Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a brushless DC motor and its method for commutation and control, in particular, to the armature winding and its arrangement construction with the commutating devices which form the commutation circuit topology of the brushless DC motor and to the method for commutation by turning on and turning off the commutating devices and to the speed controls for such motor.

2. Description of the Related Art

Always for DC motor except for the commutation problem, the other characteristics all are perfect or tend to be perfect. In order to overcome the commutating disadvantages, the brushless DC motor has appeared and well known in the art. A brushless DC motor is a synchronous machine typically, which is powered by alternating current, and operated in such a way as to behave like a DC motor. Mainly because it needs to rely on the commutating devices with the ability of self-turn-off, and the price of said commutating devices with the ability of self-turn-off is far higher than that of the commutating devices with no ability of self-turn-off such as thyristor, in order to reduce the using quantity of the commutating devices, brushless DC motor mostly moved towards the path of using the variable frequency driving synchronous motor in the basic electromagnetism structure and the principle of work, namely, the electronic commutation sine or the nonsine driving synchronous motor, and the armature winding is in three-phase mostly.

Brushless DC motor has overcome the defect of the original electric slide contact commutator while maintaining the fine external characteristics of DC motor basically. These technical projects have obtained great successes in various fields as we know and are also in the process of high speed developing. However, they are not suitable for large power utilization, for they are restricted by the commutating devices with the ability of self-turn-off in technology and economy.

There are some patents having disclosed some polyphase armature winding brushless DC motors, but the arrangement ways of the armature winding have not improved from those of the DC motor armature winding which are only suitable for small power brushless DC motors and not suitable for wide ranges of power utilization.

SUMMARY OF THE INVENTION

In accordance with the present invention, a brushless DC motor and the method for commutation and control are provided. The advantages that accrue to said brushless DC motor can use more cheap power electronic devices as commutating devices such as thyristor for commutation and control, and even for better performance (starting for example) in commutation and control can use more expensive power electronic devices which have the ability of self-turn-off such as Gate Turn-off Thyristor (GTO), Integrated Gate Commutated Thyristor (IGCT), Insulated Gate Bi-thermal Thyristor(IGBT) etc. as switches for commutation and control. With better work condition of the power electronic devices as the commutating device and the same electromagnetism structure and theory as DC motor, the present invention also has higher efficiency than previous brushless DC motor.

The present invention is mainly on the electric circuit topology of the armature windings and the power electronic devices as the commutating device. Said armature windings in structure designs are similar to DC motor, and the methods for commutation and control mainly include the succinct switch circuits

using DC motor armature winding back electromotive force (back-emf) to turn off the thyristor and commutating armature winding currents, and the running in the state equivalent with that of the magnetic field weakening speed adjustment in constant exciting magnetic field of brushless DC motor, and the running in the state equivalent with that of the magnetic field weakening speed adjustment in permanent magnetic brushless DC motor; and the reversible running method and its control. For the circuit topology of the armature windings and commutating devices of the brushless DC motor, as FIG. 1 shows, the five duplex-lap-windings are mounted in ten slots with two parallel branches, each commutator segment of the original DC motor has been replaced by a pair of commutating devices which are the upper bridge arm 10 and the lower bridge arm 11 connected in series as one half H-bridge, each said half H-bridge connects DC bus 8, 9 and respective junction points between adjacent two windings to form the commutation circuit topology. The arrangement of rotor 6 and stator is the same as that in original brushless DC motor or in the synchronous motor, that is to say, the magnetic pole places on the rotor, the armature winding places on the stator. Using the design of one slot-conductor 7 per slot is good for reducing the armature windings quantity, so as to reduce the thyristor using quantity. The armature winding design of one slot-conductor 7 per slot is unable to use the simplex-lap- winding, but can use the duplex-lap-winding and/or duplex-wave-winding and/or phase-shift-closed-loop-winding and/or combination of them. Such design has less armature windings and commutating devices quantity with the same slots quantity per pair of poles. If using the single-lap-windings which the DC motor uses generally, the armature windings and commutating devices using quantity will be doubled with the same slots quantity per pair of poles. Now the commutation of current can be completed in two slots or one armature winding per pair of poles by triggering one thyristor. Certainly, if using the topology of skein-phase-winding to joint more than one windings or winding cells as a skein-phase-winding first, then connect all such said skein-phase-windings by topology of duplex-lap-winding and/or duplex-wave-winding and/or phase-shift-closed-loop-windings and/or combination of them, the current commutation can be completed in more than two slots per pair of poles by turning on one commutating device so as to save the cost of commutating devices. The commutation will have more ripples, but it is still not similar to the commutation of the existing synchronous motor driven by square-wave voltage even if in three phase armature windings.

