Bistable Undervoltage Release Circuit for Circuit Breakers

Means are currently available for interrupting the circuit to an electric motor when the motor voltage is less than operational in order to prevent damage to the motor. U.S. Pat. No. 4,097,831 to C. L. Jencks et al. assigned to the common assignee of the instant invention, describes an undervoltage accessory for use with insulated case circuit breakers. The undervoltage facility is provided by a solenoid containing an armature which is spring-loaded for engaging the breaker tripping mechanism. The presence of a voltage of a predetermined value on the solenoid winding produces sufficient magnetic flux to hold the armature against the spring force. In the event that the voltage decreases in value, or becomes interrupted, the flux provided by the solenoid winding is insufficient to overcome the spring force causing the armature to extend and engage the breaker tripping mechanism.

It is disadvantageous for the undervoltage release trip mechanism to become activated upon a momentary undervoltage condition. This is especially true when the motor is involved in a complex manufacturing operation and time is taken to investigate the cause of the tripping occurrence. In some instances, the circuit breaker must be manually reset.

U.S. Pat. No. 4,011,484 assigned to the United States Government, describes an undervoltage release having an electrical reset for the circuit breaker. The automatic reset function is designed to start the motor when the voltage attains the correct operating value. An automatic reset function for electric motors however is not always desirable. In some instances, it could restart the motor at an inopportune time in a manufacturing process resulting in injury to personnel as well as to product. It would be more advantageous to delay tripping the motor supply upon the occurrence of temporary undervoltage conditions of short duration.

The purpose of this invention is to provide a magnetically bistable tripping mechanism in combination with a control circuit to delay the tripping operation during spurious undervoltage conditions and to trip the supply voltage circuit breaker when the undervoltage condition persists for a predetermined period of time.

An undervoltage release unit (UVR) having a bistable magnetic armature is employed with a control circuit to provide a bistable UVR for use with a circuit breaker protected electric motor for example. The UVR magnetic armature arranged within a solenoid winding is selected to have a high magnetic remnance and a low coercivity. A first pulse of current within the solenoid winding holds the armature against the bias of a charged spring. If the undervoltage condition persists for a predetermined time period, a second pulse is generated within the solenoid winding in opposite polarity to the first pulse. The electromagnetic field produced in the solenoid winding by the second pulse opposes the magnetic flux produced by the first pulse causing the armature to extend under the influence of the charged spring.

FIG. 1 is a side view, in partial section, of a solenoid bistable undervoltage release of the instant invention; and

FIG. 2 is a schematic representation of a control circuit for use with the trip unit of FIG. 1.

FIG. 1 contains a UVR solenoid 10 which includes a frame 11 having a right sidewall 11A, left sidewall 11B and a bottom 11C. Frame 11 is generally fabricated from a readily magnetizable material, such as cold-rolled steel. A solenoid 12, containing a solenoid coil 13, arranged around a bobbin 14 is used to generate an electromagnetic field within which armature 15 is concentrically positioned. A plate 16 formed at one end of the armature contacts the tripping mechanism on an associated circuit breaker when extended to the position indicated in phantom at 16' as described for example in the aforementioned patent to Jencks et al. A top member 17 is attached to a top portion of frame and retained by means of tabs 18 formed from the sidewalls 11A, 11B. A clearance hole 19 through the top member allows for reciprocal motion of the armature but retains the armature within the magnetic influence of the solenoid coil 13. The armature is retained against the bias of a compression spring 9 arranged around the armature. For purposes of providing a bistable magnetic function, a material such as high carbon steel having a high degree of magnetic remnance and low magnetic coercivity is chosen for at least some portion of the magnetic path provided through the armature and the top, bottom and side walls of the frame as indicated. The magnetic path is broken by demagnetizing the armature thereby allowing the armature to become extended by the charged spring.

