JPH04236121A - Motor protective circuit - Google Patents

Abstract

PURPOSE:To change the time required for stopping a motor in accordance with the magnitude of an overcurrent when the overcurrent flows to the motor. CONSTITUTION:The electric current flowing to a motor is detected by means of three comparators 21, 22, and 23 having different reference values. The comparator 21 evolves an output when a large overcurrent flows to the motor and the comparator 22 evolves another output when the overcurrent is small. The output time of each comparator 21, 22, and 23 is respectively measured by means of a pulse generator 27, frequency dividing circuits 28, 29, and 30, and counter circuits 31, 32, and 33. When an output is made for a relatively short period from the comparator 21, for a relatively intermediate period from the comparator 22, or for a relatively long period from the comparator 23, a protective circuit 200 is actuated and the rotation of the motor is stopped.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] This invention relates to a motor protection circuit, and more specifically, to a motor protection circuit for protecting a motor from being damaged by overcurrent when an overcurrent flows through the motor due to load fluctuation. It relates to protection circuits.

0002

2. Description of the Related Art If a motor is stopped during operation or the load fluctuates, an overcurrent may flow and cause damage to the motor. To this end, motor protection circuits have been proposed. An example of a conventional motor protection circuit is JP-A-2-
It is disclosed in Japanese Patent No. 65692. The disclosed motor protection circuit stops or reverses the motor when the motor current exceeds a predetermined reference value, thereby preventing damage to the motor.

0003

[Problem to be Solved by the Invention] The motor protection circuit described in the above-mentioned publication defines a single reference value and stops or reverses the motor based on whether the motor current exceeds the reference value. There is. Therefore, it was difficult to set standard values. This is because if the reference value is set too high, there is a risk of damaging the motor due to a large motor current, whereas if the reference value is set too low, the protection circuit will be activated immediately and the motor will be damaged by even a slight overload. This is because the system may be stopped, which poses a practical problem. In particular, when a motor is used for many years, the load gradually increases due to changes in bearing oil, etc., and the flowing current tends to gradually increase. If the reference value is set low in a conventional motor protection circuit, there is a drawback that it is difficult to sufficiently cope with such aging deterioration.

Furthermore, since the characteristics of motors vary from product to product, when trying to set appropriate reference values,
Standard values must be adjusted for each product, which is time-consuming. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve the above-mentioned technical problems and to provide a motor protection circuit that can stop the motor only when an overcurrent that causes damage to the motor flows. .

[0005]

[Means for Solving the Problems] The present invention provides a protection circuit for connecting to a motor, which includes a motor current detection means for detecting the amount of current flowing through the motor, a plurality of mutually different reference values, and a protection circuit for detecting the amount of current flowing through the motor. Measurement is started when the detection output of the setting means and motor current detection means exceeds a reference value, and the time that has exceeded reaches the limit time set corresponding to the reference value. The present invention is characterized in that it includes a determining means for determining whether or not a limit time has been reached, and a motor stopping means for stopping the motor in response to the determining means determining that the limit time has been reached.

The present invention also provides that in the motor protection circuit, the discrimination means includes a plurality of comparison means provided for each reference value and a clock means for measuring time in response to the output of each comparison means. This is a characteristic feature.

[0007]

[Operation] According to the above-mentioned structure, since a plurality of reference values are set, even if the current flowing through the motor increases due to load fluctuation, it is possible to deal with it in accordance with the amount of increase in the current. Furthermore, if the motor current exceeds one of the reference values, the motor will be stopped only when the exceeding time reaches the limit time set corresponding to that reference value. Even if an overcurrent flows, if the motor does not suffer any loss due to the overcurrent, the protection circuit will not operate.
The protection circuit is activated only if substantial damage to the motor occurs.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a diagram of a motor control circuit equipped with a protection circuit according to an embodiment of the present invention. The motor control circuit shown in FIG. 1 includes a drive circuit 100 and a protection circuit 200. The drive circuit 100 has a transistor bridge including four transistors 1, 2, 3, and 4, and the servo motor M
can be rotated forward and reverse. Note that a diode 5 is connected in parallel to each transistor 1, 2, 3, and 4.
, 6, 7, and 8 are protection diodes for protecting each transistor 1, 2, 3, and 4 from induced currents generated when the servo motor M decelerates, and 9, 10, 11, and 12 are emitter voltage resistances. be.

A rotation direction switching signal is applied to the input terminal 13 of the drive circuit 100. When this switching signal is at a high level, the servo motor M rotates in the forward direction, and when at a low level, the servo motor M rotates in the reverse direction. Further, the motor drive signal is given to the input terminal 14,
It becomes one input of the AND gate 15. and gate 15
The output of the protection circuit 200 is given to the other input. As will be described later, the output of the protection circuit 200 is at a high level under normal conditions, so the drive signal applied to the input terminal 14 is output as is through the AND gate 15 during normal conditions.

