Motor control system and method

A method and apparatus for catching a rotating motor is described. Switches (typically the lower switches) of an inverter of a motor drive circuit are short-circuited for a brief period of time. The pulses generated by this process are analyzed to determine the speed and direction of rotation of the motor. The pulses are checked to determine whether they are orthogonal. If they are, the motor velocity determination is deemed to be reliable and then motor is magnetized and normal motor control can be resumed. If not, the motor velocity determination is deemed to be unreliable and a magnetization pulse before the velocity determination step is repeated.

TECHNICAL FIELD

The present invention relates to a motor control system and method.

BACKGROUND

Adjustable speed drives are known for use in the control of multi-phase electric motors. In some cases, the state of an electric motor being driven by an adjustable speed drive may be unknown. Specifically, the angular position and/or the speed of rotation of a rotor of the motor may be unknown. This can happen for many different reasons. Such a situation can occur, for example, following a short power outage. Alternatively, this situation can arrive when a rotor is already spinning before motor drive control has commenced (i.e. on startup of the motor drive).

Thus, a motor may have both mechanical and electrical energy stored therein before being under the control of a motor drive. Algorithms are known that seek to enable a motor drive to take control of a motor without creating a large disturbance in the energy stored in the machine and to do so without taking all the energy out of the motor. However, there remains a need for improved and alternative motor control systems and methods.

The present invention seeks to provide an alternative method to those already available.

SUMMARY

The present invention provides a method (of catching or controlling a motor) comprising: estimating a frequency of rotation of a motor (i.e. motor speed) by repeatedly short-circuiting at least some windings of the motor in order to generate a plurality of short circuit current pulses, identifying the peaks of the short circuit current pulses and using the identified peaks to estimate said frequency (and typically also direction) of rotation; determining whether the estimation of the frequency of rotation of the motor is reliable; and in the event that the estimation of the frequency of rotation of the motor is determined to be unreliable, applying a magnetising pulse to the motor and repeating the steps of estimating the frequency of rotation of the motor and determining whether the estimation of the frequency of rotation of the motor is reliable.

The present invention further provides a controller (e.g. a motor controller/flystart module) comprising: a first input for receiving a plurality of short-circuit current pulses from at least some windings of the a motor; a control module configured to identify peaks of the short circuit current pulses and using the identified peaks to estimate said frequency (and typically also direction) of rotation of the motor, the control module further configured to: determine whether the estimation of the frequency of rotation of the motor is reliable; and in the event that the estimation of the frequency of rotation of the motor is determined to be unreliable, to instruct the application of a magnetizing pulse to the motor and to repeat the steps of estimating the frequency of rotation of the motor and determining whether the estimation of the frequency of rotation of the motor is reliable. The controller may further comprise a frequency estimator (e.g. incorporating a phase locked loop) configured to generate the estimate of the frequency (and direction) of rotation of the motor. The frequency estimator may be further configured to determine whether signals indicative of cosine and sine vectors of measured currents are orthogonal.

Determining whether the estimation of the frequency of rotation of the motor is reliable may comprise determining whether signals indicative of cosine and sine vectors of measured currents are orthogonal. More specifically, determining whether the signals indicative of the cosine and sine vectors are orthogonal may include determining whether a dot product of the cosine and sine vectors is below a threshold (e.g. close to zero). In one form of the invention, the cosine and sine vectors are the Clarke's transform of the peaks of the short circuit current pulses.

The invention may further comprise determining that the motor is at standstill in the event that repeating the step of determining whether the estimation of the frequency of rotation of the motor is reliable results in a determination that the frequency of rotation of the motor estimate is still unreliable.

In some forms of the invention, applying a magnetising pulse to the motor comprises injecting a ramping multi-phase (typically 3-phase) current into the motor (typically at a predetermined rate). This is typically done until the rated current of the machine is reached. A current regulator may be used to generate the magnetization pulse.

Some forms of the invention include increasing the magnetizing level within the motor (and then exiting the method), if the estimation of the frequency of rotation of the motor is determined to be reliable. The step of increasing the magnetizing level within the motor may comprise injecting a multi-phase (typically three phase) current into the motor (e.g. until the motor is fully magnetised).

The present invention yet further provides a motor drive circuit comprising a controller as set out above and further comprising an inverter under the control of said controller.

DETAILED DESCRIPTION

FIG. 1is a block diagram of a system, indicated generally by the reference numeral1, including an adjustable speed drive (ASD). The system1comprises an AC power supply2, an ASD4and a load6(such as a three-phase motor). The ASD4includes a rectifier8(often a diode-based rectifier, as shown inFIG. 1, although alternatives, such as advanced front end rectifiers are known), a DC link capacitor10, an inverter12and a control module14.

