Small-sized industrial rated electric motor starter switch unit

This is related to U.S. patent application Ser. No. 09/375,872 entitled Breaker/Starter with Auto-Configurable Trip Unit filed concurrently herewith which is incorporated herein by reference.

In the field of motor control, a conventional motor starter typically consists of a contactor and a motor overload relay. The contactor is typically a three-pole switch, which is usually operated by a continuously energized solenoid coil. Since the contactor controls the operation of the motor, i.e., the starting and stopping, it must be rated for many thousands of operations.

The overload relay, on the other hand, provides overload protection to the motor from overload conditions. Overload conditions occur when equipment is operated in an electrically undamaged circuit in excess of normal full-load rating, or when conductors carry current in excess of rated ampacity. Overload conditions persisting for a sufficient length of time will damage or overheat the equipment. Overload conditions do not include faults, which require instantaneous protection such as a short circuit or ground fault or a loss of a phase. The terms “overload,” “overload protection” and “overload relay” are defined in the National Electrical Manufacturers Association (NEMA) standard ICS2, which is herein incorporated by reference.

To protect a motor from faults, which require instantaneous protection, circuit breakers, e.g. instantaneous trip circuit breakers, are typically used. Additionally these circuit breakers function as a manual disconnect switch (disconnect) which serve to isolate the motor during a maintenance operation.

Devices which combine the instantaneous protection of a circuit breaker as well as the motor starter functions in a single enclosure are known as combination starters. However, the current-carrying components of instantaneous trip circuit breakers are constructed of heavy copper bars and large-sized tungsten contacts to protect against thermal damage upon short circuit interruption. This is especially so when the circuit breakers are used in circuits containing electric induction motors. The large size of the components increases the size of the circuit breaker to the extent that such circuit breakers do not fit within certain standard Asian and European circuit breaker enclosures. Therefore prior art combination starters utilizing instantaneous trip circuit breakers are bulky and expensive.

U.S. Pat. No. 4,754,247 entitled “Molded Case Electric Switch Accessory Enclosure” describes an arrangement whereby electric switch accessories can be inserted within the electric switch enclosure for electrical connection with the electric switch control circuit. U.S. Pat. No. 4,884,164 entitled “Molded Case Electronic Circuit Interrupter” describes an industrial-rated circuit breaker having an electronic trip unit that is adjustable over a wide range of long and short time over-current conditions. U.S. Pat. No. 4,149,216 entitled “Fused Unitized Combination Starter” describes the early use of a switch, contactor and fuse unit with the fuse unit connected in series between the switch and contactor. U.S. Pat. No. 5,822,164 entitled “Pre-Engineered Combination Starter for Multiple Motor Sizes” describes the use of an electronic overload relay, a contactor and a fuse for motor protection.

The use of a positive temperature coefficient device in series with an electronic circuit breaker to provide improved circuit protection to circuits including electric inductive motors to prevent the circuit breaker trip unit from responding to over-current motor start-up current is described within a more recent U.S. Pat. No. 5,539,370 entitled “Inductive Motor Protective Circuit Breaker.”

It has since been determined that adequate short circuit protection can be provided to low ampere-rated electric switches having smaller-sized moveable contact arms and contacts by the use of an electronic trip circuit for motor overload and phase loss conditions in induction motor protective circuits, a fuse for short circuit interruption, and a motor contactor unit for turning the motor ON and OFF. A deterrent to the use of replaceable fuses with electronic circuit protective devices is the inconvenience of locating the fuse and replacing the fuse before re-closing the electric switch contacts.

A further deterrent to the use of a motor contactor unit in combination with electronic trip units is the requirement of supplemental power for operation of the electronic trip units when the switch contacts are opened. This is because electronic trip units compensate for thermal memory effects through software algorithms. Thermal memory, i.e., operating memory, is defined in NEMA standard ICS2 as the cumulative heating effect in a motor circuit resulting from motor operation or overload, and the cooling effect after the motor circuit is deenergized. However, the ability to compensate for thermal memory is lost in prior art electronic trip units when power is interrupted.

A recent attempt to use the electronic trip unit in a “smart” circuit breaker incorporating an application specific integrated circuit (ASIC) microprocessor in combination with a motor contactor unit is found in U.S. Pat. No. 5,657,193 entitled “Electronic Control Module for Motor Controller Units” wherein a smart circuit breaker trip unit is interconnected with a contactor unit to employ the current sensing and circuit interrupting function of the circuit breaker in a motor protection circuit.

Thus there is a particular need for a molded case combination starter having an electronic trip unit for motor overload conditions, a fuse for short circuit interruption and a manual disconnect switch.

The molded case combination starter housing has an enclosure which includes a motor contactor control circuit within one enclosure for long motor overload, and a replaceable fuse unit within a separate enclosure for handling short-circuit type faults. Upon the occurrence of a short circuit within the protected circuit, the fuse rapidly responds to interrupt the short circuit. The fuse is conveniently replaced before power is restored to the protected circuit.

In an exemplary embodiment of the invention, a manual disconnect switch is provided. The disconnect isolates the motor during a maintenance operation, e.g., changing of the fuse.

