Note: Descriptions are shown in the official language in which they were submitted.
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CAPACITOR-TYPE MOTOR SPEED CONTROLLER
B. Gershen
E. Krajci
B. Neiger
The present invention relates to motor speed control and, more
particularly, relates to a capacitor-type motor speed controller.
AC motors may be electrically connected directly to an AC source
such that AC signals derived therefrom drive the motor at a particular speed.
The
motor speed may be controlled or adjusted by controlling the amplitude of the
AC
signal supplied to the motor. For example, if the magnitude of a signal
driving the
motor at a speed S is reduced by 20 %, the motor speed will be reduced by an
amount that is substantially linearly related to the 20 % reduction in motor
drive
signal amplitude. The amplitude of the drive signal can be reduced by placing
a
known impedance (e.g., a resistor) in series with the motor. Further, by
switching
a known resistance or combination of resistances into and out of the series
circuit
formed with the motor, the speed of the motor is varied with the switching.
There are drawbacks, however, to attenuating a motor drive signal
with a pure resistance in series with the motor. For instance, resistors
generate
heat in proportion to the voltage induced across them (and the current flowing
through them). In addition, at the time of switching, troublesome transients
can be
generated in various parts of the motor drive circuit which might be better
regulated with an impedance which is not purely resistive.
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A second known method for varying an amplitude of a motor speed
control signal for controlling motor speed includes utilizing a tapped auto
transformer or tapped motor windings to control the power delivered to the
motor.
Inductor based solutions, however, tend to be quite noisy during switching
rendering such methods unsuitable for particular applications.
It is therefore an object of the present invention to provide a motor
speed controller which overcomes the above-mentioned shortcomings of the prior
art.
It is another object of the present invention to provide a motor speed
controller which does not rely solely on a resistive element to control
attenuation
of a signal driving the motor.
It is another object of this invention to provide a motor speed
controller which utilizes a variable capacitive reactance in series with the
motor to
vary the amplitude of a motor speed control signal driving the motor.
It is yet another object of the present invention to provide a motor
speed controller which switchingly attenuates the motor drive signal in such a
way
that motor speed may be switched substantially noiselessly.
Accordingly, the present invention provides a motor speed controller
for quietly controlling a speed of an AC motor driven by an AC source. The
controller includes a first impedance placed in series with the motor relative
to the
AC source. A second impedance is preferably included which is electrically
connectable to the first impedance. By switchingly controlling the connection
between the first and second impedances, the attenuation of the motor drive
signal
and consequently the motor speed is switchingly controlled.
In a preferred embodiment, the first and second impedances are
defined by one or more capacitors which are electrically connected either in
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parallel or in series. Additional impedances may be combined via switching to
provide any reasonable number of switching speeds for AC motor operation.
Because of a possibility of large surge currents at switching, a resistor is
preferably
included in series with each capacitor. In addition, a resistor may be shunted
across the capacitor/series resistor combination to limit current during
capacitor
discharge.
While the invention provides for the switched interconnection of
various impedances, e.g., capacitive reactances, with a first impedance to
vary the
total impedance placed in series to the motor, the invention also envisions
switching a first impedance, e.g., a capacitive reactance, into and out of
connection
with the motor to even more widely vary the amount of the motor drive signal
provided across the motor.
The invention also includes a method for controlling the speed of an
AC motor driven by an AC source such that minimal audible noise is generated
in
the speed controller. The method includes selectively interposing a first
impedance between the motor and the AC source in order to variably attenuate a
motor drive signal provided to drive the motor. A second impedance, which is
switchably connectable to the first impedance, may be intermittently
electrically
connected to the first impedance such that an attenuation of the AC motor
drive
signal is varied in accordance with the switching such the the motor speed is
quietly switched.
Preferably, the first and second impedances include capacitive
reactances. The currents charging and discharging the capacitors which define
the
capacitive reactances are preferably limited by current limiting resistors.
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CA 02178026 2005-07-05
Accordingly, very little, if any, audible noise is contributed by the motor
speed
controller.
IN THE DRAWING FIGURES
Fig. 1 is a schematic diagram of one embodiment of the present invention;
Fig. 2A is a more detailed schematic diagram of the Fig. 1 embodiment;
Fig. 2B is a variation on the Fig.2A embodiment; and
Fig. 3 is a schematic diagram of an alternative embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
A first embodiment of a capacitor-type motor speed controller (hereinafter
"the controller") 10 of the present invention will now be described with
reference to
Fig. 1. The controller 10 is shown in the figure electrically connected in
series to a
motor 12 which is electrically connected to an AC source 14. The source is
typically
120-Volt AC source. The controller includes a first capacitor 18 to which a
second
capacitor 20 is electrically connectable in parallel through a switch 16. The
second
capacitor 20 is thereby selectively connected in shunt to the first capacitor
18 via
switch 16.
