Field

The present invention relates to a load dump, for example of the type used with a direct current (DC) link connection for sharing regenerated power between two or more electrically driven units. More particularly the invention relates to a load dump for connection to a dynamic braking resistor. For example, when several motor drives and motors are operating together, are interconnected in one system.

Background

There are increasing incentives to improve the efficiency of electric power conversion equipment. Conventional electric induction motors are said to consume approximately 70% of all electricity used in industry and about 45% of all the electricity used globally. Efforts are being made to improve the operation and efficiency of electrical motors.

Examining the DC link connection highlights other issues. For a conventional motor drive it is the peak value of the supply voltage that is usually of concern. However connect together two DC connections, via a mains rectifier and any input filtering that may be present, can cause severe problems if all power of the connected drives is routed through the input circuit of one of the drives as this leads to current and power overloading issues.

Prior Art

Conventional motor drives do not cope well when connected together (via a so- called DC link) or when connected with regeneration equipment. Consequently elevated voltages can occur during motor deceleration.

The design of a conventional motor drive tends to include a rectified front end and direct connection to a DC link capacitor (charged to the peak of the input voltage) with a 3 phase half bridge output circuit connected to this. In order to use the DC link it is standard practice to ignore any integral rectified front end and use an external front end that is capable of running a total load of all the drives and motors connected to it. So far as handling motor deceleration is concerned, the usual technique is to operate the motor as a generator by altering the phasing of applied waveforms to the motor in the case of permanent magnet or switched or variable reluctance motors, and altering the applied waveform frequency in the case of induction motors. The consequence of this is that the voltages across the motor and motor drive increases beyond their normal operating voltages. As such motor winding insulation has to be able to cope with this additional voltage stress and the voltage rating of the drive components have to be rated higher than their normal operating voltage in order to cope with this.

Consequently this so-called overvoltage on regeneration, caused by the deliberate braking of the motor, requires careful regulation so as not to exceed motor or component peak voltage limitations or cause triggering of any circuitry that is designed to shut down in case of an overvoltage for other reasons.

This is one reason why the DC link capacitor has to be large, as it can accommodate the voltage ripple that occurs during braking, and so that the instantaneous peak braking voltage does not drop below the input voltage, thus wasting input power in the braking resistor.

Another factor that systems had to protect against was to guard against input voltage peaking at excessively high voltages, risking over voltage issues (both for the motor and components) as well as risk triggering automatic over voltage protection systems.

At best it can be said that the DC link and sharing of regenerated energy between drives has been handled by using extra components and connections, but was not always a reliable solution to many of the problems encountered in regenerative braking systems. Furthermore a drive powered directly from a DC bus, for example in an aircraft, may have additional restrictions on current flowing back into a local 'DC grid'. Such applications require a motor drive configured to have a one way current capability. There are also significant penalties attached to voltages present in such motor drives under all operating conditions. These are, for example, fail safe issues, voltage margin ratings, resistance to cosmic ray degradation and reduced ionisation voltages at elevated altitudes. EP 3012936 (General Electric) discloses a power generation system with an induction generator coupled to a prime mover and configured to generate electrical energy. An inverter is connected to the generator and controls a terminal voltage of the generator during a grid-loss event. A power dissipating device is coupled to the inverter for dissipating power during the grid-loss condition.

WO 2013/013678 (Vestas) teaches a device for dissipating power from a generator in a wind turbine. The device has a plurality of dissipating units, a plurality of semiconductor switches, a trigger circuit for switching the semiconductor switches and a control unit for controlling the operation of the trigger circuit, thereby controlling the switching of the semiconductor switches. An aim of the invention is to provide a reliable motor braking system without high voltages being developed either across the motor or within the motor drive itself.

Summary of the invention

According to a first aspect of the invention there is provided a load dump for connection to a load includes: an energy sink and a controller, the controller actuates a switch to divert current to the sink when a threshold is exceeded, so as to dissipate excess energy from the load to the sink, characterised in that the switch is actuated when excess power from the load causes an increase in an intermediate voltage.

Considering a single motor being driven by a single motor drive then the motor drive topology used allows for the braking resistor to be placed between the variable voltage stage and the 3 phase half bridge output stage. This allows the midpoint clamping of voltage to occur and it is this midpoint clamping of voltage within the drive which achieves a motor braking system at low voltages.

Under normal conditions an intermediate voltage is at a potential less than the input voltage. As the phase angle or frequency is altered, to effectively brake the motor, the motor becomes a generator and the voltage rises. The input variable voltage stage is controlled under these conditions so as not to allow this increased voltage to be passed back to the input of the variable voltage stage.

Advantageously the switching device that is connected to the motor braking resistor is switched into operation to dissipate this braking energy without excessive voltage build up occurring. This so-called dynamic regeneration has a significantly greater tolerance range than conventional braking techniques.

At its limit the midpoint voltage is typically between zero volts and a maximum peak rating that is permitted by lesser rated components and the motor insulation. A typical value is between 50 and 80% of the nominal maximum drive supply voltage. Comparing this with a conventional drive, where the clamping voltage is in the region of 1 15 to 120% and the overvoltage trip is around 125% of the nominal peak supply voltage so as to accommodate supply voltage tolerances.