The output of the AC power source2is connected to the input of the rectifier8. The output of the rectifier8provides DC power to the inverter12. As described further below, the inverter12includes a switching module used to convert the DC voltage into an AC voltage having a frequency and phase dependent on gate control signals. The gate control signals are typically provided by the control module14. In this way, the frequency and phase of each input to the load6can be controlled.

The inverter12is typically in two-way communication with the control module14. The inverter12may monitor currents and voltages in each of the three connections to the load6(assuming a three-phase load is being driven) and may provide current and voltage data to the control module14(although the use of both current and voltage sensors is by no means essential). The control module14may make use of the current and/or voltage data (where available) when generating the gate control signals required to operate the load as desired; another arrangement is to estimate the currents from the drawn voltages and the switching patterns—other control arrangements also exist.

FIG. 2shows details of an exemplary implementation of the inverter12.

As shown inFIG. 2, the inverter12comprises first, second and third high-sided switching elements (T1, T2and T3) and first, second and third low-sided switching elements (T4, T5and T6). Each switching element may, for example, be an insulated-gate bipolar transistor (IGBT) or a MOSFET transistor. As shown inFIG. 2, each of the switching elements (T1to T6) is associated with a corresponding free-wheeling diode (D1to D6).

The exemplary inverter12is a three-phase inverter generating three outputs: U, V and W. The three phases of the inverter12provide inputs to the three-phases of the load6in the system1described above. Of course, the inverter12could be modified to provide a different number of outputs in order to drive a different load (such as a load with more or fewer than three phases).

The first high-sided switching element T1and the first low-sided switching element T4are connected together between the positive and negative DC terminals. The mid-point of those switching elements provides the U-phase output. In a similar manner, the second high-sided switching element T2and the second low-sided switching element T5are connected together between the positive and negative DC terminals with the mid-point of those switching elements providing the V-phase output. Furthermore, the third high-sided switching element T3and the third low-sided switching element T6are connected together between the positive and negative DC terminals with the mid-point of those switching elements providing the W-phase output.

The inverter12is a 2-level, 6 transistor inverter. As will be apparent to those skilled in the art, the principles of the present invention are applicable to different inverters, such as 3-level inverters. The description of the inverter12is provided by way of example to help illustrate the principles of the present invention.

FIG. 3is a flow chart of an algorithm, indicated generally by the reference numeral20, in accordance with an aspect of the present invention.

The algorithm20starts at step22, during which step, initialization and calibration may occur, if required. The algorithm then moves to shorted windings estimation step (step24).

At step24, a number of switches of the inverter12are closed for a brief period of time in order to generate short-circuit pulses. In one form of the invention, each of the low-side switching elements (T4, T5and T6in the exemplary inverter12) is closed periodically. In general, the short-circuiting must be long enough to get a measurable current that can be used later in the algorithm, but short enough to keep the inverter in a discontinuous mode of operation.

FIG. 4is a graph, indicated generally by the reference numeral40, showing current waveforms on motor windings in response to the short circuiting of the low-side switching elements as described above.FIG. 4shows three pulses and the 3 phase current waveform looks like a set of sawtooth waveforms whose envelope is 3 phase sinusoidal.

Taking the peaks of the sawtooth waveform allows for the envelope to be separated and used for frequency estimation.FIG. 5shows envelopes (indicated generally by the reference numeral50) of current waveforms in accordance with an aspect of the present invention. The sawtooth waveforms visible inFIG. 4can be filtered out by sampling the short-circuit currents when the switches are open, thus obtaining the peak of the sawtooth waveform.

The frequencies of the envelopes shown inFIG. 5indicate the frequency of rotation of the respective phases of the motor6.FIG. 6is a graph, indicated generally by the reference numeral60, showing estimated frequency over time in accordance with an aspect of the present invention. The frequency estimation is based on the envelopes shown inFIG. 5and corresponds to the frequency of rotation of the motor6. In the example ofFIG. 6, the frequency estimate settles at about 325 radians/second.

At this stage, the shorted windings estimation step24of the algorithm20has been used to obtain an estimate of the speed of rotation of the motor.

As noted above, the frequencies of rotation of the envelopes shown inFIG. 5are equal to the electrical frequency rotation of the motor. The envelopes are used as the input of a Phase Lock and Loop, PLL.

The idea behind the PLL is that the sine of the difference between the actual input vector angle θ and its estimate {circumflex over (θ)} can be reduced to zero using a proportional-integral (PI) controller, thus locking the detected phase to the actual angle. The estimated frequency {circumflex over (ω)} is then integrated to obtain the angle {circumflex over (θ)}.FIG. 7shows the block diagram of a phase locked loop (PLL), indicated generally by the reference numeral70, that may be used in the present invention.

