Motor Protection with Magnetic Only Breaker

[Ingenieur]

Here's your breaker
what code is the motor or the locked rotor amps?

https://www.downloads.siemens.com/d...fct=downloadasset&pos=download&id1=BTLV_40374

400% 3-7 sec

as was previously said try a 100 A cb or a breaker with etu
adjust instantaneous and perhaps toc with some time delay

 

[templdl]

I'm not sure how far down the rabbit trail this is going to get but from what I understood the OP has a Siemens QP 60at TM breaker that is tripping that installed in a panel board which has been sized to protect a feeder #6 wire. He said he selected he #6 wire for 74.5a @ 90degC based upon 125% the motor:'s FLA. Please note that he sized the wire based upon 90degC and not 75degC for which the terminals are rated. If the wire size is based upon 75degC he should have used #4wire.which is rated 85a @ 75degC. If so he can could have used a QP90at TM breaker.
Saying that the existing QP60 is nuisance tripping on start up it is a matter of what is tripping the breaker. My guess is because of the prolonged staring current and the instantaneous trip for a QP should be 600a or more without looking at a trip curve assuming thstthe LRA should be about 7x the FLA. As such that is why I think the QP60 is tripping on overload because off the prolonged starting time.
By sizing the wire based upon 75degC allows him to use #4 wire rated at 85a and use a 90at trip breaker in lieu of a 60at breaker.
If the feeder supplies motor control then consider if there is a motor control issue such as with a combination motor starter.
If if is a combination starter with an MCP that is nuisance trippinng that is the issue I may have missed that. But it was that QP60 that I though was tripping.

 

[kwired]

That really doesn't tell us much
assume lrc 300 A
probably much less than that with long conductors
assume 250
that is 400% (4 ir) of a 60 A cb
in the thermal region of the curve, 6 sec is not out of line

typ curve
400% is 6-10 sec
http://static.schneider-electric.us...100-400 A Frame FA-LA/FA-FC-FH/0600DB0105.pdf

I'm not sure how far down the rabbit trail this is going to get but from what I understood the OP has a Siemens QP 60at TM breaker that is tripping that installed in a panel board which has been sized to protect a feeder #6 wire. He said he selected he #6 wire for 74.5a @ 90degC based upon 125% the motor:'s FLA. Please note that he sized the wire based upon 90degC and not 75degC for which the terminals are rated. If the wire size is based upon 75degC he should have used #4wire.which is rated 85a @ 75degC. If so he can could have used a QP90at TM breaker.
Saying that the existing QP60 is nuisance tripping on start up it is a matter of what is tripping the breaker. My guess is because of the prolonged staring current and the instantaneous trip for a QP should be 600a or more without looking at a trip curve assuming thstthe LRA should be about 7x the FLA. As such that is why I think the QP60 is tripping on overload because off the prolonged starting time.
By sizing the wire based upon 75degC allows him to use #4 wire rated at 85a and use a 90at trip breaker in lieu of a 60at breaker.
If the feeder supplies motor control then consider if there is a motor control issue such as with a combination motor starter.
If if is a combination starter with an MCP that is nuisance trippinng that is the issue I may have missed that. But it was that QP60 that I though was tripping.

The other problem that has been mentioned is that the motor apparently doesn't even turn during this six seconds. There is either too much voltage drop during starting or the motor doesn't have enough torque to overcome the pressure against it when starting against loaded pump or likely a little of both.

 

[Smart $]

The other problem that has been mentioned is that the motor apparently doesn't even turn during this six seconds. There is either too much voltage drop during starting or the motor doesn't have enough torque to overcome the pressure against it when starting against loaded pump or likely a little of both.

And getting the voltage reading while the motor is not turning (and humming) should reveal how much of a contribution each is making...

 

[templdl]

The other problem that has been mentioned is that the motor apparently doesn't even turn during this six seconds. There is either too much voltage drop during starting or the motor doesn't have enough torque to overcome the pressure against it when starting against loaded pump or likely a little of both.

Yes, ends up tripping the QP60 the thermally.

 

[kwired]

Yes, ends up tripping the QP60 the thermally.

I agree it would be thermal trip and not instantaneous trip, now if the motor would at least turn it may not trip or at least current would start dropping as speed increased and maybe a 80 to 100 amp breaker holds long enough to get the motor accelerated.

 

[ibarrola]

I agree it would be thermal trip and not instantaneous trip, now if the motor would at least turn it may not trip or at least current would start dropping as speed increased and maybe a 80 to 100 amp breaker holds long enough to get the motor accelerated.

Sorry for the blackout... We were not able to measure voltage during humming of motor (I would have liked to see the value).

