Over-load and light-load protection for electric machinery

An apparatus for protecting machinery driven by electric motors from operating during over-load and light-load conditions, comprising: a current transformer operatively associated with an electrical supply line for the induction motor; a current-voltage transforming circuit for generating a control signal having a magnitude proportional to current flowing in the supply line; an upper limitation magnitude comparator for generating an over-load signal output whenever the magnitude of the control signal exceeds a predetermined upper safety value; a lower limitation magnitude comparator for generating a light-load signal output whenever the magnitude of the control signal is less than a predetermined lower safety level; an inhibitable protection timer for producing a fault persistence output signal whenever one of the over-load and light-load signal outputs is generated for a period of time longer than a predetermined protection time; a steady state timer for inhibiting an enable output signal for a predetermined period of time after start-up and during shut-down of the electric motor; a start-up/shut-down comparator for producing an output signal for enabling operation of the protection timer and the steady state timer whenever the magnitude of the control signal exceeds a predetermined reference value corresponding to an electric current lower than the unloaded current of the electric motor; and, an output relay for interrupting operation of the electric motor when both the protection timer and steady state timer produce time coincident output signals.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the field of protective devices for electric 
machinery, in general, and in particular, to protective devices which 
detect over-load and light-load conditions in electric machinery. 
2. Prior Art 
Many kinds of machinery, driven by electric motors or other kinds of 
electrical drive mechanisms, usually require over-load and light-load 
protection. In the case of over-load conditions, excessive heating and 
current draw can result in destruction of the machinery, and in some 
cases, a general fire hazard. Under light-load conditions, such machines 
will tend to operate at unsafe higher speeds or rates of rotation, 
engendering thermal damage and structural failure. In either case, the 
risk of explosion or other catastrophic failure is a distinct possiblity. 
A good example is that of a ram which is operated under a no-load 
condition, that is, at a decreased water level. The pump itself is not 
only damaged, but the cooling unit to which the water is otherwise fed 
from the pump is unduly heated as well. Another example is that of an 
operating chain apparatus in a sludge water processing equipment. Such 
chains are immersed in the sludge water and are particularly subject to 
corrosion. When such chains snap, as expected from time to time, the 
breaking of the chain should be sensed in order to sound an alarm. 
In the conventional type of protective devices for electric machinery known 
heretofore, for use in protecting against the effects of over-load and 
light-load conditions, a contact in the control unit of an electric motor 
is closed simultaneously with start-up of the motor, starting a timer, in 
order to prevent the protective device from operating during the start-up 
period and the opening of a contact in the control unit is utilized in 
order to distinguish the no-load operation of a motor from shut-down 
thereof. In such systems, the protective device must be designed and 
installed fully in relationship with said contacts in the control unit for 
the electric motor of the particular machinery. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a protective device which 
eliminates the need for interconnection and interrelationship with the 
motor and/or drive mechanism of the machinery. Instead, means are provided 
for sensing an electric current in one of the wires supplying the electric 
motor, the sensed signals being utilized to provide the desired over-load 
and light-load detection. 
In a presently preferred embodiment, a current transformer is so disposed 
relative to an electric supply line for an induction motor as to develop a 
voltage signal corresponding to the magnitude of current flowing through 
the supply line. This voltage level is compared to an upper reference 
voltage signal and to a lower reference voltage signal. Voltage levels 
above the higher level and voltage levels below the lower level indicate 
one of an over-load or light-load condition. In order to avoid spurious 
indications from transient conditions a first timer is preferably employed 
with regard to assuring that such over-load or light-load condition 
persists for a minimum length of time. A second timing circuit is 
preferably provided in order to render the system ineffective during the 
known transient conditions which exist when the electric machinery is 
first started, or is stopped. 
