JPH03214859A - Image reader - Google Patents

Abstract

PURPOSE:To easily reduce speed variance peculiar to a stepping motor to efficiently suppress the speed variance by switching excitation of an exciting winding at intervals of the time to vary it in accordance with speed variance data read out of a storage device to rotate the motor. CONSTITUTION:When the output of a speed meter 75 rises, an arithmetic circuit 74 reads out speed variance data corresponding to the output of the speed meter 75 from a storage circuit 73 and the time interval of output pulses is controlled based on this output and read-out speed variance data. The variance of the pulse rate of this pulse interval has a phase approximately opposite to that of speed variance data read out of the storage circuit 75. Output pulses of the arithmetic circuit 74 are given to a distributing circuit 72, and a base signal to turn on/off power transistors 61 and 62 of an exciting circuit 71 in a prescribed order is outputted from the circuit 72. Consequently, a rotor 51 of a stepping motor 12 starts rotation, and the speed variance peculiar to the motor is reduced to efficiently suppress the speed variance.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) This invention reads a document placed on a document table, for example, and outputs the read image signal to an external device such as a computer. The present invention relates to an image reading device.

(Prior art) Conventionally, image reading devices can be controlled by digital signals from the CPU, and although they are open-lube systems, the speed can be easily adjusted, high-precision positioning is possible, and the direction of rotation can be changed freely. For this reason, stepping motors are used for scanning optical systems such as light sources and mirrors, and for moving optical systems such as lenses.

However, since the stepping motor is structured so that rotational force is obtained through continuous step motion, speed fluctuations, that is, rotational irregularities occur, and in even worse cases, large vibrations are caused. These speed fluctuations (rotation unevenness) and vibrations generated by the stepping motor appear as image blur and image unevenness in images read by the document reading device, adversely affecting reading accuracy and further causing vibrations.

In other words, a typical stepping motor is provided with a rotor having a plurality of small teeth at equal pitches on its outer circumferential surface, a stator core arranged on the outside of this rotor, and a number of stator protrusions corresponding to the number of phases on the inner surface of the stator core. A plurality of small teeth are provided at equal pitches on the surfaces of these salient poles facing the rotor, and an excitation winding is attached to the outer periphery of each stator salient pole. Then, the drive device sequentially switches and excites each excitation winding according to a predetermined sequence.
As a result, an attractive force or a repulsive force is applied to the rotor-side small teeth opposing the stator-side small teeth to perform a stepping operation.

By the way, in a stepping motor configured as described above, speed fluctuations occur due to stepwise movement of the rotor in the low speed range, and although there is no stepwise change in the high speed range due to a decrease in frequency response characteristics, the output torque of the excitation phase changes. Variations cause speed fluctuations. This speed fluctuation will be further explained using FIGS. 13 to 16. FIG. 13 shows the torque characteristics when a hybrid type (same polarity type) five-phase stepping motor is driven at low speed. This example is a case where the excitation sequence is repeated in 10 steps, and there is no variation in the torque characteristics between the excitation phases. Now, if the excitation is started from the initial state of stopping at point P, when the next step of excitation is performed from the stopping point, the torque curve indicated by 1 will switch to the 1 torque curve indicated by 2. At this moment, torque is generated, and the rotor advances by one step and stops at point Q.Furthermore, if the excitation is sequentially switched according to the drive sequence, the rotor moves continuously. As can be seen from this figure, the output torque fluctuates greatly as shown by the thick line. Therefore, speed fluctuations that match the excitation switching frequency occur in the rotor.

The above-mentioned example is an example in which there is only variation in output torque between five excitation phases, but in reality, there is always variation in output torque between each excitation phase. Therefore, as shown in FIG. 14, if there is variation in the torque curve from 1 to 0, a speed variation equal to the variation frequency of the variation will be superimposed.

On the other hand, when driven at high speed, the following phenomenon occurs.

That is, Fig. 15 shows the speed fluctuation when the above-mentioned hybrid five-phase stepping motor is switched to excitation at 150 IHz (this is called a pulse rate of 1500 ppS), and Fig. 16 shows the power spectrum at that time. As is clear from Fig. 16, the peak level of remarkable speed fluctuations similarly appears at 150Hz, which corresponds to 1/10 of the excitation frequency.Such speed fluctuations This is caused by variations in the output torque between the excitation phases.

