Note: Descriptions are shown in the official language in which they were submitted.
f~lECTRIC ~IOTO~ SPE~D CONTROLLF.R ~MD Mr.T~lOD
B~ G~OUND Ol Tlll~ INVf:~lTION
__ __ _ _
A) l~ield of the Invention
___ _ __ _~_
This invention relates to speed controllers for electric
motors anc'l especially relates to speed controllers for electric
motors, particularly series-field rnotors, used to dri-ve battery
powered electric vehicles.
B) Ilistory of the I'rior Art
In the prior art, motors, especially DC electric motors,
were generally controlled by the use of an adjustable large
current series resistance. This ~ethod of controlllng the speed
of an electric rnotor, particularly those used to drive electric
vehicles, has been generally unsatisEactory because torque and
speed are almost completely interclependent. The interdependency
results in very ~oor speed regulation. In addition, such
resistance controllers are e~temely ineEficient c]ue to high
dissipation of electrical energy through the controller
resistance elements. This energy dissipation marlcedly reduces
battery llfe and the allowab]e time between charges and also
increases the minimum acceptable initial size, weight and cost o-f
the battery.
~ urthermore, such controllers did not result in Eully
proportional speed control since frequently the controllers had
to be almost in the fully on position before sufficient torque
could be developed to start an electric vehicle from a stopped
position. The controller would then have to be backed ofi to
reduce the torque after motion started. Such controllers
therefore resulted in inconsistent non-uniform motor operation
~q
givlng a "jerky" feel to the movernent oE a vehicle.
More recently, an attempt has been made to control the speed
of an electric motor by placing a power transistor as an
elec-tronic switch between the battery and the motor and
ac~ivating the transistor by supplying pulses of curr~nt to the
transistor base. In general, these pulses were of constant
duration and the speed was controlled in only a few stages by
chopping a current at only a few different speeds~ The resulting
motor operation is non-uniEorm, and electrlcal energy ls wastecl
slnce current ls supplied only at a Eew levels.
BRIEF DESCRIPTION OF T~IE INVENTION
In accordance wlth wlth the present lnventlon, there ls
provlded an electrlc motor speed eontroller circuit which
overcomes essentially all of the previously mentioned
disadvantages of prior art controller circults. In particular,
the electric motor speec] controller circuit, in accordance with
the present invention, is interposed between the motor and a
power supply sufficient to drive the motor~ The speed
controller circuit comprises at least one power transistor
connected through its emitter and collector in series between the
motor and the power supply to form a power circuit such that
current flows between the power supply and the motor through the
transistor only when a signal current is supplied to the base of
the transistor. The speed controller further comprlses a speed
control whlch provldes a variable resistance, lnduc~ance or
capacltance within a control circuit and a control circuit having
a power requirement substantially lower than the power
requirement to operate the motor which supplies pulses of a pulse
width rnodulated signal current to the base of the power
transistor, the i~idth of the pulse being proportional to the
value of the variable resistance, inductance or capacitance of
the speed control.
The invention Eurther comprises a method for controlling the
speed of an electric motor. The method comprises varying the
variable resistance, inductance or capacitance of a speed
control; and supplying a pulse width modulated signal current,
having pulse widths proportional to the value of the variable
resistance, inductance, or capacitance, to the base of at least
one power transistor to activate the transistor which is
connected by its emitter and collector as an electrical switch in
series between the motor and a power supply.
The invention further comprises a variable speed motor
connected with and controlled by the electrical motor speed
controller of the invention and an electrical vehicle powered by
a motor controlled by the electric motor speed controller of the
invention.
~RIEF DESCRIPTION OF THE DP~AWINGS
Figures 1, 2 and 3 are representative graphs showing the
relationship between the EMF's of an elec-trical voltage
proportional to the resistance, capacitance or inductance of a
speed control, the electrical voltage of a comparison saw tooth
wave generated by the control circuit and the electrical voltage
of a pulse modulated output signal.
FiguLe ~ is a schematic diagram of a preferred embodiment of
the electric motor speed controller of the invention.
Figure 5 is a schematic layout of a 1/~ mile test course.
Figure 6 is a power curve showing the power used by a golf
car controlled with the solid state controller oE the invention
as the golf car moves around the test course shown in Figure 5;
and
Figure 7 is a power curve showing the same golf car as
Figure 6 wherein a resistor controller is used as the golf car
travels the test course of Figure 5.
DETAILED DESCRI~TION OF T~IE NVENTION
The electric motor controlled by the speed controller
circuit oE the invention may be any electric motor but is usually
a direct current (DC) electric motor which most often has a
series-field.
