GB1569653A - Arrangements for controlling the speed of ac asynchronous motors - Google Patents

Description

(54) ARRANGEMENTS FOR CONTROLLING THE SPEED OF A.C. ASYNCHRONOUS MOTORS (71) We, LOHER G.M.B.H. ELEKTROMOTORENWERKE, a body corporate organised and existing under the laws of the Federal Republic of Germany, of 8399 Ruhstorf/Rott, Federal Republic of Germany, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to arrangements for controlling the speed of A. C. asynchronous motors.

A preferred embodiment of the invention described hereinbelow controls the speed and changes in speed of such a motor according to a predetermined programme, particularly for achieving gentle starting and gentle braking, preferably for use with passenger lifts.

The starting and braking behavious of lifts is an important criterion for the subjective determination of the quality of travel. The acceleration and increase in acceleration during starting and the retardation as well as increase in retardation during braking are decisive for the quality of travel. Uncontrolled drives for lifts are provided with a flywheel which is responsible for the conditions of travel. Nevertheless, in the case of uncontrolled lifts these properties are to a large extend dependent on the load. Further, lifts that are driven in an uncontroller manner tend to produce jolting during the transition between, for example, driving and braking. In addition. the accuracy of lifts having uncontrolled drives is poor and to a large degree depends on the load and the properties of the brakes.

According to the invention there is provided an arrangement for controlling the speed of an AC asynchronous motor having two windings, a first of which drives the motor when fed with alternating current and the second of which brakes the motor when fed with direct current, the arrangement being operative to feed alternating and direct current to the first and second windings. respectively, to adjust the driving and braking effects whereby the speed of the motor varies with time in a predetermined manner, the arrangement including a desired speed value generator operative to generate a desired speed output voltage that varies with time and represents the desired speed of the motor, the generator comprising differentiator means responsive to an instruction to accelerate the motor to provide a first voltage of a first polarity which gradually decreases in magnitude, and speed control means also responsive to the instruction to accelerate the motor to produce a second voltage of a second polarity which, in the absence of the differentiator means, would rise at a predetermined adjustable rate to a maximum value, the speed control means being responsive to the first voltage in such a manner that the rate of rise of the second voltage, which constitutes said desired speed output voltage, is rounded off in that it gradually increases until it is substantially equal to said predetermined adjustable rate.

An arrangement embodying the invention will now be described, by way of example, with reference to the accompanying schematic drawings, wherein: Figure 1 shows two windings of an asynchronous motor and regulating elements (thyristors) for controlling energisation of the windings; Figure 2 shows the basic construction of the arrangement, including an actual speed value generator, a desired speed value generator. a regulator. a distance-time computer and rounding-off means; Figure 3 shows the basic construction of the desired speed value generator and rounding-off means: Figure 4 shows the basic construction of the distance-time computer: Figure 5 illustrates two typical time-dependent desired value curves for lifts; Figure 6 shows the basic construction of the arrangement for producing a control voltage lead; and Figure 7 shows the basic curve for the control voltage with and without lead.

It is known that the torque of A.C. asynchronous motors can be reduced by decreasing the current supply. The current supply to the motor can, for example, be controlled by way of controlled rectifiers. Such circuits are known.

An asynchronous motor can be braked by feeding a coil thereof with direct current. The intensity of the brakmg effect depends on the magnitude of the direct current supplied, which may be controlled by way of controlled rectifiers. Such circuits are likewise known and Figure 1 shows one of these in which an asynchronous motor is provided with two separate coils so that it will function as a driving element as well as a braking element.

Driving current is fed to a winding 14, 15, 16 via resistors 1 to 3 and thyristors 4 to 9, and braking current is fed via a controlled bridge rectifier 10 to 13 to another winding, 17. 18, 19 of the motor.

In general, the windings 17 to 19 can be combined to form a single coil; in particular, use can be made of conventional motors having a high-speed and a low-speed winding which are separated from one another from the point of view of voltage. The driving function is allocated to the high-speed winding and the braking function to the low speed winding.

By means of the method as just described, the speed of an A.C. asynchronous motor can be controlled within certain time limits as is envisaged in the present case and advantageous for lift drives. By constantly comparing the actual motor speed with a predetermined desired speed value, one can prescribe the speed behaviour for the entire time of travel.

