Patent Application
20070236845
The present invention relates generally to circuits used in power sensing applications. More particularly, the present invention relates to circuits used for detecting open neutral configurations in power circuits for recreational vehicles.
Recreational vehicles (RVs) use the same type of power as residential housing. Namely, both recreational vehicles and residential housing are supplied by public utility power (240/120 VAC). For example, as shown in
When a generator is used to supply power, it is necessary to inhibit the flow of power to the utility power grid. In order to minimize the risk of putting power from the generator into the power grid, a switch over box switches from utility power to generator power. A common switch over box 28 is shown in
In one embodiment, the line contactors 36 are 50 Amp rated contactors while the line coil 44 which actuates the line contactors 36 is a 125 VDC coil. The neutral contactors 38 are 50 Amp rated contactors while the neutral coil 46 which actuates the neutral contactors 38 is a 125 VAC coil.
When power is supplied from the generator source, the transfer delay board 42 delays the opening and closing of the line contactors 36 and the neutral contactors 38, generally for 15 to 20 seconds so that the generator source may stabilize. Once the delay has passed, the line coil 44 is energized, and opens the line contactors 36 on the lines 30 and 32 from the utility and closes the line contactors 36 on the lines 30 and 32 from the generator. The line coil 44 also closes the auxiliary contactor 46 so that power will flow to the neutral coil 48. When the neutral coil 48 is energized, the neutral contactors 38 change state and power flows from the generator to the AC main power panel 40. When the generator is turned off, the coils 44 and 48 are de-energized and power flows from utility to the AC main power panel 40 as the line contactors 36 and neutral contactors 38 which are normally closed are returned to a closed state and the de-energized line contactors 36 and neutral contactors 38 which are normally open are returned to an open state.
The use of a mechanical connection to the utility power grid creates potential problems over time as the mechanical connections tend to degrade or erode causing an unreliable power source. It is also possible that the power plug may be improperly inserted into the utility supply receptacle. When either problem exists, an open neutral condition may occur. An open neutral condition can allow the full voltage differential between lines L1 and L2, that is 240 VAC, to be applied to all devices within an RV. Many devices within an RV are designed only for 120 VAC sources. When 120 VAC devices are energized with 240 VAC supplies, failure of the devices and possible injury to the RV and the owner may occur, including the ignition of RV materials.
An open neutral sensing circuit, comprising a first sensing resistance contacting a first line and a neutral; a second sensing resistance contacting a second line and the neutral having a value greater than the first sensing resistance; and a control switch contacting the first line and the neutral configured to switch from a first state to a second state when the voltage drop across the first sensing resistance is equal to the voltage drop across the second sensing resistance.
Turning now to
Turning now to
Power cannot flow from the L1 line 72 and the L2 line 74 to neutral 76. Instead, the power flows from L1 line 72 to the L2 line 74, and the voltage drop between L1 line 72 to the L2 line 74 will be 240 VAC. Because the loads are in series, the voltage drop across the TV 82 and the microwave 84 will total 240 VAC. Thus, the drop across the TV 80 will be 24 VAC while the drop across the microwave 82 will be 216 VAC. The voltage across the microwave 82 may overload the microwave 82 and the voltage drop across the TV 80 may also damage the TV 80.
Turning now to
The switch over box 86 includes L1 line 90, L2 line 92, and neutral lines 94. Line contactors C196 and neutral contactors C298 mechanically couple the power sources (utility source and generator source, respectively) to an AC main panel 100. A pair of secondary neutral contactors 101 couple the neutral lines 94 to the generator source. A transfer delay board 102 delays power to the main panel from the generator source and converts the AC power from the generator source to a DC current. A line coil 104 actuates the line contactors 96. An auxiliary contactor 106 is also controlled by the line coil 104. A neutral coil 108 actuates the neutral contactors 98.
In one embodiment, the line contactors 96 are 50 Amp rated contactors while the line coil 104 which actuates the line contactors 96 is a 125 VDC coil. The neutral contactors 98 are 50 Amp rated contactors while the neutral coil 106 which actuates the neutral contactors 98 is a 125 VDC coil.
