GB1574255A - Rotary electrical machine - Google Patents

Description

(54) ROTARY ELECTRICAL MACHINE (71) We, MOTEURS ELECTRIQUES POLICO, a French Body Corporate of 73, rue de Lagny, 75020 Paris, Seine, France, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to rotary electrical machines of the kind operable as a motor or as a generator, and having the advantage that when it is used as a motor it can operate as required in accordance with either of two different torque-speed curves, one of these curves corresponding to high torque with low speed and the other curve to low torque with high speed.

There is a need for motors of this kind when using magnetic tapes which while being read have to pass in front of a magnetic read head at a very constant low speed, which requires high torque, and which during rewinding must be able to travel at a high but not necessarily precise speed, which can be done with low torque.

In United States Patent No. 3,753,068 there is described an electrical machine which can be used as motor having two different torque-speed curves when it comprises an elongated wound rotor which is subjected to the magnetic field of a stator consisting of two stator parts following one another in the longitudinal direction. One of these stator parts consists of permanent magnets disposed circumferentially around part of the length of the rotor; the other stator part comprises stator windings and poles which are spaced circumferentially around the remainder of the rotor.

According to the invention there is provided a rotary electrical machine operable as a generator and as a motor having at least two different torque-speed curves, comprising a wound rotor surrounded by a stator having pole pieces associated with windings and with permanent magnets, these pole pieces each having at least one cavity extending longitudinally and opening in the direction of the rotor, each cavity containing a permanent magnet having two opposite pole faces wherein each cavity and each magnet has a rectangular cross-section the longer side of which is disposed radially relative to the rotor, and each permanent magnet is fixed in its cavity with a pole face close to the rotor, the other pole face being situated at the bottom of the cavity.

A preferred embodiment of the invention is a two-speed motor having a wound rotor which is surrounded over its entire length by the pole pieces which are spaced circumferentially and associated with stator windings.

Preferably, the magnets are then elongated bars all of whose faces are plane faces.

In certain circumstances, when it is thought desirable to give better protection to the permanent magnets against the risk of demagnetisation, there may be provided an additional winding including conductors housed in grooves formed in the pole face of the pole shoes on each side of the permanent magnets, which conductors extend circumferentially between each pole shoe of a pole piece and the nearest pole shoe belonging to the adjacent pole piece, whereby the total rotor current of the motor passes through the whole of this additional winding in order to produce a magnetising effect cancelling the demagnetising effect of the transverse armature reaction. The entire armature current passes through the additional winding in the opposite direction to the current circulating in the armature.

When very good speed stabilisation is desired it is also possible to replace each of the field windings, which are each respectively connected to one pole piece, by two separate windings one of which is connected in parallel with the rotor windings and the other in series with the rotor windings.

In order that the invention may be more clearly understood some embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional view of a rotary electrical machine according to the invention, in a longitudinal plane passing through the axis of the machine, Figure 2 is a partial view in section on the line II-II in Figure 1, of the stator of the machine, and Figure 3 is a partial view in section similar to Figure 2, showing a modified form of the stator.

The rotary electrical machines described by way of example consists of the usual parts, which will not be described in detail with the exception of the parts, or portion of those parts, which are concerned with the invention. In an outer yoke 1 there is supported a shaft 2 on which is fastened a wound rotor 3, known per se of a motor.

The shaft 2 and rotor 3 are free to rotate. In the embodiment the stator of the motor comprises four elongated pole pieces which are fixed to the inner surface of the yoke 1 by screws 5. Each pole piece 4 extends over the entire length of the rotor 3, and may be made in known manner of mild steel or from laminations which are cut out to shape and assembled together. Each pole piece 4a has overall a known cross-sectional shape comprising a body 4a and pole shoes 4b which extend circumferentially around the rotor 3, which is not shown in Figures 2 and 3.

Windings 6 of insulated wire surround the body 4a of each pole piece 4, and the windings 6 rest on the pole shoes 4b.

The pole face 7 of the pole shoes which is close to the rotor 3 has a concave curvature, and the radius of curvature of the pole face 7 is such that it defines, with the outer face of the rotor 3, the smallest possible air gap. In this way the magnetic field produced by the windings 6 and pole pieces 4 affects the entire length of the rotor 3.

Each pole piece 4 has an elongated cavity 8 which preferably extends over the entire length of the pole piece and which is open in the direction of the rotor 3.

That is, each cavity 8 opens into the pole face 7 of the pole piece in which the cavity is formed. Because of the fairly large dimensions of each pole piece 4 in the radial direction, each cavity 8 can have a rectangular cross-section the longer side 9 of which is disposed in the radial direction relative to the rotor.

Each of the cavities 8 houses a permanent magnet 10 of rectangular cross-section having opposite pole faces respectively situated close to the rotor 3 and at the bottom of the cavity 8, as can be seen in Figures 2 and 3.

