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Title:
A SOFTSTARTER FOR CONTROLLING AN ASYNCHRONOUS THREE-PHASE MOTOR
Document Type and Number:
WIPO Patent Application WO/2011/063851
Kind Code:
A1
Abstract:
The present invention relates to a softstarter (2) for starting and stopping an asynchronous motor (1) having three phases (L1, L2, L3), comprising two pairs (3A, 3C) of semiconductor devices of the type turning off at zero-crossing of the current therethrough, wherein each of said two pairs of semiconductor devices is connected in anti-parallel, and the first pair of the semiconductor devices is adapted to control the voltage of one of the phases of the motor and the second pair of the semiconductor devices being adapted to control the voltage of another of the phases of the motor, a DC reducing unit (16) associated with said two pairs (3A, 3C) of semiconductor devices for reducing a DC component in a load current of the said asynchronous three-phase motor during start of the motor, a first voltage measuring unit (8A, 8C) for measuring voltages across the two pairs of semiconductor devices, and a first zero-crossing detecting unit (14) configured for detecting zero-crossings of the measured voltages across the two pairs (3A, 3C) of semiconductor devices and providing zero-crossing signals (X1, X3) to the DC reducing unit.

Inventors:
EK, Magnus (Ryttene Vallby 6, Västerås, S-725 92, SE)
LENDENMANN, Heinz (Föreningsgatan 27, Västerås, S-722 18, SE)
KRISTENSSON, Jesper (Karlfeldtsgatan 35B, Västerås, S-722 22, SE)
Application Number:
EP2009/066026
Publication Date:
June 03, 2011
Filing Date:
November 30, 2009
Export Citation:
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Assignee:
ABB RESEARCH LTD. (Affolternstrasse 44, Zürich, CH-8050, CH)
EK, Magnus (Ryttene Vallby 6, Västerås, S-725 92, SE)
LENDENMANN, Heinz (Föreningsgatan 27, Västerås, S-722 18, SE)
KRISTENSSON, Jesper (Karlfeldtsgatan 35B, Västerås, S-722 22, SE)
International Classes:
H02P1/26; H02P1/28; H02P3/18; H02P3/24
Domestic Patent References:
WO2008033088A1
WO2007123458A1
Foreign References:
US5008608A
JP2009303323A
Attorney, Agent or Firm:
KOCK, Ina (Abb Ab, Intellectual PropertyIngenjör Bååths Gata 11, Västerås, S-721 83, SE)
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Claims:
CLAIMS

1 . A softstarter (2) for starti ng and stoppi ng an asynchronous motor ( 1 ) having three phases (L^ L2, L3) , comprising

· two pai rs (3A, 3C) of semiconductor devices of the type turning off at zero-crossing of the current therethrough, wherein each of said two pairs of sem iconductor devices is connected in anti-paral lel , and the first pair of the sem iconductor devices is adapted to control the voltage of one of the phases of the motor and the second pai r of the sem iconductor devices being adapted to control the voltage of another of the phases of the motor,

• a DC reduci ng unit ( 1 6) associated with said two pairs (3A, 3C) of sem iconductor devices for reducing a DC component i n a load current of the said asynchronous three-phase motor during start of the motor,

· a first voltage measuring unit (8A, 8C) for measuring voltages across the two pairs (3A, 3C) of sem iconductor devices, and

• a first zero-crossi ng detecti ng unit ( 1 4) configured for detecting zero- crossings of the measured voltages across the two pairs (3A, 3C) of semiconductor devices and providing zero-crossing signals (X1 , X3) to the DC reducing unit, and

characterized in that the softstarter further comprises,

• a second voltage measuring unit (7) for measuring input voltages in a first and second of the phases or between the first and second phases,

• a second zero-crossing detecting unit ( 1 2) configured to detect zero- crossings of the input voltages between the first and second phases and providing zero-crossi ng signals (Χ1 ' , Χ3') to the DC reducing unit, and

• the DC-reducing unit ( 1 6) is configured to calculate phase delays between the zero-crossings of the measured/calculated input voltages between the first and second phases and the zero-crossings of the voltages across the two pairs (3A, 3C) of sem iconductor devices, to calculate an average phase delay based on the two latest phase delays or a m ultiple of two of the latest phase delays, and to adjust firing angles of the semiconductor devices based on the zero-crossings of the meas- ured/calculated i nput voltages between the first and second phases and the calculated average phase delay during start and stop of the motor.

