JPS6159044B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides protection for effectively protecting a small-capacity electric motor driven at low voltage and a circuit for driving the electric motor from accidents such as burnout due to abnormal load conditions. Regarding equipment.

Electric trolley transport devices used in factories, buildings, etc. have low voltages, such as 24 V DC powered by trolleys, as electric motors to drive the trolleys.
In addition, a small electric motor with a small capacity, for example, about 100 watts, is used. When starting a small motor like this, the rated voltage is applied directly, so the starting current is large, and when the motor is forced to stop while the drive current is still applied, the lock current is equal to the rated current. It is about 200% to 300% and does not cause excessive current, but its low efficiency causes large internal loss, and its small size and small heat capacity means that it will burn out within 30 seconds to 1 minute if left in a locked state. There is.

Furthermore, as described above, since the driving current for this motor is often supplied from a trolley or the like, short-circuit accidents are likely to occur.

Therefore, it is desirable to realize a protection device that can effectively prevent the burnout accident, power short circuit accident, etc. of the small electric motor as described above.

Traditionally, this type of protection devices include thermal relays, fuses, electromagnetic shields, and disconnectors, but all of them are either thermal type that uses the heat of the detection part or electromagnetic type that uses position. It has the disadvantage that it takes time to return after activation, and furthermore, carts using the aforementioned small electric motor generally start and stop frequently, and also move not only horizontally but also vertically, resulting in large load fluctuations. Malfunctions tended to occur due to large currents at start-up and current fluctuations due to loads.

However, in order to prevent malfunctions as mentioned above,
If the protective operation level setting value is increased, the current value when a small motor is locked is a relatively low multiplier of several times the rated current, as described above, so a protective operation failure may occur. , which is generally undesirable because the protection operation time becomes longer.

On the other hand, if the set value of the protective operation level is decreased in order to prevent the malfunction and ensure protection of the small electric motor, malfunction is likely to occur due to the load fluctuations as described above.

Because of these conflicting problems, there has been no device that can effectively protect a circuit including a small electric motor subject to large load fluctuations.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a protection device which can solve the above-mentioned problems.

Generally speaking, the motor protection device according to the present invention reliably separates and detects the current level during overload and the current level when the motor is locked, which are relatively close to each other, and performs protective actions accordingly. In order to A device with a relatively low multiplier level (motor lock level) of several times the rated current level and a short time constant (short time limit) is installed in parallel to ensure reliable operation only in each operating range, and further as necessary. A device that operates instantaneously at a high response level (short circuit) is also installed, and the charge stored in devices with long and short time constants to determine the time constant is shortened at the same time as the input voltage drops. Provide a circuit that discharges in time,
It can effectively and reliably protect all loaded circuits, including the power supply, from accidents such as burnout due to overload and short circuits, and can quickly return to the initial state even if the protective operation fails or after the protective operation. It's something you get.

FIG. 1 is a schematic circuit diagram showing an embodiment of a motor protection device according to the present invention. In the figure, 1 is a circuit to be protected, including a motor M, an electromagnetic shield, and a circuit breaker.
Contains NFB etc. 2, 3, and 4 are the detection parts of the protective device, respectively. 2 is a long time characteristic for low level overcurrent, 3 is a short time characteristic for low magnification level overcurrent, and 4 is an instantaneous operation characteristic for high level overcurrent. It is configured to have. Four of these circuits may be deleted if they can be replaced with electromagnetic shields or disconnectors. In addition, 5 is a rectifier for rectifying the current detection signal from the current detection current transformer CT, and CP is 2 and 3.
and 4 are comparators that compare the output signals from each circuit with a predetermined reference level.

The operation of this protection device is as follows. Taking the detection unit 2 as an example, the detection current from the current transformer CT is rectified by the rectifier 5 and applied to one input terminal of the amplifier Q _L via the input resistor R.
When the detection current increases, the input voltage V _IN to Q _L also increases, and when this input voltage V _IN exceeds the level setting voltage V _LL applied to the other input terminal of Q _L , the output terminal of Q _L V _L = (V _IN - V _LL ) x V _LT /
An output voltage V _L of R appears. Then, due to this voltage V _L , the terminal voltage V _CL of the capacitor C _L becomes the time constant T _L of the time constant circuit consisting of R _L2 and C _L = R _L2・C
When this voltage V _CL reaches the reference level of the comparator CP, _the transistor TR becomes conductive.
The protective relay RY is activated, closing its contact Ry ₁ and energizing the trip coil TC, which causes the electromagnetic disconnector NFB to close.
Protective operation is performed by opening the .

However, if the detected current decreases before the voltage V _CL reaches the reference level of the comparator CP, the voltage V CL
Since _IN also decreases, voltage V _L also decreases. and V _C
When _L >V _L , the charge stored in the capacitor C _L is discharged through a discharge circuit consisting of a resistor R _L1 and a diode D _L1 , as indicated by the broken arrow. Therefore, if the resistance value of R _L1 is set to a low resistance value to the extent that Q _L is not destroyed by the current, this discharge will occur with a small time constant of T = R _L1 · C _L , and the voltage V _CL will rapidly increase. descend. Here, it is assumed that the output resistance R _L2 of the amplifier Q _L and the input resistance of the comparator CP are sufficiently high values compared to the discharge resistance R _L1 .

