Vehicle Generators Grounding & Protection

[coconutalley]

A 3-phase AC generator is mounted in a vehicle, say a truck and it is driven by the engine through a power take off. This generator provides power for the vehicle but it also can export power to external loads which can be grounded. The vehicle is metallic and it is required to be grounded as well. Does anyone know what is the best practice as cited by relevant standards (NEC, IEC etc) for: 1) short circuit protection and 2) grounding?
I know both are related: the ground is needed to clear circuit breakers for faults and also to prevent vehicle chassis/frame from developing high voltage during lightning storms. The other consideration: what about when the vehicle is on the move and still uses the generator for its internal electrical loads? Another question is the what about leakage currents in the chassis,
is there a limit?
Thanks in advance for any insights.

 

[waross]

Hypothetical question? The system that you describe will not work very well in practice. Although it may be technically possible, using a truck engine to drive a power alternator while the truck is being driven has issues that are nor worth solving in practice.
Le Tourneau tried this with earth movers about 50 years ago and proved conclusively that it was a bad idea. He put an alternator behind a diesel engine and a clutch and 5 speed transmission behind the alternator to drive the machine. All the functions that are normally done by hydraulics including steering were done with electric motors.
As a youth I was working on substation grounding while a group of 5 of these machines finished the civil work. Maintenance included changing one or two electric motors a week due to under frequency burnout. I haven't seen this type of system used since then. Le Tourneau was later more successful with machines with both an alternator and a DC generator. Propulsion was by DC wheel motors. The system was not suitable for feeding other lads.
Grounding, this should be done with the equipment that is supplied by a generator. Ground rods may have to be driven. As a general rule, one generator should be able to be disconnected and another connected without disturbing the grounding of the equipment.
This statement notwithstanding, there may be instances where it is prudent to drive a ground rod adjacent to the generator for grounding the generator vehicle, rather than using the fixed equipment ground to ground the vehicle. In most areas, this will be subject to codes and may be subject to additional requirements of the AHJ.
Check the NEC and/or CEC for carnivals and midways.
When in motion, the unit should be wired so that there are no ground currents or any other currents through the frame in normal operation. There may be currents through the frame during faults. The vehicle should be inspected for existing ground jumpers. These may be from the frame to the body across resilient mounts. These may have to be replaced with jumpers of greater ampacity to avoid burnout or fusing during fault conditions. I have seen these jumpers fused due to improper connection of a jumper battery.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

If you are talking about a small generator running something briefly I'd skip the ground. How many thousands of contractors do this daily? Perhaps use a GFI.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ccjersey]

More common and practical approach would be a stand-alone generator set in the vehicle. Still the same questions of grounding and protection in that situation.

With the modern electronic controls, LeTourneau's design is being revisited by JohnDeere in a midsized tractor shown in europe only at this point. It has a 3 phase alternator between engine and transmission and powers many PTO and accessory functions with 230/400 VAC power. The basic electric drive concept of his machines has also been used extensively in very heavy haul mining trucks for many years and now in the new Caterpillar D7E tractor which was recently unveiled. Both new tractors are expected to be released in 2009.

 

[waross]

The point was; using the propulsion engine for generation in a road vehicle is problematic.
Yes it may be done, but the speed demands on a truck engine and a generator engine are so different that solving the problems is usually not economical. It is easier to solve the problems with a tractor, a haul truck or a bulldozer than with a road truck.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ccjersey]

Agreed!

I think the manufacturers are depending on the controls to keep them out of the hot water that LeTourneau found himself in. As they say though, "some folks like it hot"!

R.G. LeTourneau stayed in the hot end of the pool a lot of the time and ended up creating much of the modern earthmoving equipment in use today. He must have had a tendency to think electric while others tended to think mechanical or hydraulic when confronted with a problem.

 

[jghrist]

If the loads are on the vehicle or cord connected to receptacles on the vehicle, then NEC does not require a grounding electrode. An equipment grounding conductor must be supplied to the load. This will carry return fault current. See NEC 250.34(B).

 

[coconutalley]

Thanks for the replies.
Vehicle will not be grounded when moving but will be grounded when stationary because it will power off vehicle loads which they may or may not be grounded.
The design of the 3-phase generator has two possibilities, keep the neutral floating inside or bring it out and connect it to chassis.
Any arguements for either?

 

[waross]

Please give us an indication of the size of this set and the possible uses.
Ground the neutral to the chassis. In the case that an external load is connected and externally grounded, any fault to ground in the generator or on the vehicle will result in the vehicle chassis becoming energized in respect to earth.
In such a case, anyone standing beside the vehicle and touching the vehicle may be subject to a fatal current.

Bill
--------------------
"Why not the best?"
Jimmy Carter