Motor Protection with Magnetic Only Breaker

[kwired]

Sorry for the acronym, Hydraulic Power Unit... It has one 15HP motor and it reaches 3000PSI, when pressure reduces to 2700PSI, the motor starts and that's when it trips. So they have to manually relieve the pressure to let's say 2000PSI and then start the motor in order to get to 3000PSI.

The nameplate would be at the motor... HP 20//15...Volts 230/460//190/380... Amps 48/24//44/22...SF 1.15, NEMA B

Thanks Smart$

As was mentioned this appears to be a 20 HP motor if connected to 60Hz. My bet is you need at least a 100 amp breaker to ensure it holds during starting, that is what I would put on it in most applications, and is what my Square D slide motor calculator recommends, don't know what Siemens recommends but would also bet it is no less then 90 and possibly up to 125.

If it is 20 HP connected to 208 volt three phase - NEC FLC is 62.1 amps and minimum size 75C copper conductor is 4 AWG.

Whether or not this motor is designed to operate on 208 is another issue, but in many instances it probably will be fine at 208.

60 amp breaker is not enough for starting 20 HP across the line, 60 amp breaker is common size used @ 480 volts for 20 hp motors.

 

[Jraef]

He's technically installing it as a 230V motor, as the motor is rated for 208V. Would not the 230V value (54A) be the required value to use for rating determination?

Actually, I'm not sure what you do there when the motor nameplate doesn't actually say it's OK to use at 208V, you may be right. On the smaller motors that are "triple rated" 208/230/460, they give you an actual 208V FLA value, so I use the 208V column in the chart. Not sure here, I wouldn't do it, I'd boost it to 230V 'cuz I hate replacing motors...

If it is 20 HP connected to 208 volt three phase - NEC FLC is 62.1 amps

That's the 200V column by the way... DEFINITELY out of the -10% range for a 230V motor!

 

[Smart $]

He's technically installing it as a 230V motor, as the motor is NOT rated for 208V. Would not the 230V value (54A) be the required value to use for rating determination?

Correction in bold red.

 

[Jraef]

Correction in bold red.

I knew what you meant... I had been thinking about it too :thumbsup:

 

[kwired]

Actually, I'm not sure what you do there when the motor nameplate doesn't actually say it's OK to use at 208V, you may be right. On the smaller motors that are "triple rated" 208/230/460, they give you an actual 208V FLA value, so I use the 208V column in the chart. Not sure here, I wouldn't do it, I'd boost it to 230V 'cuz I hate replacing motors...


That's the 200V column by the way... DEFINITELY out of the -10% range for a 230V motor!

I guess I have always had pretty fair luck with 230 volt motors on 208 nominal systems, but actual operating voltage of a 208 volt system in these parts is usually close to 215 volts, if nameplate actually says 220 then we are even closer to nameplate voltage.

 

[templdl]

Templdl,

It is a non adjustable 60A 3P Siemens Type QP breaker on a panel, specs say its a thermal magnetic bkr. Looks like breakers that we install on house panels.

To me the overload protects the motor from exactly that, and the breaker is there for any short circuit, but I always thought that the cable's ampacity would need to be higher then the breakers trip rating.
Between the overload and the breaker, there is a run of 3/C 6AWG.

Cable sized per 125% of FLC (knowing now that we have a 20HP at 208V which FLC = 59.4A) so 1.25 x 59.4A = 74.25A minimum and we have our #6 rated at 75A (90C), so we are good with that.
Overload max of 125% of FLC (for SF 1.15) so 1.25 x 59.4 = 74.25A, overload is set to 75A.
Breaker selected was 60A non-adjustable n... Now knowing the above, I can select a max of 250% of FLC (2.5 x 59.4A = 148.5A) which the next standard size of 148.5 would be a 150A breaker.