Figure 2 illustrates the arrangement of an actual speed value generator 20, a regulator 21 and a desired speed value generator 6. The regulator 21 is preceded by a starting bridge 22 and is connected to a driving regulating amplifier 23 controlling the thyristors 4 to 9 and to a braking regulating amplifier controlling the bridge rectifier 10 to 13, the arrangement being such that current flowing to the windings of the motor is controlled in a sense to maintain equal signals provided by the generators 20 and 26. The functioning of the desired speed value generator 26' is also influenced by the required and appropriate guide parameters, such as a rounding-off means 25 which is responsible for producing a gentle transition on receipt of acceleration and deceleration commands.Further, the desired speed value generator 26 is preceded by a distance-time travel curve computer 27, the function of which will be desribed later. The speed desired value generator 26 aims to meet the requirements of an optimum travel curve, in particular for a lift.

Fast and slow movement command switches 28 and 29. respectively. are connected as shown.

Figure 3 shows the basic circuitry of the desired speed value transmitter 26 and of the rounding-off means 25.

A contact 30 is closed on commencement of travel. Accordingly, a voltage +U is applied to the circuit whereby a voltage U2 set at a potentiometer 33 becomes effective and is applied to one input of an amplifier 40 via a resistor 38b. The other input of the amplifier 40 is earthed via a resistor 51, and a resistor 39 connects such other input to the output. By way of a resistor-diode circuit including diodes 43, 44. resistors 47a and 47b, and potentiometers 45 and 46. the output of the amplifier 40 reaches an input of an amplifier 49 which has a feedback capacitor 48 and functions as an integrator. The other input of the amplifier 49 is earthed. The integrator 49 is inversely connected. i.e. the polarity of the output voltage is opposite to that of the input voltage.By reason of the integration behaviour of the amplifier 49, its output voltage rises slowly until feedback to the amplifier 40 becomes effective through a resistor 50. If the current through the resistor 50 is equal to the opposite current through the resistor 38b, the input voltage and thus also the output voltage of the amplifier 40 becomes zero. This means that no further integration can take place by way of the amplifier 49.

It will be known that the integration behaviour of the amplifier 49 depends on the input voltage and on the associated circuit component values according to the equation:

wherein: Ua is the output voltage, Ue is the input voltage, R is the resistance on the input side. and C is the feedback capacitance.

By influencing Ue, one can influence the speed gradient (i.e. the rate of rise) of the output voltage U.,, which is here done by means of the potentiometer 46. The output voltage Ua represents the desired speed of the motor and is passed to the regulator 21. The influence of the potentiometer 45 is excluded by way of the diode 43. The potentiometer 46 thus serves to set the acceleration of the motor at an adjustable value on starting.

If the contact 30 is opened and a contact 31 closed, a voltage -U is applied to the circuit and a voltage U1 becomes effective at a potentiometer 32 and is applied to the amplifier 40 via a resistor 38a. The voltage kU1 is in general lower than the voltage U. Accordingly, the desired speed value voltage Ua will also be lower in accordance with what as previously been described. The speed of travel will- thereby also be reduced in accordance with the control conditions to a value that serves for positioning.

In order that the voltage Us is reduced to the required value when the contacts 30 and 31 are switched over the output voltage from the amplifier 40 is temporarily reversed as governed by the feedback condition of the resistors 38a and 50. This voltage becomes negative: The influence of the potentiometer 46 is exclused by the diode- 44, but the potentiometer 45 is effective. The degree of retardation during braking can therefore be set by means of the potentiometer 45. If the contact 31 is switched over at the end of travel, the voltage set at the potentiometer 32 becomes negative. This will also reverse the voltage Ua at the output of the amplifier 49: the desired value increases towards 'more intensive braking'.This means-that the motor is braked not only up to standstill, but is also held during standstill until the mechanical- brake that is normally provided on a lift drive becomes effective. This ensures that there will be no jolt even upon stopping.

