Main breaker tripping on startup, with a twist

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milemaker13

Senior Member
Funny thing we have noticed with our dust collectors. one is 75hp, 480v, full voltage starter. The other is 60 hp, 480v with a soft starter. Both come from the same panel using 100 amp breakers. That panel is fed from MDP 250amp breaker. When these are running they only actually draw about 45-60 amps depending on filter condition.

In warmer weather we had been starting the 75hp (full volt) first, then the soft start 60hp. Now that it is cold, the full volt starter trips the 250a breaker in the MDP. We kinda figured cold grease or whatever.

So we started the soft starter first and then tried starting the full volt. Came up no problem. This confuses me. I figured that would only add more load to the MDP breaker... so why does it seem to help starting the full voltage collector?

Just curious if anyone has an explanation? :?
 

milemaker13

Senior Member
Let me simplify....

When we start the 75hp (full voltage starter) first, the main breaker blows. If we start the 60hp (soft starter) first, then the 75 hp there is no problem. Main breaker does not blow.

Any ideas?
 

templdl

Senior Member
Location
Wisconsin
The breaker trips because of one of 2 reasons, either it sees a current with a magnitude h which is high enough to trip the breaker magnetically (instantaneously) of a current that is high and long enough in duration to trip the breaker thermally (overload).
If you are there at the time the breaker trip try relatching the breaker (resetting). If it latches it most likely tripped instantaneously. If it doesn't and you have to wait a bit before you can it tripped thermally.
It is normally not a good practice to reclose a breaker if it tripped magnetically because there is a good possibility that you would be closing it back into a fault which can be dangerous. However, I doubt if it is an actual fault in your case if it tripped magnetically.

I would be more inclined to assume that the breaker is tripping thermally because of a starting current that is too long in duration which is tripping the breaker thermally.
If so you might try using a clamp on ammeter to see what the current is which would give you an idea what your dealing with. When breaker doesn't trip and you have a normal start you may want to measure the current again to compare to the other. If you are experiencing unusually high stating current then you must looked at the cause.

Also, what is the magnetic setting of the main breaker? More likely than not when they are supplied the magnetic setting is set in the lowest position and quite often never changed.
If so there is not reason why you can't raise that setting, There is nothing that says that you can't.

This may not be the complete answer but it is a place to start. To jump around this may cause you to be guessing at the cause and then you may end up shooting at shadows having to still go back and define how the breaker is tripping and measuring current anyway.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Cold air is denser than warm air and makes the motor take longer to get up to speed. This should not happen if the mechanical designer did all of the calculations correctly and picked the correct horsepower for the worst case.
Does the 60 hp move air though the same system as the 75 hp?
 

ronaldrc

Senior Member
Location
Tennessee
Is your voltage maybe a little higher than usual?


Sometimes the power utility will raise there voltage in the winter time to compensate
for low voltage, there is another reason but I want go there.
:)
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
I figured that would only add more load to the MDP breaker.
Full load is minor compared to inrush currents.
To trip instantly the 250A breaker probably needs to see a minimum of 5x - this is 1250A. At full load your 60HP motor only draws 77A.
 

milemaker13

Senior Member
The 250a breaker trips instantly, and resets with out cooling. These are 2 seperate air systems.

The funny thing isn't that the breaker trips due to the inrush of the full volt starter... I get that.
Nor is it that the soft start does not trip the breaker. I understand that as well (much lower initial inrush using soft starter)

The funny thing is... the breaker does not trip if the 60hp is already running. Then the 75hp slaps in and away we go! No breaker tripped.

Why would having an additional load on the line prevent the breaker poping?

Its not really a problem we are having, I am just curious and want to understand what electrical theries (sp?) may apply to this situation.:dunce:
 

zog

Senior Member
Location
Charlotte, NC
Is your voltage maybe a little higher than usual?