2. The softstarter according to claim 1 , wherein the second zero-crossing detecting unit is f urther configured to calculate input voltages between the first and second phases based on the measured voltages in the first and second phases.

3. The softstarter according to claim 1 , wherein the softstarter is also adapted for smoothly starti ng and stoppi ng said motor and comprises,

• a current measuring unit for measuring currents in at least two of the phases,

• a voltage estimating unit configured to estimate an input voltage between a third phase and one of the other phases based on at least one of the measured input voltages measured by the second voltage measuring unit,

• a power computing unit configured to calculate an instantaneous power supplied to the motor based on at least one estimated input voltage and the measured currents, and

• a torque control unit incl uding a torque computing unit configured to calculate an electromagnetic torque of said motor based on the calculated instantaneous power, and the torque control unit being adapted to control the motor torque during start and stop of the motor based on the calculated electromagnetic torque and a reference torque value for the electromagnetic torque.

4. The softstarter according to claim 1 , wherein the current measuri ng unit is configured to measure currents i n the fi rst and second phases and the power computing unit is configured to calculate the instantaneous power based on the measured currents of the first and second phases and estimated i nput voltages between the first and third phases and between the second and third phases.

5. The softstarter accordi ng to claim 1 , wherein the softstarter further comprises a current estimating unit configured to estimate a current of one of the phases based on the measured currents of the other two phases and the power computi ng unit is configured to calculate the instantaneous power based on one of the measured currents, one estimated current and the measured input voltage between the first and second phases.

6. The softstarter according to claim 1 , wherein the voltage estimating unit configured to estimate the input voltage between a third phase and one of the other phases based on present and historically measured voltages received from the second voltage measuring unit.

7. The softstarter according to claim 1 , wherein the power computing unit is configured to carry out the calculation of the power with a first frequency and the torque computing unit is configured to carry out the calculation of the electromagnetic torque of said motor with a second frequency which is significantly less than the fi rst frequency.

8. The softstarter according to claim 6, wherein the first frequency is higher than 250Hz and the second frequency is lower than 250Hz.

9. A method for reducing DC component during starti ng and stoppi ng an asynchronous motor havi ng three phases, wherei n two of the three phases are controlled by a softstarter comprisi ng two pairs of sem iconductor devices of the type turning off at zero-crossi ng of the current therethrough , each of said two pairs of semiconductor devices being connected in anti-parallel , and the first pair of the semiconductor devices being arranged to control the voltage of one of the phases of the motor and the second pair of the sem iconductor devices being adapted to control the voltage of another of the phases of the motor, the method comprising,

• measuring voltages across the two pairs of semiconductor devices, and • detecting zero-crossings of the measured voltages across the two pairs of sem iconductor devices and sendi ng the detected zero-crossings as signals, and

characterized in that the method further comprises,

· measuring input voltages in a first and second of the phases or between the first and second phases,

• detecting zero-crossings of the measured input voltages between the first and second phases and sending the detected zero-crossi ngs as signals,

• calculating phase delays between the zero-crossings of the meas- ured/calculated i nput voltages between the first and second phases and the zero-crossings of the voltages across the two pairs of semiconductor devices upon receiving the sent signals,

• calculating an average phase delay based on the two latest phase delays or a m ultiple of two of the latest phase delays, and

· adjusting firing angles of the sem iconductor devices based on the zero- crossings of the calculated input voltages between the first and second phases and the calculated average phase delay during start and stop of the motor. 1 0. The method according to claim 1 0, wherei n the method further comprises a step of calculating input voltages between the first and second phases based on the measured voltages in the first and second phases.

1 1 . The method according to claim 1 0, wherei n the method further comprises the following steps for smoothly starting and stoppi ng said motor,

• measuring currents in at least two of the phases,

• estimating an input voltage between a thi rd phase and one of the other phases based on at least one of the measured input voltages,

• calculating an instantaneous power supplied to the motor based on at least one estimated input voltage and the measured currents,

• calculating an electromagnetic torque of said motor based on the calculated instantaneous power, and thereon controlling the motor torque during start and stop of the motor based on the calculated electromagnetic torque and a reference torque value for the electromagnetic torque.

