Non-contact switching system for DC motors.

This invention relates to electrical machines, in particular, to DC motors, and can be used for current winding control.

The DC motors are rather multipurpose machines with a high torque. Besides, they are capable of operating in a broad rpm range. However, a switching system, which employs brushes sliding over a sectional copper ring, is used in a traditional DC motor for controlling each current winding. Brushes and the sectional ring develop the required switching of the armature winding current with the corresponding rotor winding in the preset time. The switching system ensures that each drive generate the torque.

In traditional electric DC motors, the winding of the internal rotor is coiled in a way so that current would pass virtually all wires of the rotating rotor and the torque would be generated.

The application of the said design is explained by the fact that the armature winding current produces a magnetic flux in the rotor, whose direction is perpendicular to that which is generated by the excitation system (either a set of windings in the outer stator, or permanent magnets). The magnetic flux of the armature tends to orientate itself towards the magnetic flux of the stator, thus generating the motor torque and providing the rotation of the armature. However, due to the availability of the collector, the rotor flux permanently returns to the perpendicular state to provide torque even during the rotation.

To ensure the DC motor operation, it's necessary to establish switching of currents in the armature winding. For this purpose, a ring comprising insulated copper plates (bars) is used; each bar is properly connected with the armature winding. Two brushes ensure a contact between the power unit and the rotating collector.

In case of the typical armature coiling method, two current-ways are emerged between the two brushes, and the current passes through each wire (or virtually all wires) in a certain direction and generates the electromagnetic torque. In this configuration of current ways, the preset counter-electromotive force emerges at a lower current in the winding.

It's also necessary to mention that, while engineering the motors, special attention in the switching process is paid to the current reduction in the winding to avoid brush sparking. Generally, the magnetizing flux in a certain winding is decreased for this purpose (preferred option - by introducing an additional pole winding).

The DC motor is typically controlled by changing the motor voltage to decrease the armature current (in some case, it's possible to control current in the stator winding). The prior-applied method called for the installation of a regulating resistor at the motor input line. The resistor's influence on the motor voltage was in the current-caused voltage drop in the resistor itself. This method complicates the control over high-capacity motors, since a significant amount of heat is released in the regulating resistor. A semiconductor regulator can be used for breaking the supplied DC voltage.

Two types of semiconductor devices could be contemplated: MOS- transistor-based system (MOSFET) (including insulated-gate bipolar transistors (IGBT)), or a thyristor (or bidirectional triode thyristor) based system.

A field-effect MOS-transistor is employed to supply voltage to the motor over a limited period of time, i.e. to control intermediate voltage at the motor. The use of a transistor for DC voltage breaking is rather convenient, since current is available only in case if a relevant control signal is sent to the transistor gate.

To ensure breaking capability, a thyristor can also be implemented. Key advantage of using a thyristor instead of a field-effect MOS-transistor is that

this semiconductor item is capable to maintain higher currents and higher voltages, which makes this device more suitable for high-capacity systems. However, it's wiring diagram is much more complicated, since it's necessary to ensure the thyristor current cut-off at the end of each breaking cycle. It's necessary to isolate the thyristor current for a while with the aim to terminate the conductivity of the said; for this purpose, it's possible to invert voltage for a short period of time, for instance, by using a resonance circuit which is started up each time whenever it necessary to stop the main thyristor. An additionally mounted thyristor connected in-series with an inductance coil, which forms a parallel connection with a capacitor and the motor, can be used to control the above-mentioned additional resonance circuit. When the additional thyristor is switched on, current enters the inductance coil leading to a resonance effect; now, for a short period of time, the resonance current in the LC-system makes it possible to reduce down to zero the current flowing through the main thyristor and thus, to switch it off immediately.

It's worth mentioning that current passing through this motor and its regulator is switched over twice: by electronic system to reduce average applied voltage, and by a mechanical switch (collector) inside the motor.

