Low current ground fault relay

• US 6,327,124 B1
• Filed: 02/04/2000
• Issued: 12/04/2001
• Est. Priority Date: 02/05/1999
• Status: Expired due to Term

• Current Assignee: SMC Electricial Products Incorporated (Accton Technology Corporation)
• Original Assignee: SMC Electricial Products Incorporated (Accton Technology Corporation)
• Inventors: Fearing, Oliver H., Nojima, Geraldo
• Primary Examiner(s): SHERRY, MICHAEL JUDE
• Application Number:
  US09/498,581
• Time in Patent Office: 669 Days
• Field of Search: 361/23, 361/29, 361/31, 361/42, 361/47, 361/78, 361/79, 318/434, 318/445, 318/798, 318/806
• US Class Current: 361/31
• CPC Class Codes: H02H 11/005 in case of too low isolation resistance, too high load, short-circuit; earth fault
• CPC Class Codes: H02H 3/33 using summation current transformers H02H3/347 takes precedence

Abstract

A ground fault relay, especially useful to detect motor leakage currents due to insulation failure. The invention operates in two (2) modes. The first mode is a “look-ahead” mode where possible motor circuit insulation failure is detected prior to the electrical motor being turned on, and the second mode is “real time” mode which detects leakage currents after the motor is turned on and during running of the motor. Processing circuitry is disclosed which enables a user to set predetermined values of leakage current to be detected and to trip the relay after comparison of the leakage current and the predetermined value, if the former is larger than the latter. This invention is able to accurately detect extremely small values of ground leakage current without experiencing nuisance tripping due to extraneous noise.

Claims

1. A method for sensing a ground fault in a load control system, including a motor coupled to at least three conductors with a current transformer integral therewith to measure current therein and a ground fault relay for controlling the energization of the motor, the method comprising the steps of:

energizing a resistance fault relay to initiate a resistance measurement circuit while maintaining an open circuit to the motor;

measuring the resistance between a winding circuit in the motor and to ground;

comparing the measured resistance to a predetermined value set in the ground fault relay;

maintaining the open circuit condition to the motor until the relay compares the measured resistance to the predetermined value;

generating a signal to close the circuit to the motor in response to data indicating the measured resistance is lower than the predetermined set value;

closing the circuit to the motor to thereby energize the motor;

changing a mode switch from a resistance measuring mode to a current measuring mode of the motor;

measuring the load leakage current to ground;

comparing the measured load leakage current to ground to a predetermined value set in the ground fault relay; and

generating a signal to open the circuit to the motor in response to data indicating the measured load leakage current is higher than the set predetermined value.

2. The method of claim 1, further comprising the steps of:

injecting a current into the motor conductor to ground circuit,

producing an analog voltage drop signal which is proportional to the resistance between the conductor and ground,

providing a differential amplifier, amplifying the voltage drop signal,

changing the voltage drop signal from an analog signal to a digital signal, and

comparing the digital signal to the predetermined value set in the ground fault relay, thereby measuring the effective resistance between the motor conductor to ground.

3. The method of claim 2, further comprising the step of:

providing a plurality of gain settings in the differential amplifier for amplifying the voltage drop signal prior to the motor being energized.

4. The method of claim 2, further comprising the step of:

providing a plurality of gain settings in a resistance measuring circuit for varying the current injected into the motor conductor prior to energizing the motor.

5. The method of claim 1, further comprising the steps of:

generating a pulsating regulated current, injecting the pulsating regulated current into the auxiliary winding of the current transformer,

producing a periodical test signal, and

detecting the periodical test signal to verify the integrity of the current transformer and the connections of the current transformer to the ground fault relay.

6. The method of claim 5, further comprising the steps of:

providing a control processing circuit to detect the periodical test signal while the motor is energized, and

activating an error condition to deactivate the motor indicating the current transformer to be defective.

7. An apparatus for sensing ground faults in a motor control system comprising:

means for energizing a resistance fault relay to initiate a resistance measurement circuit,

means for measuring the resistance between a winding circuit in the motor and to ground,

means to compare the measured resistance to a predetermined value set in the control system,

means to generate a signal to close the circuit to the motor in response to the measured resistance,

contact means to close the circuit to the motor to energize the motor,

means to change a mode switch from a fault resistance measuring mode to a fault current measuring mode,

means to measure the load leakage current to ground,

means to compare the measured load leakage current to ground to a predetermined value, and

means to generate a signal to open the circuit to the motor in response to data indicating the measured load leakage current is higher than the set predetermined value.

8. The apparatus as claimed in claim 7 further comprising:

means for producing an analog voltage drop signal between the conductor and ground,

a differential amplifier means to amplify the voltage drop signal,

a converter means to convert the analog voltage drop signal from the amplifier means to a digital signal, and

means for comparing the digital signal to the predetermined value of resistance set in the ground fault relay.

9. An apparatus as claimed in claim 8 further comprising:

means to provide a plurality of gain settings to the differential amplifier for amplifying the voltage drop signal prior to the motor being energized.

10. An apparatus as claimed in claim 9 further comprising:

means for setting a plurality of gain parameters in a resistance measuring circuit for varying the current injected into the motor conductor.

11. An apparatus as claimed in claim 10 further comprising:

means for generating a pulsating regulated current,

means for injecting the pulsating regulated current into an auxiliary winding of the current transformer, and

means for producing a periodical test signal to verify the integrity of the current transformer and the connections of the current transformer to the ground fault relay.