Why do cabs reduce PTO horsepower in some tractors?

[Labrat]

LRA is Locked Rotor Amps in the event the compressor is locked up. your running amps will be more like 17-18 running. 18 amps times 220 volts = 3960 watts divided by 746 watts per HP = 5.3 HP on your 5 ton unit. But that may be a scroll compressor that is more efficient than a recip.

Also tractor compressor will be turning much less compressor speed than a car engine so it will draw less power.

 

[Magicman710]

I've been looking around on the brochures for the JD compacts and I havn't found one that lists a cab model as having less PTO HP as the OOS version.

Seems only the 5E series is listed as having different HP's, and it makes since the AC compressor is the reason. I guess the HP is the exact same in the cab models if the AC is turned off or the heater on.

 

[nc redneck]

A lot of dealers will not sell less than 50 HP cab tractor, probably a good reason why.

 

[Henri88]

LRA is Locked Rotor Amps in the event the compressor is locked up. your running amps will be more like 17-18 running. 18 amps times 220 volts = 3960 watts divided by 746 watts per HP = 5.3 HP on your 5 ton unit. But that may be a scroll compressor that is more efficient than a recip.

Also tractor compressor will be turning much less compressor speed than a car engine so it will draw less power.

Hey Labrat, this is right up your alley! :thumbsup:

 

[xtn]

Also tractor compressor will be turning much less compressor speed than a car engine so it will draw less power.

How do you figure that? PTO operation speed on compact tractors is usually within an engine rpm range that is higher than what most car engines will be turning when cruising 65mph down the highway. Now figure in the fact that pulley ratios exist and so you have no basis whatsoever to even guess at what nominal rpm range a given manufacturer has elected to design his compressor installation for.

xtn

 

[xtn]

If an automotive compressor requires that much HP (12) to cool such a small space, how do whole house units do it with such small motors. My 5 ton house unit is rated at 23 LRA amps @ 220 volts. Per formula on internet, that comes to 4.4 HP. That size unit would have snow flakes coming from the vents on a car, so why do auto/tractor compressors require so much power?

AC systems for vehicles often have as much capacity as a small house requires, despite the much lower volume of space to be cooled would imply. The reasons for this include:

1. A vehicle cab usually has a MUCH higher ratio of glass area to non-glass area than a house.
2. Those parts of a vehicle cab that are non-glass are usually MUCH less insulated than a typical house roof or wall.
3. The fenestrations (openings, like doors and windows) of a vehicle cab usually comprise a MUCH higher percentage of the envelope area than is typical for a house.
4. There is a desire to cool a vehicle cab from 130degF down to 75degF in a few minutes. This ability is not designed into home HVAC systems, as they would be friggin HUGE!

Also consider that the systems designed for vehicles must maintain reasonable efficiency over a wider range of rpms and ambient conditions while being packaged into a smaller space, while your residential unit can have a compressor that runs at one design rpm (at one selected point on its efficiency curve) and be less concerned about its dimensions and weight.

xtn

 

[mikehaugen]

On cars, you can hear the idle speed fall a bit when turning on the AC but on my tractor, I don't notice any change at all in the engine speed.

I would guess because cars don't have a governor system really, yeah there is electronic idle control, but it isn't really the same as the mechanical governors at least on the older tractors that are designed to keep consistent rpms under varying loads. I am not sure how the newer computerized tractors work, but I would guess it's similar.

 

[Tx Jim]

AC systems for vehicles often have as much capacity as a small house requires, despite the much lower volume of space to be cooled would imply. The reasons for this include:xtn

Also there's a lot of heat generated from engine,trans & hyd systems on tractors that building AC units don't have to contend with.

 

[xtn]

Also there's a lot of heat generated from engine,trans & hyd systems on tractors that building AC units don't have to contend with.

I said that: "Also consider that the systems designed for vehicles must maintain reasonable efficiency over a wider range of rpms and ambient conditions...."