The control circuit 20 shown in FIG. 2 supplies the current pulses to the solenoid coil 13 and has a pair of terminals 21, 22 across which the control voltage supplied to the controlled motor circuit is also applied. The circuit consists of a plurality of linear resistors R.sub.1, R.sub.3 -R.sub.11, non-linear resistor R.sub.2, capacitors C.sub.1 -C.sub.3, diodes D.sub.1, D.sub.2, zener diodes D.sub.4, D.sub.5, thyristors Q.sub.1 -Q.sub.3, transistor Q.sub.4 and solenoid coil 13. Thyristors Q.sub.1 -Q.sub.3 are connected in the following manner. The anode of Q.sub.1 is directly connected to the gate of Q.sub.2 and the cathode of Q.sub.1 is connected through capacitor C.sub.2, resistor R.sub.5, trip coil 13 to the cathode of Q.sub.3. The gate of Q.sub.1 is directly connected to the anode of Q.sub.2 and with the anode of Q.sub.3 through resistor R.sub.6. The gate and cathode of Q.sub.3 are connected respectively to the collector and emitter of transistor Q.sub.4. The base of Q.sub.4 is connected through resistor R.sub.10 to the cathode of Q.sub.3 and to the cathode of Q.sub.1 through the series combination of resistor R.sub.9 and zener diode D.sub.4. The base of Q.sub.4 is connected to the cathode of Q.sub.2 through the series combination of resistor R.sub.11 and diode D.sub.5. The cathode of Q.sub.2 is connected to the gate of Q.sub.2 through capacitor C.sub.4. Resistor R.sub.8 and capacitor C.sub.3 are connected in parallel with the collector and emitter of Q.sub.4 and with the gate and cathode of Q.sub.3. Resistor R.sub.7 is connected between the collector of Q.sub.4 and the cathode of Q.sub.1. The cathode of Q.sub.3 is connected with the cathode of Q.sub.1 through diode D.sub.3. Resistor R.sub.4 is connected between the cathode of D.sub.3 and one side of trip coil 13. Diode D.sub.2 and resistor R.sub.3 are connected in parallel across the cathode and gate of thyristor Q.sub.1. Capacitor C.sub.1 is connected in parallel with the anode of Q.sub.1 and the anode of D.sub.3, and in series with diode D.sub.1 and resistor R.sub.1 back to terminal 21. Non-linear resistor R.sub.2 is connected to terminal 22 at one end and to the connection between one end of resistor R.sub.1 and the anode of D.sub.1. When terminals 21, 22 are connected across the control voltage applied to the protected motor, capacitor C.sub.1 becomes charged to the positive peak value of the control voltage. As long as the voltage across C.sub.1 exceeds the voltage V.sub.1 across zener diode D.sub.5, current flow through diode D.sub.5 and resistor R.sub.11 turns on transistor Q.sub.4 which functions as, and can be replaced with, a PNP and thyristor Q.sub.2 transistor, wherein the gate of Q.sub.2 serves as the emitter, the cathode of Q.sub.2 serves as the base, and the anode of Q.sub.2 serves as the collector of the pnp transistor. Current flow from gate to cathode of Q.sub.2, through diode D.sub.5, causes current to flow from gate to anode of Q.sub.2 and through resistor R.sub.3 and the gate of thyristor Q.sub.1. Thyristor Q.sub.1 becomes conductive and charges capacitor C.sub.2 through Q.sub.1, resistor R.sub.5 and the solenoid coil 13 with a current pulse sufficient to retain armature 15 in a non-trip condition. When the control voltage across terminals 21, 22 is reduced to a value less than V.sub.1 but greater than the voltage V.sub.2 across zener diode D.sub.4, current no longer flows through thyristor Q.sub.2 and diode D.sub.5. Current flow through diode D.sub.4 and resistor R.sub.9, however, maintains transistor Q.sub.4 conducting and saturated. When the control voltage V.sub.c drops below V.sub.2, current no longer flows through diode D.sub.4 and transistor Q.sub.4 while thyristor Q.sub.3 becomes triggered by current flow through resistor R.sub.7, and capacitor C.sub.2 discharges through resistor R.sub.6, thyristor Q.sub.3, trip coil 13 and resistor R.sub.5. The current flow through the solenoid coil produces an electromagnetic force in opposition to the remnant magnetic force causing armature 15 to become extended under the influence of spring 9 to trip the breaker. When the breaker is manually reset, armature 15 is retained by the magnetic field produced by the energized solenoid winding provided that the control voltage V.sub.c exceeds voltage V.sub.1.

When the UVR is used in combination with a shunt trip type circuit breaker, a signal is applied to solenoid coil 13 from the trip unit circuit to trip the breaker upon over current conditions only upon command. It is readily understood that a single bistable UVR can be multi-functionally employed for both overcurrent tripping on command as well as for providing undervoltage release within the same circuit.