Systems and method for controlling electrodynamic machines with a variable frequency drive

Systems and method for controlling an alternating current (AC) electrodynamic machine (390) with a variable frequency drive (VFD) (380) include a control system (300) with a phase-locked-loop (PLL) circuit (382) for providing a stator flux angle signal (338) to the VFD (380), the PLL circuit (382) comprising a proportional integral (PI) regulator (332) providing an output signal (334); and a feedforward generator (350) in communication with the PLL circuit (382), wherein the feedforward generator (350) tracks a stator flux position of the AC electrodynamic machine (390) such that the feedforward generator (350) determines a stator frequency signal (352) based on stator flux signals (308, 310, 312) and supplies the stator frequency signal (352) downstream of the PI regulator (332), and wherein the stator frequency signal (352) is summed with the output signal (334) of the PI regulator (332) to provide a dynamically adapted output signal (335) of the PI regulator (332), and wherein the adapted output signal (335) is used to determine the stator flux angle signal (338).

BACKGROUND

Aspects of the present invention generally relate to electrodynamic machines, which include for example electric motors, such as alternating current (AC) asynchronous motors, for example induction motors, and AC synchronous motors, as well as electric generators, and more particularly to a system, apparatus and method for controlling an induction motor with a variable frequency drive (VFD).

2. Description of the Related Art

When starting large, e.g., medium voltage, AC motors via direct connection to a utility power source, one or more problems may occur. For example, a large AC motor may draw four to six times its rated current (known as inrush current) at a low power factor upon startup. This may cause significant transient voltage drops in the network of the utility power source, which may adversely affect other equipment and systems connected thereto. Also, the AC motor may undergo severe thermal and mechanical stress during a direct on-line start, which may shorten the life of the motor and/or limit the number of starts in a given period. Furthermore, during acceleration of a large AC motor, large torque pulsations may occur that can excite system torsional resonances that have been known on at least one occasion to cause a broken motor shaft.

To overcome the aforementioned problems, large AC motors may be “soft started” with a variable frequency drive (VFD). A VFD may controllably increase the magnitude and frequency of voltage applied to an AC motor during start-up. The voltage magnitude and frequency may start at very low values and may then increase to the rated voltage of the AC motor and to the frequency of the utility power source, e.g., 60 hertz, as the AC motor reaches its rated speed.

In order to provide a precision speed control of the AC motor, the VFD may comprise a control system with a phase-locked loop control circuit, herein also shortly referred to as PLL. Phase-locked-loop techniques are known and well suited to provide the precision speed control by phase locking the AC motor to a stable and accurate reference frequency. When the rate of change of frequency is very slow, for example less than 1 Hz per second, the dynamic accuracy of the PLL is acceptable. But when the frequency of the AC motor changes rapidly, for example 60 Hz per second, the dynamic accuracy of the PLL is not acceptable any more. Thus, a need may exist to provide an improved control system with a PLL for rapidly changing frequencies of an AC motor.

SUMMARY

Briefly described, aspects of the present invention generally relate to electrodynamic machines, which include for example electric motors, such as AC asynchronous motors, for example induction motors, and AC synchronous motors, as well as electric generators, and more particularly to systems and method for controlling an induction motor with a variable frequency drive (VFD).

A first aspect of the present invention provides a control system for controlling an alternating current (AC) electrodynamic machine comprising a phase-locked-loop (PLL) circuit for providing a stator flux angle signal to a variable frequency drive (VFD), the PLL circuit comprising a proportional integral (PI) regulator providing an output signal; and a feedforward generator in communication with the PLL circuit, wherein the feedforward generator tracks a stator flux position of the AC electrodynamic machine such that the feedforward generator determines a stator frequency signal based on stator flux signals and supplies the stator frequency signal downstream of the PI regulator, and wherein the stator frequency signal is summed with the output signal of the PI regulator to provide a dynamically adapted output signal of the PI regulator, and wherein the adapted output signal is used to determine the stator flux angle signal.

