DE3009090A1 - Control circuit for drill and suction device motors - uses comparator to operate triac in supply line to regulate start-up - Google Patents

Abstract

The circuit for limiting the starting current for two electric motors, e.g. the motor driving a drill and a second motor for a suction device, includes a transformer connected across the mains supply, followed by a full wave rectifier, smoothing capacitor and variable resistance to supply a reference current to a comparator. In one of the supply lines is a triac, preceded by the primary of a transformer for a full wave rectifier with an ohmic resistance across it. This supplies a signal corresponding to the motor current to the other input of the comparator. The output of the comparator is fed to a logic circuit followed by a control circuit with a control connection to the triac. The two motors are started one after the other, the starting current for one being higher than that for the other. The supply to the triac is reduced to a given value when the comparator has established a larger instantaneous value for this current during a certain period of time.

Description

Circuit to limit the motor

lassstrom Description The invention relates to a Circuit for limiting the motor starting current according to the preamble of the claim 1.

It is known that when an electric motor is switched on, it is used for draws a particularly high starting current for a short period of time. If the engine is from one via household automatic circuit breakers or line plug-in switches according to UDE 0641, hereinafter always referred to as household automatic circuit breakers for short, powered network operated it may happen that the size and duration of the starting current are sufficient to to have the domestic automatic circuit breaker respond. Working with such a Motor is extremely annoying and time consuming, depending on the operating conditions of the Household automatic circuit breaker responds once and once when the engine is started does not respond, but when it has responded, it is switched on again every time must become.

To inadmissibly high starting currents from the household AC network to To avoid operating electric motors, circuits are already limiting of the starting current has become known. With such a circuit, the motor current felt, and if the permissible limit current is exceeded, the supply voltage for the motor is reduced. These circuits are only at Switching on the motor is effective.

In a circuit known from DE-OS 26 31 418 for limiting of the starting current is switched on when the motor is connected but the motor switch is open Current regulator activated in such a way that when you turn on the motor operating switch a The motor voltage is reduced from the outset, which corresponds to the Discharge curve of a capacitor increases. This circuit also has a Control circuit on which a set-actual value comparison for during engine operation carries out the operating current carried by the motor and the motor current if it is exceeded of a permissible value, but reacts relatively sluggishly.

The known circuits are all unbranchable when the consumer consists of two or more electric motors that are switched on one after the other and in which the second or subsequent motor has a significantly higher one Starting current as the first motor draws. As the well-known protective circuits when starting of the first engine respond, ignore starting the second engine, like this that then in the described, undesirable way of the household automatic circuit breaker can address.

The invention is based in particular on this disadvantage to avoid and a circuit to limit the starting current according to the preamble of claim 1 mentioned type in such a way that the circuit in any case when starting a second or further motor, its starting current is limited without to prevent a fuse from responding in the event of persistent overload conditions during operation.

This task is achieved by the characteristics of the characterizing part of the Claim 1 solved.

The invention has the advantage that if after turning on the first motor a second or further motor is switched on, a current limitation also takes place when switching on the second or further motor and not how in the known circuits, the starting current of the second motor is ignored or is detected too late. The motors are preferably fed from an alternating current network AC motors. Especially in drilling rigs with a drill stand with a Electric drill motor and a vacuum suction device to fix the drill stand on the borehole, where a second electric motor is used to generate the negative pressure is provided, the invention can be used to advantage. Such drilling rigs are often used in buildings with domestic automatic circuit breakers secured AC power lines operated. Becomes after switching on the weaker Motor for generating the negative pressure to fix the drill stand on the drilling substrate the more powerful drilling motor is switched on, so its starting current is limited to such an extent that that the household automatic circuit breaker does not respond. By the fact that after exceeding the maximum permissible current through the starting current is the supply voltage for the motors is initially lowered to a specified value, at which it is ensured that that a household automatic circuit breaker does not respond, and then during a given Period of time is gradually increased back to its setpoint, occurs very quickly a current limitation, but without this in the case of prolonged overload conditions the triggering of the fuses would be prevented.