Sometimes the power utility will raise there voltage in the winter time to compensate
for low voltage, there is another reason but I want go there.
:)

A given load will draw less current at a higher voltage
 

rcwilson

Senior Member
Location
Redmond, WA
Could the panel voltage be lower when the 60 HP motor is running due to votlage drop through the feeders to the panel? That voltage drop might be just enough to keep the 75 HP inrush below the breaker's trip point.

While possible, this doesn't seem likely because the starting inrush current on the 75 HP is much more than the 60 HP running current. (60 HP runing amps = 70+/-, 75HP inrush = 6 x 96 A = 576+/- A).
 

ronaldrc

Senior Member
Location
Tennessee
Maybe Charle could answer this, but couldn't the 60 HP motor already running act as a supplemental
Generator thereby keeping the voltage up?

I know I have worked at several factories where one of the mains might be open and the smallers
motors run fine and you never know theres a problem untill a very large motor tries to start. :)
 

hurk27

Senior Member
Maybe Charle could answer this, but couldn't the 60 HP motor already running act as a supplemental
Generator thereby keeping the voltage up?

I know I have worked at several factories where one of the mains might be open and the smallers
motors run fine and you never know theres a problem untill a very large motor tries to start. :)

works that way with roto-phase converters, start the smallest motors first then bring the larger motors online.
 

templdl

Senior Member
Location
Wisconsin
Now that it has been determined that the breaker is tripping magnetically. If the breaker it set in the lowest setting a 5X I personally would have no problem with increasing the setting to the max. There is nothing that says you can't. If you are concerned should raise the setting gradually, keep raising it little by little and see is the problem eventually goes away.
I look at it this way; the magnetic setting can cause nuisance tripping should it be set too low where a spurious spike in current occurs. The magnetic trip of the common breaker is there to detect fault current. Whether the setting is 5 or up to 10 times when there is a fault quite often it doesn't matter where it is set. I'm sure there often are experiences where a main with trip magnetically when there is a fault in a branch circuit protected by a smaller breaker. One would expect the branch to have tripped but when in fact the fault was high enough for both the branch and the main to see and then it became a race and the main won. In other words trying to coordinate the magneting setting of breakers is not assurance that there will be coordination.
I guess what I'm saying that you may be splitting hairs and you may not have a problem at all if the magnetic setting of the breaker is nuisance tripping because it is set too low and you are experiencing spike of current that may be harmless.

The only place where a magnetic pickup is adjusted is with MCPs in a combination motor starter where you would like them to be set just high enough to permit the motor to start without a nuisance trip. If the breaker trips down the road a bit one may suspect that the motor is starting to go south, that there is a winding failure in the works causing an arcing fault to ground and I don't see that as an issue in this application.

I think it's the dynamics of the starting condition that is causing an unusually high current spike which may not be a problem but is nuisance tripping a breaker where the magnetic setting is just set too low. If it's set in the highest position I would be very concerned though if it's tripping the breaker though.
 

milemaker13

Senior Member
Maybe Charle could answer this, but couldn't the 60 HP motor already running act as a supplemental
Generator thereby keeping the voltage up?

I know I have worked at several factories where one of the mains might be open and the smallers
motors run fine and you never know theres a problem untill a very large motor tries to start. :)

This is really the only thoughtful awnser so far... Thank you Ronaldrc.

Its like you guys arn't really reading the question... Just see a few key words you know and go off on a tangent. Just sayin...

I don't have a problem. There is no over load. Both dust collectors are running just as they should. No motors going south, ect.

I'm just curious, thats all.:thumbsup:
 

milemaker13

Senior Member
Could the panel voltage be lower when the 60 HP motor is running due to votlage drop through the feeders to the panel? That voltage drop might be just enough to keep the 75 HP inrush below the breaker's trip point.

While possible, this doesn't seem likely because the starting inrush current on the 75 HP is much more than the 60 HP running current. (60 HP runing amps = 70+/-, 75HP inrush = 6 x 96 A = 576+/- A).