The power supply is any power supply sufficient to drive the
motor and for a DC motor is a DC power supply which may have a
uniform or variable EMF. Usually the power supply is a battery
of any suitable type including one or more packaged batt~ries in
series or parallel. Suitable batteries include rechargeable
batteries such as lead-acid batteries; nickel-cadminum batteries
and the newer lithium-chloride batteries.
~ s previously discussed, the speed controller circuit
comprises at least one power transistor connected through its
emitter and collector in series between the motor and the power
supply to form a power circuit such that the current flows
through the transistor from the power supply to the motor only
when a signal current is supplied to the base of the transistor.
The power transistor is a single transistor or coupled
transistors which act as a single transistor. The po~Jer
transistor is clesirably a bistable transistor, i.e., a transistor
whose base currellt controls both on and oEE switching but ~mder
certain circumstances may be a thyrister or in the case of an
al-ternating current, a ~riac. Examples of power transistors are
power Darlington connected transistors~ Examples of suitable
particular transistors are Motorola's MJ11028 Darlington and
~C~'s Series SE~3570 through SK3582.
A plurality of such power transistors can be used in
parallel to increase the current carrying capability oE the
circuit.
The speed control, which is usually manually operated, i.e.,
operated by human action such as a footpedal or a hand operated
accelerator, provides a variable resistance, inductance or
capacitance within the control circuit. The speed control
usually provides a -variable resistance within -the circuit by
means of a rheostat.
The control circuit is designed to have a power requirement
substantially lower than the power requirement to operate the
motor. The control circuit usually requires less than 10 and
preferably less than 5% of the power requirement of the motor.
The control circuit supplies pulses of a pulse width modulated
signal current to the base of -the power transistor or
transistors, the width of the pulse being proportional to the
value of the variable resistance, inductance or capacitance of
the speeci control.
Pulse width modulation (PWM) is a:Lso known as pulse duration
modulation (PDM) or pulse length modulation (PL~I). In general,
pulse width modulation is that modulation oE a signal at an
essentially constant frequency where a time length or width of
pulses of a constant EMF` signal are varied in relation to an
input signal. The longer the width oE the modulation, the more
current is supplied with the signal and the shorter the width of
the mod1llation, the less current is supplied with the signal
Mumerous methods are known to those skilled in the art for
creating a pulse width modulated signal sincc the signals have
been examined in the area of electronics communication and to a
lesser extent in the area of electronic controllers in other
applications (see references on page 1~-51 of the Electronics
Engineers' Handbook edited by Donald G. Eink, published by
McGraw-~lill, 1975).
In accordance with a preferred embodiment of the present
invention, the control circuit comprises a means for generating a
constant freq-uency electrical vol-tage wave Eorm biased so that
the lowest voltage of the wave is greater than zero; a means Eor
generating a voltage proportional to the-variable resistance,
inductance or capacitance of the speed control and a means for
comparing the voltage of the wave form with the generated
proportional voltage and for outputing a pulse width modulated
signal to the base of the power transistor when the gcnerated
proportional voltage is greater than the voltage of the
electrical wave -form. The constant frequency electrical voltage
wave form is most desirable a saw tooth wave form so that the
lengths of the outputted signal pulses are directly proportional
to the generated proportional voltage, although other constant
frequency wave forr~ls can be used so that other proportional
relationships can be obtained between the generated proportional
voltage ancl the lengths of the outputted signal pul~ses.
"E'roportional", as used herein, means having the sarne or
constant ratio or a ratio which varies in accordance with a set
mathematical function. In general, the proportional
relationship, in accordance with the preEerred embodiment of the
invention, is a direct proportion and optionally, a proportion
which varies exponentially, logarithmically or trigonometrically.
Figure 1 of the drawings shows a graph of voltage against
time for a generated constant Erequency saw tooth electrical
voltage wave, (a), a voltage proportional to the variable
resistance when the resistance is exceedingly high thus producing
a proportional voltage (b) which is lower than the lowest voltage
of the constant frequency electrical voltage wave (a) and the
resulting outputted pulse width modulated wave ~c) which has no
pulses (pulse width zero) since at no time is the generated
proportional voltage greater than the voltage oE the constant
frequency electrical voltage wave.
Figure 2 shows essentially the same curve as Figure 1 except
that the proportional voltage (b) ~iS sometimes greater than and
sometimes less than the voltage of constant frequency electrical
voltage wave (a) which resul-ts in pulses (x) which are o-utputted
to the base of the power transistor in original or amplified
form, the length of pulse (x) representing the current flow to
the base of the transistor. The width of pulse (x) of wave (c)
is determined by the length of time that the propor-tional voltage
is greater than the voltage of the constant frequency electrical
voltage wave form. Figure 3 again shows essentially the same
relationship except that generated proportional voltage (b) is
always greater than the voltage of constant frequency electrical
voltage wave form (a) thus the generated current pulses (x) of
wave (c) have essentially merged into a constant high current
flow to the base of the power transistor.