For example, the circuit for limiting the motor starting current occurs due to a short circuit or a very high motor current due to jamming of the Drill in the drilling subsurface in function, then occurs in the last phase of the lifting the motor supply voltage has a correspondingly high motor current for a sufficiently long time, to trigger an overload protection.

While the supply voltage after being lowered after a response the circuit for limiting the motor starting current preferably again in a ramp shape is increased, the voltage can also be increased according to another function, as long as it is ensured that the response of the domestic automatic circuit breaker is suppressed due to the starting current.

The invention is of course also applicable to other types of fuse than domestic automatic circuit breakers applicable if these permit a high overload current for a short period of time, without addressing.

Preferred refinements of the invention are set out in the subclaims claimed.

The following is a preferred embodiment of the invention with reference to the drawings, with reference to the disclosure because of the great simplicity and clarity of the drawings this is referred to.

Fig. 1 shows a circuit diagram partially shown as a block diagram an embodiment according to the invention; Figures 2a, b and c show Curves for the course of the motor current after rectification and the motor voltage; 3 shows an exemplary time / current diagram for the response behavior of a household circuit breaker.

Reference is initially made to FIG. 1. After that are two single-phase AC motors 1 and 2 of a drilling device, not shown, are provided. The drill owns a drill stand, which is connected to the drilling subsurface by means of a negative pressure announcement device can be sucked in, so that the drilling device can thereby be used for the drilling process on the drilling substrate can be set. The first motor 1 is provided to reduce the negative pressure in the Generate suction device.

The drilling device also has the second motor 2 as a drilling drive motor who has a higher power and also a higher starting current than the Engine 1.

The motor 1 is via a switch 3 and the motor 2 via a switch 4 connected to an alternating current network 5, which is via conventional household automatic circuit breakers is secured. A current limiting value circuit 6 has one his Primary side connected to the network voltage converter 7, one to the secondary side of the converter 7 connected full-wave rectifier 8 and one at its output lying smoothing capacitor 9. By means of the grinder one parallel to the Capacitor 9 lying variable resistor 10 can have a limit current Ilimit can be set as reference current. The grinder of variable resistance 10 is connected to the positive input of a comparator 11. In the power supply line A current transformer 12 is switched on with its primary side for motors 1 and 2, to the secondary side of which a resistor 13 is connected in parallel as a burden. Behind the resistor is connected to a full-wave rectifier circuit 14, the direct current output of which is led to the negative input of the comparator 11. The converter 12, the resistor 13 and the full-wave rectifier circuit 14 together form a sensing circuit 12a for the current carried by motors 1 and / or 2.

The output of the comparator is to an input of a logic circuit 15, the output of which is in turn fed to the input of a control circuit 16 is. The logic circuit 15 and the control circuit 16 will be described below in individually explained.

The output of the control circuit 16 forms a voltage control signal for an actuator forming and in the power supply line to the motors 1 and 2 lying triac 17, with the help of which the motor supply voltage for motors 1 and 2 can be changed.

The voltage set by the triac 17 is transmitted via lines 18 and 19 fed back to the control circuit 16.

The circuit described above works as follows: From the motors 1 and 2 of the drilling rig, the motor 1 is first activated by means of its operating switch 3 switched on.

As a result, a negative pressure is generated in the suction device and that The drilling rig can be fixed to the drilling substrate using the suction device. Of the Motor 1 usually remains switched on during the entire drilling process to during to fix the drilling device during the entire drilling process via the vacuum created in this way. There the power of the motor 1 is small, its starting current is also not very large, so that as a rule it does not need to be limited. Is a starting current however once greater than the limit current Ilimit, then its starting current will continue in the limited manner described below for the motor 2.

After the drilling rig has been fixed to the drilling substrate, the Drill motor 2 switched on by means of its operating switch 4. The drill motor 2 has a higher power consumption and therefore pulls you in the moment it is switched on larger to let current than the motor 1. This occurs with current peaks, the usual Can address household safeguards.

The total current (motor current of motor 1 and starting current of the motor 2) is sensed by the sensing circuit 12a and the negative input as the actual value of the comparator 11 is supplied.

