Patent Description:
An inverter is an inverting device that electrically converts DC to AC. An inverter used in the industry receives power supplied from a commercial power supply and varies a voltage and frequency of the power and supplies the varied power to a motor. Accordingly, the inverter may control a operation speed of the motor.

A dual high voltage inverter is an inverter whose voltage specification is in a range of <NUM> to 11000V. The dual high voltage inverter is used to control a rotation speed of a large capacity motor with a capacity ranging from 600kVA to <NUM>. 5MVA, or to control a output torque.

In a system that drives a motor using such an inverter, various reasons may lead to restarting the inverter while the motor is rotating. For example, while a commercial power is cut off, such as in a momentary power failure, and thus the motor is free-running, the commercial power is re-input such that the inverter is restarted. This is called a flying start.

In this connection, the inverter has an excessive current, which causes a noise, etc. which is to be suppressed. In severe cases, a power element of the inverter may be damaged. Therefore, a method is required by which the inverter is easily restarted when the power is restored while the motor is in a free running state.

In a conventional case, an output voltage of the motor was measured and analyzed to control the motor based on a change in an active current. However, there is a problem in this approach in that when a residual counter electromotive force exists in the motor, it is difficult to analyze an output current of the motor, which disallows the flying start.

Accordingly, a method for controlling the motor by estimating a frequency of the counter electromotive force of the motor is used. However, in this approach, when a magnitude of the counter electromotive force is small to disallow the frequency estimation, the frequency estimation time becomes longer because a conventional flying start method should be employed.

<CIT> elates to AC generator sensor-less vector control method and control device thereof. A direct current or a direct-current voltage is applied to the alternating-current motor in the free running state before it is restarted, and a secondary current, that flows across the rotor of the motor at this time, is employed to estimate the rotational direction and the velocity of the alternating-current motor.

<CIT> relates to a method and an apparatus for determining motor rotation status. A motor drive unit includes a voltage inverter, a controller, and reconnect logic. The voltage inverter provides motor drive signals to an associated motor. The controller is operable to generate demand signals for at least two control axes for controlling the voltage inverter. The reconnect logic is operable to direct the controller to inject a current into a first control axis. The reconnect logic is further operable to monitor a voltage of a second control axis to detect zero crossings and determine a speed of the associated motor based on the detected zero crossings.

In order to solve the problem, a purpose of the present disclosure is to provide an inverter-controlling device that allows a flying start by estimating a rotation speed of a motor even when a counter electromotive force of a motor is very small,.

The present invention is defined in the independent claim <NUM> with preferred embodiments disclosed in the dependent claims.

According to the present disclosure, even when it is difficult to accurately analyze the counter electromotive force of the motor, the motor rotation speed can be estimated quickly, such that the inverter can be restarted in a short time.

Further specific effects of the present disclosure as well as the effects as described above will be described in conduction with illustrations of specific details for carrying out the invention.

Hereinafter, a device for controlling an inverter in accordance with the present disclosure will be described with refence to the accompanying drawings.

For simplicity and clarity of illustration, elements in the figures, are not necessarily drawn to scale. The same reference numbers in different figures. denote the same or similar elements, and as such perform similar functionality. Also, descriptions and details of well-known steps and elements are omitted for simplicity of the description. Furthermore, in the following detailed description of the present disclosure, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. However, it will be understood that the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present disclosure.

The above objects, features and advantages will become apparent from the detailed description with reference to the accompanying drawings. Embodiments are described in sufficient detail to enable those skilled in the art in the art to easily practice the technical idea of the present disclosure. Detailed descriptions of well-known functions or configurations may be omitted in order not to unnecessarily obscure the gist of the present disclosure. Throughout the drawings, like reference numerals refer to like elements.

Unless defined otherwise, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. When the terms used herein are in conflict with a general meaning of the term, the meaning of the term is in accordance with a definition used herein.

Examples of various embodiments are illustrated and described further below. It will be understood that the description herein is not intended to limit the claims to the specific embodiments described.

It will be further understood that the terms "comprises", "comprising", "includes", and "including" when used in this specification, specify the presence of the stated features, integers, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, operations, elements, components, and/or portions thereof. Expression such as "at least one of" when preceding a list of elements may modify the entire list of elements and may not modify the individual elements of the list.

