FR2668662A1 - Method of protecting an electric motor against overloads - Google Patents

Abstract

Method of protecting an electric motor against overloads, in which the rotational speed of the motor is monitored with respect to a preselected reference value and, if the rotational speed of the motor is lower than the reference value, the voltage applied to the motor is cut off so as to cease the drive to this motor and to protect it against overheating or damage. The rotational speed of the motor is detected in the form of a number of pulses in order to monitor an associated pulse frequency, this pulse frequency being, for example, converted into an output voltage level which constitutes one of the elements for detecting the state of overload of the motor without having recourse to a microcomputer. … …

Description

Method for protecting an electric motor against overloads La

  The present invention relates to a method for detecting the overload state of an electric motor and preventing overheating or

  damage to it due to this overload.

  Motor controls are found in many technical fields, in particular in the automobile field, for example for controlling a seat, a window, mirrors or other elements.

mobiles of an automobile.

  In such motor control systems, there is an overload protection system to detect an overcurrent due to an overload state of the motor and immediately cut the current from

  overload to protect the motor from overheating or damage

  due to this overload state.

  Figure 8 shows the electrical circuit of a

  conventional overload protection, which is based on the characteristics

  torque T / current I of the motors, as shown in FIG. 7 According to this circuit, resistors 112 for detecting a current

  are connected in series with the two motors, respectively

  ment Ml and M 2 Thus, when the motors undergo an overload due for example to the fact that the mechanical elements which are associated with them are blocked in their movements for one reason or another, an increase in the value of the current or the voltage at the level of

  overload current detection resistor 112 terminal is de-

  sensed by the corresponding overload current detection circuit 114 This value is compared by a comparator with a voltage of

  reference in circuit 114 Such a system is used in the

  motor control positive 110 having a central processing unit and which is normally equipped with limit switches 152, 154 These limit switches serve to stop the movement of an associated movable member when the latter comes into contact with

  the switch and for it, thus stopping the motor drive.

  Instead of limit switches, you can use

  a timer which is used to stop the engine according to a pre-stop time

set in the timer.

  However, the aforementioned conventional motor control systems have the disadvantage that a voltage supplied by a power source is consumed in the resistor 112 for detecting the overload current, resulting in a lowering of the voltage.

  applied to the motor, so that the motor does not have a rotational speed

  sufficient torque and therefore does not reach the required nominal level.

  Another disadvantage is that the circuit for detecting the

  rant of overload 114 is difficult to assemble in the device 110,

  because it requires a complex structure taking into account the

  conditions, the mounting conditions and the ambient conditions of the motors to be used In addition, if a discontinuity such as

  burn or cable break occurs in the lines between the circuit

  cooked overload current and resistance detection, the voltage level is lowered at the resistance terminal to a level below the reference voltage, causing an overcurrent

  in the engine and therefore overheating and damage to it.

  The timer also has drawbacks in that it allows the motor to continue to be driven until the time has passed beyond a predetermined stop time, even if an overload occurs. in the motor, which means that there is a possibility of overcurrent in the motor until said time

is over.

  Even if a thermal protection device, such as a resistor

  thermal resistance, is incorporated in the motor as protection against over-

  loads due to limit switches, an overload current flows through the motor until the motor temperature rises to a preselected limit level determined by the thermal protection device, which adversely affects operation and the duration of

engine and battery life.

  Given the above-mentioned drawbacks, the main object of the present invention is to provide a method of protecting an electric motor against overloads which eliminates unnecessary voltage consumption and complicated conditions for the motor, in order

  to allow very effective protection of the motor against overloads.

  To achieve this object, and in accordance with the present invention, the steps are provided which consist in: (a) detecting the speed of rotation of the motor and (b) determining the overload state of the motor if the speed of

  rotation is reduced by an increase in engine torque.

  The engine speed is detected by means of pulses.

  sions generated by a position detector provided on the motor The pulse frequency is controlled by a microcomputer control system which compares the pulse frequency with a preselected reference value to determine if the motor is in an overload state yes Par further, it is not necessary to provide a resistor for detecting the overload current and the system adapts itself to the microcomputer used A nominal voltage may be applied to the motor

without being consumed.