The operation is similar in the detection unit 3, but the input level setting voltage V _SL of the amplifier Q _S is set higher than the voltage V _LL , and the charging time constant T _{S =}
Let R _S2 ·C _S be smaller than the time constant T _L .

In the detecting section 4, the input level setting voltage V _IL of the amplifier Q _I may be set as high as possible within a range that can ensure operation in the event of a short-circuit of the power supply, and no time-limiting element may be provided.

Furthermore, as shown in the figure, if the input level setting voltages V _LL , V _SL , and V _IL of each detection section are made variable, and the amplification degree of the amplifier is made variable using resistors V _LT , V _ST , etc., it is possible to The input setting level and operating time of each part can be changed accordingly.

Next, the overall operation of this embodiment will be explained with reference to FIG. Figure 2 shows the fluctuations in the detected current I due to various load conditions on the motor, and the associated voltages V _CL , V _CS , V in each detection section in Figure 1.
Graphs showing changes in _I , where the horizontal axis is time t and the vertical axis is current I or voltages V _CL , V _CS , V _I
shows.

In FIG. 2, first, when starting the motor, as shown in waveform 11, the detection current I is such that the V _SL level occurs but the V _IL level does not occur.
flows, but because the time is short, the capacitor voltage V
_CL and V _CS do not reach the reference level of comparator CP, as shown in waveforms 21 and 31, respectively.

Next, in an overload condition as shown in waveforms 12 and 13 where the detection current I flows, which causes V _LL but does not cause the V _SL level, the capacitor voltage V _CL increases as shown in waveform 12. comparator
If the overload condition is removed before reaching the CP reference level, the capacitor voltage V _CL will quickly return as shown in waveform 22, and if the overload condition continues for a predetermined period of time as shown in waveform 13. If this continues, the capacitor voltage V _CL will decrease as shown in waveform 23.
reaches the reference level of comparator CP, at which point a trip command is issued and a protective action is triggered. Furthermore, when the motor is locked, a detection current I flows that causes the V _SL level but not the V _IL level, as shown in waveform 14, and if this continues for more than a predetermined time, the capacitor voltage V _CS As shown by waveform 32, the voltage quickly rises to reach the reference level of the comparator CP, and a trip command is issued to trigger a protective operation. Furthermore, when a power supply short circuit occurs, if a detection current I that causes the V _IL level flows as shown in waveform 15, voltage V _I rises instantaneously as shown in waveform 41, and a trip command is issued. is released and a protective action is triggered.

FIG. 3 is a diagram showing the anti-time characteristic (solid line and dashed line) of the protection device according to the present invention in comparison with the anti-time characteristic (broken line) of the conventional protection device. As can be seen from this figure, the inverse time characteristic of the protection device according to the present invention is discontinuous at a predetermined current value. This also has the effect of ensuring protection against overcurrents at low magnifications, such as in the case of Furthermore, as shown in this figure, the characteristics of long time and short time can be changed by adjusting the resistance _VLT and/or resistance _VST , so it can be easily adapted to the characteristics of the applied equipment. can. Further, since a discharge circuit is provided, false detection will not occur even if the motor is started and stopped frequently. In addition, the protective operation relay shown in Figure 1
If the protection circuits below RY are made to be able to recover automatically, it becomes possible to automatically recover immediately after the protection condition is recovered.

[Brief explanation of the drawing]

FIG. 1 is a schematic circuit diagram showing an embodiment of a motor protection device according to the present invention. FIG. 2 is a graph for explaining the protection operation of the circuit in FIG. 1. FIG. 3 is a graph showing the inverse timing characteristics of the protection device according to the present invention using the circuit of FIG. 1 as an example. 1: Protected circuit, 2: Detection section (long time characteristic),
3: Detection section (short time characteristic), 4: Detection section (instantaneous characteristic), 10: Detection current graph, 20: Capacitor terminal voltage graph (long time characteristic), 30: Capacitor terminal voltage graph (short time characteristic), 40 : Amplifier output voltage graph (instantaneous characteristics).

Claims (1)

[Scope of Claims] 1. A device for overcurrent protection of a motor that is frequently started and stopped and whose load fluctuates frequently, comprising: a current detection unit that detects a current supplied to the motor; The voltage corresponding to the current detected by the motor is input, and this input voltage exceeds the operating level set to a value that is smaller than the input voltage due to overcurrent when the motor is locked and larger than the input voltage due to overcurrent when the motor is overloaded. a short time limit device that operates based on its inverse time limit characteristic and issues a trip command when When this input voltage exceeds an operating level set to a value smaller than the input voltage due to overcurrent at the time of motor overload, a long time limit device operates based on its inverse time limit characteristic and issues a trip command, and a motor power supply circuit. The short time limit device and the long time limit device each have an input level setting voltage and an amplification degree independent of each other. It has an adjustable sense amplifier and a time constant circuit for timed operation, each of which has a capacitor that can be discharged when the input voltage falls below the operating level. What is claimed is: 1. A motor protection device characterized in that a discharge circuit with a small time constant is provided, thereby making it possible to individually set each inverse time characteristic and repeating starting and stopping operations.