Electronic brake for AC motors

A circuit for electronically braking an AC motor. The circuit includes a controlled rectifying means for applying DC to the motor and a control means for turning the rectifying means on after deactuation of the motor for a timed interval determined by the charging rate of a timing capacitor through a variable resistance.

The present invention relates to electronic circuits for braking AC motors, 
and more particularly to circuits for braking motors employed in 
dispensing systems. 
In certain applications for AC motors, it is desirable that the motor not 
be allowed to coast to a stop. For example, in a dispensing system where a 
motor and pump are powered for a given length of time to dispense a fixed 
volume of liquid, if the motor is allowed merely to coast to a stop, 
variations in sample size will occur. Mechanical brakes have been used but 
these have suffered due to wear and heating, especially where the motor is 
started and stopped repeatedly in a short span of time. Therefore, it is 
desirable to use a brake that does not depend upon friction. 
Prior proposed electronic braking circuits include an SCR switch which is 
turned on for a timed interval which is determined by the value of a 
capacitor. The capacitor is discharged directly into the gate of the SCR 
switch which rectifies the AC power applied to a motor. Thus the timed 
interval is very dependent upon the particular capacitance value of the 
capacitor and the particular gate trigger characteristics of the SCR 
switch employed. The time needed to brake the motor (hereinafter the 
braking time) is related to the duration of the timed interval. The 
particular capacitance value of the capacitor may vary due to capacitor 
aging. Therefore, the timed interval (and thus the braking time) is also 
subject to the effects of capacitor aging. In addition, due to normal 
manufacturing tolerances, the capacitance value may vary by as much as 10% 
from capacitor to capacitor in production quantities resulting in the 
timed interval also varying by that amount. Furthermore, different motors 
and motor loads require different timed intervals. Thus, since capacitors 
suitable for this purpose are manufactured at certain standard values, it 
may not be possible to obtain the timed interval required by a particular 
application. In addition, these circuits are not readily adaptable to 
changing the timed interval should it become desirable to change the 
interval due to a new application for the AC motor or to restore the 
interval due to capacitor aging or variations in SCR characteristics. 
It is an object of the present invention to provide an electronic braking 
circuit obviating, for practical purposes, the above mentioned limitations 
heretofore present.

Other objects and advantages of the invention are more particularly set 
forth in the following detailed description, and in the accompanying 
drawing which is an electrical schematic diagram showing a preferred 
embodiment of the present invention. 
With reference to the FIGURE, a circuit is shown in accordance with an 
embodiment of the present invention for electronically braking an AC motor 
10, such as a shaded pole or capacitor start AC motor for example, 
operated by an AC power source (not shown). The circuit generally 
comprises a means 12 for removing the AC power source from motor 10, a 
variable resistance 14, and a timing capacitor 16 to be charged from a DC 
voltage source 18 through variable resistance 14 such that the charging 
rate of capacitor 16 is capable of being varied by variable resistance 14. 
The circuit further comprises a controlled rectifying means 20 for 
rectifying the AC power to DC power, means 22 for applying the DC power to 
the motor, and control means 24 for turning on the controlled rectifying 
means, for comparing the voltage level across capacitor 16 with a 
predetermined voltage level, and for enabling the controlled rectifying 
means to turn off after a timed interval defined by when the voltage 
across capacitor 16 reaches the predetermined level so that the timed 
interval is a function of the variable resistance 14. 
In the preferred embodiment, the present invention is used to brake a motor 
10 which powers a pump 11 in a dispensing system. Thus the braking time 
may be controlled so that the volume of liquid dispensed during braking 
may be controlled. Here, means for supplying AC for powering motor 10 
comprises a switch 26 having input lines 28 and 30 from an AC power 
source. Means 12 for removing the AC power source from a shaded pole AC 
motor 10 is comprised of double pole-double throw relay 32 (hereinafter 
relay 32) which is powered by step-down transformer 34. Transformer 34 is 
coupled to the AC power source and is also coupled to relay 32 by means of 
normally open, momentary contact switch 36. Motor 10 is directly connected 
to line 28 of the AC power source and can also be connected to line 30 of 
the AC power source by means of stationary contact 38 and movable contact 
40 of relay 32. 