  The use of pulses to detect the rotational speed of the motor greatly contributes to eliminating the aforementioned need for an overload current detection resistor to ensure detection

  the motor overload state, because the pulses themselves

  are a clear indication of the motor drive state, which eliminates the need to provide limit switches on

  the movable element associated with the engine.

  Overall, the circuit is very simplified.

  A secondary object of the present invention is to provide another method of protecting an electric motor against overloads, which does not

  no microcomputer control system required.

  To this end and according to a first aspect of the present invention, the frequency of the pulses is transformed into an output voltage level by means of a differentiator circuit including a diode and a smoothing circuit, so that the voltage of each of the pulses spawned is

  transformed into a trigger pulse by the differentiation circuit

  the trigger pulses are then subjected to an operation

  smoothing tion to produce a corresponding output voltage which is

  proportional to the frequency of the pulses.

  In this context and according to another aspect of the invention, the above process is carried out by means of a saturated transformer and a rectifier, so that the frequency of the pulses is transformed into a certain level of output voltage According to this provision, the pulses are applied to the transformer which is used to generate a corresponding output voltage level 4 in cooperation with the rectifier, the output voltage level being of course proportional to the frequency of the pulses. In the two embodiments above, the output voltage level obtained is compared with a preselected value in order to determine whether the motor is in an overload state or not. These two systems are advantageous because they avoid having

  to use a microcomputer and its associated integrated circuits or the like.

  Several embodiments of the invention are described below.

  after, by way of example, reference to the appended drawings in which FIG. 1 is a block diagram of a device for controlling an electric motor using a microcomputer, in accordance with the present invention; Figure 2 is a flow diagram illustrating the process of controlling the motor with the device of Figure 1; Figure 3 is a schematic circuit of a second embodiment of the invention; Figures 4 (A) and 4 (B) show the input and output waveforms of the circuit of Figure 3; Figure 5 is a schematic circuit of a third embodiment of the invention; Figures 6 (A) to 6 (D) show input and output waveforms of the circuit of Figure 5; FIG. 7 is a graph showing the characteristics of torque T / current I and torque T / speed of rotation N of a motor; and

  Figure 8 is a block diagram of a conventional motor control device.

  Figures 1 and 2 show a first embodiment of a control device according to the present invention Reference 10 designates the motor control device comprising two position detectors 16, 18 associated with two electric motors, respectively Ml and M 2, a central unit 20, a manual switch 22, a memory switch 24 and relays RL 1 and RL 2 The motors Ml and M 2 are preferably direct current geared motors and are functionally connected to a movable element making part for example of a device for adjusting the position of a motorized seat, which is not shown in the drawings since it does not constitute the essential characteristic of the

present invention.

  The position detectors 16, 18 comprise in this embodiment and as shown rotation sensors having a structure with a disc-shaped magnet 26 which is fixed on the output shaft (not shown) of the motor and which is connected to the central unit 20, so that the pulses generated are counted and stored in the central unit As will be explained below, the number of the pulses thus stored represents a memory position of the aforementioned movable element and the number of counted pulses represents the current position of this mobile element Preferably, the rotation sensors 16, 18 are each constructed so as to generate four pulses per rotation of the motors Ml, M 2, so that the corresponding position of the mobile element

  can be determined in quarter turn increments.

  As shown in FIG. 1, the manual switch 22 comprises a switch 30 for controlling the motor M1 which will be designated hereinafter by switch M 1, and a second switch 32 for controlling the second motor M 2 which will be designated hereinafter by switch M 2 Each of these two switches Ml and M 2 is of the rocker and automatic return type with as shown a midpoint and two contact points which are connected to the central unit 20 Such a rocker switch causes the switching of the relay contacts 40 a, 42 a, 44 a and 46 a respectively associated with the two relays RL 1 and RL 2, so as to change the direction of rotation of the corresponding motors Ml and M 2 The flow of current, the direction of switching and the direction of rotation of the two motors can be easily understood by

  referring to the block diagram of FIG. 1, so that it is not necessary

  Please describe in detail the relationships between relay systems 36, 38, RL 1, RL 2, 44 a, 42 a, 44 a, 46 a and switches 30, 32 or

with motors Ml, M 2.