When the contact of switch 16 is in the open position, the impedance
provided in series with the motor 12 and power supply 14 combination is merely
the capacitive reactance of the first capacitor 18 as driven by the AC source
14.
When the switch 16 contact is in the closed position, the first and second
capacitors
18, 20 are electrically joined in parallel. The total capacitive reactance
(i.e.,
impedance) of the parallel combination 18, 20 is seen by the motor AC source
combination. Such an arrangement changes the magnitude of the drive
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CA 02178026 2005-07-05
signal provided through the impedance across the motor. In other words, by
switching varying impedances into and out of the circuit, different voltages
appear
across the motor terminals for varying speed control. Very little, if any,
audible noise
is emitted from a controller which defines its voltage output thereby,
rendering the
controller ideal for use with fan speed control motors.
The inventive concept disclosed herein is simple. In order to provide
different
voltages and therefore different motor speeds, various capacitor combinations
are
arranged in accordance with one or more switch settings. The impedances formed
by the switched capacitive combinations limit the voltage signal impressed
across
the motor's terminals. The capacitive combinations may be series or parallel
connected, depending on the switching.
An inherent problem exists, however, in the controller shown in Fig. 1. At the
point of switching, high currents may flow into/out of the capacitors. Such
possibility
exists because the voltage of the source, as compared to the voltage across
the
capacitor, at the instant of switch closure is indeterminate rendering
indeterminate
current which will flow in response to the voltage. High currents generated
thereby
could be large enough to damage the capacitor(s). For example, assuming that
capacitor 20 is discharged with switch 16 open and a peak voltage present
across
the first capacitor 18. When the switch contact is closed, the second
capacitor 20
will be charged with the discharge of first capacitor 18 and the current
available
through the motor 12. Because there is no current limiting impedance in series
with
capacitors 18, 20, the charging current can be very high. In particular, if
the
capacitors are a metallized film construction, the surge current may be enough
to
damage the capacitors.
The aforementioned problem may be even more acute if the second
capacitor 24 is initially charged to a polarity opposite that of first
capacitor 18 at
switching. Such a case could arise where previous operation charged second
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capacitor 20 when the switch 16 was closed. When the contact closes, the
voltages
would add resulting in an even greater surge current flow to establish
equilibrium
conditions.
Fig. 2A shows an embodiment of a controller 30 the present invention, where
a surge current which could flow to charge either of the first or the second
capacitors 18, 20 is limited by the addition of first and second resistors 22,
24
electrically connected in series with the first and second capacitors 18, 20
respectively. Because the first and second resistors 22, 24 respectively, must
be
capable of transient high current operation, carbon composition, wirewound or
cermet resistors are preferable. The value of the resistors is selectable, by
those
skilled in the art, based on the steady state load current which will flow in
the
controller.
Fig. 2B shows an alternative embodiment 30' to the controller of Fig. 2A. Two
additional resistors, a third resistor 26 and a fourth resistor 28 are
provided to shunt
the first capacitor l8/first resistor 22 and the second capacitor 20/second
resistor 24
combinations. These third and fourth shunt resistors 26, 28 are included to
limit the
discharge current flowing from the first and second capacitors 18, 20, when
one or
the other are switched out of the circuit. This provides two benefits to the
controller
30'. First, after the current stored, for example, in second capacitor 20 is
discharged
through its shunt resistor 28, the maximum voltage available for current flow
during
equilibrium is equal to the maximum voltage across first capacitor 18 and
first
resistor 22. Second, a user would not receive an electric shock when removing
a
controller from service because all residual charge on controller capacitors
18, 20
would be discharged. It should be noted that the third and fourth shunt
resistors 26,
28 may be arranged to merely shunt the first and second capacitors 18, 20
respectively, not each of the first capacitor 18/first resistor 22 and second
capacitor
20/second resistor 24 combination.
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Fig. 3 is an embodiment of an controller 40 of the present invention in which
a number of additional switches, such as switch 36, are included for switching
additional capacitors, such as Nt" capacitor 34, into the controller circuit
to further
vary the impedance provided in series with the motor 12. Such an arrangement
could theoretically provide unlimited switchable variations on a maximum speed
defined by the maximum power supply potential.
It is clear that the above description of the preferred embodiment in no way
limits the scope of the present invention which is defined by the following
claims.