In a presently preferred embodiment, an apparatus for protecting electric 
machinery driven by electric induction motors from operating under 
over-load and light-load conditions comprises: inductive means, 
operatively associated with an electrical supply line for the induction 
motor, for generating a control signal having a magnitude proportional to 
current flowing in the supply line; an upper limitation magnitude 
comparator for generating an over-load signal output whenever the 
magnitude of the control signal exceeds a predetermined upper safety 
value; a lower limitation magnitude comparator for generating a light-load 
signal output whenever the magnitude of the control signal is less than a 
predetermined lower safety level; an inhibitable protection timer for 
producing a fault persistence output signal whenever one of the over-load 
and light-load signal outputs is generated for a period of time longer 
than a predetermined protection time; a steady state timer for inhibiting 
an enable output signal for a predetermined period of time after start-up 
and during shut-down of the electric motor; a start-up/shut-down 
comparator for producing an output signal for enabling operation of the 
protection timer and the steady state timer whenever the magnitude of the 
voltage signal exceeds a predetermined reference value corresponding to an 
electric current lower than the unloaded current of the electric motor 
and, an output relay for interrupting operation of the electric motor when 
both the protective timer and the start-up/shut-down timer produce output 
signals. The protective device may also comprise a test circuit for 
generating voltage test signals in place of the voltage signal generated 
by the inductive means, one of the voltage test signals having a magnitude 
greater than the upper safe value, one of the voltage test signals having 
a magnitude less than the lower safe value yet greater than the reference 
voltage signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A presently preferred embodiment of a protective device according to this 
invention is shown in block diagram form in FIG. 1. Inductive means, in 
the form of a current transformer 18, is operatively associated with an 
electrical supply line of a 3-phase induction motor (not shown) for 
driving machinery. The current signal (I) is transformed to a stable DC 
voltage (V) control signal A in a current-to-voltage transducer or 
transforming circuit 1. The voltage signal A forms an input to an upper 
limitation magnitude comparator 2 and a lower limitation magnitude 
comparator 3. Upper limitation magnitude comparator 2 generates an 
over-load signal output whenever the magnitude of control signal A is 
greater than a predetermined upper safety value B. The upper safety value 
B is set by voltage level setting unit 4, and corresponds to that level of 
current which would ordinarily be expected to be present in the supply 
line if the induction motor operates under over-load conditions. The lower 
limitation magnitude comparator 3 generates a light-load signal output 
whenever the magnitude of the control signal A is less than a 
predetermined lower safety value C. The predetermined lower safety value C 
is provided by voltage level setting unit 5. The lower safety value C 
corresponds to the value of current below that which the induction motor 
would operate under light-load conditions. Upper limitation magnitude 
comparator 2 produces an output signal G whenever the magnitude of signal 
A exceeds the magnitude of signal B. Lower limitation magnitude comparator 
3 produces an output signal H whenever the magnitude of signal A falls 
below the magnitude of signal C. The output signals G and H form inputs to 
an OR gate 6, which produces an output signal I whenever one of the output 
signals G and H is produced. Accordingly, whenever the current drawn by 
the induction motor, as sensed by the current transformer, exceeds a 
predetermined value, indicating an over-load condition, or falls below a 
predetermined value, indicating a light-load condition, the output signal 
I of OR gate 6 signifies a load fault condition. 
A start-up/shut-down magnitude comparator 9 has as one input the output 
signal A produced by the current-voltage transducer circuit 1. A reference 
voltage setting unit 8, powered by DC power supply circuit 7, produces a 
reference voltage output signal D which corresponds in magnitude, with 
regard to the expected range of values of output signal A, to a current 
level lower than a non-load current of the induction motor, for example 
20% to 30% of the rated current. Reference voltage signal D is a second 
input to comparator 9. Start-up/shut-down magnitude comparator 9 always 
produces an output signal E, the polarity of which depends upon the 
relative magnitudes of signals A and D. If the magnitude of signal A is 
greater than the magnitude of signal D, the induction motor is presumed to 
be operating at a loaded condition. In this instance, the polarity of 
output signal E is positive. Whenever the magnitude of signal A falls 
below the magnitude of signal D, the motor is presumed to be shutting 
down. In this instance, the polarity of output signal E is negative. 
Depending upon the nature of the integrated circuits being utilized, 
output signal E of magnitude comparator 9 could also switch between a 
logical level "0" and a logical level "1". 
Signal E forms an enable/inhibit control line for a protection timer 10. A 
positive signal E also triggers a starting timer 11. The protection timer 
10 has as one input fault signal I. Whenever fault signal I is present for 
a period of time which exceeds the predetermined time-out of protection 
timer 10, an output fault persistence signal J is produced. The period of 
timing-out for protection timer 10 is adjusted by time setting unit 12. 
Accordingly, provided that the polarity of signal E is positive, whenever 
a fault signal I is generated, a predetermined time period begins to run 
by means of protection timer 10. Whenever the fault condition persists for 
a period of time longer than the time-out, the fault persistence output 
signal J is generated. If at any time the polarity of signal E becomes 
negative, the operation of protection timer 10 is inhibited, and no output 
signal J may be generated. 