Brushless DC motors have found a broad application recently. The design of this motor is based on the principle of operation of a synchronous AC motor. Generally, three windings are installed in the three-phase synchronous AC motor at an angle of 120°. When alternating current is applied to the three-phase voltage system, the rotating magnetic flux is equal to a sum of independent fluxes, which are offset from each other at an angle of 120° (electrical). In this case, the rotating flux interacts with the rotor magnet and ensures the orientational coincidence of the rotor magnet with the rotating flux (occurs at a zero rotational torque of the motor). As the rotor torque grows, an angle between the rotating flux and the rotor magnet forms. Maximum torque generated by the electric machine, corresponds to a 90° lag

angle. One could imagine a brushless DC motor as a synchronous AC motor upgraded to allow the system operation at a 90° displacement angle at any torque (as it takes place for traditional DC motor).

To ensure reliable operation and effective control of this kind of motors, an angular position sensor, which allows continued determination of the rotor angular position, is used. Based on the measurement results, control electronics supply required voltages to three winding in a way so that the angle between the rotating flux and the rotor would be equal to 90°.

In the brushless DC motor, it's necessary to use an electronic drive control system, which generates output voltages for each of the coils to allow them jointly develop the required magnetic flux. These three voltages must be perfectly controlled by amplitude and have proper synchronization. Often configuring of this electronic system is a challenging task.

In particular, one of the prior arts (RU, patent 2091969, 1994) is a non- collector DC motor, comprising a stator with Z-numbered similar evenly- distributed teeth, on which m-phase winding is mounted, and a rotor with 2p alternating poles made of permanent magnets magnetized in the radial direction. The number of the stator teeth (Z), which is divisible to the number of phases (m), and the number of the rotor poles (2p), is selected to provide a minimum difference between them, which is equal to 2K, where K is a positive integer; the m-phase winding in the form of a concentric winding, whose each phase consists of the 2K coil groups, is wound up in a way so that the coiling direction on the stator teeth would alternate within each coil group and the coiling direction on teeth of one phase, shifted by 1807K, would coincide at odd-numbered K and be opposite at even-numbered K.

Another prior art (SU, author's certificate 972634, 1980) is a motor, comprising a rotor fixed at the motor shaft, at least one fixed armature mounted at the stator, with the winding on the said armature, whose sections are connected to the output of the motor rotor position sensor, and a tacho-

generator whose rotor is aligned with the rotor of the motor rotor position sensor and fixed at the motor shaft; magnetic field sensitive elements (Hall's elements) are located on one side of the tacho-generator rotor, whilst the stationary armature with the tacho-generator winding is located on the other side of the rotor; the number of the sensor poles is equal to the number of pole pairs of the motor.

Another prior art (SU, patent 1419531, 1988) is motor comprising a rotor fixed at the motor shaft, at least one fixed armature located at the stator with the armature winding, whose sections are connected with the output of the first electronic switching device, whose control circuits are connected to the output of the motor rotor position sensor, and a tacho-generator, whose rotor is aligned with the rotor of the motor rotor position sensor and is fixed at the motor shaft; position sensors - magnetic field sensitive elements - are located on one side of the tacho-generator rotor, whilst the stationary armature with the tacho-generator winding is located on the other side of the rotor; the number of the position sensor poles is equal to the number of pole pairs of the motor. The position sensor rotor is made as a disc on which permanent magnets are installed; magnetic axes of the said permanents magnets are oriented in parallel to the shaft; the number of phases of the tacho-generator' s armature winding is proportional to the number of phases of the tacho- generator' s armature winding which is connected to the second electronic switching device.

A complexity of their design is the disadvantage of the known motors.

The engineering task to be reached through the implementation of the suggested engineering solution aims to replace mechanical switching system with a set of solid-body switches (electronic keys, semiconductor keys, inverter, ((electronic switching device»), which switch on in the prescribed time and provide required switching.

Technical result to be achieved through the implementation of the suggested system includes the simplification of the DC motor design, with a simultaneous reduction in fabrication-related labor-intensity and weight of the finished motor.

