U.S. patent application number 09/759040 was filed with the patent office on 2001-09-20 for speed control.
Invention is credited to Bosma, Gaatze Bareld, Dijkstra, Jacob, Ettes, Wilhelmus Gerardus Maria, Marinus, Harry.
Application Number | 20010022507 09/759040 |
Document ID | / |
Family ID | 8170933 |
Filed Date | 2001-09-20 |
United States Patent
Application |
20010022507 |
Kind Code |
A1 |
Marinus, Harry ; et
al. |
September 20, 2001 |
Speed control
Abstract
A motor speed control for controlling the speed of an electric
induction motor by modulating the amplitude and frequency of the
driving voltage. When the load increases the speed control controls
the speed so as to be relatively constant with respect to the
frequency of the driving voltage. Preferably, the speed control
controls the amplitude of the driving voltage in proportion to the
square of the frequency.
Inventors: |
Marinus, Harry; (Drachten,
NL) ; Ettes, Wilhelmus Gerardus Maria; (Drachten,
NL) ; Bosma, Gaatze Bareld; (Kollum, NL) ;
Dijkstra, Jacob; (Veen Wouden, NL) |
Correspondence
Address: |
Corporate Patent Counsel
U.S. Philips Corporation
580 White Plains Road
Tarrytown
NY
10591
US
|
Family ID: |
8170933 |
Appl. No.: |
09/759040 |
Filed: |
January 11, 2001 |
Current U.S.
Class: |
318/727 |
Current CPC
Class: |
H02P 23/03 20130101;
H02P 25/04 20130101; H02P 27/047 20130101; H02P 27/08 20130101 |
Class at
Publication: |
318/727 |
International
Class: |
H02P 001/24; H02P
001/42; H02P 003/18; H02P 005/28; H02P 007/36 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 20, 2000 |
EP |
00200221.0 |
Claims
1. A speed control for controlling the speed of an electric
induction motor by modulating the driving voltage of the motor,
particularly the amplitude and frequency of this voltage,
characterized in that the speed control is adapted to control the
amplitude V of the driving voltage in accordance with the formula
V=b.fx.sup.x when the frequency f varies, in which formula x has a
value greater than 1 and smaller than 3 and b is a constant.
2. A speed control as claimed in claim 1, characterized in that the
value of x is greater than 1.5 and smaller than 2.5, and is
preferably 2.
3. A speed control as claimed in claim 1, characterized in that the
driving voltage for the motor is a single-phase voltage.
4. A speed control as claimed in claim 1, 2, or 3, characterized in
that the driving voltage is approximately sinusoidal.
5. A speed control as claimed in claim 1, 2, 3, or 4, characterized
in that the driving voltage is modulated by pulse width
control.
6. A speed control as claimed in claim 5, characterized in that the
pulse width is modulated by the output voltage of a digital signal
processor.
7. A speed control as claimed in claim 6, characterized in that the
digital signal processor supplies an output voltage having at least
six discrete voltage levels which approximate a sinewave
voltage.
8. An electric induction motor having a speed control as claimed in
any one of the preceding claims.
9. An electric induction motor as claimed in claim 8, characterized
in that the motor has a squirrel-cage armature.
10. An electric induction motor as claimed in claim 8 or 9,
characterized in that the motor is of the shaded-pole type.
11. An electric induction motor as claimed in claim 8, 9, or 10,
characterized in that the motor drives a load torque which
increases as the square of the speed.
12. A fan, a pump or an air cleaner having an electric induction
motor as claimed in claim 8, 9, 10, or 11
Description
[0001] The invention relates to a speed control for controlling the
speed of an electric induction motor by modulating the driving
voltage, particularly the amplitude and frequency of this
voltage.
[0002] Such a speed control is known from U.S. Pat. No. 4,327,315.
Said patent specification describes an induction motor in which an
LF motor voltage (a voltage of a comparatively low frequency of
approximately 1-100 Hz) is obtained by means of an HF pulse width
control (pulse width control with a comparatively high frequency of
approximately 1-100 kHz). The speed control is then based on the
so-called V/f principle. This means that the motor voltage is
raised proportionally as the speed (f) increases. In this way, the
amount of flux in the motor coil, expressed in V.s (volts second),
remains constant and the maximum motor torque remains constant.
