GB2493308A

GB2493308A - Controlling a DC series motor using a three phase inverter

Info

Publication number: GB2493308A
Application number: GB1219965.9A
Authority: GB
Inventors: Peter Barrass
Original assignee: Sevcon Ltd
Current assignee: Sevcon Ltd
Priority date: 2012-11-06
Filing date: 2012-11-06
Publication date: 2013-01-30
Anticipated expiration: 2032-11-06
Also published as: GB201219965D0; GB2493308B

Abstract

A three phase inverter circuit designed for controlling an AC motor is used to control the current through the armature winding and field winding of a DC series motor. A DC series motor has a first current supply to the armature 4 of the motor and a second current supply to the field winding 6. A controller is configured to control the current through one of the armature and field winding based on the respective other one of the current through one of the armature and field winding. A three phase inverter may be used to control a DC series motor provided with only three terminals. The armature may be coupled between the first and second node of the inverter, and the field winding may be coupled between the second and third nodes. The controller may also control the current through the armature and field winding of the motor, where the armature current is based on a required output torque. The first and second current supplies may be configured to be controlled independently.

Description

DC SERIES MOTOR APPARATUS

The present invention relates to control of DC motors, and control circuitry for controlling DC motors.

Various types of DC electric motors are known, and each having associated advantages and disadvantages. One type of DC motor is a DC series motor, which is illustrated schematically in Figure 1. In a DC series motor the armature winding 4 and the field winding 6 of the motor 2 are connected in series such that the same current passes through each winding. Compared with other motor types, this means that the field winding carries much higher currents than usual, and must be made of heavy duty wire.

As the amount of current passing through the windings, and in particular the field winding, determines the torque that can be developed by the motor, DC series motors generally provide high starting torques, making them able to move relatively high shaft loads when first energized. One common use of a DC series motor that relies on this high torque characteristic is as a starter motor to start an internal combustion engine. The high torque characteristics of series motors have also led to consideration of these motors in traction applications such as in electrically powered vehicles.

One characteristic of DC series motors is that reversing the direction of current through the motor does not change the direction of rotation of the motor. This is because reversing the direction of current through the motor, reverses the current in both the field winding and the armature winding, resulting in reversal of both magnetic fields. This characteristic of DC series motors has led to these motors being considered universal' motors as it allows them to be powered using either DC or AC current.

In order to reverse the direction of rotation of a series motor, it is necessary to reverse the current flow through one of the field winding or the armature winding, but not both. Thus, in order to allow reversing of the motor, DC series motors supplied for traction uses commonly provide more than two terminals at which a voltage can be applied to the motor. For example, many commercially available DC series motors are provided with four terminals associated with the terminals Si, S2, Al and A2 shown in Figure 1. Other designs of series motor allow control for reversing the motor using three terminals.

A circuit for motor reversal (for example as shown in Figure 2) is commonly employed to control the direction of current in one of either the field or armature windings, and therefore allow the motor to be operated in either direction. The circuit for motor reversal is coupled to the four terminals Al, A2, Si and S2 provided for the DC series motor. In order to manage the high current flows in the motor, a circuit for motor reversal may use mechanical switches to direct the current through one of the windings depending on the required direction of rotation. In the example circuit of Figure 2, switches 8a and 8b may be operated (i.e. closed) together to direct the current through the armature winding 4 in a first direction, while operating switches lOa and lOb together will reverse the direction of current through the armature, thereby reversing the direction of rotation of the motor. However, these mechanical switches can be unreliable and increase the cost of manufacturing and servicing the motor/motor controller assembly.

Another known type of DC motor, is the separately excited motor or SEM. In a SEM, the current flowing through the field winding and the armature winding of the motor can be controlled (excited) separately. In contrast to the DC series motor, the field winding of an SEM is not expected to carry the same current as the armature, and therefore the field winding and associated control circuitry will be rated for much lower currents.