To achieve the above-stated technical result, it's suggested to apply the non-contact switching system developed for DC machines comprising a set of solid-body switches, which ensure bi-directional current switchover in the motor armature windings and which are located in a stationary part of the motor; besides, the system includes a set of position sensors (in particular, Hall's sensors), connected either mechanically or through a magnetic field of permanent magnets (in case if Hall's sensors are used as rotor position sensors, then the link with the rotating part of the motor is established through the influence of the magnetic field of permanent (or alternating) magnets on sensors themselves; in case if, for instance, sine-cosine transformers (resolvers) are used as rotor position sensors, then this sensor is (directly) mechanically linked with rotating parts of the motor, i.e. the rotating part of the resolver is mechanically connected to the motor rotor and rotates together with it) with rotating parts of the motor and electrically - with the above- mentioned switches (sensor outputs are connected to the switch (or key) inputs, possibly, through some intermediate linking elements, e.g. transistor IGBT drivers, pulse transformers, amplification circuits, voltage-ratio dividers, etc.), at least, one magnet fixed at the rotating part of the motor, and an electronic circuit which controls operation of the said switches. Field-effect transistors or thyristors are used as switches.

The invention characterizes a switching system for traditional DC motors, with solid-body devices to be used instead of brushes sliding along an ordinary mechanical collector. The switchover effect is provided by using a set of solid-body switches, which replaced brushes sliding along the collector bars. Position sensors (e.g., Hall's sensors), connected with rotating parts of

the motor, are applied to turn on the switches. The newly-developed switching system does not have a mechanical collector which is a source of sparking and carbon dust (from brushes). The solid-body switch is perfectly adapted for the use in DC motors with a fixed armature and external rotating magnet.

The solid-body switch can be used for launching the DC motors with a switchover delay to reduce the time-average current (or applied voltage). The availability of this system ensures control over the motor torque, similar to the winding switching system. This system proved to be very effective in the design of a reversible motor, in which the armature core is fixed and the external magnetic system rotates.

The newly-developed switching system can be widely used in motors with a small number of the armature windings (e.g., motors of submersible electric pumps) as well as in motors operating at non-critical temperatures (e.g., oil-filled submersible motors for well operation at temperatures of no more than 125°C).

In electric motors with the implemented newly-developed switching system, the winding of the main armature is coiled up similar to the traditional DC motor. It is commonly supposed that the magnetic field in the motor is induced by a permanent magnet. Obviously, inductive windings can also generate the magnetic field.

Similarly to a traditional armature coiling method, two current-ways (symmetrical against the armature bore) emerge between the switchover points. The newly-developed switching system ensures current switchover through the application of solid-body devices instead of traditional brushes and a rotating ring separated into bars (collectors of traditional DC motor). Each bar of the traditional collector is replaced with the oppositely-located switches, which provide bi-directional current switchover. An electric circuit that allows each solid-body switch to pass through current when required, controls the gate of each switch. This target can be reached by using solid-

body Hall's sensors which are installed in proper position as soon as the magnet fixed at the motor's rotating part approaches them.

In the first and the simplest design option, field-effect transistors (MOSFET or IGBT) can be used as switches. The application of solid-body devices simplifies the driving mechanism: the switch conducts current as soon as the preset voltage is applied to its gate. One of possible embodiments of the motor switching system is the application of Hall's sensors for controlling solid-body switch gates - when the magnet is near the sensor and the magnetic flux is properly directed, the sensor unlocks the switch gate and the switch start conducting current. And vice- versa, when the magnet goes away, the gate is closed and the field-effect transistor does not conduct current any longer. Structurally, this engineering solution shall be implemented in such a way that Hall's sensor would activate only if the rotating magnetic field against the sensor is properly directed. To ensure that the sensor is in the preset state only when the magnet occupies a restricted (and defined) angular position, a magnetic flux focusing system can additionally be used.

Besides, an angular focusing of the magnetic field can also be obtained due to the application of two small magnets oriented towards the motor's magnetic flux. Similar to a traditional mechanical collector, the switch remains in the current-conducting status equivalently to the motor rotor's angle of rotation, as it took place during the contact of the collector bars with brushes. It should be mentioned that the same process proceeds at two switches (oppositely situated in the switch module) to ensure the connection with the both poles of the power supply unit (VDC+ and VDC-).

