Title: Electronic device for controlling a synchronous motor with permanent-magnet rotor
Abstract: Electronic device for controlling a synchronous motor with permanent-magnet rotor, comprising an alternating voltage current source at mains frequency connected in series with said synchronous motor (
Patent Number: 6,885,161 Issued on 04/26/2005 to de Nanclares, et al.
| Inventors:
|
de Nanclares; Eduardo Beltrán (Mondragon, ES);
Mitxelena Alzuri; José M. (Mondragon, ES)
|
| Assignee:
|
Fagor, S. Coop. (Mondragon, ES)
|
| Appl. No.:
|
461762 |
| Filed:
|
June 12, 2003 |
Foreign Application Priority Data
| Jun 18, 2002[ES] | 200201408 |
| Current U.S. Class: |
318/254; 318/430; 318/721 |
| Intern'l Class: |
H02P 006//20 |
| Field of Search: |
318/138,254,430,431,437,439,720-722,724
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Ro; Bentsu
Attorney, Agent or Firm: The Kline Law Firm
Claims
1. An electronic device for controlling a synchronous motor with a permanent-magnet
rotor comprising:
an alternating voltage current source at mains frequency connected in series
with said synchronous motor;
at least one static switch connected in series with said synchronous motor; and
an electronic circuit acting on said static switch, wherein said electronic circuit
calculates the firing instants for the static switch for each half-cycle, taking
as reference the zero-crossing of the mains voltage and applying a firing time
starting from said reference, said firing time being the firing time of the previous
half-cycle corrected according to the position of the rotor with respect to the
current flowing through the motor in the previous half-cycle.
2. An electronic device according to claim 1, wherein the electronic circuit
determines the direction of rotation of the rotor according to the polarity of
the mains voltage and the position of the rotor, firing the static switch, at the
firing instants calculated, provided that the values of the mains voltage and the
position of rotor are such as to help rotation in the direction chosen.
3. An electronic device according to claim 2, wherein the electronic circuit
calculates the firing time by correcting the firing time of the previous half-cycle
according to the phase shift produced between the current flowing through the motor
and the position of the rotor of the motor in the previous half-cycle.
4. An electronic device according to claim 3, wherein the electronic circuit
determines the firing time at a given instant i calculated by the following formula:
##EQU5##
where tr
x is the phase shift between the current in the static switch
and the position of the rotor in a given half-cycle x, D is a predetermined constant
phase shift and k is a given constant value, so that the firing time at a given
instant i+1 is the firing time of the previous half-cycle i corrected according
to the following formula:
##EQU6##
5. An electronic device according to claim 4, wherein the electronic circuit
calculates the phase shifts produced between the current flowing through motor
and the position of the rotor of the motor, measuring the phase shifts between
the voltage in the static switch and the signal from a position sensor adapted
to detect the position of the rotor of the motor.
6. An electronic device according to claim 5, wherein the position sensor is
placed at the central point of the free space between the poles of the stator of
the motor.
7. An electronic device according to claim 2, wherein the electronic circuit
calculates the firing time by correcting the firing time of the previous half-cycle
according to the phase shift produced between the position of the rotor of the
motor and the mains voltage in the previous half-cycle.
8. An electronic device according to claim 7, wherein the electronic circuit
calculates the firing time in a given instant i using the following formula:
##EQU7##
where tr
x is the phase shift between the position of the rotor and
the mains voltage in a given half-cycle x, D is a predetermined constant phase
shift, and k is a given constant value, so that the firing time at a given instant
i+1 is the firing time of the previous half-cycle i corrected according to the
following formula:
##EQU8##
9. An electronic device according to claim 8, wherein the position of rotor of
the motor is measured from the signal from a position sensor.
10. An electronic device according to claim 9, wherein the position sensor is
placed at the central point of the free space between the poles of the stator of
the motor.