In this way, when the stepping motor is operating at low speed, speed fluctuations occur for each step, and in the high-speed region, speed fluctuations occur with cycles equivalent to several steps due to variations in output torque between excitation phases. . Such speed fluctuations are a major drawback of stepping motors, and in particular, when used as a drive source for precision equipment, the stepping motors often fail to meet required performance due to speed fluctuations. The above explained the speed fluctuations caused by the stepping motor, but when it is incorporated into an actual system, speed fluctuations due to the load, such as mass imbalance, may also occur.
These superimposed velocity fluctuations occur in the entire system.

Therefore, in order to reduce speed fluctuations (uneven rotation) and vibrations that adversely affect the image quality of reading, conventional methods have been to attach a dynamic damper to the rotor shaft or load shaft of a stepping motor, or to install a dynamic damper in the middle of the torque transmission mechanism. Techniques used include inserting damping material to suppress rotation, or using large flyholes to smooth rotation. In addition, as an electrical method, one step is divided into several to several dozen steps by increasing and decreasing the applied current to different excitation phases in stages and shifting the excitation stop point of the rotor in stages. Attempts have also been made to reduce speed fluctuations (unevenness) and vibrations using a so-called microstep drive system.

However, the method of reducing speed fluctuations (rotational unevenness) and vibrations of the stepping motor by providing external additional elements not only increases the size and weight of the apparatus, but also has the problem that it is not possible to expect a significant improvement in image quality. Furthermore, even if a microstep drive method is adopted for the stepping motor, if there are variations in torque characteristics between different excitation phases, it is not possible to expect an improvement in reading accuracy.

Therefore, it is not possible to easily and effectively reduce the speed fluctuations of the stepping motor, that is, uneven rolling and vibration, and it is not possible to improve the reading accuracy. There is a drawback.

Therefore, as a means for reducing the speed fluctuations inherent in the stepping motor, variable pulse control has been proposed in which the time interval at which excitation of the stepping motor is switched is periodically varied in response to the fluctuations.

However, in order to drive a stepping motor at a variable pulse rate in an image reading device, the speed fluctuation specific to the stepping motor must be measured in advance.
When installing a stepping motor into a device, unique speed fluctuation data is written into memory and used. Therefore, there is a problem that the stepping motor, the timing belt of the transmission source, etc. must be replaced, or the speed may fluctuate due to aging.

(Problems to be Solved by the Invention) As described above, in conventional image reading devices, when reducing the speed fluctuation inherent to the stepping motor without requiring external additional elements, it is difficult to reduce the There was an inconvenience that the timing belt etc. had to be replaced and the speed fluctuated due to aging.

Therefore, with this invention, speed fluctuations inherent to the stepping motor can be easily reduced without requiring external additional elements, and speed fluctuations that occur due to aging can be easily reduced by replacing the stepping motor or the timing belt of the transmission source, etc. The purpose of the present invention is to provide an image reading device that can suppress this phenomenon.

[Structure of the Invention] (Means for Solving the Problems) An image reading device of the present invention includes an optical system that guides light from an object on a reading surface irradiated with light, and a light guided by the optical system. A stepping motor having a photoelectric conversion means for photoelectric conversion, a scanning means for scanning the optical system, and a plurality of excitation windings for driving the scanning means, and a stepping motor provided on the reading surface for measuring speed fluctuations of the stepping motor. A measurement chart, the light from the measurement chart obtained by driving the scanning means by rotation with the stepping motor is photoelectrically converted by the photoelectric conversion means, and the speed of the stepping motor obtained by the photoelectrically converted output. detection means for detecting speed fluctuation data corresponding to the fluctuation;
A storage means for storing the speed fluctuation data detected by the detection means, and a plurality of excitation windings provided in the stepping motor are switched and excited in a predetermined order, and the speed fluctuation data read from the storage means is stored. The stepping motor is constituted by a rotating means for rotating the stepping motor by correspondingly varying the time interval at which excitation of the excitation winding is switched.