Thus, a first element, component, region, layer or section described below could be termed a second element, component,.

In addition, it will also be understood that when a first element or layer is referred to as being present "on" a second element or layer, the first element may be disposed directly on the second element or may be disposed indirectly on the second element with a third element or layer being disposed between the first and second elements or layers. It will be understood that when an element or layer is referred to as being "connected to", or "coupled to" another element or layer, it can be directly on, connected to, or coupled to the other element or layer, or one or more intervening elements or layers may be present. In addition, it will also be understood that when an element or layer is referred to as being "between" two elements or layers, it can be the only element or layer between the two elements or layers, or one or more intervening elements or layers may also be present.

Hereinafter, a conventional flying start technique will be described with reference to <FIG>. Then, an inverter-controlling device according to one embodiment of the present disclosure will be described with reference to <FIG>.

<FIG> shows a configuration of a typical high-voltage motor system.

As shown in <FIG>, in the conventional high-voltage motor system, even when the high-voltage inverter <NUM> fails, a commercial power line is connected to a motor <NUM> to allow the motor <NUM> to operate using a commercial power. That is, a first switch <NUM> and a second switch <NUM> are configured for connecting the motor <NUM> and the high-voltage inverter <NUM> to each other. A third switch <NUM> is configured for connecting the motor <NUM> directly to the commercial power line.

While the motor <NUM> is being driven via the third switch <NUM> being connected to a commercial power source, the commercial power is not supplied to the motor due to an instantaneous power failure or the like and thus the motor <NUM> is free-running. In this case, when the commercial power is again supplied to the motor via electric power restoration, the high-voltage inverter <NUM> is connected to the motor <NUM> via the first and second switches <NUM> and <NUM>, thereby to restart the motor <NUM>. In this connection, the third switch <NUM> is in an off state.

<FIG> shows an example of a change in a rotation speed and a counter electromotive force of a motor in a free-run state when a voltage supply to the motor is interrupted. As shown in <FIG>, when the voltage supply to the motor <NUM> is cut off, both the counter electromotive force and rotation speed of the motor <NUM> are decreased.

<FIG> is a graph for illustrating a flying start operation of a conventional inverter. <FIG> is a graph of an example of a change in an active current of the motor in the operation as in <FIG>.

At a start point 3A of the flying start operation, an output frequency f of the high-voltage inverter <NUM> begins to decrease from a maximum frequency, while an output voltage V of the high-voltage inverter <NUM> begins to increase from a minimum voltage.

At a completion point 3B of the flying start operation, the output frequency f of the high-voltage inverter <NUM> becomes equal to a rotation speed of the motor <NUM>, while the output voltage V of the high-voltage inverter <NUM> starts to increase at a V/f ratio. A subsequent operation is controlled according to the V/f ratio.

Thus, a flying start control performs independent control between a voltage magnitude and a frequency during control of variable voltage variable frequency (VVVF) of the high-voltage inverter <NUM>. When there is present a counter electromotive force in the motor <NUM>, an internal circuit of the high-voltage inverter <NUM> may not operate. Thus, when the counter electromotive force is below a certain magnitude, the internal circuitry of the high-voltage inverter <NUM> begins to operate. The frequency of the high-voltage inverter <NUM> decreases continuously from the highest speed. When the frequency reaches an actual rotation speed of the motor <NUM>, the frequency remains constant.

The V/f based control refers to a control scheme for keeping an internal magnetic flux of an induction motor <NUM> constant. In general, the V/f ratio is set at a ratio between a rated voltage/rated frequency. A user may change the ratio as a parameter.

A speed detection in this conventional flying start operation is achieved by observing an energy flow through the high-voltage inverter <NUM> and the motor <NUM>. That is, referring to <FIG>, when the output frequency of the high-voltage inverter <NUM> is faster than the rotation speed of motor <NUM>, the energy flow occurs from the high-voltage inverter <NUM> toward the motor <NUM>. Further, an active current of the motor <NUM> is detected in a positive direction.