I am concerned that you have sized you #6 wire using the 90degC column. You may find that your terminations are rated 75degC which would mean that you should be using #4 rated at 85a@75degC. As such you could step up to a 90at breaker.
In my opinion the breaker is tripping thermally. The LRA should be long enough is duration that you should be able to measure it with a clamp on ammeter as there is probably a brief delay after attempting to start the motor before the breaker trips.
If so revisit the wire size which would permit you to use a higher Ampere rated breaker.as my understanding that you are experiencing nuisance tripping that 60at breaker.
Make sense?

 

[kwired]

I am concerned that you have sized you #6 wire using the 90degC column. You may find that your terminations are rated 75degC which would mean that you should be using #4 rated at 85a@75degC. As such you could step up to a 90at breaker.
In my opinion the breaker is tripping thermally. The LRA should be long enough is duration that you should be able to measure it with a clamp on ammeter as there is probably a brief delay after attempting to start the motor before the breaker trips.
If so revisit the wire size which would permit you to use a higher Ampere rated breaker.as my understanding that you are experiencing nuisance tripping that 60at breaker.
Make sense?

Conductor size is not a limit for short circuit/ground fault protection with motors. This motor could easily go up to 175 amp breaker before it has to fail to hold during starting to increase it even more.

 

[templdl]

Conductor size is not a limit for short circuit/ground fault protection with motors. This motor could easily go up to 175 amp breaker before it has to fail to hold during starting to increase it even more.

I don't think that I implied that is was. The breaker is a common 60at panel mounted breaker protecting #6 wire. My concern was that the wire was sized per the 90degC column based on the motor FLA. As my post stated the wire size should be based upon the 75degC column. The breakers is tripping which I would guess it is a thermal trip as a result of prolonged LRA starting current. It is not a mag only breaker which would be a code violation to as a branch circuit protection device.

 

[ibarrola]

I knew what you meant... I had been thinking about it too :thumbsup:

Guys, thank you for all of your comments and help...
I was served with a problem and an installation that was already done which had some incorrect information beforehand.

Checking now the photos and the correct information, it is as follows....

At the moment we have a transformer (75KVA) supplying 208V to a panel (for the HPU skid - hydraulic power unit). This panel has the Siemens TypeQP 3P 60A breaker (like Jraef mentions, it looks like a residential breaker, see photo below), Siemens webpage mentions it's a thermal magnetic breaker.... From there we run a single cable of #6 for a lenght of 210ft (not sure of temp rating, need to find out, but true, I'm assuming 90C)... The cable connects to a junction box on the skid which has the motor contactors and an overload which the minimum setting is 75A...



Motor is dual voltage 230/460, 48/24, SF1.15... and at the bottom it does say "USABLE @ 208V with a current of 52A"...



With this being said, if we are running the motor at 208V then per NEC we have a FLC of 59.4A (FLA namplate= 52A). This would give us a max overload of 125% x 52A = 65A, overload is too high per NEC, unless they used the exception and multiplied by 140% to cover for inrush, which is still high. As for the cable (assuming 90C), a 6AWG 3/C would be good for 75A, which is above FLC of 59.4A x 125% = 74.25A...
As far as the breaker, since it's thermal magnetic, we use 250% for design B motor on inverse time breaker and the max would be 148.5 (size up to 150A).

One problem I see is that there's a voltage drop of about 10V with the 210ft lenght (siemens voltage drop calculator), so that would give us 197V at motor terminals.. They mentioned they measured 203V at motor terminals and running current of about 35A.


With no pressure at the hydraulic unit, motors starts and pressurizes the unit to 3000psi, when the pressure decreases to 2500psi, the motor starts, or at least tries to start, it hums for about 6 seconds and the breaker at the panel trips.

I know that the motor is usable at 208V, according to nameplate, but the unit was successfully factory tested at 240V.... My suggestion is to install a transformer that outputs 240V (or see if this same transformer can be wired delta-delta), where our cable lenght would give us a voltage drop of 10V and have 230V at the motor leads... As well as replacing the breaker with a thermal-mag that has an adjustable trip (like Jraef mentioned) rated at 150A so we can dial it back to 100A and go from there. Cable run would stay the same... Overload hmm, not sure... I guess that needs to be replaced per nameplate's FLA and NEC.

Any thoughts?