[0003] In an electric motor the driving voltage builds up a
rotating field in the stator, which is followed by the rotor. By
modulating the driving voltage the speed of the motor can be
influenced. In general, it holds that the speed will increase by
increasing the driving voltage and/or by increasing the frequency
of the driving voltage. In a no-load condition the rotor will run
in synchronism with the rotating field. When loaded the motor
rotates at a speed which can be up to approximately 80% as low as
the rotating field frequency. An induction motor develops a motor
torque only by virtue of a difference in speed of the rotor and the
frequency of the rotating field built up between the poles of the
stator (the so-called slip). Hereinafter, frequency is to be
understood to mean, unless otherwise stated, the frequency of the
rotating field built up in the stator, i.e. the frequency of the
driving voltage.
[0004] The speed assumed under load is referred to as the operating
point. When the motor torque is constant this operating point will
be shifted toward increasingly lower speeds as the load
increases.
[0005] Induction motors for electrical appliances are popular in
the industry because they can be manufactured comparatively
cheaply. However, the possibilities of using motors of this type
are limited for uses at lower speeds, particularly in the cases
where the load torque at low speeds is comparatively small. As a
result of the difference between the motor speed and the rotating
field frequency the rotor is alternately accelerated and
decelerated in one cycle of the rotating field. The frequency of
this pulsation is twice the frequency of the rotating field. As a
result of this pulsating torque the motor runs less steadily and
produces more noise at lower speeds.
[0006] Such a situation occurs, for example, in the case of a fan
or a pump. With such appliances the load torque at low speeds is
substantially smaller than the motor torque, as a result of which
the motor begins to "pound" and no longer runs steadily owing to
the pulsating torque and saturation effects. Known speed controls
are unsatisfactory in particular in the case that an electric
induction motor is used in the fan of an air cleaner. Such an air
cleaner feeds the air through a number of cleaning filters at a
comparatively low velocity. The sound effects produced by the air
stream are then comparatively small. Nevertheless, the noise level
produced by the air cleaner owing to the presence of the fan is
fairly high. This effect is experienced as annoying.
[0007] It is an object of the invention to eliminate said drawbacks
and to provide a speed control which makes the electric induction
motor run silently and steadily at low speeds. It is another object
of the invention to provide an electric motor having such a speed
control, which can be manufactured simply and cheaply, particularly
for use in a fan or pump. In a speed control of the type defined in
the opening paragraph this object is achieved in that the speed
control is adapted to control the amplitude V of the driving
voltage in accordance with the formula V=b.f.sup.x when the
frequency f varies, in which formula x has a value greater than 1
and smaller than 3 and b is a constant.
[0008] In a preferred embodiment the value of x is greater than 1.5
and smaller than 2.5, and is preferably 2. This speed control has
the advantage that at low speeds the maximum motor torque is much
smaller than with the known V/f control, as a result of which the
motor runs better and more silently. Since the motor torque is
always optimized for the load saturation effects at low speeds are
less audible. Harmonic distortion in the rotating field in the
stator is less likely to occur, as a result of which the motor is
more silent. When an electric motor is used as a fan or pump the
load torque is theoretically a square-law function of the speed.
This means that the load torque at lower speeds is much smaller
than the motor torque of a motor which is operated with a motor
voltage proportional to the speed, as is the case in the said
United States patent specification. The speed control then controls
the amplitude of the driving voltage proportionally to the square
of the frequency, as a result of which the value x is 2.
[0009] When the speed control is used in an electric motor of an
air cleaner this cleaner can operate very silently at lower
speeds.
[0010] The invention can be used with a multi-phase driving
voltage, for example a three-phase driving voltage. However, this
type of driving voltage is not customary with motors for domestic
uses, particularly with motors for an air-cleaner fan. In addition,
the effect is less pronounced in the case of multi-phase driving
voltages because at lower speeds these motors operate more steadily
anyway. In a preferred embodiment, however, the driving voltage for
the motor is a single-phase voltage. Motors with a single-phase
driving voltage are very suitable for domestic appliances because
they can be manufactured simply and cheaply. In addition, the
required power is comparatively low for domestic uses. The use of
the speed control in accordance with the invention offers
advantages particularly with motors of this type because hitherto
these motors were less suitable for low speed drives.
[0011] Preferably, the driving voltage is approximately sinusoidal.
The rotating field which is built up by said voltage then has a
minimal distortion, as a result of which annoying vibration and
noise effects are minimal.
[0012] For realizing a speed control in accordance with the
invention various circuits are conceivable, both in analog and in
digital versions. In a preferred embodiment the driving voltage is
modulated by pulse width control. This form of modulation makes it
possible to modulate high power levels with the aid of power
transistors in a comparatively simple manner.
