Indicator lamp

[trevj]

No but it's outside in a lean-to on the side of my shop so out of consideration for my neighbors I don't like it to run when I'm not working.

Yer not supposed to have it leak down! 🙂

Been a long standing issue here on the farm. If the leaky bits get left attached, the compressor cycles. Fix the leaky bits, much less neighbor/expense drama... And, it's charged up when I roll in looking for some air....

 

[YotaBota]

@Elektrishun - good point on the hp rating. My link was just an example of what's available not what may be needed for a compressor.

 

[YotaBota]

Ref 1.5hp - Is this another one of those motor hp mystery's? The ad for the timer I posted says 30Amps@240volts, that's 7200ish watts, 1.5hp is about 1100 watts. I'm missing something here between the rated watts and the hp watts.

@David - what breaker size is on the compressor circuit?

 

[David]

@YotaBota it's on a 20A breaker.

 

[slow-poke]

Ref 1.5hp - Is this another one of those motor hp mystery's? The ad for the timer I posted says 30Amps@240volts, that's 7200ish watts, 1.5hp is about 1100 watts. I'm missing something here between the rated watts and the hp watts.

@David - what breaker size is on the compressor circuit?

Compressor motors (especially the big box store type) are often rated SPL or "special" it's the peak HP rating just as the motor starts slipping poles like crazy and can sustain that power for a brief instant.

Much like the crazy ratings on car stereos. Assuming the car stereo does not have a DC-DC stage, most don't.
12V power supply
8 Ohm speaker
P = E^2/R
Simple Ohms law dictates max power as 144/8 = 18W, or 24.5W if the car is running

Advertised calculation:
Assume 4 Ohm speakers and 14V, yields 49W
Multiply 49 by 4 because you have 4 speakers, round up a bit and you have 200W
Then ignore actual Watts and instead use peak Watts for 282W or better yet, Peak to Peak Watts at 564W

And there you have it 18 becomes > 500

I installed car stereos in high school, one of the guys had a Colt with 2 alternators and IIRC 1000 real Watts per channel. That thing was dangerous to your hearing and shook the hell out of that little car when cranked up.

 

[Elektrishun]

Ref 1.5hp - Is this another one of those motor hp mystery's? The ad for the timer I posted says 30Amps@240volts, that's 7200ish watts, 1.5hp is about 1100 watts. I'm missing something here between the rated watts and the hp watts.

@David - what breaker size is on the compressor circuit?

No mystery.

Motor loads are inductive loads and are not the same as resistive loads like your water heater. There are power surges when the load is switched OFF and ON. I believe the scenario that would be most harmful is if that timer switched-off while the compressor was running. The created surge can damage the switch's contacts.

That specific timer could probably work for awhile but clearly the manufacturer didn't design it for a 3.7 HP motor.

 

[slow-poke]

No mystery.

Motor loads are inductive loads and are not the same as resistive loads like your water heater. There are power surges when the load is switched OFF and ON. I believe the scenario that would be most harmful is if that timer switched-off while the compressor was running. The created surge can damage the switch's contacts.

That specific timer could probably work for awhile but clearly the manufacturer didn't design it for a 3.7 HP motor.

It's not really a 3.7HP motor, probably closer to 2HP continuous. Likely will produce 3.7HP for about a second just before it stalls.

IIRC measured current when running was 4.7A, so 1.5HP

The reason this type of motor draws so much current at startup is because when the motor turns at less than it's rated speed, the back emf is low so the full MAINS voltage is applied to the very low winding resistance. As the rotor approaches synchronous speed the back emf increases to close to the mains voltage and so a much lower voltage is applied to windings.

The same thing happens when the motor is heavily loaded, it slows down and the decreased back emf causes the current to increase.

 

[Elektrishun]

It's not really a 3.7HP motor, probably closer to 2HP continuous. Likely will produce 3.7HP for about a second just before it stalls.

IIRC measured current when running was 4.7A, so 1.5HP

The reason this type of motor draws so much current at startup is because when the motor turns at less than it's rated speed, the back emf is low so the full MAINS voltage is applied to the very low winding resistance. As the rotor approaches synchronous speed the back emf increases to close to the mains voltage and so a much lower voltage is applied to windings.

The same thing happens when the motor is heavily loaded, it slows down and the decreased back emf causes the current to increase.

I would still use the ratings as a guide for electrical equipment. I do not have knowledge of how the people that engineer these products come up with their recommendations but I would assume they are partly based on extreme working conditions with plenty of head room before failure.

If I were to use a horse power rated switch to control a motor I wouldn't choose one that is based what I measure, I would choose the one that has the proper rating based on the motor nameplate.

 

[JustaDB]

Don't most manufacturer's recommend draining the tank at the bottom valve when not in use? Why not just get in the habit of powering it off and draining the tank at the end of the day?

Personally, I drain the tank every time I use it. And occasionally when I'm just passing by. I also keep it pressured up all the time. It's a PITA to wait for it to pressure it up when all I may need it for is a quick blast of air to clean something off.

 

[Elektrishun]

Specific to @David 's air compressor which I believe is hardwired to the electrical panel (dedicated circuit):

CEC 28-602 Types and ratings of disconnecting means

1) A disconnecting means for a motor branch circuit shall be

a) a manually operable fused or unfused motor-circuit switch that complies with Rule 14-010 b) and has a horsepower rating not less than that of the motor it serves;


The Intermatic EH40 has a manual override so I believe that's how to apply the rule.

But if you go into it further maybe you could also apply this rule:

CEC 28-602 (1)

d) an equivalent device that opens all ungrounded conductors of the branch circuit simultaneously and is capable of safely making and interrupting the locked rotor current of the connected load;

"Locked rotor current" is 6 times the motor nameplate FLA.

If you look in the companion book to the CEC, which goes into greater detail with code rules, under CEC 28-602 (1) (d) you will read "...be capable of safely opening the motor circuit while the motor is under load."

I believe that points to what I mentioned earlier about power surges with inductive motor loads when switching OFF and ON. I am assuming the equipment is designed to give a large margin of safety. And s**t should last longer when it is used with the appropriate rating.