As can be see from these figures, the current flow to the
base of the transistor is proportionally varied depending upon
the proportional voltage which is generated in response to the
resistance, inductance or capacitance of the speecl control thus
the current flow to the base of the power transistor is also
constantly varied in relationship to the variable resistance,
inductance or capacitance and as a result, the current flow from
the power source through the power transistor to the motor ls
also constantly varied in relationship to the reslstance,
inductance or capacitance of the speed control.
The speed control therefore varies the current flow to the
motor thro-ugh the motor controller circuit and has essentially no
effect upon the voltage applied to the motor during current
pulses.
This desirable result is contrary to the results obtained
using prior art electronic motor control circuits.
Optionally and desirably, means is provided for stopping
available signal currents to the base of the power transistor
when current through the power circuit is greater than a maximum
desired level and for restoring available signal currents when
the current -through the power circuit is again below the maximum
L~ 3
desired level. Thls means is desirably present to prevent the
power circuit from being overloaded. ~ny means known to those
slcilled in the art ~or detecting current overload and shutting
off a signal in response to such detection can be used. In a
preEerred embodiment of the invention, a direct current E~I-E is
selected which will decrease through an operational amplifier as
current through a calibrated portion of the cable from a DC power
supply increases. When the selected EM~ is reduced to a
sufficiently lo~ level, another operational amplifier turns on
which forces a third operational amplifier off which in turn
clamps the generated voltage proportional to the variable
resistance, inductance or capacitance to a le-vel below the lowest
voltage of the saw tooth or triangular wave form. This results
in no pulses being sent to the base oE the power transistor thus
effectively stopping current Elow through the motor.
There is also optionally and desirably provided a means for
stopping available signal currents to the base of the
power transistor when battery voltage falls below the desired
minimum voltage to drive the motor and means for restoring
available signal currents when the battery voltage is again above
the minimum desired voltage to drive the motor. Agaln, any means
known to those skilled in the art for detecting voltage drop and
for stopping a signal to the base of the transistor as a result
of such detection can be used. In the preferred embodimen-t of
the invention, an operational amplifier is caused to turn on if
the voltage drops below a predetermined level. When the
operational amplifier is on, the generated proportional voltage
is again clamped below the lowest voltage level of the triangular
wave form, again forcing the output to the base of the power
transistor to be zero or at least too low to activate the
transistor .
The same or similar circuits can also be used as failsafe
mechanisms to turn ofE the power transistor when open circuit or
short circuit conditions are detected.
The frequency of the saw tooth or triangular wave form and
the frequency of the pulse width modulated signal is desirably
selected to be compatible with motor operating characteristics.
The method oE the invention Eor controlling the speed oE an
electric motor comprises varying the resistance, inductance or
capcaci-tance of a speed control, preferably manually, and
supplying a pulse width modulated signal current having pulse
widths proportional to the value of the variable resistance,
capacitance or inductance to the base of at least one power
transistor to activa-te the transistor which is connnected by its
emitter and collector as an electrical switch in series between
the motor and the power supply.
The controller and method of the invention are more
efficient and superior for controlling the speed of a DC motor
and, when a triac is used as the power transistor, can even be
more efficient in controlling the speed of an AC motor. The
circuit does not have ~he power loss inherent in a control
circuit using resistors or chopped pulses which are not pulse
width modulated. Contrary to the results obtained with prior art
controllers, the controller of the invention provides a variable
current supply depending upon the current requirement of the
motor in response to both motor characteristics and the variable
resistance, inductance or capacitance of the speed control.
Further~ore, the pulse widths of the signal to the base of the
power transistor or transistors can be made directly proportional
to the variance in the resistance, inductance or capacitance of
the speed control. The result is a smoothly operating motor
which uses less current than was possible using prior art
controller circuits and with less "jerk" of the motors in
response to current surges provided by prior art controller
circuits which "jerk" is undesirable for smooth operation, good
motor performance and good motor wear.
A schematic diagram of a preferred controller circuit, in
accordance with the invention, is set forth in Figure ~.
The schematic diagram of Figure 4 has been divided by dotted
boxes into various circuit components for ease of explanation.