Thanks,
Fernando

 

[Jraef]

Sounds like you have a good plan. I'd probably just use two buck-boost transformers in open delta on the output of the starter to boost it up to 240V though, it's cheaper and smaller, you don't need another isolation transformer here. Definitely change that OL relay however.

 

[ibarrola]

Sounds like you have a good plan. I'd probably just use two buck-boost transformers in open delta on the output of the starter to boost it up to 240V though, it's cheaper and smaller, you don't need another isolation transformer here. Definitely change that OL relay however.

Thanks Jraef... Where would we exactly install these transformers? At the output of the starter, so I guess, between the overload and motor?

 

[Smart $]

Thanks Jraef... Where would we exactly install these transformers? At the output of the starter, so I guess, between the overload and motor?

Forget the boost transformers. The motor is rated for 208V operation. Assuming you have not yet done so, just upsize the breaker [trip] and test it out... just like I mentioned earlier. :happyyes:

 

[ibarrola]

Forget the boost transformers. The motor is rated for 208V operation. Assuming you have not yet done so, just upsize the breaker [trip] and test it out... just like I mentioned earlier. :happyyes:

Smart, but as of right know, the motor hums for 6 seconds, then trips (when started under heavy load)... it's not like it starts turning and then trips.

 

[Smart $]

Smart, but as of right know, the motor hums for 6 seconds, then trips (when started under heavy load)... it's not like it starts turning and then trips.

Ahh... new info (or missed it earlier). Not enough torque at startup. Could be your supply has too much voltage sag. Boost trannies may not do the job either. Need to measure voltage at skid while motor is humming.

 

[ibarrola]

Ahh... new info (or missed it earlier). Not enough torque at startup. Could be your supply has too much voltage sag. Boost trannies may not do the job either. Need to measure voltage at skid while motor is humming.

Skid was tested at 240V and we are running the motor at 208V, which it mentions in the nameplate that it can operate at 208V but maybe not for the skid design. Plus, there's a voltage drop on the cable of 10V according to calculations (although they mentioned 203V at skid). Not only are we not running it at 240V or 230V as the namplate has but its 203V the motor is seeing.

The skid is a hydraulic pressure unit, so when the motor starts, there a high pressure on the pump to overcome.

 

[Smart $]

Skid was tested at 240V and we are running the motor at 208V, which it mentions in the nameplate that it can operate at 208V but maybe not for the skid design. Plus, there's a voltage drop on the cable of 10V according to calculations (although they mentioned 203V at skid). Not only are we not running it at 240V or 230V as the namplate has but its 203V the motor is seeing.

The skid is a hydraulic pressure unit, so when the motor starts, there a high pressure on the pump to overcome.

Yep, yep, yep, and yep. Did I miss anything? I understood all to be the case prior to my last post... and my recommendation to measure voltage while motor is humming, not turning at start up.

 

[Jraef]

...
One problem I see is that there's a voltage drop of about 10V with the 210ft lenght (siemens voltage drop calculator), so that would give us 197V at motor terminals.. They mentioned they measured 203V at motor terminals and running current of about 35A. ...

This was why I suggested boosting. The "OK to use at 208V" statement might be based on the assumption that it would not drop below that, 197V is way too low. 203V while running might be OK, as long as that is the LOWEST it ever gets. But most likely it is closer (or worse) to 197V on start up, which is 85% of nameplate voltage for getting full Locked Rotor Torque. So with a 15% VD on startup, that LRT value will drop by the square of the voltage change, meaning the LRT will be 72% of normal LRT. Given that LRT is about 160% of FLT, that makes the LRT just barely (115%) over FLT, which can seriously affect the motor's ability to start under load.