11. An electronic device according to claim 1, wherein, on initiating startup
of the motor, the electronic circuit fires the static switch during several cycles,
without control, to cause the rotor to move from its rest position, and then begins
to calculate the firing instants based on the firing times, and to fire the static
switch depending on said firing times.
12. An electronic device according to claim 11, wherein if, following initiation
of the startup, the time during which the rotor polarity remains the same exceeds
a predetermined value, said startup is initiated once more.
13. An electronic device according to claim 1, wherein, when the time taken by
the rotor of the motor to rotate a half turn is less than the time of one half-cycle
of the mains voltage, a delay is applied to the firing time.
14. An electronic device according to claim 13, wherein said delay equals the
difference between the time of one half-cycle of the mains voltage and the time
taken by the rotor of the motor to rotate a half turn.
Description
TECHNICAL FIELD
The present invention relates to the control of synchronous motors with a permanent-magnet rotor.
PRIOR ART
Synchronous motors with permanent-magnet rotors fed from the mains voltage
are known. Applying said voltage to the motor generates a magnetic field between
the poles of the stator of said motor, which turns the rotor of the motor over.
For example, in the case of a motor with two stator poles and two rotor poles,
when the rotor reaches synchronisation speed, i.e. in a permanent regime, the angular
positions of the rotor, from 0° to 180° (a half turn) coincide with a
given polarity of the current (current half-cycle), and the angle positions from
180° to 360° (the remaining half turn) coincide with the other current
polarity (the following half-cycle of the current). In practice, phase shifts occur
between the position of the rotor and the current signal, so that part of the current,
rather than helping to drive the rotor, produces a braking effect which is greater
the lesser the load. This causes unnecessary overheating of the motor, undesired
vibrations and noise.
On the other hand, starting is a problem with this type of motor. EP 0 574 823
B1 discloses an electronic device for starting synchronous motors which comprises
a switch in series with the synchronous motor, an alternating voltage current source
also connected in series with said synchronous motor, and an electronic circuit
acting on said switch. The electronic circuit operates on the switch when the current
flowing through the stator is approximately zero and depending on the polarity
of the alternating voltage and the position of the rotor. In this way, during startup
the current is made to flow only when said current helps the rotation in a given
direction until the rotor reaches the synchronisation speed. Once synchronisation
speed is reached, the rotor can turn without involving any type of current control.
The device described in EP 0 574 823 B1 does not solve the problem referring to
the braking effect because of the phase shift between the rotor position and the current.
EP 0 682 404 B1 describes an electronic device for starting and controlling a
synchronous motor with a permanent-magnet rotor comprising an alternating voltage
current source at mains frequency connected in series with the synchronous motor,
a static switch connected in series with said synchronous motor, and an electronic
circuit which acts on said static switch. Said electronic circuit acts on the switch
depending on the position of the rotor and the voltage applied to said switch.
The possibility is contemplated of modifying the synchronisation speed with the
introduction of delay periods in the switch firing times. No form of control is
contemplated to minimise the phase shifts produced between the rotor position and
the current.
DESCRIPTION OF THE INVENTION
The main object of the invention is to provide an electronic device to control
a synchronous motor that is simple and economical, and that makes it possible to
reduce the current used, thereby reducing motor heating and also consumption.
Another object of the invention is to provide an electronic device that also
makes it possible to reduce the vibration of the motor under no load, with the
resulting reduction of noise and consequent prolongation of the life of the motor.
The electronic device of the invention comprises an alternating voltage current
source at mains frequency connected in series with said synchronous motor, at least
one static switch connected in series with said synchronous motor, and an electronic
circuit which acts on said static switch. The electronic circuit determines the
timing of the firing of the static switch, taking as reference the zero-crossing
of the mains voltage and applying a firing time starting from said reference, said
firing time being obtained according to the position of the rotor of the motor
in the previous half-cycles. In this way, it is possible to minimise the braking
produced by the phase shift between the rotor position and the current flowing
thought the motor stator, so reducing heating and consumption.