A second aspect of the present invention provides a system for controlling an alternating current (AC) electrodynamic machine comprising a variable frequency drive (VFD) configured to be coupled to a utility power source and to provide output currents; an AC electrodynamic machine operably coupled to a VFD output of the VFD, the VFD providing the output currents controlling magnitude of stator flux and torque produced by the AC electrodynamic machine, wherein the VFD further comprises a control system comprising a phase-locked-loop (PLL) circuit for providing a stator flux angle signal to the VFD, the PLL circuit comprising a proportional integral (PI) regulator providing an output signal; and a feedforward generator in communication with the PLL circuit, wherein the feedforward generator tracks a stator flux position of the AC electrodynamic machine such that the feedforward generator determines a stator frequency signal based on stator flux signals and supplies the stator frequency signal downstream of the PI regulator, and wherein the stator frequency signal is summed with the output signal of the PI regulator to provide a dynamically adapted output signal of the PI regulator, and wherein the adapted output signal is used to determine the stator flux angle signal.

A third aspect of the present invention provides a method for providing a feedforward signal for a phase-locked-loop (PLL) circuit comprising providing stator flux signals of an alternating current (AC) electrodynamic machine; determining zero crossings of the stator flux signals; generating a train of fixed width pulses derived from the zero crossings of the stator flux signals; and converting the train of fixed width pulses to a feedforward signal, wherein the feedforward signal is supplied to a PLL circuit, and wherein the feedforward signal is summed with an output signal of the PLL circuit to provide an adapted output signal.

DETAILED DESCRIPTION

Large (alternating current) AC motors may include medium voltage AC motors, which may have a rated voltage ranging from about 600 V (volts) AC to about 15,000 V (or 15 kV) AC. The “rated voltage” of a motor is a standardized term established by the National Electrical Manufacturers Association (NEMA) that generally refers to a motor's operating voltage usually +/−10%. Large AC motors may also include high voltage AC motors and, in some cases, other types of AC motors that may have a rated voltage below the above voltage range for medium voltage AC motors.

The aforementioned problems of starting a large AC motor may be overcome by “soft starting” the AC motor with a variable frequency drive (VFD). A VFD may initially apply to an AC motor at startup a low or near-zero voltage having a low or near-zero frequency. As the AC motor speed reaches its rated speed, the VFD may controllably increase both the voltage magnitude and frequency to the AC motor's rated voltage and a utility power source's frequency. At about that point, power supplied to the AC motor may be switched from the VFD directly to the utility power source.

FIG. 1illustrates an example of a known system100for starting a large AC motor102in accordance with embodiments disclosed herein. AC motor102may be a 3-phase medium voltage AC motor having a first winding104, a second winding106, and a third winding108arranged in a star or Y-connection configuration. First winding104may have a first lead winding connection105. Second winding106may have a second lead winding connection107, and third winding108may have a third lead winding connection109. AC motor102may be coupled to a load (not shown), which may be, e.g., one or more compressors, pumps, fans, and/or other suitable equipment.

System100may also include a variable frequency drive (VFD)110and a reactor118. VFD110may have a voltage rating that is the same or substantially the same as the rated voltage of AC motor102. VFD110may be coupled to receive 3-phase power via conductors111,112, and113(one conductor per phase) from a utility power source114. VFD110may be configured to output 3-phase power having a variable peak voltage magnitude and a variable frequency via conductors115,116, and117(one conductor per phase). Reactor118, which may be a 3-phase reactor, may be coupled in series to VFD110via conductors115,116, and117. Reactor118may provide inductance (which may add impedance) to the 3-phase output of VFD110.

System100may further include a first contactor122and a second contactor130. First contactor122may include a first control switch123, a second control switch124, and a third control switch125each coupled in series to reactor118via respective conductors119,120, and121. First control switch123may also be coupled in series to first lead winding connection105via conductor126. Second control switch124may also be coupled in series to second lead winding connection107via conductor127, and third control switch125may also be coupled in series to third lead winding connection109via conductor128.

