A METHOD OF AND A DEVICE FOR BRAKING AN ASYNCHRONOUS MOTOR

Abstract: Method and device for braking an asynchronous motor by applying a direct current to the field windings. In order to achieve accurate supply time of the braking current the AC voltage appearing at the outer terminals (4, 6) of two mutually series-connected field windings (1, 2) of the asynchronous motor acting as a generator is rectified and connected to a coil (19) of a braking current relay (28) immediately after the operating current of the motor has been switched out. A terminal (R) of a braking current source (R, N) is connected by the relay (28) through a semi-conductor (24) to an outer terminal (6) of said field windings (1, 2) and through another semi-conductor (25) to another outer terminal (4) of said field windings (1, 2), a terminal (5) common to said field windings (1, 2) being connected through a braking current adjuster (26) to another terminal (N) of the braking current source (R, N). The direct current applied to the field windings (1, 2) maintains an alternating voltage in the outer terminals (4, 6) of said field windings (1, 2) and a hold current in the relay (28) until the motor rotation is completed or the motor speed is reduced to a predetermined value.

Description: A method of and a device for braking an asynchronous motor This invention relates to a method of and a device for a method of braking an asynchronous motor by applying a direct current obtained from an AC voltage source through rectification to the field windings of said motor. Previously known solutions for braking an asyn- chronous motor by means of a direct current applied to the field windings thereof are disclosed in e.g. German Offenlegungsschrifts 2,214,731 and 3,040,035. In these solutions, the duration of the braking is based on the setting of the braking time, which leads in inaccurate stops and resettings of the braking time if the speed of rotation of the motor and/or the fly mass connected to the armature is changed. Solutions for the elimination of the above disadvantages are disclosed in e.g. U.S. Patent Specification 4,392,098, German Offenlegungsschrift A1 3,110,841 and ' European Patent Specification A2 41,191, in which the control of the supply of a braking current to the field windings of a cage induction motor is effec¬ ted by utilizing the frequency of the voltage induced in the field winding, the voltage induced in the field winding or the current induced in the field winding. These solutions have frequently led to relatively expensive technical realizations including a plurality of structu¬ ral elements. The object of the present invention is to provide a method and a device which enable a controlled braking of asynchronous motors irrespective of any changes in the speed of rotation of the motor and/or in the fly mass connected to the armature thereof and which can be used for starting and braking of single-phase, two-phase or multi-phase asynchronous motors. The invention is mainly characterized by these features which are set forth in the enclosed claims. The invention will be described in the following by means of certain embodiments thereof with reference to the attached drawings, wherein Figure 1 shows a circuit diagram of a device for starting and braking of a delta-connected squirrel cage motor supplied from a three-phase net, Figure 2 shows a circuit diagram of a device according to another embodiment of the invention for starting and braking of a delta-connected squirrel cage motor supplied from a three-phase net, Figure 3 shows a circuit diagram of a braking current control unit included in the device according to Figure 2, Figure 4 shows a circuit diagram of a device provided with a safety relay and intended for connecting and controlling a braking current to a star-connected squirrel cage motor supplied from a three-phase net, Figure 5 shows a circuit diagram of a device for connecting and controlling a braking current to a squirrel cage motor supplied from a two-phase net, Figure 6 shows a circuit diagram of a device for connecting and controlling a braking current to a delta- connected capacitor motor supplied from a one-phase net, Figure 7 shows a circuit diagram of a device for connecting and controlling a braking current to a star- connected capacitor motor supplied from a one-phase net, Figure 8 shows a circuit diagram of a device for connecting and controlling a braking current to a capaci¬ tor motor supplied from a one-phase net, and Figure 9 shows the curveform of a the generator voltage generated by the embodiment of Figure 1. Similar components are indicated by the same references in all figures. Figure 1 shows terminals R, S, T, N of a three- phase net and delta-connected field windings 1, 2, 3 of a squirrel cage motor. To start the motor the terminal S is by a press-button switch 13 connected to the terminal N through a press-button stop switch 14, a press-button emergency switch 15, a contact 23 of a braking current relay 28 and a coil 7 of an operation current