Preferably the DC link voltage is operated around the midpoint between the variable voltage circuit and a 3 phase half bridge inverter which is required for the operational speed and the load of the motor.

Thus for correct operation of different motors, under different load conditions, the input voltages are different.

Preferred embodiments of the invention will now be described with reference to the Figures in which:

Brief Description of the Drawings

Figure 1 illustrates a block diagram of a quasi-sine resonant drive;

Figure 2 illustrates a more detailed circuitry of power components of the quasi- resonant drive of Figure 1 ;

Figure 3 shows a block diagram of a quasi-sine motor drive;

Figure 4 is a diagrammatical overall view of a whole motor system including: a drive module, a power drive system; and a motor system;

Figure 5A shows an example of a conventional pulse width modulated (PWM) motor drive circuit;

Figure 5B shows an additional switch to minimise input current surge in the motor drive circuit of Figure 5A;

Figures 6A to 6E are functional diagrams showing alternative system capabilities, handling energy flows, operating in different modes; Figures 7A and 7B are examples of alternative circuits for limiting in-rush current between an active front end and a DC connection; and

Figure 8 shows load dump position across intermediate voltage point.

Detailed Description of the Drawings

Figure 1 illustrates a block diagram of a quasi-sine resonant drive. Here it is shown that the output part of the circuit, consisting of the variable frequency stage, the slew rate capacitors and the motor itself forms a resonant circuit. In order for the system to operate correctly a self-adjusting turn on of the appropriate switch is required which occurs in this quasi sine form of output. Sensors may be shown connected to the motor giving an indication of speed. This can also give an indication of torque ripple if differentiated. Alternatively motor information can be calculated or derived from other measurable parameters. The variable voltage part of the circuit, shown at figure 1 , is typically also a resonant voltage conversion topology. By using these two techniques together, extremely high efficiencies can be obtained.

Figure 2 illustrates a more detailed circuit showing power components of a quasi- resonant drive circuit. Figure 2 shows a three phase half bridge frequency determining circuit with slew rate capacitors C6, C7 and C8 arranged in parallel with their outputs connected to the motor. The voltage amplitude of the generated waveforms at the output is determined by the variable voltage part of the circuit.

In operation, at the appropriate time determined by control circuitry (shown in Figure 1 ), one of the output drive transistors, Q3, (shown in Figure 2), is turned off quickly. The current that was flowing prior to switch off of Q3 transfers to charging or discharging slew rate capacitors C6 and C8 until the voltage across switching device Q4 becomes reverse biased, at which instant diode D4 switches to conduct. Diode Q4 may be either intrinsic or external to the now reverse biased switching device.

Control circuitry (shown in Figure 1 ), now turns on switch Q4, (shown in Figure 2) and maintains it on until it is switched off quickly. This repeats the resonant switching process. This same resonant process occurs on both of the other phases of the output; or as many phases that are appropriate for the motor/generator that is being controlled (Figure 2). The operation of output circuit, the variable frequency circuit part of Figure 2, is essentially determined by a controller (figure 1 ) which acts to force outputs to go off at a predetermined instant. Referring to Figure 2 switches Q3 to Q8 are switched on again by detecting the instant when the voltage across a switch is at zero potential, thereby ensuring no "shoot through" currents can occur. Therefore switch on occurs with no voltage potential across a switch. This ensures that there are no transient (voltage x current x dt) losses.

This type of operation, where the devices are turned off by the waveform frequency control mechanism (Figure 1 ) and turned back on again by the natural resonance, ensures that all component values and tolerances are automatically taken into account in order to derive optimum input parameters to drive a system (motor), for every switching transition that occurs. Further, in one embodiment, this can be achieved without the need for a microprocessor type hardware or software burden.

The variable voltage element, shown in Figure 2 of the circuit, includes switches Q1 and Q2 and associated other components which are also operated in a resonant mode. As configured the variable Voltage circuit provides a voltage step down function from the supply across C1 .

To minimise the current harmonics, so as to effectively minimise torque ripple and reduce resistive losses throughout the motor system as defined in Figure 4, several techniques can be used either on their own or concurrently. The voltage amplitude of the waveform itself can be modulated with a voltage waveform that effectively attempts to null the generation of harmonic currents.

Shunt slew rate capacitors C6, C7, C8 in Figure 2 tend to modify transitions of the voltage waveform, thus the voltage waveform (from which the motor current waveform is derived) already has a reduced harmonic content and, in combination with the motor impedances at that speed and load, the resultant current harmonics are reduced further.

Figure 3 shows optimisation of the operation of controlling power in or out of a synchronous or non-synchronous motor/generator/alternator in order to achieve maximum overall efficiency (least losses) of the combination of the drive and motor/generator/alternator consistent with other desired parameters. Figure 4 In order to understand how a motor drive system is considered, convention has arranged boundaries for the motor in context with the power connection to supply the power for it. Figure 4 shows the boundaries of a complete drive module (CDM), a power drive system (PDS) and a motor system comprising the motor itself and the attached mechanical load. This is included as a requirement of "CE Marking and Technical Standardisation Guidelines" for application to electrical power drive systems. The relevance here is that the overall efficiency of the techniques described is to be read and understood in the context of the 'motor system' in this guide.