To obtain gentle transitions (i.e. an initially gradually increasing rate of change of speed) during starting and braking. the voltage applied to-the amplifier 49 for integration is additionally subjected to a time-independent function. This is done in that on starting an amplifier 37 arranged by means of a capacitor- 34 and resistors 35 and 36 to act as a differentiating amplifier and having one input connected to the contact 30 delivers a voltage which, when the contact 30 is actuated, is of the same value but of opposite polarity to the output voltage from the amplifier 40. The other input of the amplifier 37 is earthed. The end of the capacitor 34 remote from the amplifier 37 is earthed via a resistor 52. The output voltage of the amplifier 37 is applied to the resistor-diode circuit 43 to 47b via resistors 41 and 42.The voltage applied to the amplifier 49 for integration and to the potentiometer 46 is then also zero. Depending on the components 34. 35 and 36 of the amplifier 37 the output voltage of the amplifier 37 will then gradually drop according in an e-function. To the extent to which this voltage drops the influence of the voltage at the potentiometer 46 that is necessary for integration becomes effective. This means that the voltage rise Ua from the amplifier 49 only gradually reaches the gradient - (rate of rise) prescribed by the potentiometer 46. This procedure rounds off the commencement of travel. On opening the contact 30, i.e. upon transition to braking. the procedure is initiated with opposite polarities, this influencing the voltage tapped from the potentiometer 45. This procedure rounds off the transition of the commencement of braking.

The most favourable distance-time behaviour is obtained for lifts if the speed of travel prior to switching over to braking has reached its maximum value. The higher the speed of travel. the longer is the braking distance. This distance is often so long that the lift is braked even before reaching the maximum speed in a case where it travels short distances, e.g.

when moving from one floor to an adjacent floor. This could be the cause of losing a considerable distance that would normally have been traversed at a high speed. This distance must then be made up by unnecessarily long creeping travel.

In the present apparatus. in the case of premature termination of rapid travel, a time delay of the switching-off point takes place that can be computed by electronic means. In Figure 4, a contact 56 is closed upon commencement of travel. The output voltage Ucomp of an integrator 55 connected to the contact 56 via a circuit including an amplifier 53 and a potentiometer 54 becomes positive within a very short period and a relay 59 is energised via a diode 59 because the output from a differential amplifier 57, connected to the relay 59 via a diode 58. becomes negative and the voltage U (see Figure 5) is still very low. A contact 60 of the relay 59 is in parallel with a contact 61 which is operated simultaneously with the contact 56.A relay 62 is energised and its contact 30 (which is also shown in Figure 3) is closed and initiates travel.

In the course of a gradual increase in the voltage U" according to the requirements of final acceleration. the output voltage of the amplifier 57 finally becomes zero when the two voltages Ucomp and UX, are of equal magnitude but of opposite polarity.

The relay 59 then drop out, the contact 60 opens and the relay 62 is energised only by way of the contact 61. On commencement of braking. the contacts 56 and 61 open simultaneously. The relay 62 drops out immediately and the integrator 55 is set back. The contact 30 of the relay 62 initiates the desired value for braking.

If. for example. braking is initiated very early. e.g. when the desired value has not yet reached its maximum. then at this time a proportion of the voltage Ucomp is larger than a proportion of the voltage Ua The output voltage from the amplifier 57 is positive and the relay 59 is energised, whereby the relay 62 is likewise energised by way of the contact 60.

The contact 30 is thus also switched on and the circuit in Figure 3 remains ready for increasing the desired value. The voltage proportion Ua therefore continues to rise whilst the voltage proportion of Ucorn drops. At the point where both voltage proportions are equal, the relay 59 drops out and the contact 61 opens. At this instant the relay 62 drops out and its contact 30 initiates braking.

Between the appearance of the braking command by the contacts 56 and 61 and the transmission of this command to control the desired speed value generator 26 (Figure 3), there is a time delay only if the braking command has already appeared at an instant when the desired value has not yet reached its maximum. The time delay depends on how far removed the desired value is from its maximum at the time of the braking command.

Figure 5 shows two diagrams giving typical time curves for the desired value such as may be required for lifts.