[0013] In a further preferred embodiment the pulse width is
modulated by the output voltage of a digital signal processor
(DSP). A cheap and functional embodiment is then a DSP which
supplies an output voltage having at least six discrete voltage
levels which approximate a sinewave voltage.
[0014] The invention further relates to an electric induction motor
having a speed control in accordance with one of the
afore-mentioned embodiments.
[0015] Although different types of electric induction motors are
conceivable in which the invention can be used, the motor
preferably has a squirrel-cage armature. In this type of motor
there is no electrical connection with the stationary part of the
motor, as a result of which the motor is more silent and has a
longer lifetime.
[0016] In a further preferred embodiment the motor is of the
shaded-pole type. In this type a part of the stator is screened, as
a result of which an asymmetry in the rotating field is produced.
This enables the motor to be started from standstill, without the
additional cost of an auxiliary winding or capacitor. The
efficiency of a shaded-pole electric motor is comparatively low.
However, this need not be a drawback in the case of the
comparatively low power ratings required for a fan or air
cleaner.
[0017] The invention also relates to a fan, a pump or an air
cleaner having an electric induction motor in one of the
embodiments described herein.
[0018] The invention will be described in more detail hereinafter
with reference to the drawings, in which:
[0019] FIG. 1 shows a motor characteristic representing the
relationship between speed and motor torque of an electric
motor;
[0020] FIG. 2 shows a diagram with motor characteristics in the
case of an increasing speed, the amplitude being increased in
proportion to the frequency of the motor voltage;
[0021] FIG. 3 shows a motor characteristic for an electric motor,
the amplitude of the motor voltage being increased and the
frequency thereof being maintained constant;
[0022] FIG. 4 shows a motor characteristic with a speed control in
accordance with the invention;
[0023] FIG. 5 is a simplified representation of an electronic
circuit arrangement including a digital signal processor for
controlling an electric motor by means of pulse width
modulation;
[0024] FIG. 6 is a diagram which shows the output voltage of the
digital signal processor and the driving voltage applied to the
electric motor;
[0025] FIG. 7 shows a sound and vibration spectrum of a known air
cleaner with a voltage-controlled speed control, at approximately
1000 r.p.m.;
[0026] FIG. 8 shows a sound and vibration spectrum of a known air
cleaner with a speed control in accordance with the invention, also
at approximately 1000 r.p.m.
[0027] Referring to FIG. 1, it will be explained hereinafter how
the motor torque T.sub.motor of the induction motor develops at an
increasing speed and how this is related to the load torque
T.sub.last. In the diagram the speed r is plotted on the horizontal
axis as a fraction of the rotating field f and the motor torque
T.sub.motor and the load torque T.sub.last are plotted on the
vertical axis as a fraction of the maximum torque T.sub.max. The
load torque T.sub.last is shown as a broken line and the motor
torque T.sub.motor is shown as a solid line. The motor of a fan
experiences an increasing resistance owing to the air stream. This
results in a load torque T.sub.last which increases as the square
of the speed r:
T.sub.last.apprxeq.K.multidot.r.sup.2
[0028] In this equation K is a constant factor.
[0029] In a stationary situation the motor torque T.sub.motor and
the load torque T.sub.last are equal to one another. This
equilibrium is reached at the intersection of the load line
T.sub.last and the motor characteristic and is referred to as the
operating point. In this operating point the motor has a speed
r.sub.n. This speed lies between 80-100% of the frequency f of the
rotating field. In a stationary situation the following
relationship is valid:
f=a.r.sub.n
[0030] In the equation a is a constant.
[0031] At zero load the speed r.sub.n approaches the frequency of
the rotating field f and, as consequence, no motor torque is
produced. However, if the load torque is increased the operating
point is shifted towards a lower speed at which a higher motor
torque can be produced. In the operating range L the motor torque
increases linearly at a higher load and a lower speed. Outside this
range the motor torque increases to a decreasing extent up to the
point where the maximum torque is produced; this maximum torque is
referred to as the pull-out torque. It can be demonstrated that the
maximum torque is inversely proportional to the square of the
frequency:
T.sub.max.apprxeq.K.multidot.Error!
[0032] Herein, T.sub.max is the maximum torque; V is the motor
voltage and f is the frequency thereof (the rotating field
frequency); K is a constant factor.
