Note: Descriptions are shown in the official language in which they were submitted.
CONTROL CIRCUIT FOR REDUCING AC HUI!~ IN SPLIT
PH~SE ELECTRIC MOTORS
This invention relates to an improved control circuit
for con-trolling the speed of split phase electric motors, and
more particularly to a circuit which permits operation of
permanent split phase capacitor motors at low speeds without
any objectionable AC hum.
There are numerous appliances in the modern-day
household which are operated by small, split phase electric motors
of the fractional horsepower variety. Many such items (for
example, refrigerators, exhaust fans, ceiling fans, etc.) are
within earshot of the members of the household, when in use, and
it is therefore most desirable that their associated motors operate
- as quietly as possible.
One of the principal problems heretofore encountered
in connection with electric motors of the type described is that,
when used with electronic speed controls~ and particularly at lower
speeds, they tend to develop an objectionable hum, which can be
very distressing to any one within hearing distance of the motorn
Efforts have been made in the past to minimi~e signal distor-tion
inherent in electronic speed controls and consequent undesirable
humming during low speed operation of motors of the type
described, but such efforts have not proved to be successful.
It is an object of this invention, therefore, to
provide an improved motor control circuit which substantially
eliminates any objectionable hum heretofore encountered during
low speed operation of split phase motors of the type described.
Another object of this invention is to provide for
motors of the type described an improved, adjustable control
circuit which/relatively simple and inexpensive to manufacture,
and which is capable of operating fractional (and larger)
horsepower motors at variable speeds, and with a minimum of
signal distortion and substantially no undesirable hum.
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`^"`` 3~:17iS~7
A more speci~ic .objec~ of this invention is to provi.de
for a permanent split phase c~pacitor motox an improved speed
control circuit wh.ich permits ope~ation of the motor a~ variable
speeds, but without an~ undesirable signal distortion or humO
Accordingly, the present invention provides a control
circuit for a variable speed, permanent split phase capacitor mo-
tor of the type having a main stator winding, an aux;.liary stator
winding, and a capacitor connected in series with said auxiliary
winding, means for substantially eliminating signal distortion
and consequent audible hum from the motor during low speed opera-
tion thereof, comprising, a transistor with a base and main elec-
trodes consisting of a collector and emitter, having its emitter=
collector circuit connected in series with said auxiliary winding
for supplying power thereto from an ~C power supply, means for
connecting one of the main electrodes of said transistor to one
side of the AC power supply~ and to the base of said transistor,
and a ~irst variable resistor connected between the base and the
other main electrode of said transistor, and adjustable to control
the r~rrent flow through the emitter-collector circuit of said
transistor during operation of said motor and thereby the current
flow through said auxiliary winding.
Other objects of the invention will be apparent herein-
after from the specification and from the recital o~ the appended
claims, particularly when read in conjunction with the accompany-
ing drawing.
In the drawing;
Fig. 1 is a wiring diagram illustratiny the windings of
a conventional permanent split phase capacitor motor, together
with a control circuit made according to one embodiment of this
invention, and shown connected in series with the auxiliary wind-
ing of the motor to permit operation .thereof at ~Jariable low
speeds; and
-- 2
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'7
Fig. 2 is a wiring diagram illustrating this control in
greater detail.
Referrin~ now *o the drawing by numerals of reference,
and first to Fig. 1, Wl and W2 represent the main and auxiliary
windings, respectively, of a conventional permanent split capa-
citor motor, which is of the type that is frequently employed to
drive household ceiling fans, exhaust fans, etc. Opposite ends
of the main winding Wl are connected by lines Ll and L2 to one
side of a conventional On-Off switch Sl, the opposite side of which
is connected to an AC power supply, which may be of the usual
120 Volt, 60 Hertz household variety.
The auxiliary winding W2 is connected between lines Ll
and L2 in parallel with the main winding Wl, and in series with
a capacitor Cl and a control circuit 10, by means of a line L3
which connects one end of winding W2 to line L2, by a line L4
- 2a
~P7B65~ ,
which connects the opposite end of winding W2 to one side of
the capacitor Cl, by a line L5 connecting -the opposite side of
the capacitor Cl to the control circuit 10, and by a Iine L6
which connects circuit 10 to L1. As a consequence, when switch
Sl is closed, the control circuit 10, the capacitor Cl and
auxiliary winding W2 are connected in series across the AC
power supply. Also at this time, of course, the winding Wl
will also be connected directly across the power supply.