[0010] The output of the AND gate 15 is at a high level,
When a high level signal is applied to input terminal 13, transistors 1 and 2 are turned on, current flows in the direction of arrow a to servo motor M, and servo motor M rotates normally. On the other hand, when the output of the AND gate 15 is at a high level and a low level signal is applied to the input terminal 13, transistors 3 and 4 are turned on, and a current flows in the direction of arrow b to the servo motor M. is reversed.

Note that when the output of the AND gate 15 is at a low level, energization of the servo motor M is prohibited;
Servo motor M stops. Next, the configuration and operation of the protection circuit 200 will be explained. Drive circuit 100
is grounded via a current detection resistor 19. Therefore, in proportion to the amount of current flowing through the servo motor M, the potential at the connection point A between the drive circuit 100 and the current detection resistor changes.

Drive circuit 100 and current detection resistor 19
A protection circuit 200 is connected to the connection point A. For example, the protection circuit 200 includes three comparators 21 and 2.
2 and 23 are provided, and the voltage detected at the connection point A is given to three comparators 21, 22, and 23, respectively. Each comparator 21, 22, 23 converts the detected voltage into a reference voltage Vref1, Vref2, Vref, respectively.
3, and outputs an output when the detected voltage exceeds the reference voltage. Each comparator 21
, 22, 23 are outputted from AND gates 24, 23, respectively.
At 25 and 26, the AND with the reference pulse generated from the pulse generator 27 is taken, and the frequency dividing circuits 28 and 2
Given to 9,30. Each frequency dividing circuit 28, 29, 30 is set with a different frequency division ratio T1, T2, T3, and the output frequency divided by each frequency dividing circuit 28, 29, 30 is used as a clock input to the corresponding counter circuit. 31, 32, 3
given to 3. Three counter circuits 31, 32, 33
are all circuits that derive a count-up output when counting clocks of "4", for example. The output of each counter circuit 31, 32, 33 is the OR gate 3
4, is inverted by an inverter 35, and is applied to the AND gate 15 of the drive circuit 100.

The frequency dividing circuit 28 in the protection circuit 200
, 29, 30 and counter circuits 31, 32, 33 constitute three timer circuits, and three time periods are measured as will be described later. FIG. 2 is a timing chart for explaining the operation of the circuit shown in FIG. 1, particularly the operation of the protection circuit 200. Next, a description will be given with reference to FIGS. 1 and 2. The potential at the connection point A changes depending on the amount of current flowing through the servo motor M. That is, the detected voltage V is the value obtained by multiplying the current flowing through the servo motor M by the resistance value of the current detection resistor 19. The detection voltage V is determined by three comparators 2
1, 22, and 23, the reference voltage Vref
1, Vref2, and Vref3.

Here, the reference voltage Vr of the comparator 21
In ef1, the servo motor M is restrained and the servo motor M
The voltage is set to be close to the voltage that appears at connection point A when a stall current flows through the terminal. Therefore, servo motor M
An output is derived from the comparator 21 only when a stall current flows through the comparator 21. In addition, the reference voltage Vref2 of the comparator 22 indicates that although the servo motor M is not restrained, the servo motor M is subjected to a considerably higher load than the normal load, and the current flowing through the servo motor M is considerably larger than the normal current. It is set to a value such that an output is derived from the comparator 22 when this happens. Furthermore, the reference voltage Vref3 of the comparator 23 is
The value is set such that an output is derived from the comparator 23 when a load slightly larger than the normal load is applied to the servo motor 100 and the current flowing through the servo motor M becomes slightly larger than normal.

Furthermore, the three frequency dividing circuits 28, 29, and 30 have mutual frequency division ratios such as T1<T2<T3.
The frequency division ratio is set to be smaller for a frequency divider circuit that is provided with a comparator output having a relatively higher reference voltage. Also,
As described above, the three counter circuits 31, 32, and 33 to which the outputs of the frequency dividing circuits 28, 29, and 30 are applied derive count-up outputs when they all count the same number of clocks.

Therefore, as shown in FIG. 2, when the output is derived from the comparator 21, the count-up output of the counter circuit 31 is derived when time t1 has elapsed.
The high level signal outputted through the OR gate 34 is inverted by the inverter 35, one input of the AND gate 15 becomes low level, and the servo motor M is stopped. Further, when an output is derived from the comparator 22, a count-up output of the counter circuit 32 is derived at the time point when time t2 has elapsed, and the servo motor M is stopped. Further, when an output is derived from the comparator 23, a count-up output from the counter circuit 33 is derived after time t3 has elapsed, and the servo motor M is stopped.