Steady state or starting timer 11 begins a predetermined time-out period 
whenever the polarity of signal E is positive. The time-out period is set 
to a time longer than that which is necessary for the induction motor to 
achieve a steady state operating condition, in which transient start-up 
conditions have disappeared. The predetermined time is adjusted by time 
setting unit 13. At the end of the time-out period, an output signal F of 
positive polarity (or logical level "1"), is produced. Output signal F is 
not generated when the polarity of signal E is negative. 
Fault persistence signal J and steady state signal F form inputs to AND 
gate 14. Accordingly, even if a fault persistence signal J has been 
generated, it will be inhibited from further promulgation in the circuit 
if the induction motor is sensed to still be in a start-up condition with 
attendant transient conditions. In this way, such transient conditions 
will not be inadvertently mistaken for a real fault condition. 
Whenever a fault persistence signal J is produced during the presence of a 
positive level steady state signal F, the output of AND gate 14 energizes 
an output relay 15, which interrupts power to the electric motor and/or 
sounds an alarm that the machinery is over-loaded or light-loaded. The 
output relay 15 may be reset by a reset button 16. 
A test circuit 17 may form an alternative input to the current-voltage 
transformer circuit I, in addition to current transformer 18. The test 
circuit should be able to produce at least a first test signal having a 
magnitude greater than that of upper safety value signal B and at least a 
second test signal having a magnitude less than the lower safety value 
level C, but greater than the magnitude of reference signal D. 
In the embodiment of FIG. 1, a single output relay 15 is provided, which 
operates in response to either an over-load signal or a light-load signal. 
Alternatively, in place of using OR gate 6, it would be possible to 
utilize two protection timers 10, each of which was enabled by signal E, 
and two AND gates 14, each of which was enabled by steady state signal F, 
each of the resulting circuits being utilized to drive one of two relays 
15. In such an alternative embodiment, the alarm signal could then 
indicate which of the over-load and light-load conditions caused 
interruption of operation. 
A detailed circuit diagram of an apparatus according to this invention is 
shown in FIG. 2. A current transformer 18 is operationally disposed in the 
main circuit of an electric motor. The current-voltage transducer or 
transforming circuit 1 includes a full-wave rectifier 19 and a shaping 
circuit including condenser 20, and produces voltage signal A at resistor 
21, which is connected between power supply lines L.sub.1 and L.sub.0. The 
current-voltage transducer circuit also includes stable Zener diodes 23 
and 24 across the output of the rectifier and an ammeter 22 for showing at 
least a percentage of the electric current being drawn by the electric 
motor. The signals B and C representing the upper and lower safety values 
respectively are tapped off variable resistors 27 and 28 respectively. 
Resistors 27 and 28 are connected to the power supply line L.sub.2 of DC 
power supply circuit 7 through the voltage divider formed by resistors 25 
and 26, respectively. The magnitudes represented by signals B and C are 
compared with the magnitude of signal A in comparators 2 and 3. Output 
signal G of comparator 2 is produced whenever the magnitude of signal A 
exceeds the magnitude of signal B, signal G being of negative polarity. 
Signal H is produced whenever the magnitude of signal A falls below the 
magnitude of signal C. Signal H is also a negative polarity signal. Output 
signals G and H are hard-wired together at point a, point a being 
connected to a pull-up resistor. The output of the resulting hard-wired OR 
gate 6 forms an input to comparator 31. The other input of comparator 31 
is the output of a voltage divider network formed by resistors 29 and 30. 
Comparator 31 produces a fault signal I whenever the voltage at point a is 
decreased by the presence of output signals G or H. 
The DC power supply circuit 7 is connected to the secondary winding of a 
power transformer 32, the primary winding of which is connected to an AC 
power supply. The power supply circuit 7 includes a full-wave rectifier 
33, smoothing and shaping condenser 34, Zener diode 35 and voltage 
regulator 36. The Power-On state is displayed by light-emitting diode 37. 
The DC voltage is applied to a voltage dividing circuit formed by 
resistors 38 and 39, the output of which forms reference voltage signal D. 