As can be seen from Fig. 2a, when the second is switched on Motor 2 has a very high actual current Imax for some periods, which is above the limit current limit set at the variable resistor 10 border lies. The limit current f is in turn considerably higher than the nominal current n der occurs in the course of operation of the two motors. With the variable resistance 10 such a limit current #limit is set to ensure that that a common household circuit breaker for a given amperage in does not appeal to the practice.

When the motor 2 is switched on, the maximum permissible current Ilimit the comparator indicates that the limit actually determined is exceeded 11 sends the signal L to the logic circuit 15 at its output.

The logic circuit 15 ensures that only a first overshoot of the current Ilimit through the current Iist is detected during a predetermined period T and that further exceedances of 1 by Iist while limit is the predetermined Duration T, the beginning of which is determined by I when I limit is exceeded for the first time is to be ignored. After the signal L from the comparator 11 to the logic circuit 15 was given, this in turn gives an output to the control circuit 16, whereupon this the supply voltage by means of the triac 17 to a predetermined minimum value Umin can drop and the supply voltage by means of the triac during a predetermined Time T can increase approximately ramp-shaped from Umin back to Umax. The start-up the voltage during the period T from Umin to Umax is a pure control process, which, once triggered, from the current carried by motors 1 and 2 is not affected.

The control voltage of the control circuit 16 is fed to the triac 17, with the help of which according to the control voltage of the control circuit 16 at the beginning of the starting process of motor 2, the motor supply voltage is reduced and is ramped up to the nominal voltage.

In Fig. 2b it is shown how the operating current of motors 1 and 2 is lowered after the response of the control circuit. It can be seen that the maximum permissible current Ilimit exceeded during the first two half-waves but the operating current is then reduced to a value below 1.

limit From Fig. 2c it can be seen how the supply voltage for the Motors by means of phase control to lower the starting current initially is decreased and then increased again approximately in the form of a ramp by means of the triac 17.

3 shows an exemplary time / current diagram for the response behavior a household automatic circuit breaker. It can be seen from this that the household automatic circuit breaker the longer an overcurrent can carry without it being claimed, the smaller it is is the ratio of the conducted current I to the tripping current IaUsl. From the diagram it can also be seen that the automatic household circuit breaker, without addressing, can also carry currents up to 10 times the tripping current, if only the duration during which the overcurrent is conducted is sufficiently short.

This means that the current limiting circuit for the two motors 1 and 2 of FIG. 2 an excessively large starting current when switching on the engine 2 only has to limit it quickly enough to avoid unnecessary activation of the domestic automatic circuit breaker to avoid.

L e r s e i t e

Claims (6)

Circuit for limiting the motor starting current with a circuit for sensing the motor current, an actuator for adjusting the Motor supply voltage, a circuit for controlling the actuator and a comparator, the current sensed by the sensing circuit with a predetermined value for the Motor current compares that of the circuit two or more motors (1, 2) are connected downstream to limit the starting current, which are started one after the other and of which the second or each subsequent one Motor (2) draws a higher starting current than the first motor (1) that the circuit to control the actuator (17) the supply voltage via the actuator to one predetermined value (Umin) decreases when the comparator (11) has a larger instantaneous value of the current as the specified value (Ilimit) for the motor current limit and a control circuit (16) which controls the supply voltage via the actuator (17) back to theirs after they have been lowered for a predetermined period of time (T) Setpoint (Umax) increases. 2. A circuit according to claim 1, d a d u r c h g e k e n n z e i c h n e t that as an actuator (17) semiconductor elements for loss-free adjustment the motor supply voltage are provided. 3. A circuit according to claim 2, d a d u r c h ge e n n z e i c h n e t that the semiconductor elements have a triac (17) for phase control. 4. A circuit according to claim 2, d a d u r c h g e k e n n z e i c h n e t that the semiconductor elements have thyristors for phase control. 5. Circuit according to one of claims 1 to 4, d a d u r c h g e it is not indicated that the control circuit supplies the supply voltage via the actuator (17) can increase approximately ramp-shaped after lowering the supply voltage. 6. Circuit according to one of claims 1 to 5, d a d u r c h g e it is not indicated that the control circuit supplies the supply voltage via the actuator over a period of about 1 to 10 seconds, preferably about 3 seconds, increases.