The ability to control the field and armature currents independently allows an SEM to be more flexible, and allows more advanced control of the torque and speed characteristics of the motor to be achieved. However, the circuits required to individually control the field and armature currents are often expensive. In an aspect there is provided an apparatus comprising: a DC series motor; and a first current supply configured to supply a first current to an armature of the DC series motor; and a second current supply configured to supply a second current to a field winding of the DC series motor, preferably further comprising a control means configured to control the second current supply to provide the second current based upon the first current. In an embodiment the first and second current supplies are derived from a single three-phase inverter circuit with each current supply being provided by one or more legs of the inverter circuit. In one example, the first current supply is provided by a first leg of the inverter circuit and the second current supply is provided by second and third legs of the inverter circuit. In another example the first current supply is provided by the first and second legs of the inverter circuit and the second current supply is provided by the third leg of the inverter circuit. The inventors in the present case have appreciated that by applying appropriate control signals, it is possible to use a three phase inverter circuit (e.g. a standard six switch inverter, usually applied to AC motors) to control a DC motor. This enables AC controllers to be retrofitted to existing DC motors to improve performance without the additional cost and resources required to replace a DC series motor with a conventional SEM.

Further aspects and examples of the invention are set out in the claims.

Examples of the invention may comprise control means implemented in software, middleware, firmware or hardware or any combination thereof. Embodiments of the invention comprise computer program products comprising program instructions to program a processor to perform one or more of the methods described herein, such products may be provided on computer readable storage media or in the form of a computer readable signal for transmission over a network. Embodiments of the invention provide computer readable storage media and computer readable signals carrying data structures, media data files or databases according to any of those described herein.

Apparatus aspects may be applied to method aspects and vice versa. The skilled reader will appreciate that apparatus embodiments may be adapted to implement features of method embodiments and that one or more features of any of the embodiments described herein, whether defined in the body of the description or in the claims, may be independently combined with any of the other embodiments described herein.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 schematically illustrates a DC series motor; Figure 2 schematically illustrates a DC motor and a circuit for motor reversal; Figure 3a schematically illustrates an arrangement for power circuitry for a four terminal DC series motor; Figure 3b schematically illustrates a further arrangement for power circuitry for a four terminal DC series motor; Figure 4 schematically illustrates an arrangement for power circuitry for a three terminal DC series motor; Figure 5 illustrates a control algorithm for generating armature and field currents according to embodiments of the invention; and Figure 6 illustrates a method of determining motor speed according to an embodiment of the invention.

Embodiments of the invention aim to allow a commercially available DC series motor to be operated in a similar fashion to a separately excited motor, providing a greater level of control of the motor operating characteristics while maintaining the ability to provide a high starting torque associated with DC series motors.

Figures 3a and 3b illustrate two arrangements for connecting a four terminal DC series motor to a three phase inverter. Three phase inverter circuits are known and recently inverters capable of managing high currents have become commercially available at reasonable cost, such as the Sevcon (RIM) Gen4 (RIM) AC motor controller. Each leg of the inverter provides the same current capacity allowing equal currents to be provided to the armature and field windings. In the arrangement of Figure 3a, a first leg of the three phase inverter comprises transistors 12a and 12b coupled in series between positive (V+) and negative (V-) supply voltages. The armature 4 of the DC series motor is connected between the positive supply voltage and a first node between transistors 12a and 12b. Second and third legs of the three phase inverter comprise transistors 14a and 14b, and 16a and 16b respectively, coupled in series between the positive and negative supply voltages. Field winding 6 is coupled between a second node between transistors 14a and 14b, and a third node between transistors 16a and 16b. While the armature 4 is shown coupled to the positive supply voltage, it will be recognized that it could equally be coupled to the negative supply line.

In operation, a control signal comprising a series of control pulses is applied to the gate couplings of transistors 12a and 12b to modulate the current flowing through the armature winding 4. It will be recognized that for each leg of the three phase inverter, the two transistors (a and b) will be controlled in complementary fashion so that only one of the transistors will conduct at one time. Thus, pulse width modulating the control voltage applied to transistors 12a and 12b allows the armature current I to be controlled. As the field winding 6 is coupled between two legs of the three phase converter, it is possible to reverse the direction of current I through the field winding 6, to thereby control the direction of rotation of the motor, by controlling the transistors 14a, 14b, 16a, 16b.