These magnets may be of any desired kind, for example, it is possible to use a bar of special alloy which has a rectangular crosssection, or else a composite sintered powder magnet, sometimes called a ceramic or ferrite magnet. One of the advantages provided by the invention is that it permits a fairly wide choice of type of magent. This is possible because of the rectangular crosssection with a relatively long longer side 9 which separates the two poles from one another. This gives good protection against the demagnetising field resulting from the fact that the magnet is placed inside a pole piece.

The pole face 11 of each magnet 10 which is close to the rotor may have concave curvature such as to be continuous with the incurved pole face 7 of the pole shoes 4a and 4b. However, the invention has the advantage in that it is not necessary that the pole face 11 of each of the magnets 10 should have a curved shape. It is possible to use magnets having flat pole faces and a rectangular cross-section which are simple parallelepipeds. Production of such magnets is simple and their cost is low. The magnets 10 are fixed in the cavities 8 in any suitable way, for example by adhesive bonding.

Preferably all the magnets 10 also extend over the entire length of the rotor 3, so that the whole of the rotor is subjected to the constant magnetic field which they produce.

From one pole 4 to the next the polarity of the magnets 10 is reversed so that the pole faces near the rotor will in succession be N-S-N-S....

The construction of the motor requires accurate centering of the rotor 3 relative to the pole pieces 4, as in a conventional motor. The use and the installation of the magnets 10 in the cavities 8 do not give any additional difficulty.

Means known per se, which are outside the machine and are not shown in the drawing, control the direction and intensity of the current circulating in the windings 6.

These means may be electrical or electronic.

The magnetic field applied to the rotor 3 can thus be modified, for example by producing an additional flux which reinforces the magnetic field produced by the magnets 10, thus making it possible to operate the machine, as a motor according to two different torque-shaped curves.

The invention is not to be limited to the number of poles or to the variation of magnetic flux which are indicated above by way of example. If desired, the speed of the motor of the invention may be varied in any known way for this type of motor.

The machine of the invention has length dimensions which are as short as possible, because there is no addition in the axial direction of the length of the magnets 10 and pole pieces 4. Moreover, the diameter is not increased because the magnets 10 are housed in the radial thickness of the pole pieces 4. In addition, since these pole pieces 4 extend along the entire length of the rotor 3 it is possible to use a rotor having just the required length, thereby reducing the starting inertia of the motor. Conversely, if a longer rotor is acceptable it is possible to use smaller windings 6.

It has been found in practice that the motor of the invention has overall dimensions and starting inertia which are reduced by about 30% relative to a motor in which the windings and magnets constitute extensions in the longitudinal direction. The concentric mounting of the rotor and of the two stator parts, that is the permanent magnets 10 and the windings 6, is achieved without additional difficulties; in other words, it is no longer necessary to meet the difficult requirement of providing two stator parts which are separate in the longitudinal direction and which must be exactly coaxial, as is the case in the construction described in the patent mentioned above.

Two modifications may be made to the machine just described.

Instead of making each winding 6 in a single piece it is also possible to provide windings consisting of two parts 6a and 6b, as shown in chain-dotted lines in Figure 3, around each pole piece. The windings 6a may be connected in parallel with the rotor windings and the windings 6b in series with the rotor windings. Thus the machine of the invention can also have the advantages of compound machines. One or the other form of field winding may be adopted with another modified embodiment which will now be described with reference to Figure 3.

On each side of the permanent magnets 10 grooves 12 are formed in the pole faces 7 of the pole shoes 4b. These grooves receive conductors 13 of an additional winding. The conductors 13 extend longitudinally of the machine in the grooves 12 over the entire length of the pole pieces 4, and they extend circumferentially between each shoe of a pole piece and the next adjacent shoe of the next pole piece. This winding is a compensating winding which is connected in such a manner that the entire armature current will flow through it. This construction will be readily understood and will not be described further. The conductors of this additional winding are arranged so that the current will flow through them in the opposite direction to the direction of the current in the rotor windings which are situated under the pole shoes. This additional compensating winding is designed to have a magnetising effect which cancels the demagnetising action of the transverse armature reaction.

By means of this modification it is possible, when desirable, to obtain better protection for the magnets 10 against demagnetisation due either to normal operation or to transient excess currents; in addition, the characteristic curves of the machine are more linear.

When the machine constructed in accordance with Figure 3 is used as a generator, overcompensation may be provided with the aid of the additional winding, which is particularly advantageous for wind generators.

The machine of the invention may also be used as a permanent magnet alternator. The commutator shown in Figure 1 is then replaced by collecting rings and the rotor is wound with a polarity equal to the number of poles of the stator.

WHAT WE CLAIM IS: 1. A rotary electrical machine operable as a generator and as a motor having at least two different torque-speed curves, comprising a wound rotor surrounded by a stator having pole pieces associated with windings and with permanent magnets, these pole pieces each having at least one cavity extending longitudinally and opening in the direction of the rotor, each cavity containing a permanent magnet having two opposite pole faces, wherein each cavity and each magnet has a rectangular cross-section the longer side of which is disposed radially relative to the rotor, and each permanent magnet is fixed in its cavity with a pole face close to the rotor, the other pole face being situated at the bottom of the cavity.