When power is supplied from the generator source, the transfer delay board 102 delays the closing of the neutral contactors 98, generally for 15 to 20 seconds. Once the delay has passed, the line coil 104 is energized, and switches the state of the line contactors 96, opening the normally closed line contactors 96 and closing the normally open line contactors 96. The coil 104 also closes the auxiliary contactor 106 so the power will flow to the neutral coil 108. When the neutral coil 108 is energized, the neutral contactors 98 switch states, opening the normally closed neutral contactors 98 and closing the normally open neutral contactors 98. Power flows from the generator to the AC main power panel 100. When the generator is turned off, the coils 104 and 108 are de-energized and power flows from utility to the AC main power panel 100 as the line contactors 96 and the neutral contactors 98 which are normally closed are returned to the closed state and the de-energized neutral contactors 98 and the line contactors 96 which are normally open are returned to the open state.
Regardless of the source of power, the power flows from the power source to the AC main power panel 100 through the open neutral sensing circuit 88. The open neutral sensing circuit 88 includes a control relay 110, a power relay 112, a neutral relay 113, a pair of normally open power actuators 114, a normally open control actuator 116, a high load resistor 118 and a low load resistor 120. The low load resistor 120 is placed between the neutral 94 and the L1 line 90. The high load resistor 118 is placed between the neutral 94 and the L2 line 92. One of the normally open power actuators 114 is placed on the L1 line 90, and the other normally open power actuator 114 is placed on the L2 line 92. The control relay 110 is placed between the neutral 94 and the L1 line 90, and is parallel to the low load resistor 118. The normally open control actuator 116 and the power relay 112 are placed in series between the neutral 94 and the L2 line 92, and are parallel to the high load resistor 118. The neutral relay 113 is placed parallel to the power relay 112.
The open neutral sensing circuit 88 senses an open neutral by measuring the voltage drop across the load resistors 118 and 120. When the neutral 94 is closed (i.e., properly attached to the power source and AC main panel 100), the voltage drop across both load resistors 118 and 120 is 120 VAC. The control relay 110 is energized and the relay closes the normally open control actuator 116. When the normally open control actuator 116 closes, then power flows to the power relay 112 and the normally open power actuators 114 close. Power then flows from the power source to the AC main panel 100.
When the neutral 94 is open, the voltage drop across the low load resistor 118 is too small to actuate the control relay 110. Because the control relay 110 does not close the normally open control actuator 116, the power relay 112 does not energize and power does not flow to the AC main panel 100. The neutral relay 113 is placed in parallel to the power relay 112 to close the neutral lines 94 to the generator when the neutral is properly sensed. The neutral relay 113 keeps the neutral 94 of the generator from interfering with the neutral of the generator.
As an example, the resistive loads 118 and 120 may be set to 270 kOhms and 2.7 MOhms. The ten fold difference in resistance between the two loads 118 and 120 allows the voltage drop difference between the low load resistor 118 and the high load resistor 120 to be markedly different when the neutral 94 is open. Moreover, by setting the resistances 118 and 120 relatively high to the resistances inside the RV, the current to the AC main panel 100 will be minimally effected by the open neutral sensing circuit 88.
The control relay 110 may be chosen as a 110 VAC relay. This allows the control relay 110 to close when the voltage drop across the low load resistor 118 is at least 110 VAC. Such a condition exists only when the neutral 94 is closed. The power relay 112 and neutral relay 113 may also be a 110 VAC relay. Because the power relay 112 closes contactors 114 that are placed along the L1 line 90 and the L2 line 92, the power relay should be rated to high currents. In this example, the relay 112 is rated to 50 Amps. The neutral relay 113
Turning now to
While the invention has been shown in embodiments described herein, it will be obvious to those skilled in the art that the invention is not so limited but may be modified with various changes that are still within the spirit of the invention.