  The central unit 20 comprises a microcomputer 34 which processes the signals of the various abovementioned elements and provides an appropriate control signal in accordance with a given program. The control signal circulates in the circuit shown in FIG. 1 according to the maneuvers

  briefly described below.

  With the structure described above, the operation of the switch Ml causes the central unit 20 to generate a control signal which controls the drive of the motor Ml so as to move the associated movable element, thus making it possible to adjust its position, while the operation of the switch M 2 similarly causes the drive of the motor M 2 via the central unit 20, to allow movement

  and adjusting the position of another associated movable element not shown

felt on the drawings.

  The memory switch 24 comprises a selection switch 48 and a memory call switch 50, each connected to the central unit and to an automatic return push-button. In order to briefly understand the operation of these two switches 48, 50 , we will take the following example: when the selection switch 48 is operated, while the associated movable element is located in an arbitrary position, a signal 15 is supplied by the switch 48 to the central unit 20 and then this unit central unit stores in memory the pulses counted from a reference position up to this arbitrary position as memory position of the movable element On the other hand, with the movable element located in a position other than the position of memory, actuation of the memory call switch 20 causes the central unit 20 to recognize the current position of the movable element by the counted pulses, performs a e comparison between the number of pulses counted for the current position (current count value) and the number of pulses counted for the memory position (merotized value), and gives the instructions to the motors Ml and M 2 so that those -these function until the current counted and memorized values are brought to the same value, from

  so that the movable member is returned to the memory position. The reference 56 designates a power source or a battery for supplying a voltage to the device 10.

  Referring now to Figure 7, the motor control device 10 of the present invention is based on a characteristic

  ristic torque T / rotational speed N, that is to say a characteristic of the relationship between the rotational speed and the torque of each of the motors Ml and M 2, Dar opposite to a characteristic torque T / current I, 35 i.e. a relationship between the torque of the motors and a current, used

  7 in the engine control of the prior art The character-

  teristics torque T / speed of rotation N is defined as meaning that the speed of rotation of the motors Ml and M 2 is reduced in inverse proportion to increase the torque of the motors. 5 According to the embodiment described here, the pulses generated by the rotation sensors 16, 18 have a certain period and this period is controlled by the central unit 20, so that the corresponding rotation speeds of the two motors Ml and M 2 are detected Consequently, if an irregularity occurs in the pulse generation period, in other words if this period is longer than the normal period, the rotational speed of the motors Ml, M 2 will be reduced in a small proportion, as a result of which, on the basis of the characteristic torque T / speed of rotation N, the corresponding torques of these motors will be higher As a result, the overload state of the motors Ml and M 2 is detected by the central unit e 20 which then provides a control signal to stop the drive of the motors. This eliminates the possibility that the motors Ml and M 2 are in an overload state and are damaged. In this context, it should be noted that the periods of generation of the pulses of the two rotation sensors 16, 18 are expressed in the form of pulse frequencies f 1, f 2, by calculation in the central unit 2 (In accordance with In the present invention, these pulse frequencies fl, f 2 are compared with a fixed reference frequency f O established in the central unit 20, in order to determine the overload state of the motors. 25 In other words, when the frequency d pulses fl, f 2 and the reference frequency f O are in a relationship such that fl tf O and / or f 2 (LDO, then the overload state of the motors Ml, M 2 is noted. In this kind of command by pulses, it is desirable that a timer (not shown) be incorporated in the central unit 20 to delay the above-mentioned comparison between the pulse frequencies fl and f 2 and the reference frequency f O by a delay time by 0.5 seconds, because such a delay gives the time necessary for the frequencies fl, f 2 to rise beyond the frequency f O after the motors Ml, M 2 have started to be driven, which allows the corresponding rotational speeds of these motors to rise up to a determined value without this being considered as a state of overload with respect to the engines in activity. The overload protection process described above

  will now be explained step by step with reference to Figure 2.