Description:
A SOFTSTARTER FOR CONTROLLING AN ASYNCHRONOUS THREE-PHASE MOTOR

FI ELD OF TH E I NVENTI ON

The present invention relates to a softstarter for controlling start and stop operation of an asynchronous motor having three phases. The softstarter controls two of the three phases of the motor. P R I O R ART

A softstarter for an asynchronous three-phase motor, comprisi ng semiconductor devices for controlling voltages applied to the motor i n one, two or three of the phases by adjusting fi ring angles of semiconductor devices of the type turning-off at zero-crossing of the current therethrough , wherein two sem iconductor devices connected in anti-parallel with each other in each phase, has been used for some time. By controlling the voltage in the respective phase a reduction of the torque and current provided to the motor is achieved during start and stop operation . The firing angle of the sem iconduc- tor devices is used for controlling the amount of energy supplied to the motor.

Each of these types of softstarters is normally provided with three such pairs of semiconductor devices, such as thyristors. However, the sem iconductor devices have become a cost-determining factor of such a softstarter, so a soft- starter having only two pairs of such semiconductor devices for three of the phases is used for controlli ng the voltage applied to three-phase motors. This means that the remaining third phase is in the form of a conductor, which cannot be switched . The present invention is related to this type of softstarters. It is desired that a softstarter with the above-mentioned type is able to start and stop motors in a desired torque ramp in order to minimize electrical surges in the electrical supply and/or overheating in the motors and to reduce mechanical shocks or vibration which m ight otherwise occur under start/stop conditions causing wear and damage to the pumps, fans etc. driven by the motors. Such damage may result in higher mai ntenance costs and also a shortening of the l ifetime on the pipe-system . However, when the control is lim ited to only two phases, undesired effects in the form of , for example, a DC component in the motor current appear during starting and stopping operation . During start-up or shut-down operation a smooth and gradually increasing or decreasing torque is desired , but this DC component appearing during start-up or shut-down sometimes generates a braking or oscillating torque. This can be both disturbi ng and damaging to the controller system , the electric motor, and to the motor driven load .

Furthermore, it is costly to carry out measurements on the third phase that is in the form of a conductor since an extra measuring unit and connection point have to be built only for the purpose of the measurement.

A patent application , with a publication number WO 2008/033088, describes a method and a softstarter for reducing such a DC component during start and stop of a motor having three phases, only two of the three phases being con- trolled by two pairs of sem iconductor devices, wherein firi ng angles of the sem iconductor devices are adjusted at a zero-crossing of the current that flows through the semiconductor devices. However, this i nvention may still have some error in the tim ing when triggering the semiconductor devices, so that DC components can not be reduced efficiently.

A patent application , with a publication number WO 2008/046885, describes a control method and motor starter device, wherein a motor has three phases and each of the three phases is controlled by a pair of sem iconductor devices and the motor torque is controlled in dependence on a torque error signal , based on the calculated difference between the motor torque and a reference torque value, so that the motor torque displays a rate of change with respect to time, duri ng a stopping or starting time interval . However, this invention is related to a softstarter comprising three pairs of sem iconductor devices, each pair controlling each of the three phases of the motor.