Similarly, the magnitude of the field current If can be controlled by modulating the control voltage of the transistors 14a, 14b, 16a, 16b, for example by using pulse width modulation.

The arrangement of Figure 3b differs from that of Figure 3a in that the armature 4 is coupled between the first node and the second node, while the field winding 6 is coupled between the third node and the positive supply. As for Figure 3a, the field winding could instead be coupled between the third node and the negative supply.

In operation of the circuit shown in Figure 3b, the current a in the armature 4 is reversed, by switching of appropriate ones of the transistors of the three phase inverter, in order to reverse the direction of rotation of the motor.

As will be recognized, a three phase inverter circuit such as illustrated in Figures 3a and 3b will include further features that have been omitted to increase clarity of the figures. In particular. a freewheeling diode would be expected to be associated with each of the switches 12a, 12b, 14a, 14b, 16a, 16b.

In some circumstances, the arrangement of Figure 3b may have advantages over that shown in Figure 3a. In particular, the field winding generally has a much higher associated inductance than the armature, and therefore it is generally quicker to reverse the armature current when attempting to reverse the direction of rotation of the motor. Furthermore, the arrangement of Figure 3b ensures that it is always possible to control the armature current 1a even if the back e.m.f.

becomes negative, e.g. during freewheeling of the motor. In contrast, in the arrangement of Figure 3a, a negative e.m.f may result in a short circuit forming across the armature via a freewheeling diode associated with the transistor 12a, resulting in the armature current becoming uncontrolled.

Figure 4 illustrates an arrangement in which the three phase inverter is used to control a DC series motor provided with only three terminals. In the arrangement of Figure 4 the armature 4 is coupled between the first node and the second node of the three phase inverter, and the field winding 6 is coupled between the second and third nodes.

Figure 5 illustrates a control algorithm for providing the armature current 1a and the field current If based on a torque demand (TDeflafld). The algorithm of Figure 5 can be used in conjunction with any of the arrangements shown in Figures 3a, 3b or 4. It will of course be appreciated that the switches will be controlled differently in the different examples. For example, in the case of Figure 3a switches 12a and 12 b will be controlled to control the armature current whereas in Figure 3b switches 12a, 12b, 14a and 14b will be controlled to control the armature current.

The following description with reference to Figure 5 relates to the circuit shown in Figure 3a.

According to the illustrated algorithm, a torque demand is received at a first function element 50 which limits the torque demand, e.g. so that the torque demand does not exceed a maximum forward or maximum reverse torque. The limits may be predetermined or calculated based on measurements of the motor, for example temperature to protect the motor from excessive heat, or based on other criteria. The output of the function element 50 is a torque demand (Thm) that is to be provided by the motor.

The torque demand output (him) is provided to a first amplifier 52 which converts the torque demand to signal representing an armature current demand corresponding to the current required to provide the requested torque output. The current demand signal is then input to a feedback control loop consisting of a first differencer 58, first PID controller 54 and second amplifier 56. The output of the feedback loop (from the second amplifier 56) is configured to regulate the armature current by switching transistors 12a, 12b (in the example shown in Figure 3a) to provide the armature current 1a for the motor based on the current demand signal from the first amplifier 52.

The armature current l is also supplied to a second function element 60. Second function element 60 includes a torque-flux table. The torque-flux table is used by the second function element as a look-up table to convert the supplied armature current value to a field curient demand value. The torque-flux table may be generated empirically, by characterisation of the motor or may be based on calculated performance of the motor. The output field current demand value is then input to a further feedback loop comprising second differencer 62, second PID controller 64, and third amplifier 66. which regulates the output field current If based on the field current demand value. The output of this feedback loop (from the third amplifier 66) is configured to regulate the field current by switching transistors 16a, 16b to provide the field current If for the motor based on the current demand signal from the third amplifier 66.

It will be appreciated that the use of a torque-flux table is just one way of deriving If and that other ways known in the art may be used to derive I from 13 and other parameters.

