circuit when a certain voltage divider ratio is reached whereby the motor is switched off by the switching relay, the divider further including a thermoswitch having a switching temperature which is substantially lower than the transition temperature arranged so as to lock the control circuit in the activated state during a substantial cooling period.

In the circuit arrangement of the invention, therefore, the low mass of the highly sensitive PTC-resistor temperature sensor on the one hand is used for rapidly switching off the motor in the event of a fault, whilst on the other hand the high mass of the machine initiates the reconnect operation through the thermoswitch, but only after a certain time interval (corresponding to the cooling of the machine). The need to reduce the reconnect frequency in the event of a persisting fault is satisfied in this way.

One example of an embodiment of the invention is illustrated in the accompanying drawings, wherein: Figure 1 is a basic circuit diagram of a circuit arrangement according to the invention, Figure 2 is a temperature-time diagram of the circuit arrangement in the absence of the thermoswitch T, and Figure 3 is a temperature-time diagram of the circuit arrangement in the presence of the thermoswitch T.

The illustrated circuit arrangement for protecting a motor (not shown) against thermal overloading essentially comprises a switching relay dl, a Schmitt trigger acting as control circuit for the switching relay and at least one PTC-resistor temperature sensor PTC which is arranged in good thermal contact with the winding of the motor to be protected.

The Schmitt trigger consists of the transistors pl and p2 and of the resistors r2, r3, r4 and r5.

The tripping device G is supplied with direct current at the terminals 4, 5 (the tripping device may of course also contain a built-in d.c. power supply unit).

The motor contactor and, optionally, an indicator are connected to the terminals 11, 12 and 14.

The PTC-resistor temperature sensor PTC lies between the terminals 1 and 2. The thermoswitch T and a protective resistor r7 are connected in series between the terminal 1 and a terminal 3 which is connected to the collector of the transistor p2.

Finally, the circuit contains a resistor rl which, together with the PTC-resistor temperature sensor PTC, forms a voltage divider which lies between the feed voltage terminals 4, 5 and whose tap (terminal 1) is connected to the base of the transistor pl.

The thermoswitch T is arranged for example in terminal boxes of the machine housing. It has a considerably lower switching temperature than the PTC-resistor temperature sensor PTC.

In order to explain the mode of operation of the circuit, its function will first of all be described for the case where the thermoswitch T is not provided (in this case, therefore, there is no connection between the terminals 1 and 3).

In the normal operation of the motor, the transistor pl is non-conductive and the transistor p2 conductive. The switching relay dl is excited; the motor is switched on.

The PTC-resistor temperature sensor PTC has a low resistance value so that the voltage drop occurring between the terminals 1 and 2 is not sufficient to make the transistor pl conductive.

If then the motor reaches its cut-off temperature of, for example, llO"C (cf. Fig. 2), the resistance value of the PTC-resistor temperature sensor PTC suddenly increases.

The voltage present at the terminals 1 and 2 exceeds the switching threshold of the transistor pl. This transistor pl thus becomes conductive whilst the transistor p2 is blocked. The switching relay dl drops out and switches off the motor.

If then the motor temperature falls for example to only around 105"C, the resistance value of the PTC-resistor temperature sensor PTC decreases to such an extent that the transistor pl becomes non-conductive again whilst the transistor p2 becomes conductive, so that the motor is switched on again. In this case, therefore, the motor is rapidly switched on and off between the temperatures of 105"C and 110"C (cf. Fig.

This high switching frequency, which is undesirable in many cases, is now prevented by the thermoswitch T additionally provided in the circuit arrangement according to the invention.

When the motor is started up, the thermoswitch T is initially open because its response temperature, for example 70"C, has not yet been reached. If this response temperature is reached during operation, the closure of the thermoswitch T is initially inconsequential because the PTC-resistor temperature sensor PTC has not yet activated the Schmitt trigger. If then the PTCresistor temperature sensor PTC reaches its switching temperature (110 C in the assumed case), it activates the Schmitt trigger T in the manner already explained and switches off the motor through the switching relay dl.