OBJ ECTS AN D S U M MARY OF TH E I N VE NTI ON

The object of the present invention is to provide a softstarter for smoothly starting and stopping an asynchronous motor having three phases, wherein only two phases are controlled by two pairs of sem iconductor devices. This object is achieved by a softstarter as defined in the preamble of claim 1 , characterized in that the softstarter further comprises a second voltage measuring unit for measuri ng input voltages in a f irst and second of the phases or between the first and second phases, a second zero-crossing detecting unit configured to detect zero-crossings of input voltages between the first and second phases and providing zero-crossing signals to the DC-reducing unit, and the DC-reducing unit is configured to calculate phase delays between the zero-crossings of the measured/calculated input voltages between the first and second phases and the zero-crossings of the voltages across the two pai rs of sem iconductor devices, to calculate an average phase delay based on the two latest phase delays or a multiple of two of the latest phase delays, and to adjust firing angles of the semiconductor devices based on the zero-crossi ngs of the measured/calculated input voltages between the first and second phases and the calculated average phase delay duri ng start and stop of the motor. A phase-phase, i .e. input voltage between the first and second phase lines, is used as a reference for tim ing the triggering of the two pairs of sem iconductor devices. The input voltage between the first and second phase lines may be measured directly or calculated based on the measured voltages in the first and second phases. The second zero-crossing detecting unit is arranged to detect zero-crossi ngs of the input voltages between the first and second phases and the fi rst zero-crossing detecti ng unit is arranged to detect zero- crossi ngs of the voltages across the two pairs of sem iconductor devices. The detected zero-crossings are provided as digital signals to the DC-reducing unit. The signals are used as references for firing the sem iconductor devices and to calculate a phase delay, for either a positive or a negative half period , from a zero-crossing of the input voltage between the first and second phases to a zero-crossing of the voltage across the two pairs of sem iconductor devic- es is calculated .

To adj ust a triggering time of a sem iconductor device, a more stable zero- crossi ngs of the input voltages between the first and second phases is used as a reference point for firing the sem iconductor device. Because the phase delay is calculated for a half period that is either a positive or a negative half period , the average of the two latest phase delays or a m ultiple of two of the latest phase delays is calculated to even out a time difference between a positive half period and a negative half period , which cancels out the ti ming error, i .e. the time difference between the phase delays in a positive and negative half periods, during the f irings of the sem iconductors. Consequently, DC components that would appear during start and stop operation can be reduced more efficiently, as compared to the method described in prior art with a publication number WO 2008/033088, wherein the zero-crossings of voltages across the sem iconductor devices, i .e. the zero-crossings of actual currents, are used as reference point.

Due to the fact that the invention is able to adj ust the triggering time of the sem iconductor devices, and the firing angles of the sem iconductor devices are equivalently adjusted , the invention results in an efficient and reliable reduc- tion of DC components during start and stop of the motor, which makes it possible to smoothly start and stop an asynchronous three-phase motor in a desired start and stop ramp, respectively.

According to an embodiment of the invention, the softstarter is also adapted for smoothly starti ng and stopping said motor and comprises a current measuring unit for measuring currents in at least two of the phases, a voltage estimating unit configured to estimate a input voltage between a third phase and one of the other phases based on the measured voltages measured by the second voltage measuring un it, a power computing unit conf igured to calculate an instantaneous power supplied to the motor based on at least one estimated input voltage and the measured currents, and a torque control u nit including a torque computing unit configured to calculate an electromagnetic torque of said motor based on the calculated i nstantaneous power, and the torque control unit being adapted to control the motor torque during start and stop of the motor based on the calculated electromagnetic torque and a reference torque value for the electromagnetic torque. To be able to calculate an electromagnetic torque of the motor, an instantaneous power supplied to the motor needs to be calculated first. Due to the fact that the i nvention makes it possible to estimate the input voltage between the third phase and one of the first or the second phase, the instantaneous power supplied to the motor can be calculated based on the estimated i nput voltages and measured/estimated currents.

It is an econom ic advantage to perform measurements of currents and voltages in as few phases as possible. This, however, requires estimating currents and voltages for those phases that are unmeasured in order to calculate the instantaneous power to be supplied to the motor and thereon further calculate the electromagnetic torque of the motor.

Depending on the type of the second voltage measuring unit, the measured voltages may be either an input voltage between the first and second phases, or the voltages measured in the f irst and second phases. In a latter case, an input voltage between the first and second phases is calculated based on the measured voltages which may be phase-ground voltages. The i nput voltage between the third phase and one of the first and second phases is estimated based on the measured or calculated input voltage to enable the power com- puting unit to calculate the instantaneous power. Preferably, the measurement of i nput voltages is arranged in the phase li ne that is controlled by a pair of sem iconductor devices, where a measuring device is usually already arranged for the purpose of controlling of the semiconductor devices. This means that a single set of measurement instruments may be used for several purposes, which is an economic advantage.