  The first block 60 indicates the start of the overload protection function Manual and memory switches 22, 24

  are then in a waiting state. The actuation of these switches

  due to the following decision block 62 o it is determined which motor is driven, M 1 or M 2 The next step is constituted by blocks 64, 74 o the timer is reset to zero and the motors Ml and M 2 are driven in the desired direction of rotation as represented by blocks 66, 76 At this instant and as represented by blocks 68, 78, the timer

  starts counting time up to 0.5 seconds from the start of the

  dragging of motors Ml and M 2 After a time of 0.5 seconds has elapsed, a "YES" is emitted by blocks 68, 78 and we find ourselves in the next step consisting of blocks 70, 80 o it is determined whether the pulse frequencies fl, f 2 are smaller than the reference frequency f O or not The positive response "YES" stops the drive of the two motors Ml, M 2 as represented by blocks 72, 82 o the engines are stopped

  through the switching of relay contacts 40 a, 42 a.

  According to this first embodiment, the overload of the motors M1, M 2 is detected by means of the pulse control of the central unit 20 on the basis of the torque T / speed of rotation characteristics N of the motors, this which simplifies the circuit structure of

  motor control with respect to the circuit of the prior art which comprises

  was a resistor for detecting the overload current with its related circuit This improves the adaptation between the microcomputer and the motor control circuit and there is no voltage consumption, since there is no of overload current detection resistor In addition, the fact of not providing in the present invention an overload detection circuit simplifies the structure of the circuit since it is not

  necessary to take into consideration the nature of the motors.

  Furthermore, if a discontinuity occurs in the lines between the rotation sensors 16, 18 and the central unit 20, the pulse frequencies fi, f 2 are determined in the central unit 20 as being

  zero, which means that they are lower than the reference frequency

  rence f O, with the result that a voltage is not supplied to the motors Ml, M 2 so as to prevent the drive of said motors in the case where the central unit 20 does not control the overload state of those - This provides protection with high security of the

motor control 10.

  We will now refer to Figures 3, 4 (A), 4 (B) which represents

  feel a second embodiment of the present invention that

  the pulses of the rotation sensors 16, 18 are transformed into a cer-

  a certain voltage level in proportional relation to this and this voltage level constitutes a criterion for determining an overload state of the

  motors Ml, M 2 In this embodiment, a detection circuit is used

  overload 84 comprising a differentiator circuit composed of a capacitor C, a resistor RL and a diode D, this circuit 84 being

  connected to the line switch 28 of the rotation sensors 16, 18.

  As a result, the rectangular pulses generated by the rotation sensors 16, 18, as shown in Figure 4, are transformed by this differentiating circuit into positive trigger pulses as shown in Figure 4 (B) A current corresponding to these pulses of

  trigger designated by IL, then flows through the differential circuit

  tiateur of Figure 3 and this current is expressed by the following formula _ t IL = E e CRL RL Thus, the time constant of circuit 84 is chosen more

  small than the first and the last transit time of the rectal pulses

  gular, so that the load current IL is proportional to the frequency of the pulses since the charge of the capacitor C is expressed by "Q = CE" and the frequency of the pulses being f, the charge current IL is expressed by "IL = f CE", ie "IL = OC f" Consequently, using an appropriate smoothing circuit (not shown), the trigger pulses corresponding to the load current IL are transformed into an average value, thus transforming the pulses at an output voltage level In this way, it can be seen that a certain level of output voltage can be obtained in relation to the frequency of these trigger pulses. Next, the output voltage level thus obtained is compared. by a level comparator (not

  shown) having a preset voltage level, so that

  ner if the voltage level obtained is lower than the preselected level in the comparator or not If it is smaller, the associated motor Ml or M 2 is in an overload state because its speed of rotation is lowered and that the pulses generated by sensor 16 or 18 become smaller in number In other words, the frequency of the pulses is reduced

  and this results in a lowering of the output voltage level.

  The arrangement described above eliminates the need to provide a resistance for detecting the overload current in the device 10,

  which simplifies the motor control circuit per se and avoids a

  unnecessary summation of energy in the circuit In addition, the arrangement according to this second embodiment does not require a microcomputer to

  detect the overload state of the motors.

