jimmy2times
Electrical
- Jun 26, 2007
- 138
We have a shaft grounding system for marine propulsion drive (electric VFD propulsion). The shaft grounding which is achieved by 4 shaft grounding brushes (brushes are of carbon-silver-carbon design, with silver segement strip being the centre of the brush and the carbon segments either side of the silver). So the brushes are kind of special design for the application. These brushes run on a stainless steel slip ring on the shaft. Two brushes in parallel across the width of the slip ring and arranged so that 2 brushes are at 10 o'clock and 2 brushes at 2 o'clock on the shaft as viewed through the section. Each brush is cross section area 8cm^2 and has current carrying capacity of 160A (20A per square cm).
The brushes have two functions, one for control of the shaft voltage for VFD circulating currents (although our bearings are insulated as well) so maybe that is belts-and-braces in that aspect of the design. The other function is to properly ground the shaftline such that the propellors are at the same (or as very close to) the potential of the hull to ensure cathodic propection via our impressed current cathodic protection system.
We are experiencing corrosion issues and our current focus is on the reliability of the shaft grounding.
The leads of the grounding brushes are brought out to an accessible location and one line of investigation has been to conduct low ohm measurements using two different types of low ohm tests (one is a megger DLRO 10 unit, which injects 10A, the other is a megger MOM2 unit which we have observed can inject up to 130A depending on the resistance it encounters). We basically test the accessible brush leads (two leads per brush) with respect to one another, and then between pairs of brush leads, i.e. testing the contact resistance of the brush on the shaft and the shaft resistance itself by measuring between the leads of the 10 o'clock brush pairs and the 2 o'clock brush pairs.
We need to achieve the lowest contact resistance possible and standards refer to no greater than 10milliohms for the shaft resistance to the hull.
Through our testing we have observed following. After a time in service with propulsion unit operating (shaft rotating), when shaft then is stationary we take our resistance measurements using the DLR10 (10A) unit and we can see resistances in the 100's-1000's of milliohm range. Then directly after this test we repeat the test with the MOM2 unit (capable up to 200A) and we see a dramatic reduction in resistance into the 10s milliohm range. Directly after this test we then repeat with the DLR10 unit and achieve the same lowe resistance measurent as the MOM2 unit? This has been observed again and again. We have done multiple tests in a row with each device to ensure we are not getting some reading error, and we are convinced that the effect is real.
To achieve good cathodic protection we normally expect to measure between 10-20A flowing in this grounded circuit. Directly after using the MOM2 unit and the propulsion system is placed back into service and we achieve what is considered to be reasonable current flow in the circuit. However the effect is short lived and after days of operation the current deteriorates to very low levels (<1A) and does not improve until we repeat our MOM2 test again and the current increases once more.
Our hypothesis is that the MOM2 unit is cleaning the surface of the brush on the slipring.
We have also found in the operating instructions of the resistance meters the following note (presumably in relation to circuit breaker contact testing?)
" Operators using 100+ A micro-ohmmeters should be aware of certain technical issues related to testing at high currents. Some operators have indicated they perform a 10 A test and then see improved resistance readings with 100+ A test currents. This difference in the measurements raises the question of whether there is a need for additional maintenance. A strict reading of Ohm’s Law does not indicate the need for the higher current to perform the measurement. In the equation R = V/I, the magnitude of the current is not defined. Is this a situation where the high current is blasting contaminants away from the contacts, and at the same time welding the contacts together? The operator should be aware that they may be masking a potential problem in a power distribution system and avoiding necessary maintenance. "
The ship's have been dry docked and the brush alignment and contact pressure has been checked and found ok, yet this problem continues?
We suspect the inherent resistance of the brush on the shaft is the actual value obtained by the DLRO 10 unit, and if not for the test of the MOM2 unit the contact resistance would remain poor.
Questions:
1. Why would the contact resistance on the shaft deteriorate so poorly over such short time?
2. Is it possible that the brushes aren't carrying enough current, they are oversized for the application?
3. Could vibration be at play here?
4. Could the VSD operation be a factor?
We are scratching our heads!! So any help or experience in this area greatly appreciated.
The brushes have two functions, one for control of the shaft voltage for VFD circulating currents (although our bearings are insulated as well) so maybe that is belts-and-braces in that aspect of the design. The other function is to properly ground the shaftline such that the propellors are at the same (or as very close to) the potential of the hull to ensure cathodic propection via our impressed current cathodic protection system.
We are experiencing corrosion issues and our current focus is on the reliability of the shaft grounding.
The leads of the grounding brushes are brought out to an accessible location and one line of investigation has been to conduct low ohm measurements using two different types of low ohm tests (one is a megger DLRO 10 unit, which injects 10A, the other is a megger MOM2 unit which we have observed can inject up to 130A depending on the resistance it encounters). We basically test the accessible brush leads (two leads per brush) with respect to one another, and then between pairs of brush leads, i.e. testing the contact resistance of the brush on the shaft and the shaft resistance itself by measuring between the leads of the 10 o'clock brush pairs and the 2 o'clock brush pairs.
We need to achieve the lowest contact resistance possible and standards refer to no greater than 10milliohms for the shaft resistance to the hull.
Through our testing we have observed following. After a time in service with propulsion unit operating (shaft rotating), when shaft then is stationary we take our resistance measurements using the DLR10 (10A) unit and we can see resistances in the 100's-1000's of milliohm range. Then directly after this test we repeat the test with the MOM2 unit (capable up to 200A) and we see a dramatic reduction in resistance into the 10s milliohm range. Directly after this test we then repeat with the DLR10 unit and achieve the same lowe resistance measurent as the MOM2 unit? This has been observed again and again. We have done multiple tests in a row with each device to ensure we are not getting some reading error, and we are convinced that the effect is real.
To achieve good cathodic protection we normally expect to measure between 10-20A flowing in this grounded circuit. Directly after using the MOM2 unit and the propulsion system is placed back into service and we achieve what is considered to be reasonable current flow in the circuit. However the effect is short lived and after days of operation the current deteriorates to very low levels (<1A) and does not improve until we repeat our MOM2 test again and the current increases once more.
Our hypothesis is that the MOM2 unit is cleaning the surface of the brush on the slipring.
We have also found in the operating instructions of the resistance meters the following note (presumably in relation to circuit breaker contact testing?)
" Operators using 100+ A micro-ohmmeters should be aware of certain technical issues related to testing at high currents. Some operators have indicated they perform a 10 A test and then see improved resistance readings with 100+ A test currents. This difference in the measurements raises the question of whether there is a need for additional maintenance. A strict reading of Ohm’s Law does not indicate the need for the higher current to perform the measurement. In the equation R = V/I, the magnitude of the current is not defined. Is this a situation where the high current is blasting contaminants away from the contacts, and at the same time welding the contacts together? The operator should be aware that they may be masking a potential problem in a power distribution system and avoiding necessary maintenance. "
The ship's have been dry docked and the brush alignment and contact pressure has been checked and found ok, yet this problem continues?
We suspect the inherent resistance of the brush on the shaft is the actual value obtained by the DLRO 10 unit, and if not for the test of the MOM2 unit the contact resistance would remain poor.
Questions:
1. Why would the contact resistance on the shaft deteriorate so poorly over such short time?
2. Is it possible that the brushes aren't carrying enough current, they are oversized for the application?
3. Could vibration be at play here?
4. Could the VSD operation be a factor?
We are scratching our heads!! So any help or experience in this area greatly appreciated.