The start-up/shut-down comparator 9 for sensing the turning on and turning 
off of the motor includes three comparators 40, 41 and 42. Reference 
signal D forms one input to comparator 40. The other input to comparator 
40 is a signal A', which is equal to the signal A, but as divided by 
resistors 43 and 44. Whenever the magnitude of signal A' is greater than 
the magnitude of signal D, an output signal E', of negative polarity, is 
produced. Point b sits at a voltage level which is equal to that of the 
power supply line L.sub.2, due to the charged capacitor 45. Whenever 
output signal E' is produced, the voltage level at point b decreases. The 
voltage level at b forms one input to each of comparators 41 and 42. The 
other inputs respectively of comparators 41 and 42 are formed by the 
output of a voltage divider formed by resistors 46 and 47. Comparators 41 
and 42 produce output signals E.sub.1 and E.sub.2 respectively, of 
positive polarity, whenever the voltage level at point b falls below the 
voltage level at the output of the voltage divider formed by resistors 46 
and 47. Signals E.sub.1 and E.sub.2 indicate that the electric motor is in 
an operating condition. 
In the circuitry of protection timer 10, the output of comparator 41 is 
connected to point C, between resistor 48 and the output of comparator 31. 
The voltage level at point c is held at zero when no output is produced by 
comparator 41. Fixed resistor 49 and variable resistor 50 connected in 
series to point c, and a capacitor 51 form a delay or time-out circuit. 
The capacitor 51 is always discharged through diode 52, whereby the 
voltage level at point d is held at zero. When signal E.sub.1 is not 
generated, the voltage level at point d is held at zero even if the signal 
I is generated, and accordingly, the over-load protection timer 10 is kept 
inoperative. However, when the signals E.sub.1 and I are produced, the 
capacitor 51 is charged, the voltage at d increasing at a rate determined 
by the timer constant of the R-C network including resistors 49 and 50 and 
capacitor 51. The voltage level at point d forms one input to comparator 
53. The other input to comparator 53 is the output of the voltage divider 
formed by resistors 29 and 30, which also forms an input to comparator 31. 
When the voltage level at point d reaches the voltage level at the output 
of the voltage divider formed by resistors 29 and 30, comparator 53 
produces a signal J, indicating that a fault condition has persisted for a 
predetermined period of time. The predetermined period of time may be 
adjusted by means of variable resistor 50, the variable resistor 50 
corresponding to the time setting unit 12 shown in FIG. 1. Its value may 
be set in accordance with the maximum amount of time for which it is 
believed the machinery can experience an over-load or light-load condition 
without being damaged, and at the same time, being set long enough so that 
transient conditions are not mistaken for fault conditions. 
Steady state or starting timer 11 has a circuit configuration similar to 
that of protection timer 10. When signal E.sub.2 is produced, the voltage 
level at point e is increased at a rate determined by the time constant of 
the R-C network including variable resistor 64 and capacitor 65, which 
together form time setting unit 13. When the voltage level at point e 
reaches the voltage level at the output of the voltage divider formed by 
resistors 54 and 55, comparator 56 produces an output signal F. 
An AND gate 14 is formed by transistor 57. When each of comparators 53 and 
56 produce output signals J and F, respectively, and simultaneously, 
transistor 57 becomes conductive. When transistor 57 becomes conductive, 
output relay 15 is energized and a self-holding switch contact point 58 is 
closed. A light-emitting diode 59 is provided for indicating that the 
output relay 15 has been operated. 
Test circuit 17 includes cooperative manual switches 60 and 61. When the 
switches are set to a point U, a voltage level higher than the maximum 
desired value of signal B is applied to the power supply line L.sub.1 
through resistors 62 and 63, resulting in the generation of signal G by 
comparator 2 and the generation of signal E' by comparator 40. The test 
circuit thereby simulates the presence of an over-load condition. 
Alternatively, when switches 60 and 61 are set to a point L, a voltage 
level lower than a minimum desired value for signal C is applied to 
resistor 21, through resistor 63, resulting in generation of signal H by 
comparator 3 and signal E' by comparator 40. The test circuit thereby 
simulates a light-load condition. 
In view of the foregoing arrangement, no electrical wiring is necessary 
other than that required for connecting the current transformer to sense 
the electric current in one of the main circuits for the electric motor 
driving the machinery. The protective relay is prevented from operation 
during start-up and shut-down of the electric motor, in order that there 
be no confusion with over-load or light-load fault conditions. 
Accordingly, positive protection against over-load and light-load 
conditions may be provided, while eliminating spurious operation due to 
transient conditions. 
This invention may be embodied in other specific forms without departing 
from the spirit or essential attributes thereof, and accordingly, 
reference should be made to the appended claims rather than to the 
foregoing specification, as indicating the scope of the invention.