transmision in big wehicles like mining dump truck

[svizoman]

They use either planetary gears with torque converter like cars do or diesel electric like diesel lcomotives.
None uses hydrostatic transmision like excavator does with variabile displacement axial piston pump and fixed displacement axial piston motor.

if we look overall transmision efficiency for:

1. electric transmision
alternator has power factor round 0.85,
inverter has efficiency round 0.9,
induction motor has power factor round 0.85
so overal efficiency is 65%

2. automatic transmision
torque converter has efficiency from 0 upto 80% (0 when stationary, 80% when speed sycronizes) and some is lost in planetary gearset as well so overall efficiency for dump truck would not exceed 60%

3. hydrostatic transmision
axial piston pump is reported to have 90% overall efficiency,
same goes for fixed displacement axial piston motor,
up to 5% is lost in the valves and pipes
so overall efficiency would be 77%

So why they prefer diesel-electric or diesel mechanical transmison over hydrostatic?

 

[MikeHalloran]

Hydrostatics work best in lawn tractors, where the pump to motor distance is basically zero.
In, e.g., a bicycle, they can't compete with roller chain.
Haul trucks need suspension travel, so they'll need hydraulic hoses, really big ones, running at high pressure, which makes them expensive and short-lived.
Not to mention all the sliding parts running with very small clearances, also expensive.

Mechanical automatics got simpler with electronic computers instead of hydraulic computers, but they still have a lot of precision moving parts.

At thousand HP levels, electric propulsion is mechanically much simpler, and has to be cheaper, especially on a lifecycle basis.


Mike Halloran
Pembroke Pines, FL, USA

 

[svizoman]

I read that variabile displacement piston pump aslo looses efficiency with plate angle and they report that overal efficiency in hydrostatic transmission in practise won't exceed 50 to 60% efficiency. As you mentioned I read that much is lost in the piping especialy on tight turns.

So mechanical over electric? Caterpillar 797 for instance uses mechanical, while Liebherr T282B uses AC electric asme goes for Hittachi EH5000.

Braking is also important on such big vehicles you can't just apply friction brakes or you'll destroy them quick.
Diesel electric locomotive uses induction motors for most of the braking, road trucks uses special valve in the exhaust port so engine takes most of the braking, how is with torque converter? Do they use same solution as road trucks with exhaust valve?

 

[itsmoked]

svizoman; I think you're making a large mistake in your assessments with regards to the diesel electric solution. You seem to be using power factor incorrectly which has little to nothing to do with efficiency. Replacing the power factors with more realistic efficiencies gives a better performing result.

1. electric transmission
alternator has power factor round 0.85 more like 98%,
inverter has efficiency round 0.9 more like 96%,
induction motor has power factor round 0.85 more like 98%
so overall efficiency is 65% really 92%

Keith Cress
kcress -

http://www.flaminsystems.com

 

[svizoman]

I thought power factor is:
Power Factor = ( watts ) / (volt amps )
Power Factor = True power / Apparent power
Efficiency = Output / Input

 

[ScottyUK]

Reactive power consumption doesn't result in increased fuel burn in the prime mover, or at least not to a first order approximation.

 

[itsmoked]

I thought power factor is:
Power Factor = ( watts ) / (volt amps )

Correct.

I thought power factor is:
Power Factor = True power / Apparent power

Correct. But this speaks only to a relationship between leading and lagging of components of the electrical power - not about power consumption.

I thought power factor is:
Efficiency = Output / Input

Correct.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[3DDave]

Kieth - isn't power factor more an efficiency problem for the generator rather than the motor, causing transmission losses that aren't seen in the typical meters? At least my understanding is that if large electrical consumers have poor power factors the power companies try to influence them to change that. It seems like it would be a small effect if the transmission lines are short.

Check it out - 40 years after the EE classes and I'm pretty close.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/powfac.html

 

[itsmoked]

It's not about efficiency at all. Power factor being bad simply means more magnetic excitation current is required. It isn't a current that requires more fuel to be consumed hence the OP's error in using it for efficiency comparisons.

The reason power companies hate low power factors is because they simply have to have BIGGER equipment to handle the excitation current. They need bigger magnetic structures in all the transformers and bigger windings and cores in their generators, so they penalize users into either paying for the premiums bad power factors cost them or it incentivizes the users to correct their power factor.

The bigger currents needed to support the excitation can at second-order level cost a wee bit more fuel to support but it's scant and really something to be left out of this discussion when people are already missing the primary point that electrical power factor has nothing to do with the primary efficiency of electromotive functions.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Hoxton]

I remember putting some 1000 kW electric motors in some Mannesman Demag excavators. They were going over to electric hydraulic drive because of the poor reliability of diesel engines
In that application. I think that these were like an automobile auto transmission,Nike a fluid flywheel.
This must have been around 1985?

These excavators were BIG I remember having a meeting in a bucket!

Nothing against Diesel engines per se, but the actual duty of a large excavator is severe.

They would blast the quarry wall, then run the excavators at the debris, scoop up the rock, run over to a equally large dump truck and drop the load in. Repeat the cycle until rock gathered up.

So you would have a long idle, short high power run (to lift the rock) then low power run,

Few of these then a long idle, while they blast.

Not a good duty for a large diesel