motor swap

[phantomrfa]

hello all ,I have been trying to work out the size of a petrol/diesel engine for my compressor . It has a 3phase 4kw motor( no 3 phase at home) . Working on that most smaller compressors use 3hp/2.2kw 1phase 240volt in the u.k. BUT FIT 5.5 h.p. petrol ,am i right in thinking 9-11 hp will fit the bill :?:confused2:confused2

 

[vshearer]

Wiser minds than I will respond but you calc looks good. The straight convert from 4Kw is 5.36HP but starting torque is the determing issue which is why petrol engines are larger. One question, does the current set-up have a compressor unloader connected? This is common on some engine driven compressors and (in addition/or with to a clutch) allows reduced load on start.

 

[phantomrfa]

straight from motor to pump/belt drive reduction .this unit started life in a small dive club and never had a petrol engine fitted .pump type is bristol bpe 5cfm

 

[CraigAClark]

When I get gas driven Bauer units in that scfm range (5-6), they come equipped with Honda 5.5s. If you would rather stay electric, why not get a single phase motor? Once you choose the motor, gas or electric, determine the full running speed and use this to determine the correct pully size:

Driven RPM (compressor) divided by the Driver RPM (motor) times the Compressor pulley diameter = Motor Pulley Size.

Craig

 

[cool_hardware52]

hello all ,I have been trying to work out the size of a petrol/diesel engine for my compressor . It has a 3phase 4kw motor( no 3 phase at home) . Working on that most smaller compressors use 3hp/2.2kw 1phase 240volt in the u.k. BUT FIT 5.5 h.p. petrol ,am i right in thinking 9-11 hp will fit the bill :?:confused2:confused2

You might want to consider keeping the current 3phase motor and "making" 3 phase at home.

The traditional options for producing 3 phase from single phase aren't very appealing. The "solid state" converter are fairly cheap, but you loose 1/3 of the name plate horsepower. The rotary converters maintain full HP, but are expensive, and complex to install. A proper installation of a "Roto-Phase" involves extra disconnects, idle motors etc. This might be reasonable if you needed to run a whole shop full of 3 phase motors, but not for a single compressor.

There is another altenative, the "digital converter" or single phase input, 3 phase output variable frequency drive.

Look here.AC Drive, DC Drive, VFD and Variable Speed Drive - PC1-75

These are actually pretty easy to implement, but you probably want an electrician to help.

The advantage to these VF drives is you retain the 3 phase stock motor, avoid the need for a petrol engine, have an easy means to vary the speed of the motor if you want to. 3 phase motors are robust, efficient, and fairly low cost. A 5 hp single phase motor will cost ~3 times what you would pay for a 3 phase. You will need to modify little or nothing in the existing compressor controls.

You need to make sure you have a large enough (amps) electrical service to power a ~5 hp motor.

If you choose to go this route take advantage of the "applications" dept of the VF drive suppliers.

Tobin

 

[pescador775]

Phantom, if you wish to exchange the existing electric motor for a gas engine, suggest you purchase a 6.5 hp Briggs & Stratton Intek. If staying with the motor, purchase a VFD.

While exchanging the motor for an engine determine the rated speed of the motor. If it is close to 3500 rpm then the same pulley can be used on the engine. Otherwise, do the calc for a different pulley. Speed is proportional to pulley diameter.

The Intek should have enough torque to start up your compressor without a clutch and enough hp to run your compressor at full output.

 

[pescador775]

Here is a pic of the BPE:
Dive Compressor. on eBay, also, Cylinders Tanks, SCUBA Snorkelling, Sporting Goods (end time 29-Mar-08 17:50:25 GMT)

About the torque issue. Typically, this type of compressor employs a very large fan pulley served by a high rpm motor or engine equipped with a much smaller pulley. This arrangement provides the advantage of "torque multiplication" and, generally, can be expected to allow use of power sources of reasonable size without being subject to starting problems. Moreover, if an engine is used, the relatively small pulley allows rpms to quickly climb into a favorable range of the torque curve.

 

[CraigAClark]

Phantom, if you wish to exchange the existing electric motor for a gas engine, suggest you purchase a 6.5 hp Briggs & Stratton Intek. If staying with the motor, purchase a VFD.

While exchanging the motor for an engine determine the rated speed of the motor. If it is close to 3500 rpm then the same pulley can be used on the engine. Otherwise, do the calc for a different pulley. Speed is proportional to pulley diameter.

The Intek should have enough torque to start up your compressor without a clutch and enough hp to run your compressor at full output.

Hey Pesky,
His electric motor is probably 50Hz and if high RPM, turns at 2800. He more than likely will have to adjust the pulley size for gas driver. Not sure Briggs are available in the UK.

Craig

 

[Isaac-1]

I did a similar swap on my 3.2 CFM Mako (Bauer mounted in a crash frame) Purus compressor, mine came with a 3 HP electric motor, although the manual states they were offered with both a 2.5 and 3 HP electric motor, I replaced it with a 4 HP Honda GC-135 gas engine. I feel the GC-135 is marginal for this application (the price was right though), as it will start to sound labored and bog down once the pressure goes over 3000psi, and does not have enough torque to restart the compressor with over about 1000 psi of back pressure, so if I run out of gas with a tank near full, I must blead the lines down to restart.

Ike

p.s. note my compressor is mounted oposite from most Purus uints therefore I had to pick an engine like the Honda GC series so the exhaust would point away from the compressor.