When the output frequency continues to decrease and then the output frequency becomes smaller than the rotation speed of the motor <NUM>, the energy flow occurs from the motor <NUM> toward the high-voltage inverter <NUM>. The active current is detected in a negative direction.

Thus, in the conventional flying start control, when the detected active current of the motor <NUM> in the negative direction is held for a certain duration, the detection of the speed is completed and the operation is performed based on the V/f ratio.

<FIG> is a graph for illustrating an operation of performing a flying start by measuring a counter electromotive force of a motor and estimating a motor rotation speed based on a frequency of the counter electromotive force in a prior art.

A conventional technique may measure a phase voltage applied to the high-voltage inverter <NUM> from the motor <NUM> and measure a counter electromotive force V of the motor <NUM>. Then, the conventional technique uses a phase-locked loop (PLL) to generate a waveform of an output voltage V having a magnitude, phase, and frequency equal to those of the counter electromotive force V. Thus, the inverter <NUM> generates a voltage equal to the counter electromotive force V of the motor <NUM> based on the thus generated output voltage V at the start of the flying start operation.

The PLL refers to a circuit that matches an input signal and a reference frequency with an output signal and an output frequency respectively. The PLL detects a phase difference between the input signal and the output signal. The PLL controls a voltage controlled oscillator (VCO) to output an accurately-locked frequency signal.

Referring to <FIG>, in a region 4A-4B, a waveform of the output voltage V that has a frequency, phase and magnitude equal to those of the counter electromotive force V due to the PLL is detected. In a 4B-4C region, a frequency of the motor <NUM>, that is, a rotation speed f thereof is fixed and a magnitude of the output voltage of the inverter <NUM> is increased. At a point 4C, after the flying start is complete, the inverter may then be controlled based on the V/f ratio.

When, as shown in <FIG>, the rotation speed f of the motor <NUM> is detected and the flying start is performed, not the output voltage of the motor <NUM> but reference information about the output voltage and output current of the high-voltage inverter <NUM> are taken into consideration.

On the other hand, when the rotation speed f of the motor <NUM> is detected based on the counter electromotive force of the motor <NUM> as shown in <FIG> to perform the flying start, the output voltage of the motor <NUM> is detected in the region where the counter electromotive force is present, thereby to detect the rotation speed f of the motor <NUM>. In this connection, the output voltage of the motor <NUM> is detected via digitization at a <NUM> sampling duration using a voltage dividing resistor.

Thus, according to the conventional flying start method as in the case of <FIG>, the output voltage of the motor <NUM> is measured and analyzed such that the motor operates according to the change of the active current. However, when there is a residual counter electromotive force in the motor <NUM>, the flying start cannot be performed because it is difficult to analyze the output current information of the motor <NUM>.

Further, according to the conventional flying start method as in <FIG>, the counter electromotive force may be used to estimate the rotation speed of the motor <NUM>. Thus, this may enable the flying start in a short time. However, when the counter electromotive force is small to disallow estimating the frequency, and disallow estimating the rotation speed, the flying start may not be implemented. In such a case, the frequency estimation time becomes larger.

The present disclosure is intended to solve the problems of the conventional technology. According to one embodiment of the present disclosure, a time duration required to estimate the motor rotation speed is shortened. Even when the counter electromotive force of the motor is very small, the motor rotation speed can be estimated quickly.

<FIG> is a configuration diagram to describe an inverter system in accordance with an embodiment of the present disclosure.

As shown in <FIG>, an inverter system in accordance with one embodiment of the present disclosure may include a rectifying module <NUM> for rectifying power supplied from an AC power supply <NUM> such as a commercial power supply; a smoothing module <NUM> for smoothing an output voltage from the rectifying module <NUM>; an inverting module <NUM> including a plurality of switching elements for converting the smoothened voltage from the smoothing module <NUM> into an AC voltage of a target frequency and magnitude; and a control device <NUM> that provides gate signals to the plurality of switching elements of the inverting module <NUM> and estimates a rotation speed of the motor <NUM> by analyzing an output current from the inverting module <NUM>. The inverter system in accordance with one embodiment of the present disclosure may further include a detection module <NUM> that measures an output current output to a motor <NUM>. The detection module <NUM> may be, for example, a current transformer CT.