Diagram A shows the time curve for the desired value Ua and the comparative voltage Ucornp in a condition where the desired value voltage Ua has reached its maximum. The contact 30 is operated at the same time as the contacts 56 and 61. In diagram B. the time curve is shown at an instant when the desired value voltage Ua has not yet reached its maximum when the braking command appears (56 and 61). The voltage U,l, continues to rise up to its point of intersection with the voltage Uce,mp when the contact 30 transmits the braking command to the desired speed value generator 26. This diagram also shows the course of the desired value voltage U for a case where the braking command reaches the desired value input without any delay.The difference in distances between the two curves is expressed by the area S. This area S would have to be made up by the undelayed input as distance S1 by means of a low speed of travel for a long period.

In the present apparatus, by means of electronic simulation of a computing process accordng to diagram B in Figure 5, a delay is obtained in the switching-over point so as to cover the longest possible distance at the highest possible speed to achieve the shortest possible time for a body to travel from one position to another.

A peculiarity of the principle of regulating an A.C. asynchronous motor is that on transition from standstill to motion a minimum torque is required. When operating the control, particularly a control with an integrating infuence. the driving current will be very small when switching on. The result of this is that the integrally-effective control makes the current supply available ith a time delay and this current may already be too high in the very next instant. Consequently there is hunting and a jolt on starting even though the applied desired value is correct.

The present arrangement aims to avoid the starting jolt as caused by the control in that the control has a leading control voltage applied to it.

Figure 6 shows the relevant circuitry. Before travel. a contact 67 is closed. At the output of an amplifier 64 provided with a feedback capacitor 65 there is a control voltage UR that is prescribed by a potentiometer 66 connected to a voltage source +U. One input of the amplifier 64 is earthed ad the other input is connected via a resistor 63 to an input voltage + Ue. If on commencement of travel the contact 67 is opened, the control voltage tends to assume values that are prescribed by the control condition. By means of the potentiometer 66 the control voltage UR can be prescribed so that at commencement of starting the required value is already applied and thus a starting jolt caused by hunting is avoided.

Figure 7 shows two diagrams illustrating the function of this equipment. In diagram A the control voltage starts from zero, first goes bevond and then oscillates to assume the required value. In diagram B the control voltage is already prescribed at the currect value by means of the potentiometer 66 of Figure 6. Swinging over and oscillations do not ocur and this meets the requirements for efficient starting without jolting or oscillations.

WHAT WE CLAIM IS: 1. An arrangement for controlling the speed of an A.C. asynchronous motor having two windings, a first of which drives the motor when fed with alternating current and the second of which brakes the motor when fed with direct current, the arrangement being operative to feed alternating and direct current to the first and second windings, respectively, to adjust the driving and braking effects whereby the speed of the motor varies with time in a predetermined manner. the arrangement including a desired speed value generator operative to generate a desired speed output voltage that varies with time and represents the desired speed of the motor. the generator comprising differentiator means responsive to an instruction to accelerate the motor to provide a first voltage of a first polarity which gradually decreases in magnitude. and speed control means also responsive to the instruction to accelerate the motor to produce a second voltage of a second polarity which, in the absence of the differentiator means. would rise at a predetermined adjustable rate to a maximum value, the speed control means being responsive to the first voltage in