The mains voltage is taken as reference signal because it is very stable, not
subject to variations caused by the actual operation of the device, as happens
for example with the voltage in the switch.
By means of the control according to the rotor position in previous half-cycles,
the operation of the motor is made stable. That would not be possible if the firing
time was calculated just as a function of the position of the rotor in the previous
half-cycle since, to do that, it would also be necessary to know the speed of the rotor.
In the electronic device of the invention, the electronic circuit can choose
the
direction of turn of the rotor. To do this, said electronic circuit takes account
of the values of the polarity of the mains voltage and the rotor position, so as
to fire the static switch at the firing times calculated, provided that these values
are such as to help rotation in the direction chosen.
Correction of the phase shift between the position of the rotor and the
current at a given time is therefore done by obtaining the firing time of each
half-cycle according to the phase shift that existed between the current and the
rotor position in the previous half-cycles. One way to obtain said firing time
is to calculate the phase shifts arising between the current flowing through said
motor and the position of the rotor of the motor. Rather than directly measuring
the current, the voltage in the switch can be measured. In this solution, the electronic
circuit needs three data: the mains voltage, the current value (or else the voltage
in the switch) and the position of the rotor.
On the other hand, it has been proved that there is a correlation between the
relative position of the rotor with respect to the current signal and the relative
position of the rotor with respect to the mains voltage signal. Therefore, a second
way of obtaining the firing time is to calculate phase shifts arising between the
position of the rotor of the motor and the mains voltage. This second solution
has the additional advantage that the electronic circuit needs only two data: the
position of the rotor and the mains voltage.
In various load situations, particularly unloaded, it may happen that, at a given
moment, the time the rotor of the motor takes to make a half turn is less than
the mains voltage half-cycle. In these circumstances, a delay is applied to the
previously calculated firing time. As a result, the rotor does not reach a speed
in excess of that required, so that the current cycles are far more even, with
a considerable reduction in vibration and noise.
These and other objects of the invention will be made clearer in the light
of the figures and the detailed disclosure of the invention.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a synchronous motor with a permanent-magnet rotor.
FIG. 2 is a block diagram of an embodiment for the control device of the invention.
FIG. 3 shows the variations in time of the mains voltage, the signal from the
rotor position sensor and the voltage in the switch in a first embodiment of the invention.
FIG. 4 shows the variations in time of the mains voltage, the signal from the
rotor position sensor and the voltage in the switch in a second embodiment of the invention.
DETAILED DISCLOSURE OF THE INVENTION
With reference to FIG. 1, the electronic device of the invention is applied
to a permanent-magnet synchronous motor
1 with a rotor
1A and a stator
1B. Said motor
1 has two rotor poles and two stator poles, and is
shown purely by way of an example.
With reference to FIG. 2, the electronic device for controlling said synchronous
motor
1 comprises:
- an alternating voltage source at mains frequency providing a voltage
Vr, connected in series with said synchronous motor 1,
- a static switch 2 connected in series with said synchronous motor
1 and comprising for example a triac, and
- an electronic circuit 3 which acts on said static switch 2.
Said electronic circuit
3 determines the timing for firing the static
switch taking as reference the zero-crossing of the mains voltage Vr and applying
a firing time Td starting from said reference, said firing time Td being obtained
as a function of the position of the rotor
1A of the motor
1 in the
previous half-cycles.
The electronic circuit
3 can choose the direction of rotation of the rotor
1A of the motor
1. To do that, said electronic circuit
3 takes
account of the polarity values of the mains voltage Vr and the position of the
rotor
1A, so that the static switch
2 fires at the firing times calculated,
provided that said values are such as to contribute to the rotation in the direction chosen.
For example, with reference to FIG. 3, in the case of motor
1 in FIG.
1, taking account of the mains voltage Vr and a signal
6 representing the
position of the rotor
1A supplied by a position sensor
4, if it is
wished for the rotor
1A to turn to the left (anticlockwise), switch
2
receives a firing signal when:
Or else when:
Should either of these combinations arise, switch
2 will receive a
firing signal once the corresponding time Td has passed after zero-crossing of
the mains voltage Vr.