Second contactor130may include a first control switch131, a second control switch132, and a third control switch133each coupled in series to utility power source114via respective conductors134,135, and136(one conductor per phase). First control switch131may be coupled in series to first lead winding connection105via conductor137. Second control switch132may be coupled in series to second lead winding connection107via conductor138, and third control switch133may be coupled in series to third lead winding connection109via conductor139.

First contactor122and second contactor130may be controlled by VFD110. That is, VFD110may control the opening and closing of first, second, and third control switches123,124, and125to connect and disconnect the output voltage of VFD110to and from AC motor102. Similarly, VFD110may control the opening and closing of first, second, and third control switches131,132, and133to connect and disconnect utility power of utility power source114to and from AC motor102. Each of conductors111-113,115-117,119-121,126-128,134-136, and137-139may be an electrical wire or cable of suitable gauge and/or size.

To start up AC motor102, system100may operate as follows: Upon or prior to startup, VFD110may cause first contactor122to connect the variable voltage output of VFD110(via reactor118) to AC motor102, while VFD110may cause second contactor130to disconnect utility power (received from utility power source114) from AC motor102. That is, first, second, and third control switches123,124, and125of first contactor122may be closed by VFD110, while first, second, and third control switches131,132, and133of second contactor130may be opened by VFD110. VFD110, which may receive 3-phase power from utility power source114, may then initially provide a low or near-zero voltage having a low or near-zero frequency to each of first, second, and third lead winding connections105,107, and109(separated by appropriate phase angles) via respective conductors126,127, and128. The application of voltage to AC motor102may cause the rotor (not shown) of AC motor102to begin rotating (in other words, the speed of AC motor102begins to increase from zero). The speed of AC motor102may be monitored by VFD110via feedback of, e.g., motor voltage and motor current. As the initial speed of AC motor102is sensed, VFD110may gradually and controllably increase both the output voltage peak magnitude and frequency. As the speed of AC motor102continues to increase, so too does the output voltage peak magnitude and frequency provided by VFD110.

As the motor speed reaches the rated speed of AC motor102, the voltage peak magnitude and frequency provided by VFD110may be at or near the voltage rating of the VFD110(i.e., at or near the rated voltage of AC motor102) and the frequency of utility power source114(which may be, e.g., 60 hertz). At about this point, power provided to AC motor102may be switched from VFD110to utility power source114. VFD110may cause second contactor130to connect utility power (received from utility power source114) to AC motor102, while VFD110may cause first contactor122to disconnect the variable output voltage of VFD110(via reactor118) from AC motor102. That is, first, second, and third control switches123,124, and125of first contactor122may be opened by VFD110, while first, second, and third control switches131,132, and133of second contactor130may be closed by VFD110. In some cases, AC motor102may be momentarily coupled to both VFD110and utility power source114. Reactor118may limit current exchanged between VFD110and utility power source114in this situation. To ensure that VFD110may be able to startup and drive AC motor102to its rated speed, VFD110may have a voltage rating that is the same or substantially the same as the rated voltage of AC motor102. For example, if the rated voltage of AC motor102is 6.9 kV AC, the voltage rating of VFD110may be about 6.9 kV AC.

FIG. 2illustrates a schematic diagram of a known VFD210in accordance with embodiments disclosed herein. In some embodiments, VFD110may have a configuration similar or identical to a VFD210ofFIG. 2. VFD210may output a voltage having a magnitude and frequency that may vary. The frequency may vary, e.g., from 0 up to the frequency of the AC input line which, as shown, may be from a 3-phase power source and may be, e.g., 60 hertz. The voltage magnitude may vary, e.g., from 0 up to about the voltage rating of VFD210. VFD210may include a controller240and a power circuit242. Controller240may control the operation of power circuit242and may be coupled to motor voltage feedback line244and motor current feedback line246. Voltage feedback line244and current feedback line246may be coupled to AC motor202. Controller240may monitor voltage feedback line244and current feedback line246to determine the speed of AC motor202and consequently determine whether to adjust the output voltage magnitude and frequency, for example in accordance with programming (e.g., a motor model) stored in and/or executing on controller240. In some embodiments, controller240may include a microprocessor or other suitable CPU (central processing unit) and a memory for storing software routines to determine motor speed and the criteria for varying the output voltage magnitude and frequency. Alternatively, controller240may transmit feedback information to another component (not shown) and receive commands from that component regarding adjustments to the output voltage magnitude and frequency. In some embodiments, power circuit242may convert received AC line voltage to a DC voltage and then invert the DC voltage back to a pulsed DC voltage whose RMS (root mean square) value simulates an AC voltage. In some embodiments, power circuit242may include a rectifier, an inverter, and/or PWM (pulse width modulation) circuitry configured to vary the output voltage of VFD210.