The DC voltage source 18 is comprised of rectifier diode 42 in series with 
current limiting resistor 44, and the parallel combination of Zener diode 
46 and filter capacitor 48. Line 28 of the AC power source is connected to 
the anode of diode 42 whose cathode is connected by means of resistor 44 
to the cathode of Zener diode 46 and to the positive end of capacitor 48. 
DC supply voltage Vcc is thus supplied at the positive end of capacitor 
48. 
Variable resistance 14, shown as a potentiometer in the preferred 
embodiment, fixed resistor 50, and timing capacitor 16 define a time 
constant of the circuit and determine the duration of the timed interval 
in which DC power is applied to brake the motor 10, as is hereinafter 
discussed. The timed interval is thus a function of potentiometer 14. 
Timing capacitor 16 is connected to Vcc at the positive end of filter 
capacitor 48 by means of limiting resistor 50 and potentiometer 14. 
Controlled rectifying means 20 for rectifying the AC power to DC power is a 
thyristor switch, or in particular, an SCR switch in the preferred 
embodiment. Means 22 for applying the DC power to motor 10 comprises relay 
32 which is powered by transformer 34 by means of switch 36 as noted 
earlier. Motor 10 can be connected to line 28 of the AC power source and 
to the anode of SCR switch 20 by means of movable contact 40 and 
stationary contact 54 of relay 32 and current limiting resistor 52. The 
cathode of SCR switch 20 is connected to line 30 of the AC power source. A 
smoothing capacitor 76 is connected between line 28 of the AC source and 
resistor 52 to provide relatively constant DC power to motor 10 while SCR 
switch 20 is turned on. 
Control means 24 for turning on the SCR switch 20, for comparing the 
voltage level across timing capacitor 16 with a predetermined voltage 
level, and for enabling the SCR switch 20 to turn off upon reversal of AC 
polarity after the voltage across capacitor 16 reaches the predetermined 
level, is a monostable, multivibrator integrated circuit 24 (hereinafter 
IC 24) having terminals 1-8 in the preferred embodiment. Terminals 4 and 8 
of IC 24 are connected to Vcc at the positive end of capacitor 48, and 
trigger terminal 2 is connected to Vcc by means of protective diode 56 and 
bias resistor 58. Input terminals 6 and 7 are connected to timing 
capacitor 16. Output terminal 3 is connected to the gate of SCR switch 20 
by means of a voltage divider, comprised of resistors 59 and 62, and diode 
60, the anode of which is connected to IC 24. The diode insures that the 
charge on the gate of the SCR can discharge thru 62 but cannot reach IC 24 
and effect its performance. Terminal 1 of IC 24 connects directly to line 
30 and terminal 5 is indirectly connected to line 30 by means of filter 
capacitor 64. 
A capacitor 66 and relay 32 produce a pulse to trigger IC 24 when it is 
desired to turn on SCR swtich 20 for the timed interval. The positive end 
of capacitor 66 is connected to Vcc by means of resistor 68 and is 
connected to trigger terminal 2 by means of capacitor 70. The positive end 
of capacitor 66 can also be connected to the negative end of capacitor 66 
by means of stationary contact 72 and movable contact 74 of relay 32, 
thereby short-circuiting capacitor 66. The negative end of capacitor 66 is 
connected to line 30 of the AC power input. 
While motor 10 is operating, switch 36 is closed, allowing relay 32 to be 
energized. Switch 36 carries only low voltage to minimize shock hazards 
for applications where an operator holds switch 36. If the low voltage 
feature is not desired, relay 32 and transformer 34 could be replaced by a 
DPDT toggle switch, of course. 
When energized, movable contact 40 of relay 32 is caused to move against 
stationary contact 38 thus completing the AC power circuit to motor 10. In 
addition, movable contact 74 of relay 32 is caused to move away from 
stationary contact 72 removing the short circuit, allowing capacitor 66 to 
charge from the DC voltage supply. Until IC 24 is triggered, timing 
capacitor 16 is short-circuited internally by IC 24 and thereby not 
allowed to charge. 
As noted earlier, motor 10 can be braked by the application of DC power. 