However, if it is wished for the rotor
1A to turn right (clockwise),
switch
2 will receive a firing signal when:
Or else when:
(
Vr=Negative) and (Signal
6=-1)
In a first embodiment of the invention, the electronic circuit
3 calculates
the firing time Td according to the phase shifts between the current flowing through
the motor
1 and the position of the rotor
1A in the previous half-cycles.
The electronic circuit
3 calculates the phase shifts produced between the
current flowing through the motor
1 and the rotor
1A measuring the
phase shifts between the voltage in the static switch
2 received along line
7, and the signal
6 from a position sensor
4 adapted to detect
the position of the rotor
1A of the motor
1 (e.g. a Hall effect sensor).
It would in principle be ideal for the phase shift between the current signal
and the position of the rotor to be zero, since that would imply that the braking
effect would also be zero, so obtaining the lowest consumption level. However,
in that situation the motor is deprived of reaction capacity, and a shift of load
may lead to a loss of synchronisation speed. Therefore, in practice, the control
consists of maintaining a phase shift that, as a maximum, equals a predetermined
constant value D. In this way, if the phase shift is greater than said value D,
said phase shift will tend to approach value D. No action is implemented on the
phase shift when it is less than said value D.
Therefore, for each half-cycle, the electronic circuit
3 calculates
the phase shift between the Vt voltage in the static switch
2 (considering
the moment at which said voltage Vt appears), and the signal
6 of the position
sensor
4, then subtracting from said phase shift the constant phase shift
D. The sum of the subtractions obtained is divided by a whole constant value k.
With reference to FIG. 3, for each half-cycle, the phase shift tr between the current
signal (represented by the letter I in FIGS. 3 and 4) and signal
6 indicating
the position of the rotor
1A will be as follows:
And the firing time Td for a given half-cycle i is calculated as follows:
##EQU1##
It is seen that the sum of all the subtractions is divided by a value k. The
value
of k is constant, and is sufficiently large for the dynamics to be slow enough
to ensure that operation is stable.
If for firing time Td a value is obtained of less than zero, it is considered
that Td=0.
It is also possible to represent the firing time Td for a given half-cycle depending
on the firing time in the previous half-cycle, corrected according to the phase
shifts in the remaining half-cycles:
##EQU2##
In this configuration, the electronic circuit
3 acts on the switch
2
according to three signals: the mains voltage Vr received along line
5,
the signal
6 from the position sensor
4, and the voltage Vt in the
switch
2, received along line
7.
Because of the correlation between the phase shift between the position of
the rotor
1A and the current signal I, and the phase shift between the position
of the rotor
1A and the mains voltage Vr, in a second embodiment of the
invention, the electronic circuit
3 calculates the firing time Td as a function
of the phase shifts produced between the position of the rotor
1A of the
motor
1 and the mains voltage Vr in the previous half-cycles. In this case,
the position of the rotor
1A of the motor
1 is also measured from
the signal
6 from a position sensor
4.
In this second embodiment, a constant phase shift D is also considered. In this
case, the control consists of maintaining as a minimum a phase shift of D. In this
way, if the phase shift is less than said value D, said phase shift will tend to
approach value D, with no action being taken on the phase shift when it is greater
than said value D.
With reference to FIG. 4, the phase shift tr between the signal
6 giving
the position of the rotor
1A and the mains voltage Vr will, for a half-cycle
i, be as follows:
And the firing time Td for a given half-cycle i is calculated, as for the first
embodiment, as follows:
##EQU3##
We can also represent firing time Td for a given half-cycle according to the
firing
time in the previous half-cycle, corrected according to the phase shifts in the
remaining half-cycles:
##EQU4##
In this configuration, the electronic circuit
3 acts on the switch
2
in accordance with two signals: the mains voltage Vr and signal
6 from the
position sensor
4, voltage Vt at switch
2 not being necessary.