The result of the interruption in the collector current of the transistor p2 hitherto flowing through the switching relay dl is that the operating voltage (positive potential of the terminal 4) appears at the terminal 3. As a result, the series circuit formed by the closed thermoswitch T and the protective resistor r7 is now in parallel with the resistor rl of the voltage divider (hitherto formed by the resistor rl and the PTCresistor temperature sensor PTC). As a result, the voltage divider ratio is displaced to such an extent that the reduction in the resistance value of the PTC-resistor temperature sensor PTC, which occurs during the subsequent cooling of the motor, does not result in reactivation of the Schmitt trigger.

Instead, the transistor pl of the Schmitt trigger remains conductive (and hence the transistor p2 blocked) during a substantial cooling period until the machine temperature has fallen to the switching value of the thermoswitch T (i.e. to a temperature of around 70"C in the assumed case). At this instant, the thermoswitch T opens so that the voltage divider formed by the resistor rl and the PTC-resistor temperature sensor PTC is reactivated, resulting in immediate activation of the Schmitt trigger so that the motor is switched on again.

This provides for the much lower switching frequency shown in Fig. 3 (switching on and off between the temperatures of 70 and 1 10 C).

WHAT I CLAIM IS:- 1. A circuit arrangement for protecting a motor against thermal overloading, comprising a switching relay, a control circuit for the switching relay, and a voltage divider including at least one PTC-resistor temperature sensor of a kind exhibiting a sharp increase in resistance above a predetermined transition temperature the divider activating the control circuit when a certain voltage divider ratio is reached whereby the motor is switched off by the switching relay, the divider further including a thermoswitch having a switching temperature which is substantially lower than the transition temperature arranged so as to lock the control circuit in the activated state during a substantial cooling period.

2. A circuit arrangement as claimed in Claim 1 in which the control circuit is in the form of a Schmitt trigger, the switching relay is arranged in the collector circuit of one transistor of the Schmitt trigger and the PTC-resistor temperature sensor is connected to the base of the other transistor of the Schmitt trigger, a protective resistor and the thermoswitch being connected in series between the collector of the first transistor and the base of the other transistor.

3. A motor protection circuit substantially as hereinbefore described with reference to and as shown in Figure 1.

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

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. resistance value of the PTC-resistor temperature sensor PTC, which occurs during the subsequent cooling of the motor, does not result in reactivation of the Schmitt trigger. Instead, the transistor pl of the Schmitt trigger remains conductive (and hence the transistor p2 blocked) during a substantial cooling period until the machine temperature has fallen to the switching value of the thermoswitch T (i.e. to a temperature of around 70"C in the assumed case). At this instant, the thermoswitch T opens so that the voltage divider formed by the resistor rl and the PTC-resistor temperature sensor PTC is reactivated, resulting in immediate activation of the Schmitt trigger so that the motor is switched on again. This provides for the much lower switching frequency shown in Fig. 3 (switching on and off between the temperatures of 70 and 1 10 C). WHAT I CLAIM IS:-

1. A circuit arrangement for protecting a motor against thermal overloading, comprising a switching relay, a control circuit for the switching relay, and a voltage divider including at least one PTC-resistor temperature sensor of a kind exhibiting a sharp increase in resistance above a predetermined transition temperature the divider activating the control circuit when a certain voltage divider ratio is reached whereby the motor is switched off by the switching relay, the divider further including a thermoswitch having a switching temperature which is substantially lower than the transition temperature arranged so as to lock the control circuit in the activated state during a substantial cooling period.

2. A circuit arrangement as claimed in Claim 1 in which the control circuit is in the form of a Schmitt trigger, the switching relay is arranged in the collector circuit of one transistor of the Schmitt trigger and the PTC-resistor temperature sensor is connected to the base of the other transistor of the Schmitt trigger, a protective resistor and the thermoswitch being connected in series between the collector of the first transistor and the base of the other transistor.

3. A motor protection circuit substantially as hereinbefore described with reference to and as shown in Figure 1.