2. An electrical machine according to Claim 1, wherein the pole face which is close to the rotor, of each permanent magnet, is a plane face.

3. An electrical machine according to Claim 1, in which the pole pieces have pole shoes which extend circumferentially around the rotor with a pole face close to the rotor, and comprising an additional winding including conductors housed in grooves formed in the pole face of the pole shoes on each side of the permanent magnets, which conductors extend circumferentially between each pole shoe of a pole piece and the nearest pole shoe belonging to the adjacent pole piece, whereby the total rotor current of the motor passes through the whole of this additional winding in order to produce a magnetising effect cancelling the demagnetising effect of the transverse armature reaction.

4. An electrical machine according to Claim 1, wherein a winding surrounding the body of the pole pieces consists of two separate windings, one of which is connected in parallel with the rotor windings and the other in series with the rotor windings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. pole pieces 4. Moreover, the diameter is not increased because the magnets 10 are housed in the radial thickness of the pole pieces 4. In addition, since these pole pieces 4 extend along the entire length of the rotor 3 it is possible to use a rotor having just the required length, thereby reducing the starting inertia of the motor. Conversely, if a longer rotor is acceptable it is possible to use smaller windings 6. It has been found in practice that the motor of the invention has overall dimensions and starting inertia which are reduced by about 30% relative to a motor in which the windings and magnets constitute extensions in the longitudinal direction. The concentric mounting of the rotor and of the two stator parts, that is the permanent magnets 10 and the windings 6, is achieved without additional difficulties; in other words, it is no longer necessary to meet the difficult requirement of providing two stator parts which are separate in the longitudinal direction and which must be exactly coaxial, as is the case in the construction described in the patent mentioned above. Two modifications may be made to the machine just described. Instead of making each winding 6 in a single piece it is also possible to provide windings consisting of two parts 6a and 6b, as shown in chain-dotted lines in Figure 3, around each pole piece. The windings 6a may be connected in parallel with the rotor windings and the windings 6b in series with the rotor windings. Thus the machine of the invention can also have the advantages of compound machines. One or the other form of field winding may be adopted with another modified embodiment which will now be described with reference to Figure 3. On each side of the permanent magnets 10 grooves 12 are formed in the pole faces 7 of the pole shoes 4b. These grooves receive conductors 13 of an additional winding. The conductors 13 extend longitudinally of the machine in the grooves 12 over the entire length of the pole pieces 4, and they extend circumferentially between each shoe of a pole piece and the next adjacent shoe of the next pole piece. This winding is a compensating winding which is connected in such a manner that the entire armature current will flow through it. This construction will be readily understood and will not be described further. The conductors of this additional winding are arranged so that the current will flow through them in the opposite direction to the direction of the current in the rotor windings which are situated under the pole shoes. This additional compensating winding is designed to have a magnetising effect which cancels the demagnetising action of the transverse armature reaction. By means of this modification it is possible, when desirable, to obtain better protection for the magnets 10 against demagnetisation due either to normal operation or to transient excess currents; in addition, the characteristic curves of the machine are more linear. When the machine constructed in accordance with Figure 3 is used as a generator, overcompensation may be provided with the aid of the additional winding, which is particularly advantageous for wind generators. The machine of the invention may also be used as a permanent magnet alternator. The commutator shown in Figure 1 is then replaced by collecting rings and the rotor is wound with a polarity equal to the number of poles of the stator. WHAT WE CLAIM IS:

1. A rotary electrical machine operable as a generator and as a motor having at least two different torque-speed curves, comprising a wound rotor surrounded by a stator having pole pieces associated with windings and with permanent magnets, these pole pieces each having at least one cavity extending longitudinally and opening in the direction of the rotor, each cavity containing a permanent magnet having two opposite pole faces, wherein each cavity and each magnet has a rectangular cross-section the longer side of which is disposed radially relative to the rotor, and each permanent magnet is fixed in its cavity with a pole face close to the rotor, the other pole face being situated at the bottom of the cavity.

2. An electrical machine according to Claim 1, wherein the pole face which is close to the rotor, of each permanent magnet, is a plane face.

3. An electrical machine according to Claim 1, in which the pole pieces have pole shoes which extend circumferentially around the rotor with a pole face close to the rotor, and comprising an additional winding including conductors housed in grooves formed in the pole face of the pole shoes on each side of the permanent magnets, which conductors extend circumferentially between each pole shoe of a pole piece and the nearest pole shoe belonging to the adjacent pole piece, whereby the total rotor current of the motor passes through the whole of this additional winding in order to produce a magnetising effect cancelling the demagnetising effect of the transverse armature reaction.

4. An electrical machine according to Claim 1, wherein a winding surrounding the body of the pole pieces consists of two separate windings, one of which is connected in parallel with the rotor windings and the other in series with the rotor windings.

5. A rotary electrical machine, substan

tially as herein described with reference to Figures 1 and 2 or Figure 3 of the accom- panying drawings.