(Function) In the present invention, light from an object on a reading surface irradiated with light is guided by an optical system, the light guided by this optical system is photoelectrically converted by a photoelectric conversion means, and the optical system is scanned. scanning means, driving the scanning means with a stepping motor having a plurality of excitation windings, and providing a measurement chart for measuring speed fluctuations of the stepping motor on the reading surface;
The light from the measurement chart obtained by driving the scanning means with the stepping motor is photoelectrically converted by the photoelectric conversion means, and the photoelectrically converted output is obtained at a speed corresponding to the speed fluctuation of the stepping motor. Fluctuation data is detected, the detected speed fluctuation data is stored in a storage means, a plurality of excitation windings provided in the stepping motor are switched and excited in a predetermined order, and the speed read out from the storage means is The stepping motor is rotated by varying the excitation of the excitation winding with a time interval for switching in response to variation data.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 2 shows a scanner as an image reading apparatus of the present invention, which reads a document and outputs the read image signal to an external device such as a computer.

That is, 1 is a scanner body, and an operation panel (not shown) is provided at the upper front part of this body 1. A document mounting table (platen glass) 2 made of transparent glass is fixed to the upper surface of the main body 1 . A document cover 1a that can be opened and closed is provided near the document table 2. As shown in FIG. The original O placed on the original table 2 is
An optical system 3 consisting of an exposure lamp 4 and mirrors 5, 6, and 7 reciprocates along the lower surface of the document table 2 in the direction of arrow a, thereby performing exposure scanning during the reciprocation. In this case, mirrors 6 and 7 move at half the speed of mirror 5 so as to maintain the optical exposure length. The light reflected from the original by the scanning of the optical system 3, that is, the light reflected from the original O by the light irradiation of the exposure lamp 4, is reflected by the mirrors 5, 6, and 7, passes through the variable magnification lens block 8, and is arranged in a column. The document O is guided to a line sensor 9 having a plurality of light receiving elements (CCD) arranged in a shape, and an image of the document O is formed on the surface of the line sensor 9.

FIG. 3 shows a drive mechanism for moving the optical system 3 back and forth. That is, the mirror 5 and the exposure lamp 4
is supported by the first carriage lla, and the mirrors 6 and 7 are supported by the second carriage 1lb, respectively, and these 11th and 11th second carriages lla and 1lb are guided by guide rails (not shown) and can freely move in parallel in the direction of arrow a. ing. That is, the stepping motor 12 drives the fixed shafts 15 of the pulleys 14, 14 via the transmission mechanism 13. This pulley 1
The rotations of 4 and 14 are transmitted to the carriages lla and llb via transmission belts 16 and 16, respectively. The transmission belts 16, 16 each have one end 16
A is fixed to the scanner body 1, and a pulley 17 is rotatably provided from one end 16A to the second carriage 1lb.
, the pulley 14, the idler pulley 18, and the second carriage llb via a pulley 19 rotatably provided, and the other end 16B is connected to the main body via a spring 20 to which a constant tension is applied. It is fixed at 1. A first carriage ll that supports the mirror 5 between the middle part 16a of the transmission belt 16 and the boley 17 and the boley 14
One end of a is fixed. Therefore, when the stepping motor 12 rotates, the rotation is transferred to the transmission belt 16 via the transmission mechanism 13, the fixed shaft 15, and the pulley 14.
, the transmission belt 16 moves, the first carriage lla moves, and along with that, the second carriage lla moves.
b also moves. At this time, since the bogies 17 and 19 serve as movable pulleys, the second carriage 11b moves in the same direction at 1/2 the speed of the first carriage lla.

Note that the moving directions of the first and second carriages lla and llb are controlled by switching the rotational direction of the stepping motor 12.

Further, the first carriage lla is moved to a predetermined position (home position according to the magnification) by driving the stepping motor 12 according to the size of the original O and the reading magnification. Then, when the start of reading is instructed, the first carriage lla first moves to the second carriage lla.
It is moved in the direction of the carriage 1lb, and then the exposure lamp 4 is turned on and moved in the direction away from the second carriage 1lb. When scanning of original O is completed, exposure lamp 4
is turned off, and the first carriage lla is returned to the home position.

Further, a detector 21 for detecting movement of the first carriage lla to the home position;
Detector 22 that detects movement of a to the limit position
, 22 are provided.

Further, a measurement chart 10 is provided on the home position side of the document table 2. This measurement chart 10 is
As shown in FIG. 4(a), the first and second carriages ll
Bars 1 are placed at equal intervals in the direction that intersects the conveyance direction of a and llb.
This is a pattern in which 0a1... is given.