Referring now to Fig. 2, the control circuit 10
comprises an NPN power transistor 21, which has its emitter
connected directly to line L5, it collector connected directly
to line L6, and its base connected by a line 22 to the
movable contact P of a 100 ohm potentiometer (pot) having a
variable resistance R1. The fixed or æero end of the pot
resistance Rl is connected by a line L8 to one end of a fixed
resistor R2, the opposite end of which is connected to line L5,
thereby placing resistor R2 in the emitter-base circuit of the
transistor 21. Line 22 is also connected through another fixed
resistor R3 and a line L9 to line L6~ so that resistor R3 is in
the collector-base circuit of the transistor. By way of e~ample,
~he resistor R2, may have a valueof approximately twenty-seven
ohms, while resistor R3 has a much larger value, for example,
in the vicinity of fifteen thousand ohms.
In operation, and assuming that -the switch Sl is
closed, the pot contact P can be adjusted manually to control
the value of Rl, and consequently the forward biasing voltage
that is developed across the emitter-base circuit of -the transistor
21. In turn, this permits manual control of the current flow
through the emitter-collector circuit. For example, assuming
that the adjustable pot contact P is moved toward the left in
Fig. 2 to reduce the value of the resistance Rl placed in the
circuit, then the forward biasing voltage between the base and
~ 6S'7
the emitter will be reduced, thereby reducing the current flow
through the emitter-collector circuit. At this stage the
transistor 21 functions as a high resiStance in parallel with
R2 and R3 between lines L5 and ~6. This, in turn, reduces
current flow through winding W2.
On the other hand, as the movable pot contact P
is moved toward the right Fig. 2, so as to increase the value
of resistance Rl, the voltage drop between the base and the
emitter will increase, and thereby effectively will increase the
current flow in the emitter-collector circuit. This permits
the transistor 21 to function more nearly as a short circuit
and allows most of the current flow to take place through the
emitter-collector circuit of the transistor, rather than through
the hi~h resistance circuit represented by R3, Rl, and R2.
At intermediate settings oE the pot contact P the
transistor 21 acts as a variable resistance between ~he emitter
and collector, either lncreasing or decreasing the fcrward
biasing voltage between the emitter and the base, depending upon
the position of the movable contact P of the pot. The feedback
resistor R3 and the biasing resistor R2 tend to stabilize the
operation of the control circuit.
In practice it has been found that this control
circuit provides relatively simple and inexpensive means for
controlling the intermediate speed ranges of the motor repre-
sented by the windings Wl and W2. For low speed operation of
the associated motor, the pot is adjusted $o that the resistance
Rl is almost negligible, thus switching the transi9tor 21 almost
to its blocking mode. This causes a decrease in current
through W2 which causes a decrease in motor tor~ue. On the
other hand, as the pot is adjusted to increase the resistance
of Rl, thereby causing a corresponding increase in the current
flow in the emitter-collector circuit of the transistor 21, the
current flow through the au~ialiary winding W2 increases which
~ s~i
causes an increase in motor tor~ue and thereby causirlg a pro-
gressive speeding up of the motor. In all such operations, however,
the control circuit 10 tends to minimize, if not substantially to
eliminate signal distortion and the consequent audible hum which
heretofore tended to accompany the intermediate and low speed
operations of permanent split capacitor motors of the type des-
cribed.
The transistor 21 may be of the type sold, for example,
by Sylvania GTE as type ECG-280, while the adjustable poten-
tiometer can be of the wire wound variety rated at one hundredohms, 2 to 4 watt, and sold for example by Clarostat under the
catalog No. A43-100. Moreover, while in the embodiment disclosed
above it has been suggested that an NPN power transistor be em-
ployed, it will be apparent to one skilled in the art that an equi-
valent solid state element, or the compliment of the solid state
element could be utilized in its place. Likewise, although speci-
fic values have been given for resistors R2 and R3, it will be
appreciated that these values also can be modified within reason
without departing from this invention. Furthermore, while this
invention has been illustrated and described in detail in con-
nection with only one embodiment thereof, it will be apparent that
it is capable of still further modification, and that this appli-
cation is intended to cover any such modifications as may fall
within the scope of one skilled in the art or the appended claims.