[0033] If the motor load is increased even further it is no longer
possible to reach equilibrium and the motor has left the linear
operating range. It is no longer possible to produce an additional
torque. The motor speed will decrease and the motor will stall.
[0034] FIG. 2 shows a motor characteristic for a motor, the motor
voltage being increased proportionally to the frequency of the
rotating field, i.e. speed control is effected in accordance with
the V/f principle as described in the patent specification U.S.
Pat. No. 4,327,315 mentioned in the introduction. The horizontal
axis and the vertical axis represent the same quantities as in FIG.
1. The load torque T.sub.last is shown as a solid line; the motor
torque is shown in broken lines T.sub.1, T.sub.2, T.sub.3. The
operating ranges L.sub.1, L.sub.2, L.sub.3 correspond to increasing
rotating field frequencies f.sub.1, f.sub.2, f.sub.3. With this V/f
control the amount of flux in the motor coil, expressed in V.s
(volts second), and the maximum motor torque remain constant. The
linear range shifts along the horizontal axis, without the maximum
torque decreasing. However, at lower speeds the maximum torque is
much greater than the load torque because this load torque
decreases quadratically. Owing to saturation effects upper
harmonics of the field then begin to contribute to an annoying
noise production, causing the motor to run less silently and less
steadily as the speed decreases.
[0035] Another method of speed control is a control in which the
amplitude of the motor voltage is increased and the frequency
thereof is maintained constant; the motor characteristics of such a
control are shown in FIG. 3. The horizontal axis and the vertical
axis again represent the same quantities as in FIG. 1. The load
torque T.sub.last is shown as a solid line and the motor torque is
shown as broken lines T.sub.2, T.sub.2. A motor torque T.sub.1 is
developed at an amplitude V.sub.1 of the motor voltage and the
motor characteristic T.sub.2 is obtained at an amplitude V.sub.2.
The speed then shifts from r.sub.n1 to r.sub.n2 along the load line
T.sub.last. Since the load torque is smaller at lower speeds this
method of speed control can be applied over a limited range.
However, a drawback of this type of control is that as the voltage
is further reduced the operating point will enter the non-linear
part of the motor characteristic; this means that the speed control
is subject to a lower speed limit. In the non-linear range an
increase of the torque causes the speed to increase, see the curve
T.sub.0 shown in FIG. 3. In this range stable control of the motor
is not possible. On the other hand, the speed cannot be higher than
the rotating field frequency f. The control possibilities of a
speed control by varying the driving voltage are therefore
limited.
[0036] FIG. 4 shows a motor characteristic for a fan motor for
which the motor voltage and motor frequency are controlled by means
of a speed control in accordance with the invention. The horizontal
axis and the vertical axis represent the same quantities as in
FIGS. 1-3. The Figure combines characteristics of FIGS. 2 and 3.
For low speeds, at which the resistance presented to the fan is
low, the developed motor torque is comparatively small. The load
torque increases quadratically as the speed increases. The
operating range of the motor is then shifted to operating ranges
L.sub.1', L.sub.2', L.sub.3' corresponding to increasing rotating
field frequencies f.sub.1', f.sub.2,'f.sub.3'. By controlling the
motor voltage and motor frequency by means of a speed control in
accordance with the invention the motor torque now also increases
quadratically and the motor keeps operating at approximately the
same relative operating point with respect to the frequency of the
rotating field.
[0037] Expressed as a formula, this is as follows: f=a.r, where a
is a constant. If the driving voltage is now chosen in accordance
with the formula
V=b.r.sup.2 the quotient Error!=Error! remains constant.
[0038] With this speed control it is achieved that the motor is
always loaded in the linear range and the motor is not saturated.
The noise production is now substantially smaller than with the
conventional speed control.
[0039] FIG. 5 shows, merely by way of example, an electronic
circuit arrangement for driving an electric induction motor 1 by
means of a speed control in accordance with the invention. This
so-called half-bridge inverter arrangement is made up of a circuit
2 for direct voltage generation and a pulse width modulation
circuit 3.
[0040] The circuit 2 has supply voltage terminals 0, 110 and 240
and further includes a Graetz bridge with diodes D.sub.1-4 and
capacitors C.sub.1 and C.sub.2. The Graetz bridge converts an
alternating voltage applied to the supply voltage terminals into a
direct voltage across the capacitors. An advantage of this circuit
is that it can operate as a voltage doubler. This enables the motor
to operate both at mains voltages of 210-240 V (across the
terminals 0 and 240) and at a mains voltage of 100-130 V (across
the terminals 0 and 110). Such a switching facility is referred to
as a "universal mains" because this arrangement allows the motor to
be used substantially world-wide.