If the windings of closed-loop-winding are odd in number, when commutating, there is only one quantity difference of the series windings in the two parallel branches, the commutation process is smooth. Originally when using the electric brush commutator, more commutator segments structures are advantageous in reducing the winding commutation inductance to reduce the commutation difficulty and the commutation spark. But when using the power electronic device such as thyristor commutator, there is no commutation spark, therefore less lead-out wires of the armature windings will be used, the less commutating devices quantity project will be more economical and advantageous.

The Commutation Principle and the Armature windings Scheme

Still the five-winding system will be taken for example to describe its complete commutation cycle situation as below.

FIG. 2 shows the schematic drawing of the thyristor commutation process of the five-winding system brushless DC motor. In order to facilitate the narration, respectively mark the five windings as 1, 2, 3, 4,

5, the thyristors as commutating devices respectively as Tl to TlO, mark the on-state thyristors with black, mark the trigging thyristor with an electric discharge symbol nearby its gate terminal, and the arrow lines express the armature winding currents of parallel branch of closed-loop-winding, the ten

circles express the ten slots and their slot-conductors of armature windings at the armature surface. Five slot-currents of said armature winding flow out under the S pole, there is a dot in the center, five slot-currents of said armature winding flow in under the N pole, there is a fork in the center. The magnetic pole rotor is in counterclockwise rotating. As FIG. 2 shows, Tl and T6 are on-state, the slot-conductors of winding 4 will leave the air gap magnetic field 15 and need to commutate, at this time trigger T8, because the anode potential of T8 is higher than that of T6, T8 will be turned on after being triggered; the winding 4 will be short circuit by T8 and T6 which are at the same side of bridge arms, and the back-emf of winding 4 will cause the brushless DC motor to change its running state from motor into generator, the current direction in winding 4 will be reversed by the back-emf of winding 4. After the current of winding 4 absorbs the current of winding 2, the current through T6 will disappear and turn off T6, i.e., the back-emf of winding 4 forms the counter pressure to T6 to cause its current to turn off, the new current established through T8 flows in winding 4 to enter the next working stage as FIG. 2a shows.

Similarly trigger T3, and Tl will be turned off by the back-emf of winding 5 after T3 is turned on. The current in winding 5 will change the direction and complete commutation.

Similarly trigger TlO, T5, T2, T7, T4, T9 and T6 orderly in the corresponding rotor positions, the rotor will turn one circle or a pair of pole distance. A complete circle of the brushless DC motor commutation will be finished. The triggering logic is Tl, T8, T3, TlO, T5, T2, T7, T4, T9 and T6. In this five windings system, there are at least 4 windings working at any time, only one is commutating in a short time of commutation period. The triggering logic has a simple rule: the thyristors of co-anode terminal and co-cathode terminal are triggered to turn on alternatively and sequentially, it is the same as the commutator segment brush system in fact. Five-winding system is not the smallest armature winding system that may depend upon the armature winding back-emf to turn off the thyristor to complete the commutation. The smallest system is the three-winding system. Certainly the performance of three-winding system cannot be so good, the electromagnetism torque and the armature current ripple will be very violent, but it is quite suitable for high speed and small power utilization. If using the skein-phase-winding when simultaneously coordinating the distribution of the air gap magnetic field 15, there will be some improvement in performance. But these characteristics and performances can be improved better by increasing the slots quantity of each pair of poles and the armature windings. It will be more convenient to describe the multi-winding system with armature winding spread diagram. FIG. 3, 4, 5 are respectively the spread diagrams of brushless DC motor of three-winding, five- winding and seven-winding. Said three- winding, five- winding and seven-winding are arranged as a closed-loop-winding respectively with the topology of duplex-lap-winding which is seldom used for the small or medium power DC motor. Of course, the duplex-wave-winding or combination of duplex-lap-winding and duplex-wave-winding which is namely the so-called frog-leg-winding are also workable, and it can not be connected to be a closed loop when the windings per pair of poles are even in number, it needs to be connected into two closed-loop-windings with a phase-shift when the windings per pair of poles are even in number, as FIG. 6 showed. To connect the windings to be the multi phase-shift-closed-loop-windings when necessary is the simple and effective method to expand the system capacity and the redundant backup. Of course, when the windings per pair of poles are more than five, they also can be connected to be the multi phase-shift-closed-loop- windings, likewise when the winding numbers of armature winding are respectively 6, 9, 10, 12, 14, 15, 18, 20, 21, 22, 24, 25, and 26 etc. FIG. 7 is an example of ten windings, of which each two adjacent windings are arranged as a