By calculating the field current It based on the armature current I the control algorithm of Figure 5 provides automatic field weakening. This is because as the back e.m.f. (electro-motive force) of the motor increases eventually the available supply voltage will not be able to provide the desired armature current to the motor. However, as the field current Iris calculated based on the armature current I, the reduction in armature current will automatically lead to a proportional

reduction in the calculated field current.

As will be understood by the skilled reader in the context of the present disclosure, Figure 5 is merely schematic and when implemented in apparatus, one or more functions illustrated as separate functional components may be provided by a single control element or may be further subdivided into multiple elements. In addition, the field current and the armature current for the motor may be regulated using pulse width modulated, PWM, control signals applied to the transistors. The signals passed between elements of the algorithm in Figure may be single values, or multi-valued parameters describing PWM signals or in some examples may comprise PWM signals.

Figure 6 illustrates a speed calculation algorithm for a DC series motor being controlled according to the algorithm of Figure 5. For many applications, it is helpful to be able to accurately determine the rotational speed of the motor, however measuring the speed of the motor directly can be difficult. The algorithm of Figure 6 provides a way of calculating the speed based on easily measuied, or known, values such as the armature and field currents applied to the motor and the armature voltage Va and battery/supply voltage Vb3t.

The speed control algorithm operates by determining a back e.m.f (electro-motive force) for the motor and dividing this by an estimate of the magnetic flux within the motor due to the field and armature windings. For a motor having independently controlled field and armature currents, the determination of the magnetic flux must take into account both of these values, providing an extra complication.

In order to calculate the back e.m.f. the armature voltage is subtracted from the battery voltage at a first differencer 68. The output of first differencer 68 is then supplied to a second differencer 70. The armature current is supplied to amplifier 76, which multiplies the armature current with the resistance of the armature to determine the IR losses in the armature (i.e. the voltage dropped across the armature due to the armature resistance). This value is then input to the second differencer 70 where it is subtracted from the output of the first differencer 68 to determine the back e.m.f value.

The armature current value I is also input to a first flux look up table 78 which outputs a first flux value associated with the armature current l. The field current It is input to a second flux look up table 80 which outputs a second flux value associated with the field current. The first and second flux values are then input to adder 82 to calculate the total magnetic flux ip within the motor. The calculated back e.m.f. value and the total flux are then input to divider 72 where the back e.m.f value is divided by the total flux to generate a speed value which is then scaled in a function element 74 to provide a calibrated speed value for the motor.

Thus, the algorithm of figure 6 is able to calculate an accurate speed value based on easily measured electrical parameters of the electrical supply to the motor.

Although the inverter has been described with reference to IGBT transistors this is merely exemplary and any voltage controlled impedance may be used, for example MOSFETs, IG-FETs or BJTs.

Embodiments of the invention are described in the following numbered clauses: 1. An apparatus comprising: a DC series motor; and a first current supply configured to supply a first current to an armature of the DC series motor; and a second current supply configured to supply a second current to a field winding of the DC series motor.

2. The apparatus of clause 1, wherein the first and second current supplies are configured to be controlled independently.

3. The apparatus of clause 1 or clause 2, wherein the first and second current supplies comprise a three-phase inverter circuit.

4. The apparatus of clause 3, wherein the armature is coupled between a first leg of the three phase inverter and a second leg of the three phase inverter, and wherein the field winding is coupled between a third leg of the three phase inverter and one of the negative and positive power supplies.

5. The apparatus of clause 3, wherein the field winding is coupled between a first leg of the three phase inverter and a second leg of the three phase inverter, and wherein the armature is coupled between a third leg of the three phase inverter and one of the negative and positive power supplies.

6. The apparatus of any preceding clause further comprising a controller configured to control the first current supply to supply the first current based on a required torque output for the motor.

7. The apparatus of clause 7, wherein the controller is further configured to control the second current supply to supply the second current based on the first l0 current.

8. A method of controlling a DC series motor comprising: controlling a first current supply to supply an armature current to an armature of a DC series motor; and controlling a second current supply to supply a field current to a field winding of the DC series motor.

9. The method of clause 8, wherein controlling the first current supply further comprises controlling the first current supply to supply the armature current based on a required torque output for the motor.

10. The method of clause 9, wherein controlling the second current supply further comprises controlling the second current supply to supply the field current based on the armature current.

