Unloader for air compressor?

[JHZR2]

Should they always have one?

While this is about my two old speedaire units, I think it’s a more universal question, thus a separate thread.

Both units are 15A at 230V. One has a motor that is listed at 110/230, the other just is rated for 230v. The pump is the same for both.

The older unit, the 115/230v unit has no unloader.

The other unit does.

Both heads have a tapped and plugged port:

Older 115/230:

Newer 230 only:

But it takes the unloader line from the main tank pipe as you can see.

The control on the older unit was changed. But there’s no obvious setup in the piping.

I get the principle but is it really necessary? Maybe that’s why the older unit is called a 2HP unit, with a big motor, while the other one is called 5HP… but again, same pump part number.

 

[Cujet]

As you noted, it is often not necessary. Especially on belt drive setups. Direct drive may be another story. And you correctly hit the nail on the head. The motor with more starting torque will likely not need an unloader.

If you have to change the switch and it has an unloader, prob best to use the same setup. Who knows what starting torque the imported 'replacement' compressor motor will have....

 

[JHZR2]

As you noted, it is often not necessary. Especially on belt drive setups. Direct drive may be another story. And you correctly hit the nail on the head. The motor with more starting torque will likely not need an unloader.

If you have to change the switch and it has an unloader, prob best to use the same setup. Who knows what starting torque the imported 'replacement' compressor motor will have....

Hopefully I’ll never buy an imported replacement compressor! These will get Amsoil compressor oil and won’t see too hard of a life.

But if I swapped the switch and found one with an unloader, I would probably try to retrofit… I think.

The motors are both the same ratings. One with the unloader has two capacitors on top, the other one that is 115/230 only has one… so I’d think that the 230 motor has a better starting assist(?). I assume one is a start cap, the other is a run capacitor, but iirc they’re both in parallel when starting up. (?).

 

[Hohn]

It really depends on how often the motor starts in your application. An unloader is intended to allow you the motor to idle at low load instead of having to expose the electric motor to the heat and stress of restarting.

The rule of thumb I got from Rolair is 4 starts per hour or <15 minutes between starts. If your electric motor is commonly starting more frequently than that, you’ll want to consider an unloader and/or an accessory tank that allows longer cycle time.

 

[oil pan 4]

Should they always have one?

While this is about my two old speedaire units, I think it’s a more universal question, thus a separate thread.

Both units are 15A at 230V. One has a motor that is listed at 110/230, the other just is rated for 230v. The pump is the same for both.

The older unit, the 115/230v unit has no unloader.

View attachment 284716

The other unit does.

View attachment 284717


Both heads have a tapped and plugged port:

Older 115/230:
View attachment 284718

Newer 230 only:

View attachment 284719
But it takes the unloader line from the main tank pipe as you can see.

The control on the older unit was changed. But there’s no obvious setup in the piping.

I get the principle but is it really necessary? Maybe that’s why the older unit is called a 2HP unit, with a big motor, while the other one is called 5HP… but again, same pump part number.

The unloader valve makes sure the motor will turn over nice and easy when the thing gets short cycled.
When the air compressor cuts off without an unloader valve eventually it will bleed the pressure back through the compressor and unload it's self.