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

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. proportion of the voltage Ua The output voltage from the amplifier 57 is positive and the relay 59 is energised, whereby the relay 62 is likewise energised by way of the contact 60. The contact 30 is thus also switched on and the circuit in Figure 3 remains ready for increasing the desired value. The voltage proportion Ua therefore continues to rise whilst the voltage proportion of Ucorn drops. At the point where both voltage proportions are equal, the relay 59 drops out and the contact 61 opens. At this instant the relay 62 drops out and its contact 30 initiates braking. Between the appearance of the braking command by the contacts 56 and 61 and the transmission of this command to control the desired speed value generator 26 (Figure 3), there is a time delay only if the braking command has already appeared at an instant when the desired value has not yet reached its maximum. The time delay depends on how far removed the desired value is from its maximum at the time of the braking command. Figure 5 shows two diagrams giving typical time curves for the desired value such as may be required for lifts. Diagram A shows the time curve for the desired value Ua and the comparative voltage Ucornp in a condition where the desired value voltage Ua has reached its maximum. The contact 30 is operated at the same time as the contacts 56 and 61. In diagram B. the time curve is shown at an instant when the desired value voltage Ua has not yet reached its maximum when the braking command appears (56 and 61). The voltage U,l, continues to rise up to its point of intersection with the voltage Uce,mp when the contact 30 transmits the braking command to the desired speed value generator 26. This diagram also shows the course of the desired value voltage U for a case where the braking command reaches the desired value input without any delay.The difference in distances between the two curves is expressed by the area S. This area S would have to be made up by the undelayed input as distance S1 by means of a low speed of travel for a long period. In the present apparatus, by means of electronic simulation of a computing process accordng to diagram B in Figure 5, a delay is obtained in the switching-over point so as to cover the longest possible distance at the highest possible speed to achieve the shortest possible time for a body to travel from one position to another. A peculiarity of the principle of regulating an A.C. asynchronous motor is that on transition from standstill to motion a minimum torque is required. When operating the control, particularly a control with an integrating infuence. the driving current will be very small when switching on. The result of this is that the integrally-effective control makes the current supply available ith a time delay and this current may already be too high in the very next instant. Consequently there is hunting and a jolt on starting even though the applied desired value is correct. The present arrangement aims to avoid the starting jolt as caused by the control in that the control has a leading control voltage applied to it. Figure 6 shows the relevant circuitry. Before travel. a contact 67 is closed. At the output of an amplifier 64 provided with a feedback capacitor 65 there is a control voltage UR that is prescribed by a potentiometer 66 connected to a voltage source +U. One input of the amplifier 64 is earthed ad the other input is connected via a resistor 63 to an input voltage + Ue. If on commencement of travel the contact 67 is opened, the control voltage tends to assume values that are prescribed by the control condition. By means of the potentiometer 66 the control voltage UR can be prescribed so that at commencement of starting the required value is already applied and thus a starting jolt caused by hunting is avoided. Figure 7 shows two diagrams illustrating the function of this equipment. In diagram A the control voltage starts from zero, first goes bevond and then oscillates to assume the required value. In diagram B the control voltage is already prescribed at the currect value by means of the potentiometer 66 of Figure 6. Swinging over and oscillations do not ocur and this meets the requirements for efficient starting without jolting or oscillations. WHAT WE CLAIM IS:

1. An arrangement for controlling the speed of an A.C. asynchronous motor having two windings, a first of which drives the motor when fed with alternating current and the second of which brakes the motor when fed with direct current, the arrangement being operative to feed alternating and direct current to the first and second windings, respectively, to adjust the driving and braking effects whereby the speed of the motor varies with time in a predetermined manner. the arrangement including a desired speed value generator operative to generate a desired speed output voltage that varies with time and represents the desired speed of the motor. the generator comprising differentiator means responsive to an instruction to accelerate the motor to provide a first voltage of a first polarity which gradually decreases in magnitude. and speed control means also responsive to the instruction to accelerate the motor to produce a second voltage of a second polarity which, in the absence of the differentiator means. would rise at a predetermined adjustable rate to a maximum value, the speed control means being responsive to the first voltage in

such a manner that the rate of rise of the second voltage. which constitutes said desired speed output voltage, is rounded off in that it gradually increases until it is substantially equal to said predetermined adjustable rate.

2. An arrangement according to claim 1, wherein the differentiator means and speed control means are responsive in like manner to an instruction to decelerate the motor from the speed corresponding to said maximum value to produce a falling output voltage of which the rate of fall is rounded off in that it initially gradually increases.

3. An arrangement acoording to claim 2. wherein the differentiator means and the speed control means are arranged such that the rate of fall of said falling output voltage, after the rounding-off, has a predetermined, adjustable value.

4. An arrangement according to any one of the preceding claims, which is used to control the speed of a motor of which a high speed winding constitutes said first winding.

5. An arrangement according to claim 4, wherein the second winding of the motor is a low speed winding.

6. An arrangement according to any one of the preceding claims. which is used to control the speed of a motor that is arranged to drive a lift.

7. An arrangement for controlling the speed of an AC asynchronous motor, the arrangement being substantially as herein described with reference to the accompanying drawings.