I asked to wire the transformer in delta-delta so we can feed 240V to the hydraulic unit. With voltage drop calculated we should be seeing a minimum of 230V for running operation.
The hydraulic unit worked as intended, the motor was able to start under pressure with now problem.

To answer Jraef's question about relieving the pressure, this unit does not do that, most hydraulic units or air compressors I've seen normally do, but this one does not have any automatic means to do so before starting the motor.

I've asked to the electrician to not use the panel and instead feed power to a MCCB breaker with adjustable trip so we can increase to 90A or so... Cable would remain the same since it's a 90C and per NEC it is rated for 75A. So a 20HP at 230V has a FLC of 59.4 per NEC table.... 125% of 59.4A = 74.25A

Overload is set to 75A per vendor, do still thing is too high. Vendor mentioned running current is 60 to 70amps when motor is under load.... Huh? I would ask vendor to confirm skid is under warranty with the overload setting being above safety values. Any comments on the overload?

Thanks,
Yogi

 

[Smart $]

... Cable would remain the same since it's a 90C and per NEC it is rated for 75A. So a 20HP at 230V has a FLC of 59.4 per NEC table.... 125% of 59.4A = 74.25A

... Any comments on the overload?

Cable is too small. At the very least, #4 is required. You can't go by 90°C rating. True that is for ampacity. But minimum size is based on [in modern context] 75°C terminations... for which #6 is only good for 65A.

I agree overload is set too high.

 

[templdl]

I agree it would be thermal trip and not instantaneous trip, now if the motor would at least turn it may not trip or at least current would start dropping as speed increased and maybe a 80 to 100 amp breaker holds long enough to get the motor accelerated.

It may but I be!ieve the problem goes farther than that. May it be s voltage drop that is occurring during startup? It appears as though the motor has some pretty hard starting conditions. As one of the others noted the OLR setting looks like it is ineffective since the QP60 is tripping (thank goodness) and not the OLR responding to the overcurrent and disconnecting the contactor. If the QP60 didn't trip the motor could fry.
After review the size of the branch circuit wire size and using the correct size OCPD to protect it as well as revisiting the OLR setting measure the voltage the motor as one of the others has suggested. That is where the rubber meets the road. If it should be adaquate enough to start the motor and still doesn't then target a motor load issue.

 

[Ingenieur]

Sorry if I missed it
how long is the witing run?

 

[templdl]

Sorry if I missed it
how long is the witing run?

As the plot thickens. We're starting to clear the dust (and not smoke yet).

 

[Ingenieur]

As the plot thickens. We're starting to clear the dust (and not smoke yet).

Even a short run of 6 awg with >200A lrc will be some serious drop

my advice get a partial hydraulic unloader if increasing the cb trip/td won't do it

100 ft at 240 A is 8%

 

[kwired]

It may but I be!ieve the problem goes farther than that. May it be s voltage drop that is occurring during startup? It appears as though the motor has some pretty hard starting conditions. As one of the others noted the OLR setting looks like it is ineffective since the QP60 is tripping (thank goodness) and not the OLR responding to the overcurrent and disconnecting the contactor. If the QP60 didn't trip the motor could fry.
After review the size of the branch circuit wire size and using the correct size OCPD to protect it as well as revisiting the OLR setting measure the voltage the motor as one of the others has suggested. That is where the rubber meets the road. If it should be adaquate enough to start the motor and still doesn't then target a motor load issue.

You also have to remember the overload is set well above nameplate rating, not surprised the marginally sized breaker trips first when motor is stalled/barely turning.

 

[ibarrola]

You also have to remember the overload is set well above nameplate rating, not surprised the marginally sized breaker trips first when motor is stalled/barely turning.

Thanks again, for all your help guys...

Talked to vendor, they'll be sending a smaller OL device were we can set values between 118% and 140%.
We'll be running a #4 to the skid and installing a proper MCCB with an adjustable trip.

 

[templdl]

Thanks again, for all your help guys...

Talked to vendor, they'll be sending a smaller OL device were we can set values between 118% and 140%.
We'll be running a #4 to the skid and installing a proper MCCB with an adjustable trip.

Great! You re reference to an adjustable trip must be for a replacement of that QP60at breaker located in that panel used to protect the branch circuit conductor. You are replacing the branch #6 branch circuit conductor with #4 which is rated 85a @ 75degC for which you can use a QP90 for your branch circuit protective device. There is no advantage to using an adjustable trip breaker which I highly doubt is available anyway.
The only other wild card that I would consider is if there may remain to be a voltage drop issue feeding your starter. If you are going to be replacing the #6 with #4 please resolve your motor starter issue to assure that you will not have a voltage drop issue with the #4 you may find it desirable to look for no more than a 10% voltage drop of the name plate voltage of the motor when attempting to start the motor.
It looks as if you have all your bases covered otherwise.