FIG. 1 schematically shows the overall control system.

That is, the CPU 30 as a control unit that controls the entire system controls the internal bus 31 and each input/output unit 32ax 32b, 3.
2c, 32ds 32es 32f via the interface circuit 33, image processing circuit 34, inverter circuit with dimming 35, motor drive circuit 36, and the above detector group 21, 22
, 22, and an operation panel 37 having a display section.

The image processing circuit 34 performs image processing on the read signal supplied from the line sensor 9 via the A/D conversion circuit 38. The image signal subjected to this image processing is outputted as a reading result to the external device 40 via the interface circuit 33. The light control inverter circuit 35 detects the amount of light from the exposure lamp 4 on the detector 3.
The light is adjusted based on the detection output of 5a. The motor drive circuit 36 rotates the stepping motor 12 at a plurality of speeds corresponding to the reading magnification. The operation panel 37 displays the input force of a plurality of instructions or a plurality of states.

The line sensor 9 is driven and controlled by a CCD driver 39.

Next, the configuration of the stepping motor 12 and its motor drive circuit 36 will be explained using FIGS. 5 to 9. That is, the stepping motor 12 is a hybrid type (same polarity type) five-phase stepping motor,
It is composed of a rotor 51 and a stator 52. The rotor 51 is connected to a mass-balanced rotating load (not shown).

As shown in FIG. 5, the rotor 51 includes a shaft 53 made of a non-magnetic material, a permanent magnet 54 attached to the outer periphery of the shaft 53 and magnetized in the axial direction, and a permanent magnet 544'.
) A toothed cup 55a155b made of a magnetic material and attached in a cap shape from both ends. In this example, 50 small teeth 56 are formed at equal pitches in the circumferential direction on the toothed cup 55a155b.

The small teeth 56 on the gear cup 55a side and the small teeth 56 on the gear cup 55b side are provided with a phase difference of 172 pitches in the circumferential direction.

On the other hand, the stator 52 is connected to the rotor 5 as shown in FIG.
1, ten salient stator poles 58 protruding from the inner surface of the stator core 57, and small teeth 59 provided at equal pitches at the tips of these salient stator poles 58. , and an excitation winding 60 wound around the stator salient poles 50. Each excitation winding 60 is attached to the opposing stator salient poles 58 and connected in series or in parallel, thereby forming a five-phase configuration divided into five excitation phases ASBSCSD and E. .

As shown in FIG. 7, the motor drive circuit 36 is broadly divided into an excitation circuit 71, a distribution circuit 72, and a storage circuit 73.
, an arithmetic circuit 74 , and a speed setting device 75 .

As shown in FIG. 8, the excitation circuit 71 connects both ends of the excitation winding constituting each excitation phase to power transistors 61.
, 62 to the power supply lines 63 and 64, and by sequentially turning on and off the power transistors located diagonally, a positive or negative current flows through the excitation winding of each phase, and the stator 52 and It is configured to generate a magnetic field between it and the rotor 51.

The distribution circuit 72 connects the power transistors 6 to the excitation windings of each phase in accordance with the output pulses of the arithmetic circuit 74, as shown in a typical excitation sequence shown in FIG.
1 and 62 are configured to output a base signal that controls on/off.

In this example, 10 steps are repeated.

In this example, the storage circuit 73 stores the inherent speed fluctuations of the optical system 3, which is the driving target. That is, in this motor drive circuit 36, speed fluctuations in a 10-step range are investigated, and a one-cycle sinusoidal fluctuation wave (speed fluctuation data) is simulated, taking into account the speed fluctuation amplitude due to this speed fluctuation. This speed fluctuation data is stored in the storage circuit 73 in advance.

In this case, if) a signal from the line sensor 9 scanning the I constant chart 10 is outputted to the speed fluctuation detection circuit 76 via the A/D conversion circuit 38. As a result, the speed fluctuation detection circuit 76 detects speed fluctuation due to the shift in read timing for each bar 10a, . . . and outputs correction data for canceling the detected speed fluctuation to the storage circuit 73 as speed fluctuation data. . As a result, the storage circuit 73 stores in advance speed fluctuation data that simulates a sinusoidal fluctuation wave for one period in which the speed fluctuation amplitude is taken into consideration.