skein-phase-winding first, and then such five skein-phase-windings are connected by means of duplex-lap- windings topology of the DC motor. It can complete commutation of current in four slots or two windings per pair of poles by triggering one thyristor. Certainly, the commutation will have more ripples with more numbers of windings arranged in a skein-phase- winding. After arranging two or more windings in a skein-phase-winding, such said skein-phase-windings then also can be connected with the topology of duplex-lap-winding and/or duplex-wave-winding and/or phase-shift-closed-loop-windings and/or combination of them; and said skein-phase-windings also can be arranged as the type of simplex-lap-winding or simplex-wave-winding or concentric-winding with more than one single winding cell. Now the current commutation can be completed in more than two slots per pair of poles by turning on just one thyristor so as to save the cost of thyristors.

Of course, in the brushless DC motor with multi-pair magnetic poles, the windings in the same phase can be connected in series or parallel so as not to increase the using quantity of commutating devices.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic drawing of five- winding brushless DC motor.

FIG. 2 illustrates commutation process of five-winding brushless DC motor.

FIG. 2a illustrates commutation process of five- winding brushless DC motor.

FIG. 3 is an armature winding spread diagram of three-winding brushless DC motor.

FIG. 4 is an armature winding spread diagram of five-winding brushless DC motor.

FIG. 5 is an armature winding spread diagram of seven- winding brushless DC motor.

FIG. 6 is an armature winding spread diagram of brushless DC motor with a pair of shift-phase-closed-loop-windings.

FIG. 7 is an armature winding spread diagram of five-skein-phase- winding brushless DC motor.

FIG. 8 is a schematic drawing of five- winding brushless DC motor powered by controlled rectifier.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 8 shows the system of five- winding brushless DC motor powered by controlled rectifier 12 with three-phase AC power supply. It can run in motor or generator state, which will be described as follows respectively.

Commutation in Starting Process or at Lower Speed Running

If the brushless DC motor merely depends upon the rotation armature back-emf to commutate, it will have no ability to commutate while starting, and its commutation ability is also bad in low speed running, it needs to first turn off the armature current and the thyristors as commutating devices with the help of the drive source, then to trigger the next pair of thyristors to build new armature current to finish commutation to work. Certainly when using the self-turn-off commutating devices which have the ability to turn off independently, it can turn on the commutating device which needs to turn on in commutation and at the same time or later to turn off the commutating device which needs to turn off to commutate, for example as FIG. 2, if the thyristors from Tl to TlO are replaced by Gate Turn-off

Thyristor (GTO), on the state when Tl and T6 are conducting, turn on T8 and turn off T6 at the same time or later to start commutation. To not be later to turn on T8 than to turn off T6 is good for reducing the over voltage forced on T6 and commutating winding 4 with the inductance of commutating winding

4 in commutation. The other self-turn-off power electronic devices include but not limited to Integrated

Gate Commutated Thyristor (IGCT), Insulated Gate Bi-thermal Thyristor (IGBT), MOS-Controlled

Thyristor (MCT), Injection Enhanced Gate Transistor (IEGT) etc.

The brushless DC motor in which all the commutating devices are thyristors has its special commutation method in starting process: the armature current is controlled by controlled rectifier 12, trigger and turn on the corresponding thyristors according to the rotor position to start brushless DC motor, after the rotor has turned with certain angle, it needs to commutate, said controlled rectifier 12 shuts off the armature current so as to turn off the conducting thyristors as commutating devices, and then trigger the corresponding thyristor of the brushless DC motor and controlled rectifier 12 starts next stage work again. In order to overcome the adverse effect of stationary current inductance 13 when turning off the armature current, thyristor 14 can be jointed with the stationary current inductance, just trigger it to turn on it when needs to turn off the armature current. This way is suitable for driving the big inertia and small or non-potential energy load, not suitable for the small inertia and big potential energy load. Reversible Running State

When the power electronic devices for commutation are un-reverse conducting devices, the brushless DC motor has a special characteristic: the armature current always has the same direction whether it runs as motor or generator, what changes is only the voltage direction, which happens to satisfy the reversible operation condition of the controlled rectifier circuit. This is deferent from the DC motor reversible control system using commutator segment and electric brush system which needs a pair of reverse parallel connection controlled rectifier circuits. And the triggering rules of the brushless DC motor for running as generator are quite simple and apparent: to trigger and turn on the commutating devices which are in the highest and lowest voltage positions; or to trigger all of the commutating devices, only the commutating devices which are in the highest and lowest voltage positions can be turned on to conduct, and this is also a workable method for said brushless DC motor running as generator. For the rotation direction of the brushless DC motor, the triggering logic can completely control it. Moreover, when the controlled rectifier system uses the self-turn-off thyristor devices at least in half bridge or in full bridge, the system will have the pulse-wide-modulation rectification and the reverse commutation ability; this will reduce the pollution to the electric network and enhance the ability of anti commutation failure.