<FIG> depicts a graph for illustrating a period where a motor rotation speed may not be figured out.

As shown in <FIG>, in a region where a magnitude of the counter electromotive force of the motor is equal to or smaller than a predetermined magnitude (for example, <NUM>), it is impossible to analyze the counter electromotive force of the motor even when the motor is rotating.

In order to quickly estimate the rotation speed f of the motor in the region where it is not possible to analyze the counter electromotive force of the motor, the control device <NUM> according to the present disclosure applies a DC current to the motor <NUM> and analyzes a ripple current resulting from the applied DC current. Further, the control device <NUM> according to the present disclosure estimates the rotation speed of the motor based on the ripple current. In this manner, the flying start is performed based on the estimated rotation speed of the motor. According to the present disclosure, the rotation speed of the motor can be estimated within about <NUM>% of the estimation duration of the conventional technique. Thus, the quick restart of the motor <NUM> may be realized.

<FIG> shows a detailed configuration of the control device <NUM> in <FIG>.

As shown in the figure, the control device <NUM> according to one embodiment of the present disclosure may include a main controller <NUM>, a current controller <NUM>, a comparison module <NUM>, and a frequency estimation module <NUM>.

The current controller <NUM> generates a gate signal to be applied to the inverting module <NUM> such that the module <NUM> generates a voltage required to apply a target command current ia *, ib *, ic * to the motor <NUM>. The current controller may control an output frequency of the inverting module <NUM> by applying the gate signal to the switching elements of the inverting module <NUM>.

The main controller <NUM> may generate command currents to be applied to the current controller <NUM>.

In this connection, the command currents may be DC currents including a phase current being a positive current, b phase and c phase currents. Each of the b phase and c phase currents may be a negative current having a half magnitude of the a phase current. Each command current as a DC current refers to a current with a certain magnitude and a frequency of zero. In this connection, the magnitude of each DC current may vary depending on the configuration of the motor <NUM>. For example, in one embodiment of the present disclosure, the magnitude of the a phase command current may be a positive current of a <NUM>/<NUM> magnitude of a rated current of the motor <NUM>. The magnitude of each of the b phase and c phase command currents may be a negative current of a <NUM>/<NUM> magnitude of the a phase command current. However, it should be understood that the present disclosure is not limited thereto. The magnitude of each command current may be varied depending on the rating of motor <NUM>.

In this connection, each command current may be injected for a predetermined time, for example, <NUM>. In one embodiment of the present disclosure, the frequency estimation module <NUM> estimates a frequency by analyzing the ripple current resulting from the application of each DC current. Therefore, the injection time of the command current is large enough to cause the ripple in the inverting module <NUM>. However, the present disclosure is not limited thereto. Depending on the rating of the motor <NUM>, the injection time of the command current may vary.

<FIG> is an example for describing a relationship between magnitudes of command currents in <FIG>.

As described above, in one embodiment of the present disclosure, the a phase command current is a positive current of a predetermined magnitude 8B. Each of the b phase and c phase command currents may be a negative current of a magnitude 8C or 8D corresponding to <NUM>/<NUM> of the magnitude 8B of the a phase command current.

Further, in order to measure ripples caused by the a phase to c phase command currents, the a phase to c phase command currents may be applied for a certain duration of 8A.

In one embodiment of the present disclosure, the comparison module <NUM> may determine a difference between each command current and an output current of the inverter measured from the detection module <NUM>, and may provide the difference to the current controller <NUM>.

In one embodiment of the present disclosure, the current controller <NUM> may be a proportional-integral (PI) controller. The current controller <NUM> may perform a proportional-integral (PI) control based on the output value of the comparison module <NUM>.

That is, the current controller <NUM> may multiply the outputs of the comparison module <NUM> by a specific proportional gain P to form first products. Then, the current controller <NUM> may multiply the products by a specific integral gain I to form second products. Then, the current controller <NUM> may integrate the second products to form an integrated sum. the current controller <NUM> may apply the sum as a gate signal to the plurality of switching elements of the inverting module <NUM>.