In both embodiments, the reaction capacity of the motor
1 depends on the
values of D and k selected. Thus, for example, for the first embodiment, with a
given value of k calculated so that the system is stable, a higher D value implies
enhanced reaction capacity and less power saving, while a lower value for D implies
on the contrary less reaction capacity and greater power saving. For the second
embodiment just the contrary occurs.
In practice, the idea is to reach a compromise when calculating the k and D values.
Simple tests can be used to determine optimal k and D values which depend, among
other factors, on the characteristics of the motor
1. The constant k can,
for example, have a value of the order of 100, and the value of D may, for a k
value of said order, fall between 0 and 2.5 ms.
During the operation of motor
1 under no or very little load, current
cycles greater than normal appear with a certain periodicity. The reason for said
greater current cycles is the advance of the rotor
1A, which causes the
phase of the counterelectromotive force to advance with respect to the mains voltage
Vr, so that the effective voltage applied to the stator
1B increases substantially,
producing such elevated current cycles and, consequently, powerful braking, which
causes vibrations. With the device of the invention, this is solved by introducing
a compensation algorithm by which a delay δ is applied to the firing time
Td when the time taken by the rotor
1A to rotate through half a turn, trot,
is less than the time Tred of a half-cycle of the mains voltage Vr. Said delay
δ equals the difference between time Tred and time trot. Therefore:
With the inclusion of said delay δ, the current cycles are much more regular
under no load or minimum load, and vibration is reduced considerably. Increased
stability due to the introduction of the compensation makes it possible to reduce
the value of k by a factor of 10, thereby significantly improving the reaction
capacity of the motor
1, so that lower D phase shifts with good reaction
capacity can be used.
With the addition of the compensation, all that has to be taken into account
is that the time to apply in each half-cycle following the zero-crossing of the
mains voltage signal Vr will, instead of Td, be Td +δ in those cases where
the delay δ effectively has to be applied. The calculation of subsequent
Td firing times does not take account of the delay δ compensations which
may have been applied in previous half-cycles.
In the embodiments described, as shown in FIG. 1, there is a single position
sensor
4 and is placed at the central point of the free space between the poles
of stator
1B of the motor
1, i.e. on the axis perpendicular to the
magnetic axis created by stator
1B. In this way, the aim is, with a single
location for the position sensor
4, to be able to start in one direction
or the other depending on the rotation direction chosen.
Because of the constructive characteristics of the motor
1, when rotor
1A is stopped, said rotor
1A is situated at an angle slightly off
the magnetic axis of stator
1B, in its rest position. In said position,
startup in one direction (to the left in the case in FIG. 1) raises no problems,
but with startup in the other direction (in the case in FIG. 1 to the right) a
block is produced. That is because one current half-cycle does not give the rotor
1A sufficient impulse to change the polarity read by the position sensor
4, so that, in the following half-cycle, the electronic circuit
3
does not act on switch
2 and rotor
1A returns to its rest position,
and the same process is repeated in the following half-cycles.
To prevent this block, on initiating startup of the motor
1, the electronic
device
3 of the invention fires the static switch
2 during several
cycles without control (a situation equivalent to a direct connection to the mains
voltage Vr) to force the rotor
1A to move from its rest position. It then
begins to calculate the firing times based on the Td firing times and to fire the
static switch
2 according to those Td firing times, depending on the direction
of turn selected, the polarity of the mains voltage Vr and position of rotor
1A.
With the current applied at the outset, the rotor
1A can be moved from
its position so as acquire sufficient inertia to begin to turn in a given direction.
Should rotor
1A have begun to turn in the direction opposite to the one
intended, the control applied will brake it in that direction, until it stops and
it begins turning in the direction chosen.
It may be that, after beginning startup, the rotor
1A returns to its rest
position and is still unable to turn. Therefore, if the time during which rotor
1A polarity remains the same exceeds a certain value, the startup is initiated
once more.
*