When the protection circuit 200 of this embodiment is provided, there are the following advantages. For example, servo motor M
When the motor is a motor for driving an optical system (a halogen lamp for document illumination, a reflector, etc.) in an electrophotographic copying machine, the motor load is a slider that reciprocates along the stay 31, as shown in FIG. Contains 32. When the slider 32 moves while being guided by the stay 31, the frictional resistance of the slider 32 varies depending on the surface condition of the stay 31, so the motor load changes. If the normal load value is set when the surface of the stay 31 is smooth, if the surface of the stay 31 becomes somewhat rough due to use etc., it becomes overload 1, and when the surface roughness increases, it becomes overload 2. .

When the motor is operated under such overload 1 or overload 2, as shown in FIG. 4, a relatively large current flows through the motor compared to when the motor is under normal load. On the other hand, as shown in Figure 5, the relationship between continuous operation time and motor temperature rise varies depending on the load value, and the larger the overload, the faster the motor temperature will rise, causing burnout of the armature winding. This may cause circuit damage, etc.

Therefore, according to this embodiment, if an abnormality occurs such as when the motor is locked, the motor can be stopped in a very short time based on the output of the comparator 21, and the motor can be stopped due to aging. The motor load changes due to distortion of the object controlled by the motor or changes in friction, etc., and even if no abnormality occurs such as the motor being restrained, an overcurrent flows to the motor and the motor is damaged. If there is a risk, the motor can be stopped.

As shown in FIG. 4, when the motor starts rotating, an inrush current (a large current almost equal to the stall current) flows, but the inrush current momentarily flows and the comparator 21
Even if the output is derived, it is instantaneous, so the counter circuit 3
A count-up output is never derived from 1. Further, in FIG. 1, a reset signal to each counter circuit 31, 32, 33 is given each time the servo motor M completes one operation pattern. For example, in the case of an optical system drive motor, a reset signal is given every time one scan is completed.

In this embodiment, as shown in FIG. 1, three comparators 21, 22, and 23 are provided to determine the magnitude of the current flowing through the servo motor M using different reference voltages Vref1, Vref2, and Vref3, respectively. The number of comparison circuits such as comparators may be plural, for example, two or four or more. Also,
In this embodiment, a pulse generator 27, a frequency dividing circuit 28,
29, 30 and counter circuits 31, 32, 33 constitute a timer circuit that measures three different times,
The timer circuit may be configured by other circuits.

Furthermore, the current flowing through the motor is not detected by converting it into a potential as in the above embodiment, but by directly measuring the current value with an ammeter, etc., and comparing it with a plurality of reference current values. You can also do this. In addition, various design changes can be made to the present invention without changing the gist of the invention. Moreover, the motor to which this circuit is applied may be used for any purpose.

[0023]

[Effects of the Invention] As described above, according to the present invention, a protection circuit is provided that stops the motor in the event that an overcurrent flows through the motor due to fluctuations in the motor load and there is a risk that the motor may be damaged. can be provided. In particular, since the time required to stop the motor varies depending on the magnitude of the overcurrent flowing through the motor, there is a risk that the motor will actually be damaged rather than immediately stopping the motor when an overcurrent flows through the motor. The motor can only be stopped if Therefore, it is highly practical, does not interfere with the normal operation of the motor, and can protect the motor only when there is a risk of actual damage to the motor.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram including a motor protection circuit according to an embodiment of the present invention.

FIG. 2 is a timing chart for explaining the operation of a protection circuit in the circuit of FIG. 1;

FIG. 3 is an illustrative diagram for explaining the state of a motor load when the motor load is an optical system of an electrophotographic copying machine.

FIG. 4 is a diagram showing the relationship between motor load fluctuation and current flowing through the motor.

FIG. 5 is a graph showing the relationship between operating time and temperature rise of the motor when different loads are applied to the motor.

[Explanation of symbols]

200 protection circuit 21, 22,
23 Comparator 27 Pulse generator 28
, 29, 30 Frequency divider circuit 31, 32, 33 Counter circuit

Claims (2)

[Claims] 1. A protection circuit for connection to a motor, comprising motor current detection means for detecting the amount of current flowing through the motor, a plurality of mutually different reference values, and a limit time corresponding to each reference value. measurement is started when the detection output of the setting means and the motor current detection means exceeds a reference value, and a determination is made to determine whether or not the time exceeding the reference value has reached a predetermined limit time corresponding to the reference value. and motor stopping means for stopping the motor in response to the determining means determining that a time limit has been reached. 2. The motor protection circuit according to claim 1, comprising:
The determination means is characterized in that it includes a plurality of comparison means provided for each reference value and a timekeeping means that measures time in response to the output of each comparison means.