The advantage to use i nput voltages between the phases is to make the esti- mation and calculation more precisely. Impedance separated ground levels for main circuit and control circuit, for example, may disturb a phase- ground measurement, while an input voltage between the phases may m inimize such a disturbance. The estimation of an input voltage between the third phase and one of the other phases is based on present and historically measured voltag- es received from the second voltage measuring unit.

According to an embodiment of the invention , the current measuring unit is configured to measure currents i n the fi rst and second phases and the power computing unit is configured to calculate the instantaneous power based on the measured currents of the first and second phases and estimated i nput voltages between the first and third phases and between the second and third phases. This may be done by a well-known method , for example, a two-watt meter method . According to another embodiment of the invention , the softstarter further comprises a current estimating unit configured to estimate a current of one of the phases based on the measured currents of the other two phases and the power computi ng unit is configured to calculate the instantaneous power based on one of the measured currents, one estimated current and the measured input voltages between the first and second phases.

According to an embodiment of the i nvention, the power computing unit is configured to carry out the calculation of the power with a first frequency and the torque computing unit is configured to carry out the calculation of the electro- magnetic torque of said motor with a second frequency which is significantly less than the first frequency. The measured/calculated input voltages are sampled and saved in order to estimate an input voltage between the unmeasured third phase and one of the two phases. It is advantageous that the first frequency is greater than 250Hz. Such an over-sampling makes the estimation more precise. The second frequency is less than 250Hz to save computational power, preferably less than 1 00Hz. The object of the invention is also achieved by a method as defined in claim 9. Such a method comprises the steps of measuring voltages across the two pairs of semiconductor devices, detecting zero-crossings of the measured voltages across the two pairs of sem iconductor devices and sending the detected zero-crossings as signals, measuring input voltages i n a first and second of the phases or between the first and second phases, detecting zero-crossings of the measured input voltages between the first and second phases and sending the detected zero-crossings as signals, calculating phase delays between the zero-crossings of the measured/calculated input voltages between the first and second phases and the zero-crossings of the voltages across the two pairs of semiconductor devices upon receiving the signals, calculating an average phase delay based on the two latest phase delays or a m ultiple of two of the latest phase delays, and adjusting firing angles of the semiconductor devices based on the zero-crossings of the measured or calculated input voltages between the first and second phases and the calculated average phase de- lay duri ng start and stop of the motor.

In order to smoothly start and stop the motor, the method may further comprise the steps of measuring currents in at least two of the phases, estimating an input voltage between a third phase and one of the other phases based on at least one of the measured/calculated input voltages, calculating an instantaneous power suppl ied to the motor based on at least one estimated input voltage and the measured currents, calculating an electromagnetic torque of said motor based on the calculated instantaneous power, and thereon controlling the motor torque during start and stop of the motor based on the calcu- lated electromagnetic torque and a reference torque value for the electromagnetic torque. The estimation of the i nput voltage between the third and one of the other phases may be based on an input voltage between the first and second phases, which may be measured directly or be calculated based on the input voltage, i .e. a phase-ground voltage, in the first and second phases.

B R I EF D ESC R I PTI ON OF TH E D RAW I NGS

The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures.

Fig. 1 a shows schematically an overview of a system comprising a softstarter according to an embodi ment of the invention , wherein a DC reducing unit is adapted to adj ust firing angles of the semiconductor devices based on calculated phase delays du ring start and stop of the motor.

Fig. 1 b shows schematically a second embodiment that is a variant of the sys- tern illustrated in Figure 1 a, wherei n a second voltage measuring unit is arranged after the sem iconductor devices.

Fig. 2a shows schematically a third embodiment that is another variant of the system il lustrated in Figure 1 a, wherein a DC reducing unit is adapted to ad- just f iring angles of the sem iconductor devices by using zero-crossings of input currents as references for calculating firing angles during start and stop of the motor.

Fig . 2b shows schematically a fourth embodiment that is a variant of the sys- tern illustrated in Figure 2a, wherein a second voltage measuri ng unit is arranged after the sem iconductor devices.