Running in the State Equivalent with that of the Magnetic Field Weakening Speed Adjustment in the Constant Exciting Magnetic Field

Because the new brushless DC motor has no commutation problem as DC motor when its brushes are far away from the physical neutral axis to cause the commutation difficulties, the commutation way of the new brushless DC motor also has and allows to use an extremely interesting control characteristic: if triggering and turning on the thyristor to commutate much more ahead of time or phase relative to armature physical neutral axis, in other words, turning on the commutating device to start commutation at much more ahead of time or phase than doing in generally running state, obviously the armature winding back-emf connected by two on-state thyristors will be weaker, the result will be equivalent with that of the magnetic field weakening speed adjustment process of DC motor, that is to say, the DC power supply voltage is constant, the brushless DC motor rotational speed will still rise, and this has special using value to the speed adjustment which includes but not limited to the permanent magnet motor, as well as to the high speed motor with short time to commutate and especially to the super high speed motor, but it doesn't fit for the system of which the power electronic devices for commutation are reverse conducting devices.

Power Electronic Devices Commutation Working Conditions

Usually when one power electronic device such as thyristor is triggered and turned on, it will only withstand the small forward voltage which has been generated to satisfy the commutation request with

the winding near to the neutral axis, and it is turned on with nearly zero-voltage; the other power electronic device such as thyristor will withstand the reverse voltage to turn off naturally. At this time the switch operation of the power electronic device as commutating device has already naturally approached the soft-switch working condition: Zero-Current-Switch and Zero- Voltage-Switch. Thus, the soft-switch turning off method of the power electronic device should also be used in self-turn-off power electronic devices as commutating devices in the present invention. Air Gap and Slot Design Characteristics

Because the armature reaction will not bring too many bad influences on the new brushless DC motor, and the armature winding inductance will not bring the commutation difficulty and more energy consumption either, the air gap of the brushless DC motor can be designed small, the corresponding pole shoe length may approach pole distance, the armature slot also may be designed to approach closed slot or use the magnetic slot wedge etc., these design characteristics will further enhance the power density and the energy efficiency of the new brushless DC motor. Rotor Position Detection

The most commonly used technical method to detect the rotor position is to use one or more magnetism sensitive Hall sensors; of course the rotation encoder etc. can also be used; to monitor the voltage or emf of the armature winding can also detect the magnetic pole rotor position; meanwhile it can provide the real-time optimization for the commutation triggering phase of the new brushless DC motor so as to enhance the brushless DC motor running performance. The optimized methods may be summarized as follows: monitor the magnetic pole rotor position or the voltage or emf of the armature winding in real-time, then adjust the commutation triggering phase to make the commutation close to the armature physical neutral axis of the brushless DC motor on the condition of guaranteeing the reliability of the commutation, when meeting the sudden events, advance the triggering time or phase preferentially guaranteeing the reliability of the commutation, of course, it is far away from the armature physical neutral axis when it is in the process of "magnetic field weakening speed adjustment". Conclusions

The new brushless DC motor completely solved the commutation problem which the traditional DC motor has faced. It maintained the succinct electromagnetism structure and the outstanding speed adjustment performance of DC motor, and the commutating device working condition is superior by using the direct current actuation. It is not necessary to use the complex sine actuation as the synchronous motor or the asynchronous motor which will inevitably cause the power line harmonic pollution and energy loss. Therefore its theoretical efficiency will be higher than that of the synchronous-motor and the induction-motor, especially higher than that of the synchronous-motor and the induction-motor which are with converter driving systems. And the armature windings controlled by thyristors are still connected in series; it is more suitable for the armature winding and the thyristor device design and using than for the common brushless DC motor. And also it is with born superiority in the aspects of the armature windings equilibrium current and equilibrium voltage and the thyristor soft switch working condition and so on. Therefore it is not only suitable for the small power utilization, but also more suitable for the large power and high efficiency utilization. Meanwhile the thyristor device is the power electronic device which is most economical with the largest control power capacity and most mature and reliable technology device.