Compressor timer switch question

[John Hasler]

The motor is "compressor-rated" at 5 hp. This seems to mean that it can deliver the same starting torque as a NEMA 5 hp motor and can deliver 5 hp for the last few seconds just before the pressure switch shuts it off without tripping the thermal overload. This rating assumes intermittent duty and a load that starts off low and ramps up.

My compressor has a similar motor. Works fine in that application. My neighbor has an old industrial compressor which has a real NEMA 5 hp motor rated for continuous duty. Looks like it weighs four times as much as mine.

Since your motor has internal thermal protection the compressor almost certainly has no motor controller. The pressure switch simply switches the line directly.

[Greg_Lewis]

Thanks John.

I just saw one of my neighbors on my morning walk and his dad is an electrician. The dad will be down from his mountain home later this week and my neighbor will get him to come over and we'll find out for sure what this motor draws. So until then this project will go on hold. I'll report back when I have some concrete info. Thanks again everyone.

[Bill Shields]

Depending on the responsiveness of the meter... you may miss actual peak. Years ago this used to need to be done by a recording oscilloscope to ensure the peak is accurately captured as analog meters did not show the accurate value.

I am not sure about how today's digital meters respond to these type of transients...and in reality it is not worth worrying about or trusting the number you see and making a decision based on it.

Just purchase a motor starter RATED for the HP of the motor and is not try to go cheap at this point in the circuitry.

[John Hasler]

A starter has overload protection ("heaters"). He doesn't need that as his motor has internal thermal protection. He just needs a contactor rated for 5 hp.

[Bentworker]

Did you get a solid answer on how to wire an excess run timer?

[Greg_Lewis]

Thanks, Bentworker. My neighbor's father is an electrician and he will be stopping by tomorrow with an ammeter. We'll find out how much current the motor draws and take it from there. John above suggested a timer from McMaster that will do the job if the amperage fits, otherwise I'll have to use a contactor downstream from the timer. I'll wire the pressure switch to the timer and then either from there to the motor or through the contactor.

My original confusion stemmed from the variety of timers offered and trying to define the terms used for them. "Interval timer" seems to be the most common name for what I need.

So I'll keep everyone posted on the result.

[Bill Shields]

i would not put any of these 'timer relays' directly in line of a 5 HP motor...no matter what the ammeter test says.

have you ever looked inside of one of these things to see how small the load conductive wiring is???

[Greg_Lewis]

i would not put any of these 'timer relays' directly in line of a 5 HP motor...no matter what the ammeter test says.

have you ever looked inside of one of these things to see how small the load conductive wiring is???

Good thought. The price of a relay is not so great that it couldn't be added.

[Bentworker]

I’d imagine you would need a 240V time delay on timer and a 240V contractor rated for 5HP

The black is what I imagine the existing wiring to be. You would have to disconnect the motor from the pressure switch and add everything in red to act as a run permissive. This should act like a one shot excess time. If it ran over whatever the timer was set to you would have to cycle the power to reset the lockout

[Greg_Lewis]

Bentworker:
GREAT! Thanks. As soon as we have some current values I'll move on this.

And just a side note..... as I look at parts, the price of a metal electrical box to put it all in is as much as some of the major components. (And in some of my other simple 12v dc projects, the box has been the most expensive part!) I should buy stock in a box maker!

[Bill Shields]

And your point is?

I do as much as I can with used surplus boxes..some are big and ugly but what the heck....

[Greg_Lewis]

Well, folks, the verdict is in. My neighbor came down with the ammeter, which was an old analog one but of professional quality. The continuous draw is about 16 amps, and the startup surge, which lasted perhaps a second, was just over 60 amps. So there will be a relay in the circuit.

It looks like I could use the timer John suggested: https://www.mcmaster.com/7630K34/
And this relay, rated for a 5HP motor at 220V: https://www.mcmaster.com/6564K726/

Of course, I would have to modify the schematic Bentworker so helpfully posted to run the timer on 120 volts and send the 240 straight to the relay and on to the motor.

Onward!