Fig . 3 illustrates a graph of the input voltage between two of the three phases, the voltage across the sem iconductor devices of one phase and the current to the motor in this phase versus the point in time when the sem iconductor devices is turned on or off for the system shown in Fig . 1 a. Fig.4 a flow chart illustration of the invented method for reducing DC component during starting and stopping operation of a motor having three phases, two of the three phases being controlled by a softstarter, according to an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The AC supply frequency to the motor may be either 50Hz or 60Hz and the range of operating currents and operating voltages supplied to the motor is extensive. In a system comprising a motor arranged with a softstarter device the angular speed of the motor is generally determined by the supplied frequency and the number of motor poles. It is to be understood that also other frequencies can be used.

Figure 1a shows a system comprising an electric motor 1 having three phases, a softstarter device 2 for controlling the current flow and voltage supplied to the motor by controlling two of the three phase lines of the motor and an apparatus such as a pump, not shown in the figure, driven by the motor 1. The mo- tor 1 is, in this case, an alternating current (AC) three-phase electric motor, such as an induction motor. The softstarter device 2 is connected in series to the motor 1. The current supplied to the softstarter device 2 is therefore equal to the current supplied to the motor. In this example, a first phase line and third phase line L 3 are controlled while the second phase line L 2 is in the form of a conductor that cannot be switched. For each of controlled phase lines and l_3, a switch 3A, 3C is provided and connected in series with the line. Each of switches 3A and 3C comprises a pair of semiconductor devices of the type turning-off at zero-cross of the current therethrough and connected in anti- parallel, i.e. back-to-back in parallel, to each other. Such semiconductors may be, for example, thyristors.

The softstarter device 2 is adapted to reduce a DC component in a load current for both start and stop of the motor by adjusting firing angles of the semi- conductor devices based on the calculated phase delays during start and stop of the motor. It comprises a DC reducing unit 1 6, a first voltage measuring unit comprisi ng voltage measuri ng devices 8A and 8C for providi ng measured voltages across the two switches 3A and 3C, step 1 00 in Fig . 4, a fi rst zero- crossi ng detecting unit 14, 0-X-Detecti , configured for detecting the zero- crossi ng of the measured voltages across the two pairs of semiconductor devices and providi ng zero-crossing signals to the DC reducing unit, step 1 1 0 in Fig. 4, a second voltage measuring unit 7 for measuring input voltages in the first and third phases or between the first and second phases, step 1 20, a second zero-crossing detecting unit 1 2, 0-X-Detect 2 , for detecting the zero- crossi ng of the input voltage between the first and third L 3 phases, step 1 30 and a trigger unit 1 5 for sending firing signals to the thyristors of each of the switches.

Each of the voltage measuring devices 8A and 8C is connected in parallel with each pair 3A, 3C of thyristors and provides at its output a digital signal X1 and X3 respectively, representing the measured voltage U 1 and U3. When the voltage in question , such as U 1 , across the thyristors of the switch 3A is essentially zero, i .e. , either when at least one of the thyristors is conducting or during a zero-crossing of the phase voltage, then the signal X1 is a logical "0". In al l other cases, i .e. , when there is a voltage across the switch , the signal X1 is a logical " 1 ". The signal X1 and X3 are forward to the DC red ucing unit. Accordingly, digital signals X1 ' and X3' generated by the second zero-crossing detecti ng unit 1 2 are also forwarded to the DC reduci ng unit, which will be used as reference poi nts for firing each of the thyristors in each of the switches 3A and 3C .

During start and stop of the motor, the DC reducing unit is active and configured to calculate phase delays based on the received signals X1 and X1 ' , X3 and X3' , respectively, to control the voltages of two of the three phases, step 140, by adjusting firing angles of the thyristors of each of the switches 3A and 3C , i .e. to make them conductive. I n this example, the voltages in the phase lines and L 3 are controlled by each 3A, 3C of the thyristor pairs, respectively-