 

[ibarrola]

Great! You re reference to an adjustable trip must be for a replacement of that QP60at breaker located in that panel used to protect the branch circuit conductor. You are replacing the branch #6 branch circuit conductor with #4 which is rated 85a @ 75degC for which you can use a QP90 for your branch circuit protective device. There is no advantage to using an adjustable trip breaker which I highly doubt is available anyway.
The only other wild card that I would consider is if there may remain to be a voltage drop issue feeding your starter. If you are going to be replacing the #6 with #4 please resolve your motor starter issue to assure that you will not have a voltage drop issue with the #4 you may find it desirable to look for no more than a 10% voltage drop of the name plate voltage of the motor when attempting to start the motor.
It looks as if you have all your bases covered otherwise.

Thanks templdl...
We won't be installing a QP, it would be a Cutler Hammer MCP on a separate panel.

As for the voltage drop, we would have a lower drop due to less cable impedance, at the moment it would be less than 3%. We are supplying 240V for a 230V motors. So I think we are good with that.


I have one other problem...:weeping:
This is what happens you get asked to get involved after the fact everything has been installed and vendor mentions motor is 15HP instead of 20HP. Is this reverse engineering?

The transformer feeding the motor is a 30KVA 4.39% with a secondary current of 72.25A. Calculating voltage drop due to motor inrush is.... high. Nema B motor, inrush about 6xFLA... so 48*6 = 288A divided by sec current (72.25A) times impedance %, it gives me a voltage drop of 17.5%. :rotflmao:

Trying to see what to do...

 

[kwired]

Thanks templdl...
We won't be installing a QP, it would be a Cutler Hammer MCP on a separate panel.

As for the voltage drop, we would have a lower drop due to less cable impedance, at the moment it would be less than 3%. We are supplying 240V for a 230V motors. So I think we are good with that.


I have one other problem...:weeping:
This is what happens you get asked to get involved after the fact everything has been installed and vendor mentions motor is 15HP instead of 20HP. Is this reverse engineering?

The transformer feeding the motor is a 30KVA 4.39% with a secondary current of 72.25A. Calculating voltage drop due to motor inrush is.... high. Nema B motor, inrush about 6xFLA... so 48*6 = 288A divided by sec current (72.25A) times impedance %, it gives me a voltage drop of 17.5%. :rotflmao:

Trying to see what to do...

You showed us the motor nameplate earlier in the thread. It is 15 HP if connected to 50HZ 20 HP if connected to 60Hz.

Actual load of driven machine could be 15 or less but all we have is the motor nameplate, and if even if not fully loaded it will have 20 HP starting characteristics if started across the line.

 

[ibarrola]

You showed us the motor nameplate earlier in the thread. It is 15 HP if connected to 50HZ 20 HP if connected to 60Hz.

Actual load of driven machine could be 15 or less but all we have is the motor nameplate, and if even if not fully loaded it will have 20 HP starting characteristics if started across the line.

True, it depends on the Hz... But, vendor mentioned on details that were given about the unit, that it was a 15HP motor. One assumes its 15HP, 60Hz... But anyways, there were more problems with the installation than the motor itself.

 

[Jraef]

Thanks templdl...
We won't be installing a QP, it would be a Cutler Hammer MCP on a separate panel.

Oops.... Can't put an MCP in a separate panel. It can only be used by a starter mfr in a tested/listed combination starter. But you can get a Thermal-Mag breaker with the exact same mag trips, that's perfectly legal.

I have one other problem...:weeping:
This is what happens you get asked to get involved after the fact everything has been installed and vendor mentions motor is 15HP instead of 20HP. Is this reverse engineering?

So common for us in this part of the industry, it's sad...

It often goes the other way too; "Yeah, I know we said 20HP and I know that's what you bought the starter for, but they had a 30HP motor in stock and gave it to us for the same price! It was a great deal, is that a problem for you?" :rant:

 

[ibarrola]

Oops.... Can't put an MCP in a separate panel. It can only be used by a starter mfr in a tested/listed combination starter. But you can get a Thermal-Mag breaker with the exact same mag trips, that's perfectly legal.



So common for us in this part of the industry, it's sad...

It often goes the other way too; "Yeah, I know we said 20HP and I know that's what you bought the starter for, but they had a 30HP motor in stock and gave it to us for the same price! It was a great deal, is that a problem for you?" :rant:

Funny and sad at the same time...


Oh man... Ok, called in and asked for a thermal mag, so basically an inverse time, the MCP is only instantaneous... correct?
Right when I was reading your post I remembered that I read this somewhere in an article. That MCP are only to be used within a starter.

Thanks again Jraef, in a way, I'm glad to have gotten involved in this simple but erroneous installation. I have learned a lot... NEC is not something I deal with, but will definitely start studying it.