Generator efficiency?

Thread Starter


Joined Oct 25, 2010
Hi all,
I've a question about efficiency in generation and motors, more electric than electronic!

I'm building a mobile milking system, and am considering the best way to power it.
I have to power;
2 x 2kw vacuum pumps
1x 2kw hydraulic pump
1x 4kw compressor
All this is 3 phase 415v

The easy way is to buy a 3 phase diesel generator, however how much energy will I lose over powering everything from one shaft driven direct from a small diesel?

If I use a shaft drive it will involve a bit of engineering which isn't a problem but there will be losses in belts which I would need to drive the pumps and in electric clutches to power them on and off.

The whole thing could ultimately end up running for 20 hours a day so savings in fuel would soon add up.

My gut says a direct drive would be most efficient but would it be enough to justify the extra work? The vacuum is mechanically regulated, as is the hydraulic system so fluctuations in shaft speed as stuff starts and stops shouldn't matter too much.
Thanks Geoff


Joined Jun 19, 2017
What size 3 Phase Diesel Generator will you need?
You need to consider the Starting Watts requirement, too = Hard
Not just the Running Watts requirement = Easy
Do all four of the devices ever turn ON at the same time or are they always sequentially / stagger started?
This makes a huge difference.
Once started, do all of the devices stay ON for 20 hours or do they Cycle On-Off?
I think, matching the Starting Watts of the 3 Phase Generator/ Diesel Motor to your loads, will be the challenge.
Electric Motors can momentarily draw 3x to 5x the Watts, especially when starting with a load.
Diesel engines do have good torque

You need to consider the Lock-Rotor Amps Rating or LR Code Letter of each motor and design for worst case scenario.

AC Generators can be greater than 90% efficient, with less then 10% lost to heat.
How efficient is a direct drive system - no AC Generator & no AC Motors?
Electrical connections are very simple.
A mechanical design using pulleys, belts and clutches will need maintenance.
Max efficiency for a Diesel Engine is usually around 80% of rated HP.
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Joined Sep 7, 2010
I think you are asking if it would be a good Idea to drive your pumps and compressor by directly coupling them to a diesel engine ... and Not produce electricity and then feed electric motors which drive the pumps and compressors ..........

You would save energy by doing this , perhaps around 20 or 30% fuel saving ... It depends how skilled you are at hands on engineering. ...
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Joined Mar 14, 2008
Flat tooth (timing) belt drives can have efficiencies in the 96-99% range, so you would save significant fuel over a generator-motor combo, which likely wouldn't have a combined efficiency of more the 75-80%.
The belt system will likely also be cheaper than the generator and four large motors.
It may be possible to drive more than one device from one belt, as they do in automobile engines.

Thread Starter


Joined Oct 25, 2010
Thanks for the replies yes that is exactly what I was asking!

I already have the pumps fitted with motors, but a good Genny isn't cheap and if I can save ~20-30% fuel that will really add up, especially if its successful it might be going for 10-15 years:eek:.

I was thinking of using those multirib belts as you can get taperlock centres for them now.

Along with clutches like you get on car aircon.

I can stagger the starting of the compressor and hydraulic pump.
A big flywheel on the shaft should smooth out the bumps!

Thread Starter


Joined Oct 25, 2010
I'm not sure I do!
I've bought 2 second hand robots with all the ancillaries. The chap I got it from had a very long run on his milk line, the compressed air is used to purge the milk line, its pumped first the purged with compressed air. As such you have to have a food grade dried air supply. The gates are also pneumatic so there is some give in them!

My milk line will be quite short so I won't need the volume of air but he gave me a good deal on the compressor:)!

There is a vacuum pump for each robot and they are 3phase but have pretty fancy vfd on them. They can be switched to single speed and there is a mechanical regulator.

As someone stated before wiring is easier than engineering in this case but I'm only doing it once! (I hope)