Air Compressor Controller

[Bluejets]

For motors it's generally better to use a fuse.
They tolerate the inrush currents better and are often faster than a breaker.

I guess my point was, that you should start by knowing exactly what your load is before designing the rest of your circuit.

Don't use fuses down this way anymore.

Very true about the loading.
Never ceases to amaze me how difficult it is to get that info out of a customer sometimes. It's like they don't want to give away any secrets.

 

[Tha fios agaibh]

The plate states 8.97A for the FLA. the NEC calls for 11A for that hp motor and voltage but the plate overrides that.

Not true, according to the NEC; motor overcurrent protection and conductor sizing is determined by the motor tables in article 430. The motor name plate (fla) is used for calculating (thermal) overload protection.

But yes, the 30A breaker is sized right.

 

[Minder]

If doing any projects such as this in the future, I would recommend obtaining a current copy of NFPA79.
It is intended to show recommended methods and code details of NEC pertaining to industrial control.
I also have the 2007 version in pdf.
Here are a sample layout from it.
M.

 

[Tha fios agaibh]

Good idea Minder. Although it's worth mentioning that wiring contained within industrial equipment that's listed by a qualified testing laboratory is not bound by Nec rules.

This control panel is not "listed" so it'd still fall under Nec Art 409. Still, not a bad idea to follow some Nfpa79 rules such as splicing methods.

A bigger issue that has not been addressed is thermal overload protection for the motor.
This depends on several things including service factor and temperature rise of the motor.
Does the motor starter have overload heaters built into the bottom of it? Is protection built into the motor?
Overload protection is not the same as the circuit breaker which acts to protects against overcurrent.
Not having properly sized overload protection can have disastrous results.

 

[Bluejets]

I assumed his "motor start with overloads" was a contactor block with an overload unit of appropriate size attached.
Fairly standard arrangement for smaller motors.
Don't see the need for inbuilt thermistors and protection relay.

 

[Minder]

I assumed his "motor start with overloads" was a contactor block with an overload unit of appropriate size attached.

That was my conclusion also.
M.

 

[Tha fios agaibh]

The op never mentioned overload protection or specs on the motor.
That's why I'm bringing it up. I also don't know if he is starting with parts from scratch, or rebuilding with parts previously used with this compressor.

I am trying to draw a circuit for an air compressor controller....
I will first tell you the parts I have.....

The overload heaters types and sizes vary widely.
Idk, maybe he has proper overload protection. But to just assume it's sized correctly is rather shortsighted. If I was a betting man, I'd say it's wrong.
If we had a picture of the motor nameplate and overload relay, it could be quickly determined if it's right.

 

[Minder]

Also as to the 120v control transformer, it is usually recommended you set up a 'local' neutral as shown in the NFPA79 typical diag.
One of the secondary terminals should be earth grounded right at the secondary and connected to the service ground.
The neutral is taken from this point.
M.

 

[Martaine2005]

Or better still, check the date of the thread you reply to.

Martin