The braking time is a function of the magnitude of the current applied and 
the duration of the timed interval that the current is applied. A small 
current is desirable since excessively large DC currents may cause motor 
overheating and would require a relay with a large contact current rating. 
Therefore, current-limiting resistor 52 is chosen sufficiently large to 
protect motor 10. 
When switch 36 is opened, relay 32 is deenergized and movable contact 40 is 
caused to move away from stationary contact 38 into contact with 
stationary contact 54 thereby removing the AC power from motor 10 and 
connecting rectifying means SCR switch 20 to motor 10. At the same time, 
movable contact 74 is caused to move into contact with stationary contact 
72 thereby short-circuiting capacitor 66 which produces a negative pulse. 
This pulse is applied to terminal 2 of IC 24 which triggers IC 24. IC 24 
thereupon releases the short circuit across capacitor 16 and produces a 
"high" output at terminal 3. This high output supplies a relatively 
constant gate current to SCR switch 20 allowing SCR switch 20 to rectify 
the current applied to motor 10 from the AC power source which causes 
motor 10 to begin to brake. Upon the release of the short circuit across 
capacitor 16 by IC 24, capacitor 16 charges from DC supply 18 at a rate 
determined by the value of capacitor 16 and the values of resistor 50 and 
potentiometer 14. When the voltage across capacitor 16 reaches two-thirds 
Vcc, IC 24 resets by short-circuiting capacitor 16 and causing the output 
at terminal 3 to go "low" which enables SCR switch 20 to turn off upon 
polarity reversal. Motor 10 may now be operated again by closing switch 
36. Since the duration of the application of DC current to motor 10 is 
dependent upon the charging rate of capacitor 16, the braking time of 
motor 10 can be easily controlled by adjustments to potentiometer 14. 
Thus, in a dispenser system, for example, the user may desire to change the 
size of the pump, or the viscosity or volume per sample of the material to 
be pumped. If any of these changes require the brake time to change, the 
variable resistance 14 need merely be reset to change the brake time. If 
desired, the potentiometer 14 in the preferred embodiment could be mounted 
on a cabinet so that non-technical personnel could change the brake time 
by adjusting the potentiometer without going inside the cabinet and thus 
exposing the circuitry to possible damage, or dust, etc. 
The present embodiment of the invention is particularly suitable for 
applications where a pump drive is started and stopped repeatedly in a 
short span of time. For example, 10 start-stop cycles in 15 seconds may be 
desired. Due to the time desired to pump the sample and the reaction time 
of the operator, the maximum time allowed for braking may be 0.5 seconds. 
By adjusting the potentiometer, the present circuit could be set for such 
a brake time. 
One desirable construction of the preferred embodiment of the invention 
illustrated in the drawing, may be constructed with the following 
component values: 
diode 46--12 V Zener diode 
diodes 42, 56 and 60--1 A diode 
SCR 20--silicon controlled rectifier 
resistor 44--4 Kohms, 5 W 
resistor 50--100 ohms 
variable resistance 14--1 Meg. ohm potentiometer 
resistor 68--270 Kohm, 1/4 W 
resistor 58--18 Kohm, 1/4 W 
resistor 62--2.2 Kohm, 1/4 W 
resistor 59--10 Kohm, 1/4 W 
resistor 52--10 ohms, 20 W 
capacitor 48--1000 mfd., 25 VDC 
capacitor 16--0.47 mfd., 200 VDC 
capacitor 66--0.47 mfd., 200 VDC 
capacitor 70--0.01 mfd., 200 VDC 
capacitor 64--0.01 mfd., 200 VDC 
capacitor 76--100 mfd., 350 VDC 
switch 26--on-off power switch 
switch 36--normally open push button switch 
transformer 34--115 V to 12 V transformer 
relay 32--12 V coil, DPDT relay 
IC 24--integrated circuit NE 555. 
It will of course be understood that modifications of the present invention 
in its various aspects will be apparent to those skilled in the art, some 
being apparent only after study and others being merely matters of routine 
electronic design. As such, the scope of the invention should not be 
limited by the particular embodiment and specific instructions herein 
described but should be defined only by the appended claims and 
equivalents thereof. 
Various features of the present invention are set forth in the following 
claims.