FIG. 3illustrates a schematic diagram of a control system300of a VFD380including a feedforward scheme for a phase-locked loop (PLL) circuit382in accordance with an exemplary embodiment of the present invention. VFD380comprises VFD output381, wherein the VFD380ofFIG. 3may be configured similar to the VFD210ofFIG. 2. VFD380ofFIG. 3and VFD210ofFIG. 2may be configured as described for example in U.S. Pat. No. 5,625,545 to Hammond which is incorporated herein in its entirety.

In order to provide a precision speed control of an AC motor390, the VFD may comprise a control system300with a phase-locked loop control circuit382, herein also shortly referred to as PLL circuit382. The control system300may be part of a power circuit controlled by a controller392of VFD380as illustrated for example inFIG. 2andFIG. 3. Phase-locked-loop techniques are well suited to provide the precision speed control by phase locking the AC motor390to a stable and accurate reference frequency. When a rate of change of frequency of the AC motor390is very slow, for example less than 1 Hz per second, a dynamic accuracy of the PLL circuit382is acceptable. But when the frequency of the AC motor390changes rapidly, for example 60 Hz per second, the dynamic accuracy of the PLL circuit382may not be acceptable since the PLL circuit382includes an error actuated signal (dynamic error) as will be described later. The control system300comprises a feedforward scheme that generates a feedforward signal which increases the dynamic accuracy and decreases a dynamic error of the PLL circuit382which is used to track a stator flux position in the AC motor390, where the frequency of the AC motor390is increasing very rapidly.

With reference toFIG. 3, a three-phase voltage output signal across lines302,304,306is generated by variable frequency drive (VFD)380, illustrated as VFD output381. The 3-phase output signal across lines302,304,306is scaled down and integrated to create stator flux signals308,310,312for each phase (3-phase stator flux signal) via 3-phase voltage sensing and integrating circuit314. The 3-phase output signal of the VFD380is scaled down and integrated using for example multiple comparators316, operational amplifiers (op amps)318and integrators320to generate the stator flux signals308,310,312. It should be noted that the comparators316, op amps318and integrators320are only shown schematically. The 3-phase voltage sensing and integrating circuit314may comprise more, less or different electronic components since there may be alternative configurations for the voltage sensing and integrating circuit314to create the stator flux signals308,310,312. As noted before, the provided control system300is specifically designed for AC motors, where the rate of change of frequency is high, for example 60 Hz per second. Therefore, the voltage output signal across lines302,304,306comprises a corresponding rapidly changing frequency (high df/dt).

The generated stator flux signals308,310,312are fed to the PLL circuit382. It should be noted that one of ordinary skill in the art is familiar with PLL techniques, which are also described in various publications. Within the PLL circuit382, the 3-phase stator flux signal308,310,312is converted into a 2-phase stator flux signal with stator flux signals324,326by a 3-to-2 phase converter322.

A direct quadrature (D-Q) transformation is performed of the 2-phase stator flux signal324,326by a D-Q transformation unit328. The 3-to-2 phase conversion by converter322is necessary before the D-Q transformation because the D-Q transformation unit328receives as third input signal the output signal (feedback)354of the PLL circuit382. Direct Quadrature (D-Q) transformation is a mathematical transformation used to simplify the analysis of a three phase circuit. In the case of balanced three phase circuits, application of D-Q transformation reduces the three AC quantities to two quantities. Simplified calculations can be carried out on these imaginary quantities before performing the inverse transformation to recover the actual 3 phase AC results. As is known, a three-phase AC motor may be mathematically represented as a two-phase AC motor with two axes of magnetic symmetry using D-Q transformation. The axis in which magnetic flux is generated is known as the direct axis (D-axis). The axis perpendicular to the direct axis is known as the quadrature axis (Q-axis).