relay 27, whereby the operation current relay picks up, the contact 8 thereof is opened and the contacts 9, 10, 11 connect phase voltages from the terminals R,S,T to the terminals 6, 5, 4 of the field windings 1, 2, 3, the coil 7 obtain- ing a hold current through contacts 14, 12, 15, 23. To brake the motor the hold current circuit of the operation current relay 27 is disconnected by means of the press- button stop switch 14 or the press-button emergency switch 15, whereby the operation current relay 27 is released, the contacts 9, 10, 11, 12 thereof open to disconnect the operation current of the motor, the contact 8 closes to connect an alternating voltage induced in the field windings 1, 2, 3 by the residual magnetism and the rotatory motion of the iron core of a short-circuit armature from the terminals 4, 6 of two mutually series-connected field windings 1, 2 to a capacitor 18 through a resistor 16 and a rectifier 17, a coil 19 of the braking current relay 28 being connected to the terminals of the capacitor, where¬ by the braking current relay 28 picks up, the terminal R of the three-phase net used as a braking current source is connected through a diode 24 to an outer terminal 6 of the mutually series-connected field windings 1 , 2 by a contact 20 and through a diode 25 to another outer termi¬ nal 4 of the mutually series-connected field windings 1 , 2 and then the terminal 5 common to the mutually series- connected field windings 1 , 2 is connected by a contact 22 through a braking current adjuster 26 to the terminal N of the braking current source. A direct current applied through the diodes 24, 25 in the same direction to the field windings 1 , 2 of the motor maintains an alternating voltage in the outer terminals of the mutually series- connected field windings 1 , 2 during the rotation of the short-circuit armature and simultaneously a hold current in the braking current relay 28, which hold current can be adjusted for different kinds of braking processes by means of a hold current adjuster 16 of the braking current relay 28 and/or the braking current adjuster 26 so that the braking current relay 28 is released and the contacts 20, 21, 22 thereof disconnect the braking current when the rotation of the short-circuit armature stops or its rate of rotation is reduced to a set value. The openable contact 23 of the braking current relay 28 disconnects the starting current circuit of the motor for the time of the braking. In Figure 9 the curveform A shows the operating voltage of the squirrel cage motor and the curveform B shows the generator voltage generated by the squirrel cage motor after disconnecting the operating voltage in the embodiment shown in Figure -1, the generator voltage being measured at the outer terminals 4 and 6 of the series-connected field windings 1 and 2. This generator voltage is generated during the currentless half-cycles of the braking current supplied and half-wave-rectified through diodes 24 and 25. Since the same voltage acts at the AC terminals of the rectifier 17 during the current- carrying half-cycles of the braking current, this braking current does not develope a voltage at the DC terminals of the rectifier 17. The generator voltage shown in Figure 9, however, is rectified to the DC side of the rectifier 17, as disclosed above referring to Figure 1. The reason for the fact that the level of the generator voltage remains relatively high, as shown in Figure 9 , is, that the iron core of the motor is magnetized during the current-carrying half-cycles of the braking current. Therefore the level of the residual magnetism and there- by the level of the generator voltage is relatively high during the currentless half-cycles of the braking current especially when the motor is still rotating at a substan¬ tial speed. Disadvantageous sparking and arching may occur in the contacts of the braking current relay of the device according to Figure 1 even if several contacts are connec¬ ted in series. This results in warming of the contacts, reduces the switching currents allowed for the contacts especially with higher operating voltages as well as the switching rate, shortens the useful time of the contacts and relays and reduce the reliability of operation of the device. These problems have been eliminated in the other embodiment of the device according to the invention, which embodiment is shown in Figures 2-8 in connection with different types of motors. Figure 2 shows terminals R,S,T, N of a three-phase net and delta-connected field windings 1, 2, 3 of a squirrel cage motor. To start the squirrel case motor the terminal S is connected to the terminal N by means of a start switch 13 through a stop switch 14, an emergency stop switch 15, a contact 23 of a braking current relay 28 and a coil 7 of an operation current relay 27, whereby the operating current relay 27 picks up, the contact 8 thereof is opened and contacts 9, 10, 11 apply phase voltages from the terminals R, S, T to terminals 6, 5, 4 of the field windings 1, 2, 3 and the coil 7 