The dotted box represented by A is the power circuit wherein
electric current from battery 10 passes through power transistors
12 to motor 14 through reversing SWitCtl 16. A free wheeling
diode l~ is provided which provides a current path Eor electrical
currents generated by motor rotation caused by coasting of the
motor and apparatus connected to the motor.
Power transistors 12 act as electronic switches which are
turned on by a sigrnal provided to transistor bases 12A~ A signal
is a wave form which is pulse width modulated in proportion to a
variable resistance which is manually altered as i-n the form of a
rheostat represented within dotted box B of Figure 40 The
rheostat, in the case of an electric vehicle, is usuall~ changed
by means of a footpedal.
In order to generate the pulse width modulated signal, a saw
tootl1 wave :Eorm is generated by means oE the circuit represented
by dotted box C. The triangular shaped circuit components,
through 8, as shown in Figure ~ are operational amplifiers which
n1ay be discrete components or one or more may be combined in a
single electronic package. The operational amplifiers used in
accordance with the embodiment shown in Figure ~ are National
quad operational amplifiers LM32~M wherein each triangle shown in
Fi.gure 4 represents one-fourth of the quad operational amplifier
package.
The frequency of the triangular or saw tooth wave form
generated by a circuit enclosed by dotted box C can be altered by
changing the values of resistance R1 and/or capacitance C1. The
triangular wave form is offset from ground preferably by about
5.5 volts with a 13 volt peak to peak swing.
The circuit represented by dotted box D generates an EMF
which is directly proportional to the val.ue of rheostat 20. The
signal from circuit D is provided to the circuit represented
within dotted box E which compares the signal -from circuit D with
the triangular wave form from circuit C. IE the voltage of the
signal from circuit D is larger than the voltage of the
triangular wave form from circuit C, a current pulse is input
from circuit E to circuit F which amplifies the pulse and
provides the amplified pulse to the bases of power transistors
12 which turns the power transistors on thus permitting c~1rrent
to flow from battery 10 to motor 1~. As was previously
explained, with respect to Figures 1, 2 and 3, the length of the
current pulse to the bases of transistors 12 frorn circuits L and
F is dependent upon the voltage of a signal from circuit D in
response to the variable resistance of circuit B.
In the preferred embodiment shown in Figure 4, safety
circuits are also provided. The circuits represented by the
dotted boxes G and ~ shut down the signal to the bases 12A of
power transistors 12 when current through the power circuit A is
too high. Similarly, circuit I shuts down the signal to bases
12~ of power transistors 12 when a brownout or low voltage
condition exists.
It is to be understood that the schematic diagram of ~igure
4 is only one of many possible circuit arrangements in accordance
with the invention and even in the schematic diagram of Figure 4,
circuit components, e.g., resistances, can be changed to balance
the circuit for the characteristics of somewhat different motors,
batteries, resistances, capacitors, speed controls, regulators,
transistors diodes and operational ampliEiers. Differen-t but
similar components may therefore be substit-uted in the circuit
with corresponding appropriate changes in other components.
In addition, if desired, a capacitive or inductive reactance
can be added in parallel with the pulse width modulated signal in
any control circuit of the invention to create a smoother
transition in signal EMF and/or current to the base of the power
transistor or optionally, if the capacitance or inductance is
large enough, the reactance may even be added in parallel with
the power circuit. The motor windings themselves add a
reasonably large inductance to the power circuit to smoothen
changes in the signal.
~ controller circuit essentially as shown in Figure 4 was
incorporated into a Melex golf car with total gross weight of
1,615 pounds (including the weight of the car). The golf car
with the electric motor speed controller circuit of the present
invention traveled over thirty miles with repeated dead stops
followed with normal acceleration and at the end of the thirty
miles, there was still sufficient charge in the batteries to keep
driving the golf car.
In a test for normal operations, a Melex golf car, having a
total gross weight of 1,493 pounds controlled with an electric
motor speed controller circuit in accordance wi~h the invention,
we able to travel thirty-six holes on a hilly golf course with
less than 50% battery discharge~
In a comparative test, a Melex golf car, having a total
gross weight of 1,615 pounds, was tested with a prior art
resîstance motor controller and the motor speed controller
circuit of the present invention around a one-fourt.h mile test
course having the configuration set forth in Figure 5. The
relative power requirement and power drain between the golf car
with the controller of the present invention and the golf car
with the resistor controller are set forth in Figures 6 and 7
respectively. The comparison of these Figures clearly shows that
at various stops at positions 1, 2, 3 and 4, the current drain
utilizing the electric motor speed controller of the invention is
consistently less and furthermore, the top speed is higher than
the golf car wherein the state of the art resistance controller
is used.
14 * Trade Mar~.
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