As described above, the signals50shown inFIG. 5are 3-phase current signals that represent the envelopes of the current signals detected in the shorted windings step24of the algorithm20. The three-phase current signals50are converted into 2-phase signals isaHighand isbHighusing a Clarke's transformation. Thus, the signals isaHighand isbHighthat are used in the PLL70are the Clarke's transform of the peak currents of the sawtooth waveform.

In normal operation, the currents vectors isaHighand isbHighare at right-angles to one another. The angle generated by joining the ends of the currents vectors together defines the vector angle signal θ. By simple mathematics, the cosine and sine of the vector angle θ are given by:

In the phase locked loop circuit70, a first function block72converts the current vectors isaHighand isbHighinto an estimate of cos(θ) using the formula:

Similarly, a second function block74converts the current vectors isaHighand isbHighinto an estimate of sin(θ) using the formula:

The PLL70generates an estimate of the vector angle. That estimate is denoted by the symbol {circumflex over (θ)}.

As shown inFIG. 7, the estimate of cos(θ) is multiplied by the sine of the estimated vector angle to give: cos(θ)sin({circumflex over (θ)}). Similarly, the estimate of sin(θ) is multiplied by the cosine of the estimated vector angle to give: cos(θ)sin({circumflex over (θ)}).

An error term is calculated as follows:
ePLL=sin(θ−{circumflex over (θ)})=cos(θ)sin({circumflex over (θ)})−sin(θ)cos({circumflex over (θ)})  (1)

A PI controller is used such that the error term (ePLL) is forced to zero. In this way, signals for estimated frequency {circumflex over (ω)} and estimated vector angle signal {circumflex over (θ)} are obtained as outputs of the PLL70.

Returning to the algorithm20ofFIG. 3, as described above, the shorted windings estimation step24is used to give an estimated frequency, {circumflex over (ω)}. The algorithm20then moves to step26, where an orthogonality test is used to determine whether the estimations made in step24are likely to be reliable. The orthogonality test step26determines whether the signals isaHighand isbHighare, in fact, orthogonal.

When the inputs of the PLL are extremely small and contain mostly noise (i.e. having a low Signal to Noise Ratio (SNR)), the cos(θ) and sin(θ) estimates will lose orthogonality. By checking for orthogonality of the cos(θ) and sin(θ) signals it is possible to determine whether the PLL estimation is valid or not. This is important because cos(θ) and sin(θ) must be orthogonal for the PLL to work properly.

The dot product of two vectors is given by:
Ā·B=|A∥B|cos()  (2)

Since the cos(θ) and sin(θ) signals swing between ±1, the equation (2). can be reduced to:
Ā·B=cos()  (3)

If cos(θ) and sin(θ) are orthogonal the angle between them is 90 degrees (cos(90)=0) which means that when cos(θ) and sin(θ) are not corrupted:
Ā·B=0  (4)

Note this would have been true even if the magnitudes were not equal to 1.

The dot product can be found iteratively by:

Thus, in step26, the PLL uses the stationary reference frame currents (isaHighand isbHigh) which should be orthogonal to each other and uses a dot product as an orthogonality test to determine whether to trust the output of the PLL (determined when the dot product is below a determined threshold i.e. close to zero). If the reference frame currents are orthogonal, the algorithm20moves to step34described further below; otherwise, the algorithm moves to step28.

At step28of the algorithm20, it has been determined that the shorted winding estimation has not provided a reliable estimation of the rotor position. This is likely to be because the level of magnetisation in the motor winding is insufficient for the short-circuit steps to provide reliable data. Accordingly, at step28, a magnetization pulse is applied to the motor.

FIG. 8shows a magnetization pulse, indicated generally by the reference numeral80, in accordance with an aspect of the present invention. The magnetization pulse80injects a ramping three-phase current into the machine at a predetermined rate until the rated current of the machine is reached. A current regulator is used to create the current ramp. The current regulator requires a ramp rate and a target current value as inputs and outputs the three phase current ramp, such as that shown inFIG. 8.

When a current ramp is applied to a rotating induction machine the back emf (bemf) signal has two components:A rampA sinusoid signal rotating at the frequency of the machine.

If the ramp is stopped before the ramp signal dominates the sinusoid signal, it is possible to get a minima of the bemf. Catching a minima of the bemf means that the current signal seen during the short winding estimation will be too low to get any useful information.

Once the magnetization pulse is applied, the step28is complete. The algorithm20then moves to step30.

Step30is a second “shorted windings estimation” step and repeats the step24described above. Following, the application of the magnetizing pulse in step28, the shorting process should result in correct measurements of rotor position.

