# 3 Phase Help



## Cow (Jan 16, 2008)

Lineman71373 said:


> Hooking up a rotary screw air compressor.
> Voltage is 240 3 phase. It trips breaker right at startup. 7.5 hp motor. Voltage is correct all the way through termination.
> Amps on startup, spikes at 150 +\-. Should be 22 +\-.
> Any advice would be greatly appreciated.


Verify the motor connections are correct.
Verify motor rotation is correct.
Verify voltage at startup is correct.
Uncouple the motor from the driven load and attempt to restart just the motor.


----------



## Lineman71373 (Mar 14, 2020)

Was the first T/S steps that was tried.
Voltage is balanced across all 3P.


----------



## gpop (May 14, 2018)

What do you know about this compressor. Was it new or used.

What do you mean by trips right at startup. Guess a time 0.1 seconds, 3 second, 5 second etc

do you here a hiss as soon as it trips like air escaping?

have you checked rotation direction and does it rotate when starting. how fast does it seem to turn.


----------



## Lineman71373 (Mar 14, 2020)

Comp, was bought used and came out of a working shop.

Breaker trips less than 5 seconds on S/U
Rotation is correct. 

Have disconnected from comp, and still trips. 

Opened all connections and checked continuity. All checked good.


----------



## telsa (May 22, 2015)

Meg it ASAP.


----------



## gpop (May 14, 2018)

how many wires from the control panel to the motor. what wires are connected at the motor.


----------



## varmit (Apr 19, 2009)

What size breaker are you using? This compressor would need a minimum 70 amp breaker to hold during start inrush. The start inrush is normally about 6 to 8 times the FLA marked on the motor. If the breaker is sized too close, the instantaneous (magnetic) function will trip the breaker on startup.


----------



## micromind (Aug 11, 2007)

Was it 240 at the old location?


----------



## Kevin (Feb 14, 2017)

Photo of the nameplate?

Sent from my new phone. Autocorrect may have changed stuff.


----------



## John Valdes (May 17, 2007)

Install the correct size breaker.


----------



## emtnut (Mar 1, 2015)

I'm with Varmit and John Valdes on this one :smile:
First thing that came to mind when reading the OP, didn't size breaker properly.


----------



## gpop (May 14, 2018)

4 to 5 seconds before tripping on start-up on a uncoupled motor with a amp reading of 150 when its name plate is 22 sound like the breaker is doing its job. (numbers come from op posts).


----------



## emtnut (Mar 1, 2015)

gpop said:


> 4 to 5 seconds before tripping on start-up on a uncoupled motor with a amp reading of 150 when its name plate is 22 sound like the breaker is doing its job. (numbers come from op posts).



Agreed, IF the 150A was measured uncoupled, and IF it's a 50A breaker tripping


----------



## glen1971 (Oct 10, 2012)

Lineman71373 said:


> Hooking up a rotary screw air compressor.
> Voltage is 240 3 phase. It trips breaker right at startup. 7.5 hp motor. Voltage is correct all the way through termination.
> Amps on startup, spikes at 150 +\-. Should be 22 +\-.
> Any advice would be greatly appreciated.


What size of breaker is feeding the compressor? The 22 amps, from what I can see online, is the FLA of the motor. Starting current will be higher, as others have said. 

Did the motor megger ok?

Was the supply voltage at the old location the same as where it lives now?


----------



## micromind (Aug 11, 2007)

Sounds sorta like a motor connected for 230 and supplied with 480.


----------



## brian john (Mar 11, 2007)

emtnut said:


> Agreed, IF the 150A was measured uncoupled, and IF it's a 50A breaker tripping


Would have to know more about the CB to see the Time Current Curve, 300% of rating (50 amps x 3 =150 amps) could be as low as 5 seconds up to 240 seconds or higher, depending on manufacture, style ETC.


----------



## gpop (May 14, 2018)

I like trouble shooting which is why i like as many detail as possible

instant trip would be:

phase to phase (would expect a cascade trip)
phase to ground
instantiations set to low (expect on new as they are set as low as possible)
safety shunt activated (phase loss)
possibly not the motor but something in the panel like a control transformer. 

5 seconds trip 

locked rotor (says it turns)
miss-wired motor (would require more than 3 leads)
safety shunt (phase loss)
starting problem (softstart, Y delta, etc)
temperamental short 
high load at starting (says he removed belts)
possible wrong voltage (not sure what that would do)
high resistance on one leg in the breaker or starter 

what ever the problem the breaker is beating the overloads which is odd for a turning motor. (then again we don't know what type of overloads)

So this one might turn out to be a real odd one especially if the information is correct. you would have expected by now that if it involved a short the time would drop after every attempt. 

Lineman could always post a bit more information and remove half the possibility just by telling us more or posting a picture of the control panel and the motor pecker head. I would also like to see the amps at starting with all 3 leads in a amp clamp.


----------



## JRaef (Mar 23, 2009)

Lineman71373 said:


> ...
> 
> Opened all connections and checked continuity. All checked good.


Continuity on a motor just means it has not (yet) caught on fire and melted the windings... You are checking the winding insulation with a 9VDC battery; even really bad insulation can hold up to that. 

You NEED to get a megger on it as tesla said a while ago. It sounds as though you have deteriorated winding insulation creating a phase-to-phase or phase-to-ground high resistance short and you are not able to see it with your DMM.


----------



## paulengr (Oct 8, 2017)

JRaef said:


> Continuity on a motor just means it has not (yet) caught on fire and melted the windings... You are checking the winding insulation with a 9VDC battery; even really bad insulation can hold up to that.
> 
> You NEED to get a megger on it as tesla said a while ago. It sounds as though you have deteriorated winding insulation creating a phase-to-phase or phase-to-ground high resistance short and you are not able to see it with your DMM.




The first thing you are measuring with resistance to ground/frame aka “megger test” is the capacitance of the motor ground wall insulation. It takes about 8-10 seconds to charge up the capacitor with the recommended voltages (500 V for all motors under 1000 V). At 9 V it will take about 4 hours. The insulation capacitance is not an indicator of anything. After that you are measuring leakage and polarization. At 10 minutes at 500 V the polarization is pretty much gone and all that is left is leakage.

So as an example that had this issue a waste water plant had a 10 HP motor. The pump contractor tested it with a multimeter and said it was fine. But when they reinstalled it, it ran for a minute before tripping the overload relay. So they swapped it out. I have a high range DMM and got 20 megaohms....again with 9 V. Then I switched to 500 V with a megger and it didn’t even reach 500 V (shorting out the meter). This is typical for marginal motors.


----------