Normally, before starting to scan the original, use this measurement chart 1.
0 is automatically read.

Fig. 4(b) and (c) show the measurement chart 10 of Fig. 4(a).
An example of read data is shown below. FIG. 6B shows data when there is no speed variation in the optical system 3, and all data intervals Δt are equal. Also, the same figure (c)
shows data when there is a speed fluctuation in the optical system 3. This data interval Δt is detected by the speed fluctuation detection circuit 76, and correction data for canceling this speed fluctuation is written into the storage circuit 73 as speed fluctuation data. The measurement chart may be read each time a document is read, or the tp1 constant chart 10 may be read only during a briscan, which is performed for the purpose of detecting a document, for example.

When the arithmetic circuit 74 receives a speed setting signal from the speed setter 75 (pulse generating means), it reads speed fluctuation data from the storage circuit 73 and outputs the output to be supplied to the distribution circuit 72 based on the set speed and the speed fluctuation data. Controls the pulse interval (step interval).

At this time, taking into account the phase of the speed fluctuation data read out from the memory circuit 73, the pulses are adjusted so that the fluctuation of the pulse rate defined by the reciprocal of the pulse time interval has almost the opposite phase with respect to the speed eating. Controlling the spacing.

With such a configuration, when a certain speed is set by the speed setter 75, the output of the speed setter 75 increases at a constant slope to a level corresponding to the set speed. Speed setting device 7
When the output of speed setting device 7 rises, the arithmetic circuit 74
The speed fluctuation data corresponding to the output of No. 5 is read out from the storage circuit 73, and the time interval of the output pulse is controlled based on the above output and the read speed fluctuation data. This variation in the pulse rate of the pulse interval is almost in opposite phase to the speed variation data read out from the storage circuit 73. The output pulse of the arithmetic circuit 74 is given to the distribution circuit 72, and from this distribution circuit 72, a base signal is sent to turn on and off the power transistors 61 and 62 of the excitation circuit 71 in a predetermined order as shown in FIG. is output.

Therefore, the rotor 51 of the stepping motor 12 starts rotating.

In this case, since the excitation timing of each excitation phase ASB and CSDSE is controlled taking into account the inherent speed fluctuations of the optical system 3, the inherent speed fluctuations can be canceled out and smooth rotation can be achieved. I can do it.

FIG. 10(a) shows excitation switching timing when the stepping motor 12 is driven to rotate at a constant speed. In accordance with this switching timing, the excitation of the excitation windings constituting each phase is switched. In this motor drive circuit 36, the time interval of excitation switching timing is set to Δt with reference to Δt.
It is varied between i+In and Δt+xax. The reciprocal of this time interval is the pulse rate, and this pulse rate varies sinusoidally with a period T as shown in FIG. 10(b).

If there is no inherent speed fluctuation in the optical system 3, the stepping motor 12 rotates with speed fluctuations at a period T as shown in FIG. 10(b). However, normal stepping motors always have unique speed fluctuations in each speed range. In this motor drive circuit 36, the period T is made to match the speed fluctuation period of the optical system 3, and the time interval of the excitation switching timing, that is, the pulse rate is varied so that the period is substantially opposite to the phase. Therefore, assuming that the amplitude is set appropriately, the speed fluctuations inherent to the stepping motor can be canceled out by the speed fluctuations accompanying the fluctuations in the pulse rate, and the rotor 51 can be rotated smoothly.

FIG. 11 shows an example in which the motor drive circuit 36 is used to reduce speed fluctuations of the hybrid five-phase stepping motor 12. This uses the motor previously explained using FIG. 18, under the same driving conditions (the pulse rate of the output pulse of the speed setting device 75 is 1500 pps, the reading magnification is 100%), and the time interval of the excitation switching timing is varied. In this example, the frequency is set to 150 Hz, and control is performed so that the above-mentioned fluctuation phase is in opposite phase to the speed fluctuation unique to the stepping motor. The power spectrum at this time was as shown in FIG. From this figure, it can be seen that the peak level at 150 Hz, which is the target for speed fluctuation reduction, has decreased dramatically compared to the case of FIG. 18, and the speed waveform has become smoother.