[0041] The pulse-width modulation circuit 3 mainly comprises a
microcontroller 4 (a digital signal processor) and two power
transistors T.sub.1 and T.sub.2. The principle of pulse-width
modulation is described comprehensively in U.S. Pat. No. 5,252,905.
The microcontroller alternately turns on the transistors T.sub.1
and T.sub.2 at a high frequency (a frequency of approximately 20
kHz). Thus, the electric motor is alternately switched at a high
frequency between the positive direct voltage of the capacitor
C.sub.1 and the negative direct voltage of the capacitor C.sub.2.
As long as the effective switching period (duty cycle) of the
positive and the negative voltage is equal the net driving power of
the motor is zero because, as a result of induction phenomena, it
cannot follow the high frequency. By modulating the pulse width the
duty cycle can be varied and a low frequency effective voltage can
be applied to the motor, by means of which the motor can be driven.
Both the frequency and the amplitude of this driving voltage is
modulated by the microcontroller to realize a speed control in
accordance with the invention.
[0042] The electric motor 1 has a squirrel cage armature. In a
motor of this type there is no electrical connection to the
stationary part of the motor, as a result of which the motor is
less noisy and has a longer lifetime. In addition, the motor is of
the shaded pole type. In this type a part of the stator is
screened, as a result of which an asymmetry in the rotating field
is produced. This enables the motor to be started from standstill,
without the additional cost of an auxiliary winding or
capacitor.
[0043] The modulation waveform generated by the microcontroller is
preferably sinusoidal, but even simpler waveforms may result in a
better and more silent operation of the motor. FIG. 6 shows a
diagram in which the output voltage of the digital signal processor
is shown as a line U and the current measured in the stator is
shown as a line I, both as a function of the time t. For the sake
of clarity the motor current I is shown as 90.degree. out of phase
with respect to the output voltage U. Although the current waveform
is not sinusoidal this six-step trapezoidal waveform is found to
exhibit only a slight distortion in comparison with a sinewave. The
noise production is proportionately small.
[0044] FIGS. 7 and 8 give results of noise production measurements.
FIG. 7 shows a sound and vibration spectrum of an air cleaner with
a conventional speed control at the lowest setting at approximately
1000 r.p.m.; FIG. 8 shows a sound and vibration spectrum, also
measured at approximately 1000 r.p.m., of an air cleaner having a
speed control in accordance with the invention. In FIGS. 7 and 8,
the sound is represented by the lines S and S', respectively, and
the vibrations by the lines V and V', respectively. The
measurements were carried out in an acoustic laboratory. The air
cleaner was installed in a so-called "dead room" and the sound
spectrum was measured by means of a microphone connected to a
spectrum analyzer. This analyzer analyzes the sound into
constituent frequency components and displays their intensities
corrected for the aural sensitivity curve of the human ear (class A
weighted). The microphone was arranged at a distance of 1.20 m from
the air cleaner. The vibration spectrum was measured by means of a
sensor mounted at the back of the motor for the detection of
accelerations.
[0045] FIG. 7 shows that peaks occur in the sound and vibration
spectrum at certain frequencies. This is particularly so at twice
the mains frequency (100 Hz) and the second and third harmonics
thereof (at 200-240 Hz and 300-360 Hz, respectively). These are the
upper harmonics of the frequency of the pulsating torque, which are
produced in the case of distortions of the rotating field in the
stator. The frequency of the rotating field is then equal to the
mains frequency, which is 50 to 60 Hz. Particularly a tone of
approximately 300 Hz is distinctly audible. This tone is the third
harmonic. Around the peak of the third harmonic sidebands are
situated, which are each spaced from the central frequency by the
frequency of the speed. The intensities of the sidebands are
influenced by the quality of the motor construction.
[0046] As is illustrated in FIG. 8, the use of the speed control in
accordance with the invention provides a substantially more
favorable sound and vibration spectrum. The motor produces a
significantly smaller amount of noise. It is to be noted in
particular that the third harmonic of twice the mains frequency is
absent. The sidebands of these harmonics still persist but to a
lesser degree. The vibration spectrum is more uniform than in the
conventional situation. A favorable additional effect is that the
contribution of these vibrations to the sound intensity is
smaller.
* * * * *