  Figures 5 and 6 show a third embodiment

  of the present invention in which is incorporated another detection circuit

  overload 86 which includes a saturated transformer 88, a rectifier

  of the bridge type and an RL resistance as essential elements

  In this embodiment, the pulses generated from the information

  line switch 28 of sensor 16 or 18 are applied to a first winding of transformer 88 and at this level each of the pulses having a voltage designated by el has a rectangular shape, as shown in Figure 6 (A) Then it occurs in transformer 88 a series of magnetic flux waveforms as shown in Figure 6 (D), which causes the generation of alternating rectangular pulses as shown in Figure 6 (C) at the second winding of the same transformer 88, these pulses having an output voltage designated by e 2. The resulting alternating pulses are then processed by the rectifier 30 90 to give an appropriate output voltage whose waveform is represented by positive rectangular pulses, as shown in Figure 6 (D), each of these pulses having a certain level of output voltage which is proportional to the frequency of the pulses En- s Then, the output voltage level thus obtained is compared with a preselected reference level using an appropriate comparator (not shown), so that it can be determined whether the associated motor Ml or M 2

  is in an overloaded state or not.

  The arrangement described above does not include any resistance for detecting the overload current and therefore avoids unnecessary consumption of the voltage applied to the motors. Furthermore, there is no need to provide a microcomputer in the control circuit which does not include

than a relay system.

  Of course, the present invention is not limited to the exemplary embodiments shown and described, and there are many that can be brought to it.

  modifications without departing from the scope of the present invention.

Claims (8)

  1 A method of protecting an electric motor against overloads, characterized in that it comprises the steps consisting in detecting the speed of rotation of said motor, comparing this speed of rotation with a preselected reference value, determining the state overloading said motor if said speed of rotation is lower than said preselected reference value, and   immediately stop supplying voltage to said motor.   2 A method of protecting an electric motor against overloads, characterized in that it comprises the steps consisting in: allowing at least one pulse to be generated by the rotation of said motor,   detecting the rotation speed of said motor from the frequency   quence of this pulse generated by the rotation of said motor, compare said pulse frequency with a preselected reference value, determine the overload state of said motor if said pulse frequency is less than said preselected reverence value, and   cease supplying voltage to said motor.   3 Method according to claim 2, characterized in that the step of comparing said pulse frequency with said reference value   The preselected interval is not carried out before the expiration of a given time by a timer, in order to delay the moment when this comparison step takes place.   4 A method of protecting an electric motor against overloads, characterized in that it comprises the steps consisting in: allowing at least one pulse to be generated by the rotation of said motor, transforming the frequency of said pulse at an output voltage level using a differentiator circuit including a diode (D), the output voltage level of which is proportional to said pulse frequency, compare said output voltage level with a reference value preselected, determining the overload state of said motor if said output voltage level is lower than said preselected reference value, and   cease supplying voltage to said motor.   5 Method according to claim 4, characterized in that said output voltage level is obtained with a certain constant level in   incorporating a smoothing circuit in said differentiating circuit.   6 Method of protecting an electric motor against over-   charges, characterized in that it comprises the stages consisting in:   allow at least one impulse to be generated by the rota-   tion of said motor, transforming the frequency of said pulse into an output voltage level by means of a saturated transformer (88), the output voltage level of which is proportional to said pulse frequency, comparing said voltage level of output with a preselected reference value, determining the overload state of said motor if said output voltage level is lower than said preselected reference value,   and20 ceasing to supply voltage to said motor.   7 Method according to claim 6, characterized in that a rectifier (90) is associated with said saturated transformer (88) so as to obtain   a substantially constant output voltage level.   8 Method according to any one of claims 2, 4 and 6,   wherein said motor is associated with position detection means (16, 18) for detecting the position of a movable element operatively associated with said motor, this position detection means counting the number of pulses generated by the rotation of said motor and detecting said position of the movable element by the counted number of said pulses, characterized in that it includes a step consisting in detecting said   pulse frequency from the generation period of said pulses   sions counted from the position detection means.