11. The method of any of clauses 8 to 10, wherein controlling the first current supply and controlling the second current supply further comprise controlling a three phase inverter circuit.

12. A method of calculating a rotational speed of a DC series motor, the method comprising: obtaining an armature current value and a field current value associated with the motor; estimating a magnetic flux for the motor based on the armature current

value and the field current value;

obtaining a back emf value associated with the motor; and dividing the back emf value by the estimated magnetic flux to calculate the rotational speed of the motor.

13. The method of clause 12, wherein estimating the magnetic flux comprises: Ii determining a first magnetic flux component associated with the armature current value; determining a second magnetic flux component associated with the field current value; and summing the first and second magnetic flux components.

14. The method of clause 13, wherein determining the first and second magnetic flux components comprises using one or more look up tables to identify a magnetic flux component associated with a current value.

15. A DC series motor assembly comprising a DC series motor having an armature and a field winding in series with the armature, a three phase inverter circuit coupled to the DC series motor and a controller to control switching of switching elements of the three-phase inverter circuit to enable the three-phase inverter circuit to control the current through at least one of the armature and the

field winding.

16. A DC series motor assembly according to clause 15, when the controller is operable to control the current through the armature and through the

field winding.

17. A DC series motor assembly according to clause 15, when the controller is operable to control at least one of the magnitude and direction of the current through at least one of the armature and the field winding.

18. A DC series motor assembly according to clause 15, when the controller is operable to control the direction of the current through one of the armature and the field winding and the magnitude of the current through the

armature and the field winding.

19. A DC series motor assembly according to clause 15, 16, 17 or 18, wherein the three-phase inverter circuit has first, second and third legs each coupled between positive and negative power supply lines of the assembly, each leg comprising first and second switching elements coupled together at a node, each switching element having a control gate controlled by the controller.

20. A DC series motor assembly according to clause 18, wherein the armature is coupled between the node of the first leg and one of the positive and negative power supply lines and the field winding is coupled between the nodes of the first and second legs.

21. A DC series motor assembly according to clause 18, wherein the armature is coupled between the nodes of the first and second legs and the field winding is coupled between the node of the third leg and one of the positive and negative power supply lines.

22. A DC series motor assembly according to clause 18, wherein the armature is coupled between the nodes of the first and second legs and the field winding is coupled between the nodes of the second and third legs.

23. A DC series assembly according to any of clauses 15 to 22, wherein the switching elements of the three-phase inverter circuit comprise semiconductor switching elements.

24. A DC series assembly according to any of clauses 15 to 22, wherein the switching elements comprise voltage controlled impedances such as insulated gate bipolar transistors (IGBT5), or MOSFETs, or IGFETs, or bipolar junction transistors (BJT5), or junction field effect transistors.

25. A DC series assembly according to any of clauses 15 to 24, wherein the three-phase inverter circuit comprises a three-phase inverter circuit designed for controlling an AC motor.

26. A DC series motor assembly comprising: a DC series motor having an armature and a field winding in series with the armature; and a controller for controlling the current through at least one of the armature and the field winding, wherein the controller comprises a three-phase inverter circuit designed for controlling an AC motor.

27. Use of a three-phase inverter circuit designed for controlling an AC motor to control the current through at least one of the armature and field winding of a DC series motor 28. A DC series motor assembly substantially as hereinbefore described with reference to and/or as illustrated in Figure 3a, Figure 3b or Figure 4 of the accompanying drawings 29. An apparatus for controlling a DC series motor comprising: a first current control configured to control the supply of a first current to one of an armature of the DC series motor and a field winding of the DC series motor; a second current control configured to control the supply of a second current to the other one of the armature of the DC series motor and the field winding of the DC series motor, wherein the first current control is configured to control the first current based on the second current.

30. The apparatus of clause 29 wherein the first current control is configured to control the supply of the first current to the armature of the DC series motor and the second current control is configured to control the supply of the second current to the field winding of the DC series motor.

31. The apparatus of clause 29 wherein the first current control is configured to control the supply of the first current to the field winding of the DC series motor and the second current control is configured to control the supply of the second current to the armature of the DC series motor.