receives a hold current through a contact 12. To brake the motor the hold current circuit of the operating current relay 27 is disconnected by the stop switch 14 or the emergency stop switch 15, whereby the operating current relay 27 is re- leased, the contacts 9, 10, 11 thereof are opened, there¬ by disconnecting the operating current of the motor, the contact 8 is closed, thereby applying a generator voltage induced in the field windings 1, 2, 3 by the residual magnetism of the iron core of a short circuit armature and the rotary motion of the armature from the outer terminals 4, 6 of two mutually series-connected field windings 1 and 2 through a rectifier 17 to a coil 19 of the braking current relay 28 and to a braking current control unit 18, whereby the braking current relay 28 picks up and contacts 20, 21, 22 thereof are closed, the contact 20 connects the terminal R of a braking current source RN through a semi-conductor 24 to the outer termi¬ nal 6 of the field windings 1, 2, the contact 21 connects said terminal R through a semi-conductor 25 to the other outer terminal 4 of the field windings 1 , 2 and the con- tact 22 connects the other terminal N of the braking current source RN through a semi-conductor switch 26 being in a non-conductive state to the terminal 5 common to the field winings 1, 2, whereafter the control unit 18 renders the semi-conductor switch 26 conductive, whereby the braking current is switched on to energize the field windings 1, 2 until the control unit 18 renders the semi¬ conductor switch 26 non-conductive, whereafter the braking current relay 28 is released and the contacts 20, 21, 22 thereof are opened. As the contacts 20, 21, 22 of the braking current relay 28 are currentless when they are closed and opened, no sparking or arching occur therein. The openable contact 23 of the braking current relay 28 disconnects the starting current circuit of the squirrel cage motor for the time of the braking. Figure 3 shows a circuit diagram of the braking current control unit 18 together with the rectifier 17, the coil 19 of the braking current relay 28 and the semi¬ conductor switch 26 for the braking current. The control unit 18 comprises a preresistor 29, a capacitor 30, a zener diode 31, a LED-current adjusting resistor 32 and a opto-coupler 33. When the operating current of the squirrel cage motor is disconnected, a generator voltage of the squirrel cage motor is connected to the coil 19 of the braking current relay 28 and the braking current con- trol unit 18 through the contact 8 of the operating current relay 27 and the rectifier 17, the braking current relay 28 picks up, the capacitor 30 being simultaneously charged through the preresistor 29. When the charge obtains a predetermined voltage level, the opto-σoupler 33 renders the semi-conductor switch 26 for example a triac conductive, whereby the braking current is applied to the field windings 1, 2. The zener diode 31 limits the charge of the capacitor 30 to a predetermined voltage level. The rate of rotation and the generator voltage of the squirrel cage motor are reduced to zero by the action of the braking and the voltage in the capacitor 30 is likewise reduced by the action of the LED-current and the hold current of the braking current relay 28, the opto- coupler 33 renders the semi-conductor switch 26 non-con¬ ductive and the braking current is disconnected, but the braking current relay 28 still receives a hold current from the capacitor 30 through the preresistor 29, until the braking current relay 28 is released as the voltage in the capacitor 30 is decreased to a predetermined level. The squirrel cage motor can be braked from the full speed of rotation thereof to a lower value by means of the control unit 18,whereby it is left freely rotating and can also be restarted. The opto-coupler 33 can be a combina¬ tion of a LED and a photoresistor or a combination of a LED and a photo-semi-conductor. The zener diode 31 can be replaced with a corresponding component for voltage limiting and stabilizing. Figure 4 illustrates switching and controlling a braking current in a star-connected squirrel cage motor energized by a three-phase net. The braking current is connected to flwo through the field windings 1, 2, 3 but it can as well be connected directly to the semi-conductor switch 26 from the terminal 5 common to the field windings 1, 2. Otherwise the switching and controlling of the braking current is similar to what is described in connec- tion with Figures 2 and 3. In the device according to Figure 4, a coil 36 of a time-delay relay 39 is connected in series with a coil 7 of the operating current relay 27. Some time after the hold current of the relay 27 has been disconnected, also the relay 39. is released, where¬ by a contact 37 is opened. This ensures that the braking 5 current is disconnected also if some of the components associated with the braking process is damaged. Figure 5 illustrates switching and controlling a braking current in a squirrel cage motor energized by a two-phase net and provided with two mutually similar 10 windings. The operation of the switching shown in Figure 5 is identical with the operation of the device according to Figure 2. Figure 6 illustrates switching and controlling a braking current in a delta-connected capacitor motor 15 energized by a one-phase net. A capacitor 34 is connected to the outer terminals 4, 6 ofseries-connected field windings 1, 2.' Otherwise the switching and controlling of the braking current is similar to what is described in connection with Figures 2 and 3. 