The proportional control refers to controlling the output frequency of the inverting module <NUM> to allow the output frequency to be proportional to the output (deviation) of the comparison module <NUM>. When the P gain is set to be large, a system response will be fast because the deviation rapidly change. However, when the P gain is set to be very large, the system becomes unstable.

The integral control is intended to correct the output frequency by integrating the deviations. For the proportional control, a large deviation may produce a large output frequency. When the deviation is small, an adjusted value of the output frequency becomes smaller. However, the proportional control cannot make the deviation zero. The integral control compensates for the limit of this proportional control. The integral correction of the output frequency may be performed by accumulating the deviations over the entire time, resulting in a deviation of zero. The I gain indicates how often the deviations are integrated.

In one embodiment of the present disclosure, the current controller <NUM> may use a P gain smaller than a P gain used for control of the inverter in a steady state, for a time duration 8A for which the command current is supplied for the current response measurement.

<FIG> is an example for describing a relationship between current responses to the command currents in <FIG>. The current responses may correspond to the outputs of the inverting module <NUM> in <FIG>.

As shown in <FIG>, the current responses to the command currents having the phase a to phase c at an output of the inverting module <NUM> may be expressed as ripple currents. A magnitude of a current response to the a phase current corresponds to twice a magnitude of each of the current responses to the b phase and c phase currents. The current response to the a phase current has an opposite direction to each of the current responses to the b phase and c phase currents. That is, the current response to the a phase current is a positive ripple current, while each of the current responses to the b phase and c phase currents is a negative ripple current.

Further, the current responses to the b phase and c phase currents are out-of-phases in an opposite direction.

A slope 9A along middle points of a sinusoidal curve in the current response to the a phase current corresponds to the P gain. This is also true for each of the current responses to the b phase and c phase currents.

The frequency estimation module <NUM> according to one embodiment of the present disclosure can estimate a frequency or a rotation speed f of the motor <NUM> using one or more of the a phase to c phase-based ripple currents.

For example, the frequency estimation module <NUM> calculates an first increment of the a phase-based current response corresponding to a predetermined time, for example, <NUM>. Then, the module computes a second increment of the a phase-based current response corresponding to a subsequent predetermined time (which is equivalent to twice derivative). Thus, the module <NUM> may obtain a sinusoidal curve of the a phase-based current response of <FIG>. The module <NUM> may estimate the rotation speed of the motor <NUM> by calculating a zero-crossing of the generated sinusoidal curve.

Alternatively, referring to <FIG>, the b phase-based current response and c phase-based current response are opposite in phases thereof. The frequency estimation module <NUM> subtracts the c phase-based current response from the b phase-based current response in <FIG> to obtain a sinusoidal curve. Then, the module <NUM> estimates the rotation speed of the motor <NUM> by calculating the zero-crossing of the resulting sinusoidal curve.

However, the manner in which the frequency estimation module <NUM> according to one embodiment of the present disclosure estimates the rotation speed of the motor <NUM> from the current responses in <FIG> is not limited to those described above. The frequency estimation may be realized in various ways.

The main controller <NUM> may perform the flying start of the inverter based on the rotation speed of the motor <NUM> estimated by the frequency estimation module <NUM>.

<FIG> illustrates an example graph for describing a flying start operation according to one embodiment of the present disclosure. This operation may be an operation of the inverter under the control of the main controller <NUM>.

As shown in <FIG>, while the supply of the commercial power is interrupted such that the rotation speed f of the motor is reduced and the counter electromotive force V of the motor is reduced, the supply of the commercial power is re-activated. In this connection, according to the conventional technique, the magnitude of the counter electromotive force of the motor <NUM> may be small and thus the flying start may not be realized.

However, in one embodiment of the present disclosure, the main controller <NUM> provides the command currents to the current controller <NUM> for a duration from a predetermined point in time, i.e., 10A to a start point <NUM> B of the flying start operation. In this connection, the time 10A at which the provision of the command currents starts may be set to a time when a magnitude of the counter electromotive force V of the motor <NUM> is equal to or smaller than a predetermined magnitude, for example, <NUM>. Further, the command current injection duration may be set to a predetermined duration, for example, <NUM>. However, the command current injection duration may vary depending on embodiments.