Because zero-crossi ngs of the voltages across the thyristors and zero- crossi ngs of actual currents are sim ultaneous, a time delay from 0-X-Detect 2 to 0-X-Detecti equals a phase delay between the zero-crossing of the input voltage between two phases and the zero-crossing of a voltage across the corresponding pair of semiconductor devices. Therefore, the triggering time of the thyristors is calculated based on the detected zero-crossings of the i nput volt- ages between two of the three phase lines, in this example the first and third phase lines, step 1 60, compensated with an average phase delay between the zero-crossing of the input voltage and the zero-crossing of the voltage across the sem iconductor devices, step 1 40 in Fig . 4, as follows,

Wherein , t trig is the triggering time of a thyristor, t uref is the time poi nt when the zero-crossing of an input voltage occurs, a is a firing angle generated by a high level algorithm , for example, a ramp generator or a torque control unit as it is in this example, and the phase delay is an average of time differ- ences/delays between the zero-crossing of the input voltage between two phases and the zero-crossings of the voltages across the two pai rs of semiconductor devices, which equals the time occurrence of a zero-crossing of an actual current subtracts the time occurrence of a zero-crossing of input voltage, i .e. t iref - t uref . To even out the time differences between a positive half period and a negative half period , the average phase delay is calculated by the two latest phase delays, or alternatively, a m ultiple of 2 of the latest phase delays, for example the latest four or six phase delays, step 1 50 in Fig. 4. Each of the latest phase delays may be kept in storage when they are calculated .

To calculate the triggering time of a thyristor, the zero-crossing of an input voltages between two of three phases instead of the zero-crossi ngs of an ac- tual currents, which is equivalent to a zero-crossing of the voltage across a thyristor pair, is used as a reference poi nt i n time. This reference point is then compensated by the average of the two latest phase delays or a m ultiple of two of the latest phase delays and the pre-generated firing angle so that the firi ng angle is adjusted. Therefore, the i nvention has a significant effect in reducing tim ing errors caused by using t iref as a reference point in a positive and negative half period , I n this way, according to the invention , the timing errors can be cancel led out almost completely, in contrast to the method described in the prior art with a Publication No. WO 2008/033088 would still re- main some of tim ing errors by using a zero-crossing of the current is used as a reference point. This is because the tim ing of the zero-crossing of an input voltage is more stable than the timing of the zero-crossing of an actual current through the thyristors. Since the latter depends on a number of factors, for example, input voltages between two of the three phases, power factor/phase delay of a motor, harmonics, and most important the way of how fi ring of thyristors is proceeded. All of these factors affect the tim ing of the zero-crossing of the actual current. The invention therefore makes it possible to decrease torque variations and oscillations, which, in turn , makes the torque control perform better because less "noises" are input into the torque control loop.

Figure 3 illustrates a graph of the input voltage between two of the three phases, the voltage across the sem iconductor devices of one phase and the current to the motor in this phase versus the poi nt in time when the sem iconductor devices are turned on or off for the system shown in Fig . 1 a.

By gradually increasi ng or decreasing the firing angle a, the voltage across the motor term inals is correspondingly adjusted .

The softstarter device 2 is adapted to smoothly start and stop the motor in a desired start and stop ramp by calculating an electromagnetic torque of the motor. It comprises a current measuring unit 8 for measuring input currents to the motor in two of the three phases, i n this example, the first and third phases ; a current estimating unit 1 0 for estimating a current of one of the phases, in this example, the current of the second phase ; a voltage estimating unit 9 for estimating an input voltage between the thi rd and one of other two phases, in this example, between the second and thi rd phases based on the measured input voltages; a power computing unit 20 and a torque control unit 30 for con- trolling the switches 3A and 3C and thereby the current by means of regulating the voltage supplied to the motor. Both current and voltage measuring units may be sensors.

The current measuri ng unit 8 provides the measured input currents to the cur- rent estimating unit 1 0 so as the current of the second phase is estimated . The estimated current is input to the power computing unit. I n this example, the second voltage measuri ng unit 7 is arranged to measure phase-ground voltages of the first and third phases and to calculate an i nput voltage ui 3 between the first and third phases. It may, however, be configured to measure an input voltage between the first and third phases directly. It is also possible to measure the voltages after the switches, i .e. to arrange the second voltage measuring unit 7 between the switches and the motor as illustrated in Figure 1 b for another embodiment. The voltage estimating unit 9 is configured to receive the measured or calculated input voltage between two phases, i n this example, the first and third phases and thereby to estimate another input voltage u 2 3 between the second and third phases. The power computing unit 20 is configured , upon receiving the measured current and voltage, the estimated current and voltage, to calcu- late an i nstantaneous power P in supplied to the motor based in dependence on the measured current and the calculated and estimated input voltages.