Further, the present invention is not limited to the shape or number of phases as long as it is a stepping motor. In addition, although the above-mentioned embodiment describes application to the case where the speed fluctuation specific to the stepping motor has a single frequency component, even if it has multiple frequency components, the frequency is higher than the stepping motor drive frequency. The present invention can be applied within a small range.

As mentioned above, each time an image is read, the inherent speed fluctuation of the optical system is automatically measured using the signal obtained from the reading data of the ap1 constant chart from the line sensor, and the system is adapted to the measured speed fluctuation. The speed fluctuation of the optical system is controlled by controlling the stepping motor to have a phase opposite to the speed fluctuation based on the speed fluctuation data. This makes it possible to replace the stepping motor and timing belt, and suppress speed fluctuations that occur due to aging, improve the reading accuracy of the image reading device used to move the optical system, and improve the reading accuracy of the image reading device used to move the optical system. Even when the magnification changes and the rotational speed of the stepping motor changes, reading accuracy can be improved. Furthermore, it is not necessary to provide speed variation data for each speed, and the storage capacity of the storage circuit can be reduced.

[Effects of the Invention] As described above, according to the present invention, speed fluctuations inherent in a stepping motor can be easily reduced without the need for external additional elements, and the timing belt of the stepping motor or transmission source can be easily reduced. It is possible to provide an image reading device that can be replaced or that can suppress speed fluctuations caused by aging.

[Brief explanation of drawings]

1 to 12 show an embodiment of the present invention, and FIG. 1 is a block diagram showing a schematic configuration of an image reading device;
FIG. 2 is a sectional view showing the structure of the image reading device, FIG. 3 is a perspective view showing the internal structure of the image reading device, FIG. 4(a) is a view showing the structure of the measurement chart, and FIG. 4(b) (c) is a diagram showing the read data of the measurement chart, Figure 5 is a longitudinal cross-sectional view of the rotor in the stepping motor, Figure 6 is a plan view of the stator incorporating the rotor in the stepping motor, and Figure 7 is the stepping motor. Figure 8 is a diagram showing the configuration of the excitation circuit in the motor drive circuit, Figure 9 is a diagram showing the operation sequence of the distribution circuit in the motor drive circuit, and Figure 10 (a) is the motor drive circuit. Diagram 1 showing an example of the pulse output of the arithmetic circuit in the circuit armor.
Figure 0 (b) is a diagram showing an example in which the same pulse output is converted to a pulse rate, Figure 11 is a diagram showing speed fluctuation results in an example in which the speed range during equal-magnification reading is controlled by a motor drive circuit, and Figure 12. is a diagram showing the power spectrum under the conditions shown in FIG. 11, FIG. 13 is a diagram showing the torque output characteristics when there is no variation in torque output between each excitation phase in a hybrid five-phase stepping motor, and FIG. Figure 15 shows the torque output characteristics when there is variation in torque output between each excitation phase in a hybrid 5-phase stepping motor. Figure 16 shows the speed fluctuation when
It is a figure which shows the power spectrum under the conditions shown in the figure. 1...Main body, 3...Optical system, 4...Exposure lamp,
5, 6, 7...Mirror 9...Line sensor, 10
...II1 Constant chart, 10a, ~... Bar 1
1a...first carriage, llb...second carriage, 12...stepping motor, 30...CPU
, 36... Motor drive circuit, 38... A/D converter, 51... Rotor, 52... Stator, 71...
Excitation circuit, 72... Distribution circuit, 73... Memory circuit,
74... Arithmetic circuit, 75... Speed setting device, 76...
・Speed fluctuation detection circuit.

Claims (1)

[Scope of Claims] An optical system that guides light from an object on a reading surface irradiated with light, a photoelectric conversion means that photoelectrically converts the light guided by this optical system, and a scanning device that scans the optical system. means, a stepping motor having a plurality of excitation windings for driving the scanning means, a measurement chart provided on the reading surface for measuring speed fluctuations of the stepping motor, and driving the scanning means with the stepping motor. a detection means for photoelectrically converting the light from the measurement chart by the photoelectric conversion means and detecting speed fluctuation data corresponding to speed fluctuations of the stepping motor obtained from the photoelectrically converted output; A storage means for storing the speed fluctuation data detected by the detection means; An image reading device comprising: rotating means for rotating the stepping motor by correspondingly varying the time interval at which excitation of the excitation winding is switched.