As others mentioned though, why is this starting under load like this? HPUs typically have a bypass (relief) valve that just dumps the fluid back into the receiver during start-up. Is that not functioning correctly? Did they design without it? There are two types, pilot actuated and a passive type that acts based on pressure alone (can't remember the term). If it's pilot actuated, there may be a problem with that circuit and the timing of it, it might be closing before the pump gets up to full speed. If it's the passive type, they are very susceptible to the hydraulic fluid viscosity and if someone put the wrong fluid in it, that can affect how it operates. In fact that may affect the pilot operated valve too. This entire issue of starting loaded is something that should be investigated and confirmed that it is EXPECTED from this HPU. It's been known to happen, but what they do is over size the pump motor. For all we know, this could have been made for a 15HP application, that's why they put in the 20HP motor (which might explain the discrepancy there). It just might be as simple as someone over sizing the motor, but not knowing that the voltage would be low TOO.

 

[kwired]

Guys, thank you for all of your comments and help...
I was served with a problem and an installation that was already done which had some incorrect information beforehand.

Checking now the photos and the correct information, it is as follows....

At the moment we have a transformer (75KVA) supplying 208V to a panel (for the HPU skid - hydraulic power unit). This panel has the Siemens TypeQP 3P 60A breaker (like Jraef mentions, it looks like a residential breaker, see photo below), Siemens webpage mentions it's a thermal magnetic breaker.... From there we run a single cable of #6 for a lenght of 210ft (not sure of temp rating, need to find out, but true, I'm assuming 90C)... The cable connects to a junction box on the skid which has the motor contactors and an overload which the minimum setting is 75A...
View attachment 15543
View attachment 15542

Motor is dual voltage 230/460, 48/24, SF1.15... and at the bottom it does say "USABLE @ 208V with a current of 52A"...
View attachment 15541


With this being said, if we are running the motor at 208V then per NEC we have a FLC of 59.4A (FLA namplate= 52A). This would give us a max overload of 125% x 52A = 65A, overload is too high per NEC, unless they used the exception and multiplied by 140% to cover for inrush, which is still high. As for the cable (assuming 90C), a 6AWG 3/C would be good for 75A, which is above FLC of 59.4A x 125% = 74.25A...
As far as the breaker, since it's thermal magnetic, we use 250% for design B motor on inverse time breaker and the max would be 148.5 (size up to 150A).

One problem I see is that there's a voltage drop of about 10V with the 210ft lenght (siemens voltage drop calculator), so that would give us 197V at motor terminals.. They mentioned they measured 203V at motor terminals and running current of about 35A.


With no pressure at the hydraulic unit, motors starts and pressurizes the unit to 3000psi, when the pressure decreases to 2500psi, the motor starts, or at least tries to start, it hums for about 6 seconds and the breaker at the panel trips.

I know that the motor is usable at 208V, according to nameplate, but the unit was successfully factory tested at 240V.... My suggestion is to install a transformer that outputs 240V (or see if this same transformer can be wired delta-delta), where our cable lenght would give us a voltage drop of 10V and have 230V at the motor leads... As well as replacing the breaker with a thermal-mag that has an adjustable trip (like Jraef mentioned) rated at 150A so we can dial it back to 100A and go from there. Cable run would stay the same... Overload hmm, not sure... I guess that needs to be replaced per nameplate's FLA and NEC.

Any thoughts?

Thanks,
Fernando

60 amp breaker is still too low of setting for that size and voltage of motor, even if you reduced the heavy starting load it still may trip once in a while.

You also must use NEC tables for FLA when determining minimum conductor ampacity - for 20 hp 208 v this is 62.1A x 1.25 = 77.625 minimum conductor ampacity.

Also your calculated voltage drop is likely based on FLA, starting amps will be much higher and will cause even more voltage drop during starting, leaving you with poor starting torque.

Can't read the nameplate in your picture, but is it a 1750 or 3450 RPM motor? High speed motor will also have less starting torque which may complicate this if that is what you have.

Can you somehow unload the motor for starting?

 

[Jraef]

... Can't read the nameplate in your picture, but is it a 1750 or 3450 RPM motor? High speed motor will also have less starting torque which may complicate this if that is what you have.

It's a 4 pole, 1765 RPM.

 

[Ingenieur]

Smart, but as of right know, the motor hums for 6 seconds, then trips (when started under heavy load)... it's not like it starts turning and then trips.

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