It worth mentioning that two switches (one switch can be connected either to VDC+, or to VDC-) are installed at each point between two windings. As through the said two switches are connected to two Hall's sensors installed face-to-face (dependent on the sensitivity to magnetic filed), they are opened either by the south-seeking pole, or the north-seeking pole. In

this design, only two symmetrically placed field-effect transistors conduct current at a particular moment and ensure current flowing from the positive pole to the negative pole through two symmetrical ways in the motor armature. The mechanism of current passage along two ways is similar to that for the traditional motor. It should be noted that the connection of an additional capacitor in parallel to the switch with the aim to limit current surge in a transient mode emerged at the time of switchover, could be required.

To restrict current surges, it's also desirable to turn on the next current- conducting switch before the preceding current-conducting switch is turned off. If this sequence is implemented, current is first distributed between the two current-conducting switches and in this case the transient process during the switchover is not so critical as before.

However, in a certain mode, with the aim of providing adequate control, first of all it's necessary to stop current in the preceding switches before the next switches are triggered. In case of higher currents (and high-capacity motor), the preferred option is to use thyristors rather than field-effect transistors, since a thyristor switch is capable of maintaining high currents at the lowest heat losses. The mechanism of the thyristor gate opening is virtually the same. However, when the thyristor is open, it's often insufficient to remove signal from the gate to get the thyristor off the current-conducting state; external tools shall be applied to stop current flowing at a preset time. Similar devices are well-known in the circuitry engineering. In particular, this issue could be resolved by using an additional capacitor or a set of capacitors in the circuit.

If the capacity of these capacitors is not enough to stop current passing in the thyristor, an additional current-quenching circuit can connected to the power supply system. The operation of this system is similar to the traditional method applied for the DC motor capacity control. A resonance circuit of this external quench circuit shall be calculated with the allowance for the

inductance of the whole electric motor, i.e., a high-capacity capacitor is generally used for this purpose.

It's worthwhile to say that the quenching thyristor is synchronized with the switch to ensure a secure ending of the switching cycle. Different time intervals can be applied to provide synchronization.

The principle of operation of the above-discussed switchover configurations (field-effect transistor and thyristor — based switches) is similar to the traditional mechanical switching system. At every instant (excluding the transient mode), current passes flows through two switches, i.e., as in the case of a mechanical switching system, when brushes were connected only with two collector ring bars. This means that the whole armature current passes through these switches, but only within a limited rotational angle, since the neighboring switch starts conducting current after a corresponding slight turn (defined by the number of switches applied).

Another embodiment can be also used; in this embodiment, switches maintain only part of the current flow (aggregate current divided by a number of independent windings). The advantage of this configuration is the possibility of current restriction in each switch. This configuration can be of particular assistance in motors intended for operation at high current values. However, the switch must withstand a full voltage of the power supply unit.

This configuration can also be useful while operating at high temperatures, since the semiconductor device's transition temperature could become a problem due to a temperature increase as a result of current passing through the switch.

The bigger the number of turns in the winding, the higher the winding inductance; this causes notable difficulties in the switch-over process. Furthermore, the bigger the number of turns in the winding, the thinner wire is required to avoid losses in the volume of ferromagnetic parts of the electric motor. This means that, at the same effective capacity, ohmic losses grow.

The suggested switching configuration allows the control of the torque generated by the motor, by a switchover delay for repeated current initiation in the armature during the switching. In case if a thyristor-based system is implemented, an external electric circuit could control the time lag, which presets a moment of the current recovery after switching. This time lag is equivalent to controlling by an average current passing through the electric motor. The average-current-based control is an integrated method for the motor torque control without the use of additional components. In this configuration, the same electronic system, which controls internal switching in the motor, is also used for the torque control.

A similar principle can be also implemented in case if a field-effect transistor is used for current switching in the motor winding. In this case, the switching process starts with the closure of the current-conducting field-effect transistor. The field-effect transistor, which should start conducting current, is triggered with a switchover time lag in a way so that the time-average current would decrease. This delay system is applicable both for the armature with series-connected windings, as well as for the armature with independent windings.