Insulated/isolated alternator

[Spyro]

What is the reason that some "marine" alternators have the earth return insulated from the casing? The earth wire goes back to the battery negative terminal and that is connected to the engine block efectively earthing the casing of the alternator. I would have thought it better to have the casing earthed and connected to the battery through the engine block in case of a failure on the earth return wire and thus the possibility of blowing the diodes on the alternator.
Any experts?

 

[VicS]

What is the reason that some "marine" alternators have the earth return insulated from the casing? The earth wire goes back to the battery negative terminal and that is connected to the engine block efectively earthing the casing of the alternator. I would have thought it better to have the casing earthed and connected to the battery through the engine block in case of a failure on the earth return wire and thus the possibility of blowing the diodes on the alternator.
Any experts?

Some engine installations have a completely isolated negative return. Ie the block is not connected to DC negative.

Done to overcome corrosion problems.. Eg Nigel Merciers MD22 which has no insulation between the engine and the saildrive leg.

Having said that there are some "halfway" setups in which the starter motor is not insulated and the block has the be connected by an earthing solenoid during starting.

IIRC even Nigel's engine is a sort of 3/4 situation that requires the block to be earthed to DC negative by a relay during preheating and stopping.

 

[Spyro]

Some engine installations have a completely isolated negative return. Ie the block is not connected to DC negative.

Done to overcome corrosion problems.. Eg Nigel Merciers MD22 which has no insulation between the engine and the saildrive leg.

Having said that there are some "halfway" setups in which the starter motor is not insulated and the block has the be connected by an earthing solenoid during starting.

IIRC even Nigel's engine is a sort of 3/4 situation that requires the block to be earthed to DC negative by a relay during preheating and stopping.

So if the engine block is earthed (connected to dc -Ve) does that mean there is no need for an insulated alternator?
Reason I ask is I've just bought a new alternator and like the old one it is insulated but my engine block is also earthed so I couldn't see what benefit there was having it insulated.

 

[sailorman]

So if the engine block is earthed (connected to dc -Ve) does that mean there is no need for an insulated alternator?
Reason I ask is I've just bought a new alternator and like the old one it is insulated but my engine block is also earthed so I couldn't see what benefit there was having it insulated.

the neg on the engine will be for the starter motor rtn & only used momentarily during starting

 

[pvb]

So if the engine block is earthed (connected to dc -Ve) does that mean there is no need for an insulated alternator?
Reason I ask is I've just bought a new alternator and like the old one it is insulated but my engine block is also earthed so I couldn't see what benefit there was having it insulated.

As Vic mentioned, isolated return alternators are often used in installations in which corrosion is a concern - especially in metal-hulled boats. If your old alternator was insulated return, there may have been a reason for the choice, although if the engine is earthed it would seem to negate any advantage. It could be argued that having a direct negative connection to the alternator avoids the likelihood of poor connection integrity through the bracketry, block, etc.

 

[VicS]

the neg on the engine will be for the starter motor rtn & only used momentarily during starting

If its like some VP engines which have an earthing solenoid.
Not sure which without checking but from memory late 200n through MD 20n0 series and ?? early MD 22

If the block is not used as negative return the instrument and warning light sensors will be two-wire types

 

[VicS]

As Vic mentioned, isolated return alternators are often used in installations in which corrosion is a concern - especially in metal-hulled boats. If your old alternator was insulated return, there may have been a reason for the choice, although if the engine is earthed it would seem to negate any advantage. It could be argued that having a direct negative connection to the alternator avoids the likelihood of poor connection integrity through the bracketry, block, etc.

My old Volvo car has a separate negative connection to the alternator. Its only earthed back to the block nearby, presumably to bridge the mountings.

 

[charles_reed]

So if the engine block is earthed (connected to dc -Ve) does that mean there is no need for an insulated alternator?
Reason I ask is I've just bought a new alternator and like the old one it is insulated but my engine block is also earthed so I couldn't see what benefit there was having it insulated.

True marine alternators are fully insulated and intended to have a direct earth return to battery (they also have SS bearings). There are two reasons - one already mentioned is reducing galvanic corrosion another is the suppression of any arcing.
Most modern marine engines, like cars, are earthed to the engine block. For you there is probably no benefit in having double insulation, but you may like having SS bearings.

 

[vyv_cox]

In some cases they seem to be intended to reduce corrosion of saildrive legs, as Vic suggests. Volvo seem to have used many different regimes of isolation to protect legs and it is difficult to keep up. Isolation of the leg from the engine can be difficult to achieve and they seem to have tried other alternatives.