Wouldn't a lower voltage (due to any reason) cause a HIGHER inrush current? I believe so...
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Wouldn't a lower voltage (due to any reason) cause a HIGHER inrush current? I believe so...
Have you ever heard of reduced voltage starting. Now that most are electronic, the common generic term is soft starters.
Motors behave differently during starting then when they are running at full load.
 

ronaldrc

Senior Member
Location
Tennessee
This is really the only thoughtful awnser so far... Thank you Ronaldrc.

Its like you guys arn't really reading the question... Just see a few key words you know and go off on a tangent. Just sayin...

I don't have a problem. There is no over load. Both dust collectors are running just as they should. No motors going south, ect.


I'm just curious, thats all.:thumbsup:


Your welcome


I wanted one of the Engineers to put it in more technical terms.

But if you decide thats what it is for sure you could always interlock the 75 HP through the 60Hp.

So the the 60 has to be running before you can start the 75 hp.

Have a Great Day :)
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Maybe Charle could answer this, but couldn't the 60 HP motor already running act as a supplemental
Generator thereby keeping the voltage up?

I know I have worked at several factories where one of the mains might be open and the smallers
motors run fine and you never know theres a problem untill a very large motor tries to start. :)
Remotely possible, but it's more likely still a related issue. If the 60Hp is already running under load, it is causing a slight voltage drop, but it happens to be just enough to keep the 75Hp breaker instantaneous trip from activating. As Jim Dungar said, similar effect to reducing voltage with a soft starter.

I'd still say the root cause however is that your instantaneous trip setting on the 75Hp breaker is too low. You don't want nuisance trips and since all it is doing is protecting against short circuits, there is no practical difference between a 5X setting and a 10X setting. The code allows that if you can demonstrate nuisance tripping, you are allowed to increase the setting until it doesn't.
 

rcwilson

Senior Member
Location
Redmond, WA
Wouldn't a lower voltage (due to any reason) cause a HIGHER inrush current? I believe so...

Only at normal operating speeds.

A running induction motor is a torque matching machine, adjusting its speed to produce the torque to match the load torque. The motors torque output increases as the rpm decreases and vice versa. At synchronous speed, 1800 rpm for a 4-pole motor, the motor has zero torque. The interaction of the spinning magnetic field and the magnetic field from the rotor creates this torque.

If the load is steady like a pump or a fan, operating at a constant rpm, the motor can be considered a constant power load. (Power = k x rpm x torque). Since power = volts x amps x pf, if voltage is less, current is more.

But during starting inrush, when the rotor is at standstill, the first instant of current flow is determined by the applied voltage and the combined resistance and impedance of the motor and the power system. Less voltage or more resistance = less current.

The 60 HP running motor may have a flywheel effect and act as an induction generator providing some current when the voltage dips during the 75HP starting inrush. But I didn't know if the soft starter would allow the 60 HP to do that. If it is supplying some current, that reduces the current seen by the upstream breaker.

I agree with the idea of raising the magnetic trip setting. A possible drawback of doing that is arc flash calculations may show a higher hazard if the calculated arcing current is now less than the new instantaneous trip level. Unlikely, but possible.
 

ronaldrc

Senior Member
Location
Tennessee
I thought most larger soft starters connected line to load through a mechanical contactor
once up to speed. Why run through the scr continuously? I'm more familiar with the old type
Transformer reduced voltage type starters.

If the Transformer that feeds this service was sized right I don't think the voltage
drop would be that much once the motor was up to speed.

I still believe its the flywheel effect of the 60 HP motor keeping the voltage from sagging
when the 75 goes into locked rotor on start up.

Either way why mess with the main current setting when you don't have to.

To whom are not familiar with starters

The lower the start voltage the lower the startup current.That is to a certain point.

The reason we use reduced voltage starters is to reduce the inrush surge current during startup.
Once the motor is up to speed the motor is put directly across the line.

:)
 
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