20 Figure 7 illustrates switching and controlling of a braking current in a star-connected capacitor motor energized by a one-phase net. A capacitor 34 is connected to the outer terminals 4, 6 of series-connected field windings 1, 2. The braking current flows through a third 25 field winding 3 but can as well be connected directly to a semi-conductor switch 26 from a terminal 5 common to the field windings 1, 2. Otherwise the switching and controlling of the braking current is similar to that described in connection with Figures 2 and 3. ~ - Figure 8 illustrates switching and controlling of a braking current in a capacitor motor energized by a one- phase net. A capacitor 34 is connected to the outer terminals 4, 6 of series-connected field windings 1, 2. Otherwise the switching and controlling of the braking 35 current corresponds to the operation described in connec¬ tion with Figures 2 and 3. The method and devices according to the invention can be used for switching and controlling of a braking current applied to the field windings of a one-phase, two-phase or multi-phase squirrel cage motor. The braking current control unit 18 can be used for controlling of a hold current of semi-conductor switches and/or direct-current relays also in connection with other devices.

Claims: Claims : 1. A method of braking an asynchronous motor by applying a direct current obtained, from an AC voltage source through rectification to the field windings of said motor, c h a r a c t e r i z e d in that the braking current is switched on after the operating current has been switched out and while the armature is still rotating by a voltage and by an electric energy generated by the asynchronous motor acting as a generator which voltage and energy is obtained from the outer termi¬ nals of two mutually series-connected field windings of the asynchronous motor, and that the supply of the braking current is maintained by said generator voltage and energy obtained from said field windings during the currentless half-cycles of the half-wave-rectified braking current until said voltage and energy have decreased to a pre¬ determined value. 2. Device for braking an asynchronous motor by applying a direct ' current obtained through rectification from an AC voltage source (RN) to the field windings (1,2,3) of said motor, c h a r a c t e r i z e d in that one outer terminal (6) of two mutually series-connected field windings (1,2) is connected to one AC voltage terminal of a rectifier (17) through a contact (8) of an operating current relay (27), which contact is closed when the operating current is switched out, another outer terminal (4) being connected to another AC voltage termi¬ nal of the rectifier (17), that a coil (19) of a braking current relay (28) and a braking current control unit (18) are connected to DC voltage terminals of the rectifier (17) in order to apply a hold current for said braking current relay (28), that one terminal (R) of the braking current source (RN) is connected through a half-wave rectifying element (24) to the other outer terminal (6) of said series-connected field windings (1,2) and through a further half-wave rectifying element (25) acting in the same direction to the other outer terminal (4) of said field windings, and that the other terminal (N) of the braking current source (RN) is' connected directly or through a third field winding (3) to a terminal (5) common to said field windings (1,2). 3. Device according to claim 2, c h a r a c t e r¬ i z e d in that a braking current adjuster (26) is provided between the other terminal (N) of the braking current source and said terminal (5) common to the field windings (1,2). 4. Device according to claim 1, c h a r a c t e r¬ i z e d in that the braking current control unit com¬ prises a capacitor (18). 5. Device according to claim 1, c h a r a c t e r- i z e d in that the braking current control unit (18) comprises a capacitor (30), a capacitor voltage stabili¬ zing element (32) connected in parallel with said capaci¬ tor, and an optp-coupler (33) controlled by the voltage of the capacitor (30) and the output of which is connec- ted to control said braking current adjuster (26). 6. Device according to claim 3 or 5, c h a r a c¬ t e r i z e d in that the braking current adjuster is a semi-conductor switch (26).

Inventors: KARJALAINEN VAEINOE, KARJALAINEN, VAEINOE
Applicants: KARJALAINEN VAINO, KARJALAINEN, VAEINOE
Application year: 1986
Main classification: H02P3
IPC codes: H02P3