From step30, the algorithm20moves to step32, where the orthogonality test (as described above with reference to step26) is repeated. If the dot product is below the relevant threshold (i.e. near zero), the algorithm20moves to step34. If the dot product is still not below the relevant threshold, it is assumed that the motor is at (or very close to) standstill and the algorithm20moves to step36.

Step34of the algorithm20occurs when it has been determined that rotor position estimations are reliable (either in step26or step32of the algorithm20). At this stage, the frequency of rotation of each phase of the motor has been deemed to have a reliable accuracy. To ensure that the magnetism level of the motor is at a suitable level (i.e. at a level such that the torque required to accelerate or decelerate the motor is available), the step34increases the flux in the machine to the appropriate level. When the flux is at the appropriate level, the algorithm20moves to step36.

The step34is a “ramp magnetization level” step. The step34injects three-phase current into the motor6until the machine is fully magnetized.FIG. 9shows an exemplary magnetization ramp signals, indicated generally by the reference numeral90.

Since the speed of rotation of the motor has been determined, the phase currents applied in step34should match the already spinning motor. If this occurs, the motor is said to be “caught”. It should be noted that if the estimation of the speed of rotation of the motor is incorrect by a small amount, then the increasing currents applied during the ramp step34will force the motor to follow the applied currents. Thus, despite small estimation errors, the motor is still “caught”. This is possible if the error is sufficiently small and has been found to work well in practice.

At step36of the algorithm20, either the motor has be “caught” at step24or step28, or it is determined that the motor is at standstill (at step32). In any event, the motor velocity is known and so normal control can commence. The algorithm36can therefore exit and normal motor control applied.

FIG. 10shows signals, indicated generally by the reference numeral100, of a motor starting arrangement in accordance with an aspect of the present invention, thereby demonstrating an application of the algorithm20. In the example ofFIG. 10, the motor6being started is initially demagnetized.

The algorithm20starts within the initialisation step22followed by the shorted winding estimation step24. The signals at this part of the algorithm are indicated generally by the reference numeral102. Since the motor6is demagnetized, the short-circuit pulses applied in step24of the algorithm20do not result is current signal being generated. Accordingly, the portion102of the signals100do not show any current pulses.

Since the short circuit pulses at step24do not lead to currents being generated, the orthogonal test (step26) fails and the algorithm moves to step28where a magnetizing pulse is applied (step28). The magnetizing pulses are clearly visible at part104of the signals100.

With the magnetizing pulse applied, the algorithm20moves to step30where short circuit pulses are again applied—this is shown at part106of the signals100. Although it appears that no current is generated in response to the short-circuiting of the windings, in fact currents are generated, but they are too small to be visible inFIG. 10.

Following the step30, the orthogonality of the current pulses is checked (step32). This should now return a positive result, so that the algorithm20moves to step34. At step34, the magnetization level is ramped up, as shown in part108of the signals100.

FIG. 11shows signals, indicated generally by the reference numeral110, of a motor starting arrangement in accordance with a further aspect of the present invention, thereby demonstrating a further application of the algorithm20. In the example ofFIG. 10, the motor6being started is initially magnetized.

The algorithm20starts within the initialisation step22followed by the shorted winding estimation step24. The signals at this part of the algorithm are indicated generally by the reference numeral112. Since the motor6is magnetized, the short-circuit pulses applied in step24of the algorithm20result is current signals being generated. Accordingly, the part112shows small current pulses.

Since short-circuit current pulses are generated from the magnetized motor, the orthogonal test (step26) indicates that the current pulses are orthogonal and so the algorithm20moves to step34. At step34, the magnetisation level is ramped up, as shown in part114of the signals100.

FIG. 12shows a block diagram of a system, indicated generally by the reference numeral120, in accordance with an aspect of the present invention.

The system120comprises an inverter12and a multi-phase motor6as described above with reference toFIG. 1. The inverter has a DC input. The DC input may, of course, be obtained from an AC supply using a rectifier in the manner described with reference toFIG. 1.

The inverter12in the system120is controlled using a switching logic controller122. The switching logic controller122provides PWM signals for the inverter switches and may obtain control information (such as voltages and/or currents) from the inverter12. The switching logic controller122is in two-way communication with a frequency estimator126and a current regulator124. The frequency estimator126may provide the functionality of the PLL70described above. The current regulator124may provide the control signals required to enable the inverter12to generate the magnetization signals described above with reference toFIGS. 8 and/or 9.

The system120is highly schematic. Clearly, many variants are possible.

The embodiments of the invention described above are provided by way of example only. The skilled person will be aware of many modifications, changes and substitutions that could be made without departing from the scope of the present invention. The claims of the present invention are intended to cover all such modifications, changes and substitutions as fall within the spirit and scope of the invention.