For the system illustrated i n Figure 1 a, the calculation of the instantaneous power P in can be done as follows,

p = * u 13 + i 2 * u 2 3

wherein,

p is an instantaneous power to be calculated ;

i 2 = - ii - i 3 , ii and i 3 are currents instantaneously measured and sampled ; u 13 is an input voltage between the fi rst and third phases, which may be instantaneously measured directly or calculated by instantaneous measured voltages in first and second phases;

u 23 is an estimated input voltage between the second and third phases.

With an instantaneous measured/calculated input voltageu 13 , u 23 can be estimated as,

u 23 (/c) = u 13 (k - x)

wherei n,

U 23(A is an estimated instantaneous input voltage between the second and third phases for the present time/latest sample;

u 13 (k - x) is an instantaneous voltage between the first and third phases measured/sampled 1 20 ° ago, x = (120/360) * (T/T s ) rounded to the closest integer, T is the period time for the input voltage and T s is the i nterval between samples. As it may be observed , this calculation of the instantaneous power needs to estimate one current and one input voltage between the second and third phases. However, other forms of calculations may be applied as well ; for example, use two measured currents and two estimated input voltages. The torque control unit 30 includes a torque computing unit 34 configured to calculate an electromagnetic torque of the motor in dependence on the calculated instantaneous power. I n this example, the torque control unit further includes a torque feedback regulator loop 40 adapted to control the torque by means of regulating the voltage supplied to the motor. The feedback regulator loop may be a proportional-integrating feedback loop, also called Pl-regulator. With this parameter, the torque control unit 30 calculates the power losses Pioss in the motor based on the relationship between nom inal current l rms and stator resistance R s tator - The motor power losses are subtracted from the calculated supplied instantaneous power P i n and a real power P re a i to the motor 1 is calculated . The elec- tromagnetic torque T 1 2 of the motor 1 is then determ ined by using the real power P rea i - The torque T 12 of the motor is, in this case, proportional to the real power P rea i supplied to the motor. The torque feedback regulator loop 1 8 is used to provide better control over the deceleration and acceleration of the motor. In the regulation loop, a difference between the estimated electromagnetic torque T 12 and a reference value T m for the electromagnetic torque is calculated . An error signal is calculated dependent on the comparison between the calculated torque and the reference val ue. The speed of the motor is controlled in dependence on the torque error signal . The reference torque T m value is obtained from a torque T set-point ramp determined from a load model based on experience, e.g. for quadratic loads such as pumps. The T set-point ramp is based on input settings related to start and stop time and a rated current , and voltage measured by the second voltage measuring unit 7. When starting a motor, the softstarter device 2 has a target torque which may be equal to be at least the nom inal input current m ultiplied by the voltage as measured . This has the effect of a torque target that is probably in excess of the torque that the motor 1 plus load attains. The rate of increase of torque is substantially l inear and preferably an even , steady increase in torque. The use of a torque target and torque control makes the acceleration smoother and more even . This principle can be applied to a stop operation of the motor with a steady decrease of torque. Therefore the object of smoothly starting and stopping a three-phase motor is achieved . It is also possible to combine the torque control unit with another DC-reducing unit to achieve the object of the invention as shown in Figure 2a, wherein , a DC-reducing component, using zero-crossings of input currents as references for calculating firing angles as described i n the prior art, is adapted to adjust firi ng angles of the semiconductor devices duri ng start and stop of the motor. Figure 2b is a variant of the system illustrated in Figure 2a, wherein the voltage measuring unit for measuring voltages in a first and second of the phases or between the fi rst and second phases is arranged between the switches and the motor. It should be noted that, although , the above describes exempl ifying embodiments of the i nvention , there are several variations which may be made to the described solution without departing from the scope of the present invention as defined in the claims. For example, measuring only the currents control led by the softstarter may be a variant.