FIG. 1 is top perspective view of a molded case combination starter having the electric switch control circuit, motor contactor control circuit and replaceable fuse in accordance with the invention; and

FIG. 2 is a diagrammatic representation of the electric switch control circuit, motor contactor control circuit and replaceable fuse used within the combination switch of FIG. 1 .

The combination starter 10 , according to the invention, is shown in FIG. 1 to consist of a molded plastic case 11 that includes a removable cover 12 for accessing the internal components whereby electric connection is made by means of the circular apertures 13 . The case is similar to that described within U.S. Pat. No. 4,754,247 entitled “Molded Case Circuit Breaker Accessory Enclosure” (which is incorporated by reference) which includes externally accessible accessory compartments 14 , 15 for housing bell alarm accessories and the like. Also contained within the case 11 is the combined electronic trip unit and contactor switch circuit 16 . In further accordance with the invention, three fuse compartments 17 which include replaceable fuses 37 such as Bussman or Gould type cartridge fuses that are capable of withstanding inrush motor starting currents and respond to actual short circuit occurrence during motor operation. Visual fuse indication status is obtained by means of the windows 18 formed in the covers of the fuse compartments 17 . The current transformers 19 for sensing circuit current are enclosed within the load side of the case 11 .

The combined electronic trip unit and contactor switch circuit 16 is shown in FIG. 2 connected with three-phase conductors L 1 and L 2 . Disconnect switch contact 34 is in series connection with induction motor 39 through fuse 37 and contactor contact 38 respectively. It should be appreciated that the portion of FIG. 2 comprising the circuit connections to motor 39 through switch contact 34 , transformer 19 , fuse 37 and contactor contact 38 is a single line diagram of a three phase circuit shown for purposes of clarity.

The current transformer 19 and manual disconnect switch contact 34 connect with conductor L 1 . The current transformer 19 connects with the control ASIC 26 via the rectifier 25 in switch control circuit 20 for determining motor overload and phase loss conditions in the manner described within U.S. Pat. No. 4,589,052 entitled “Digital I 2 T Pickup, Time Bands and Timing Control Circuit for Static Trip Circuit Breakers” (which is incorporated by reference). Upon occurrence of motor overload and phase loss conditions, a trip signal is outputted over conductor 67 and opto-isolator 65 to the coil energizing module 52 in the contactor control circuit 30 to separate the contacts 38 via the contactor coil 57 and control line 58 to interrupt circuit current conductor L 1 . Operating power is provided to the power supply 31 by means of the current transformer 19 . In accordance with the invention, supplemental operating power is provided to the power supply 31 via conductors 63 , 64 and isolation transformer 62 which connects with the voltage protection and rectification module 45 over conductors 60 , 61 as indicated. The voltage protection and rectification module 45 is similar to that contained within the CK Series Motor Contactor/Controller Unit of General Electric Company. Operating power to the contactor control circuit 30 is provided by means of multi-conductor cable 40 which connections within the three phase conductor L 1 and conductor 41 which connects with the conductor L 2 . The electric induction motor 39 connects with three phase conductor L 1 and conductor L 2 by means of the contacts 38 via control line 58 , described earlier, which connects with the contactor coil 57 and coil de-energizing module 59 for automatic separation of the contactor contacts 38 . The voltage protection and rectification module 45 further connects with the over/under voltage module 51 , control power module 48 , and coil energizing module 52 via conductors 46 , 47 , 49 , 50 and 53 - 56 , as indicated. Connection between the coil energizing module 52 and the coil de-energizing module 59 and contactor coil 57 , described earlier is made by means of conductors 68 , 69 . Connection between the coil energizing module 52 and the control ASIC 26 is made via conductor 67 and the optical isolator 65 for separation and closure of the contacts 38 . The replaceable fuses 37 within the fuse compartments 17 , shown earlier in FIG. 1, are selected to allow sufficient in-rush current to the motor 39 without interruption while interrupting upon occurrence of an overcurrent condition upon steady state motor operation in the manner described within U.S. Pat. No. 5,426,406 entitled “Induction Motor Protection Circuit Breaker Unit” (which is incorporated herein by reference). Manual motor STOP and START function is provided by means of the switches 42 and 43 respectively. An interlock contact 44 is arranged across the START switch 43 . It should be noted that conductor L 1 is connected directly to the voltage protection and rectification module 45 , such that the module 45 receives a voltage supply after the stop switch 42 is operated to deenergize interlock contact 44 . This assures that operating power will be supplied to switch control circuit 20 and contactor control circuit 30 at all times. This additionally assures that thermal memory will be compensated for in trip unit 26 at all times. It is noted that the components within the switch control circuit 20 and contactor control circuit 30 interconnect by means of the isolation transformer 62 and opto-isolator 65 to insure that the circuit current within the protected circuit does not transport therebetween to by-pass the contacts 38 .

Advantageously, the use of fuse 37 and disconnect 34 (as opposed to the use of a circuit breaker) in the combination starter 10 reduces the cost of starter 10 relative to prior art combination starters. Additionally, the disconnect 34 does not require the large current carrying components rated for short circuit interruption. Therefore the overall size of starter 10 is reduced relative to prior art combination starters.

A combined electric starter has herein been described in combination with an electric fuse to allow current-carrying contacts to be used over a wide range of motor current ratings.