After performance of the D-Q transformation, a proportional integral (PI) regulator332is fed from the Q-output330of the D-Q transformation unit328. Only the portion of the Q-axis is fed to the PI regulator332. The PI regulator332, also referred to as PI controller, continuously calculates an error value as the difference between a desired set point and a measured process variable and applies a correction based on proportional and integral terms. With the PI regulator332, the Q-portion of the D-Q transformation is regulated to zero, so that a stator flux is aligned entirely along the D-axis. An output334of the PI regulator332represents an angular frequency of the stator flux signals324,326, which is forwarded to an integrator336which integrates the angular frequency to a stator flux angle signal338. The stator flux angle signal338represents an output signal (feedback) of the PLL circuit382which is supplied to the VFD380. The stator flux angle signal338is also fed back to the D-Q transformation unit328as feedback signal354so that the stator flux angle is dynamically calculated and adapted. The purpose of the PLL circuit382is to lock onto the position of the motor stator flux of the AC motor390and provide the stator flux angle signal338to the VFD380, in particular to the controller392of the VFD380, so that the controller392causes the VFD380to generate currents in the D- and Q-axes as identified by the PLL circuit382. Current of the D-axis controls the magnitude of the stator flux, and current of the Q-axis controls the magnitude of torque produced by the AC motor390.

Because the angular frequency provided by the PI regulator336is an error actuated signal, there may be a dynamic error in the stator flux angle signal338of the integrator336which becomes zero when the angular frequency is not changing, or only changing slowly. But when the angular frequency is changing rapidly, the error in the stator flux angle signal338can grow too large. The control system300as described herein provides means to reduce the dynamic error in the stator flux angle signal338. In order to reduce the error, a feedforward generator350supplies a frequency signal352downstream of the PI regulator332for reducing the error.

The feedforward generator350provides an independent means for determining a stator frequency of the AC motor390. As described before, the 3-phase output signal across lines302,304,306is scaled down and integrated to create a 3-phase stator flux signal308,310,312, which are then processed in the PLL circuit382. In parallel, the stator flux signals308,310,312are also supplied to the feedforward generator350. In turn, the feedforward generator350supplies a stator frequency signal352downstream of the PI regulator332, where the frequency signal352is summed with the output334of the PI regulator332. Thus, an adapted output signal335of the PI regulator332is provided which represents an adapted angular frequency of the stator flux signals308,310,312,324,326.

By providing an independent frequency signal352of the AC motor390, in particular a stator frequency signal, the dynamic error in the stator flux angle signal338of the integrator336is reduced because the feedforward generator350continuously provides a signal of the current stator frequency of the AC motor390. Thus, the PLL circuit382receives current stator frequency input and the stator flux angle signal338calculated by the integrator336is adapted or modified according to the present stator frequency of the AC motor390.

The stator frequency signal352is derived by converting a pulse train, derived from flux signals zero crossings, to an analog signal, using for example pulse width modulation (PWM). Specifically, the feedforward generator350determines zero crossings of the flux signals308,310,312and generates a train of width pulses, wherein a repetition rate is proportional to the stator frequency. The train of width pulses is then filtered and/or converted into the analog frequency signal352using for example PWM.

Simulation results show that having the feedforward signal352of the feedforward generator350significantly improves the dynamic accuracy of the stator flux position signal338. Improving the accuracy of the flux position signal338improves the performance of the VFD380for highly dynamic drive systems where speed (frequency) is changing rapidly. In addition, proportional and integral gains of the PI regulator332may be increased which will also decrease the error in the stator flux angle signal338, but this approach may be limited by stability concerns.

The described control system300including the PLL circuit382and feedforward generator350includes logic which may be implemented in hardware, software or a combination thereof and may be used in connection with rotating AC electrodynamic machines as well as linear electrodynamic machines, for example linear AC motors.