FORM 2
THE PATENTS ACT, 1970 (39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION (See section 1.0 and rule 13)
TITLE OF THE INVENTION
Earth fault detector for an AC induction motor
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai - 400 030,
Maharashtra, India, an Indian Company
INVENTOR
Rawat Chandan Singh, Crompton Greaves Ltd, EDC, CG Global R&D Centre, Kanjurmarg(E), Mumbai 400 042, Maharashtra, India, an Indian national
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to
be performed:

FIELD OF THE INVENTION
This invention relates to an earth fault detector for an AC induction motor.
BACKGROUND OF THE INVENTION
An earth fault detector is a protective device for an AC induction motor. Under normal conditions of working of an AC induction motor, line current and neutral current in the voltage lines connecting the motor to a source of power supply are equal and balancing. During fault conditions of working of the motor due to leakage of current from the motor to the earth, for instance, on account of failure of insulation of the windings of the motor, there will be imbalance between the line current and neutral current and this can damage both the motor and source of power supply. In order to protect the motor against earth fault conditions, an earth fault detector is provided across the voltage lines. The earth fault detector comprises a current transformer, the primary winding of which is connected across the voltage lines and the secondary winding of which is connected to protective relay. The relay in turn is connected to a tripping device like circuit breaker or contactor connected across the voltage lines. During earth fault conditions, a current flows in the secondary winding of the transformer and develops a voltage across the secondary winding of the transformer. This current actuates the relay and the relay in turn actuates the tripping device to disconnect the power supply to the motor. The current transformer is very expensive and occupies large area.

OBJECTS OF THE INVENTION
An object of the invention is to provide an earth fault detector for an AC induction motor which detector is simple in construction and is cost effective.
Another object of the invention is to provide an earth fault detector for an AC induction motor which detector is compact and occupies reduced space.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided an earth fault detector for an AC induction motor, the detector comprising an inverter connected across the voltage lines connecting the induction motor to a source of power supply, a bridge rectifier one input of which is connected to the DC link capacitor of the inverter and the other input of which is earthed and the output of which is connected to a protective relay, the relay being connected to a tripping device which in turn is connected across the voltage lines and a Zener diode connected across the relay.
The following is a detailed description of the invention with reference to the accompanying drawings, in which the sole Fig I is a block diagram of the earth fault detector of the invention for an AC induction motor according to an embodiment thereof. The earth fault detector 1 as illustrated in Fig 1 of the accompanying drawings comprises an inverter 2 connected across the voltage lines 3 connecting an AC induction motor 4 to a source of power supply (not shown). 5 is a bridge rectifier one input of which is 6 connected to the centre of the DC link capacitor 7 of

the inverter through a DC blocking capacitor 8. The other input 9 of the bridge rectifier is earthed. The output 10 of the bridge rectifier is connected to a protective relay 11. The relay is connected to a tripping device 12 connected across the voltage lines. The tripping device includes a circuit breaker or contactor. 13 is a Zener diode connected across the relay. Under no fault conditions or normal working conditions of the motor, tine current and neutral current will be equal and balancing. When there is an earth fault in the motor, current flows through the bridge rectifier as one input of the bridge rectifier is connected to the centre of the DC link capacitor of the inverter and the other input of the bridge rectifier is earthed and the leakage current from the motor flows through the earth. The bridge rectifier rectifies the current into DC and outputs it to the relay. The Zener diode regulates the voltage across the relay due to flow of current in the relay and the relay actuates the tripping device to trip and discontinue power supply to the motor thereby protecting the motor and also the source of power supply against damage. The fault detector of the invention is very compact and occupies reduced space as compared to a current transformer. It is also less expensive as compared to a current transformer. The DC blocking capacitor is optional and the detector can work without the DC blocking capacitor. The DC blocking capacitor blocks leakage of DC into the bridge rectifier due to any malfunctioning of the inverter. It is understood that the earth fault detector of the invention can be used for detecting earth fault in any other electrical load like an uninterrupted power supply (UPS).

We claim:
1. An earth fault detector for an AC induction motor, the detector comprising an inverter connected across the voltage lines connecting the induction motor to a source of power supply, a bridge rectifier one input of which is connected to the DC link capacitor of the inverter and the other input of which is earthed and the output of which is connected to a protective relay, the relay being connected to a tripping device which in turn is connected across the voltage lines and a Zener diode connected across the relay.
2. The earth fault detector as claimed in claim 1, wherein the said one input of the bridge rectifier is connected to the centre of the DC link capacitor of the inverter through a DC blocking capacitor.