It is recognised that further development of an existing pulse width modulator (PWM) drive for motors, which already encounters and creates significant technical obstacles, results in even greater problems to be overcome in order to get it all to work correctly and these further developments may have other undesirable effects as well. In order to introduce the advantages of newer transistor materials, such as silicon carbide (SiC) and gallium nitride (GaN) switching speeds are increasing and associated switching edges are becoming sharper. This more rapid switching imposes greater constraints on design and requires drives to be more complex, mainly due to greater parasitic impedances; as well as subjecting the motor to even more aggressive waveforms than existing ones (that already cause considerable problems in motor design and installation) as a consequence of EMC.

By adopting a drive based on the fundamental principles outlined herein it is possible to revert to lower cost motor materials and also materials that give superior performance as they are only subjected to a fundamental frequency. Motor design is intended to ensure the motor runs on its fundamental frequency without having to compromise its design to cope with the issues of pulse width modulation. Improved design also allows the use of lower quality (and therefore cheaper components) and switched or variable reluctance motors (which would allow for the fundamentally cheaper and physically toughest motor design that switched or variable reluctance motors offer compared to induction or permanent magnet motors) as existing torque ripple problems are overcome.

Figure 5A shows an example of a pulse width modulated (PWM) motor drive. Figure 5A illustrates the basic building blocks of a conventional PWM motor drive. This became feasible around the mid 1980s with the development of the insulated gate bipolar transistor (IGBT). This was accepted for motor control as it allowed motors to consume less power than direct on line connection, under most conditions. However the IGBT suffered from a number of negative effects which have been accepted and worked around rather than designed out of drive circuits. The motor drive in its latest form has reached a point where it is not easy to improve it in any way. However it is a simple, reliable drive that is used in great quantities worldwide.

Figure 5B illustrates an example of a conventional PWM drive as depicted in Figure 5A but with a series resistor R1 and a shunt relay contact S1 to minimise the turn on surge current that would be drawn from the supply by the DC filter capacitors as shown in Figure 5A.

Figures 6A to 6E show diagrammatically front end current paths under different modes and shows the way that the active front end, in conjunction with some energy storage capability and the motor drive, can perform several functions. Some of the functions can occur at the same time. The size of the energy storage device (not shown) can be adjusted to suit the functions required.

Figures 7a and 7b show examples of current inrush limited circuitry added at the junction of the +ve connection between the active front end and DC link. To avoid inrush current at the instant of connection (or reconnection) of the supply voltage to the inputs of the active front end, it is necessary to incorporate some additional means to eliminate, or at least significantly reduce, the inrush current. The circuits (Figure 7a and 7b) behave in very different manner to each other.

In Figure 7a the current limiting means is placed between the output of the active Front ends and the DC link connection. Figure 7a shows a modified Buck converter that provides good control of the amount of energy that can be transferred from the active front ends to the DC link. This feature of the invention is very important as it allows reconnection and continuation during 'brown out' and blackout conditions without nuisance tripping and therefore offers significant benefits in areas where surety of supply is not always guaranteed.

Figure 7b shows an example of using a conventional resistor and relay to protect against inrush current. A disadvantage with this arrangement is that the resistance value is high in order to limit current inrush when the DC link is at a zero to low voltage at the point of application of the utility to the three phase inputs. This value is too high to allow for normal motor operation so a state exists where the motor has to be disconnected until the DC link has been established at the correct voltage. Also the resistance is present when the voltages are higher on the three phase input side than the voltage present on the DC link even when the switching devices in the 3 phase active front end are off.

Aspects of the invention have been described by way of a number of exemplary embodiments, each exhibiting different advantageous features or benefits; and it is understood that features, components or circuits from two or more of the aforementioned embodiments may be combined together to overcome specific problems or to provide a bespoke solution to a particular problem.

Figure 8 shows load dump position across intermediate voltage point, showing the position of the load dump between the variable voltage block and the variable frequency block. This allows the load dump to operate at an intermediate voltage between zero and the supply input voltage.

In a typical PWM drive (see Figure 5A) the regenerated power appears as an increase in voltage above the supplied input voltage. Thus all the components have to be of a higher voltage rating as a consequence of that. However, the circuit arrangement of Figure 8 allows this regeneration and load dumping to be done at an intermediate voltage between zero and the supply voltage. Therefore there is not a need for voltage rating of components above what is required for normal motor operation.

Variation may be made to the invention by including a connection to the load dump from an induction motor or incorporating the load dump in or on the body of a motor or its housing. The load dump may be incorporated in a permanent magnet motor or a switched/variable reluctance motor. Such modified motors may be included in electrical appliances, such as pumps, tools, drive systems or other machines or vehicles, such as electric cars, busses or trains.