32. The apparatus of any of clauses 29 to 31 wherein at least one of the first current control and the second current control is operable to control current by applying a control voltage to control at least one leg of an inverter circuit for controlling an AC motor.

33. The apparatus of clause 32 in which the inverter circuit comprises a multi-phase inverter circuit.

34. The apparatus of clause 32 in which the inverter circuit comprises a three phase inverter circuit.

35. The apparatus of any of clauses 32 to 34 further comprising the inverter circuit.

36. The apparatus of any of clauses 32 to 35 in which the inverter circuit is designed for controlling an AC motor.

37. The apparatus of any of clauses 29 to 36 further comprising a DC series motor.

38. The apparatus of any of clauses 29 to 37 further comprising a memory storing a look-up table and in which controlling the first current based on the second current comprises controlling the second current value based on the first current and the look-up table.

39. The apparatus of clauses 32 to 38 having the features of any of clauses 2 to 70119 to 25.

40. A method of adapting the control system of a DC series motor comprising providing an inverter circuit to supply current to the armature and field windings of the DC motor and providing a control means configured to control the inverter such that one of the armature current and the field current is controlled based on the other one of the armature current and the field current. l6

Claims

<claim-text>CLAIMS1. An apparatus comprising: a DC series motor comprising an armature winding, and a field winding in series with the armature winding; and, an inverter having first, second and third AC voltage couplings; wherein: each AC voltage coupling is configured to provide one phase of a multi-phase AC voltage for powering an electric motor, the field winding is coupled between the first AC voltage coupling and the second AC voltage coupling, and the armature winding is coupled between the third AC voltage coupling and a DC power supply voltage.</claim-text> <claim-text>2. An apparatus comprising: a DC series motor comprising an armature, and a field winding in series with the armature; and, an inverter having first, second and third AC voltage couplings; wherein: each AC voltage coupling is configured to provide one phase of a multi-phase AC voltage for powering an electric motor, the armature winding is coupled between the first AC voltage coupling and the second AC voltage coupling, and the field winding is coupled between the third AC voltage coupling and the DC power supply voltage.</claim-text> <claim-text>3. An apparatus comprising: a DC series motor comprising an armature, and a field winding in series with the armature; and, an inverter having first, second and third AC voltage couplings; wherein: each AC voltage coupling is configured to provide one phase of a multi-phase AC voltage for powering an electric motor, the armature winding is coupled between the first AC voltage coupling and the second AC voltage coupling, and the field winding is coupled between the second AC voltage coupling and the third AC voltage coupling.</claim-text> <claim-text>4. The apparatus of any preceding claim in which the AC voltage couplings are provided by legs of the inverter, and each leg comprises at least two voltage controlled impedances arranged to control the voltage of the AC voltage coupling.</claim-text> <claim-text>5. The apparatus of claim 4 in which the legs each have equal current capacity, thereby enabling currents of equal amplitude to be provided to the armature windingand field windings.</claim-text> <claim-text>6. The apparatus of claim 4 or 5 in which a control means is arranged for providing a control signal comprising a series of control pulses to control the voltage controlled impedances to modulate the current flowing through the armature winding.</claim-text> <claim-text>7. The apparatus of claim Sin which the control signal is pulse-width modulated.</claim-text> <claim-text>8. An apparatus according to any of claims 4 to 7 wherein the voltage controlled impedances comprise one of: insulated gate bipolar transistors (IGBTs); MOSFETs; IGFETs; bipolar junction transistors (BJTs); and junction field effect transistors.</claim-text> <claim-text>9. The apparatus of any of claims 1 to 8, wherein the inverter comprises a three-phase inverter circuit for controlling an AC motor.</claim-text> <claim-text>10. Use of a three-phase inverter circuit designed for controlling an AC motor to control the current through the armature winding and field winding of a DC series motor.ii. A method of adapting the control system of a DC series motor comprising providing an inverter circuit to supply current to the armature winding and field windings of the DC motor such that the armature winding current and the field winding current are controlled by the AC voltage outputs of the inverter as defined in any of claims ito 9.</claim-text>