During the time main controller <NUM> injects the command currents into the current controller <NUM>, that is, for a duration from a point 10A to a point 10B, the frequency estimation module <NUM> may estimate the frequency or rotation speed f of the motor <NUM>. In this connection, during the time main controller <NUM> injects the command currents into the current controller <NUM>, that is, for a duration from a point 10A to a point 10B, the main controller <NUM> may maintain the output voltage V of the inverting module <NUM> to have a constant magnitude.

When the flying start operation starts at the point 10B, the main controller <NUM> sets the output frequency F of the inverting module <NUM> to a frequency that is greater by at least a predetermined magnitude than the rotation speed of the motor <NUM> as estimated by the frequency estimation module <NUM>. The controller <NUM> decreases the output frequency F of the inverting module <NUM> by a point when the flying start operation is completed, that is, by a point 10D.

At the time point 10D when the rotation speed f of the motor <NUM> becomes equal to the inverter output frequency F, the flying start operation is completed. The motor <NUM> operates based on the output frequency F of the inverter at the time point of 10D. The main controller <NUM> increases the inverter output voltage V from zero for a duration from the time point 10B at which the flying start operation starts to the time point 10D at which the flying start operation ends.

After the end of the flying start at 10D, the main controller <NUM> adjusts the output voltage V of the inverting module <NUM> according to the V/f ratio.

As shown in <FIG>, according to the present disclosure, the flying start operation is performed at the maximum output frequency of the inverter as in the conventional technology, but the flying start operation is started based on the rotation speed f of the motor <NUM> as estimated by the frequency estimation module <NUM>. Therefore, the flying start operation is completed in a shorter time than in the conventional technique.

According to the present disclosure, the motor rotation speed is estimated by injecting the DC currents and analyzing the ripple currents resulting from the injected currents. Then, performing of the flying start based on the estimated motor rotation speed may allow the flying start operation to be completed within about <NUM>% of the duration of the flying start operation in the conventional case.

In addition, even when the counter electromotive force cannot be accurately analyzed in the conventional case, the motor rotation speed can be estimated quickly in accordance with the present disclosure so that the inverter can be restarted in a short time.

Claim 1:
An inverter-controlling device for controlling an inverter system, the inverter system comprising a rectifying module (<NUM>) for rectifying power supplied from an alternate current (AC) power supply, a smoothing module (<NUM>) for smoothing an output voltage from the rectifying module and an inverting module (<NUM>) including a plurality of switching elements for converting the smoothened voltage from the smoothing module into an AC voltage and for supplying the AC voltage to a motor (<NUM>), wherein the inverter-controlling device comprises:
a main controller (<NUM>) configured to, when the supply of the AC voltage to the motor (<NUM>) stops, generate a command current of a first predetermined magnitude;
a comparison module (<NUM>) configured to calculate a difference between the command current and an output current of the inverting module (<NUM>);
a current controller (<NUM>) configured to perform a proportional-integral (PI) control based on the difference; and
a frequency estimation module (<NUM>) configured to, while the command current is injected to the current controller (<NUM>), estimate a rotation speed of the motor (<NUM>) based on current responses to the command currents,
characterized in that
the current responses to the command currents include an A-phase current response, a B-phase current response and a C-phase current response,
wherein a magnitude of the A-phase current response corresponds to twice a magnitude of each of the B-phase current response and of the C-phase current response,
wherein the A-phase current response is a positive ripple current, while each of the B-phase current response and the C-phase current response is a negative ripple current,
the B-phase current response and the C-phase current response are out-of-phases in an opposite direction,
wherein the main controller (<NUM>) is further configured to:
when a flying start operation of the inverting module (<NUM>) begins, set an output frequency of the inverting module (<NUM>) to a first frequency greater by at least a second predetermined magnitude than the rotation speed of the motor (<NUM>) estimated by the frequency estimation module (<NUM>);
decrease the output frequency of the inverting module (<NUM>) from the first frequency to the estimated rotation speed of the motor (<NUM>); and
increase an output voltage of the inverting module (<NUM>) from zero.