30 hp 230 volt motor trips breaker intermittently on startup.

[Rsteenson]

Hey guys,

Ran in to one this last week that I wanted to run by everybody and get some ideas. I was given the job of wiring (2) 30 hp bin fan motors on some grain bins. The service is a 3-phase high-leg 230 volt system. The service conductors run about 250 feet from the pole mounted transformer and stop in a terminal box like a CT can. The service conductors from the pole to the CT cab are (2) parallel sets of 250 USE quad-plex AL direct burial cable (I DID NOT INSTALL THIS!). I tapped from the CT cabinet and installed a 200 amp QO 3-phase loadcenter to supply these two motors only. The motors bump tested fine a couple of months ago when we installed the new equipment and got everything wired. The owner called me last week after they needed them for the first time and said that they will only start intermittently. Before I even stopped out there I called to order (2) 125 amp breakers to replace the recommended size of 100 amp figuring it was a start-up current issue, only to find out that in the 3-phase line they do not manufacture a QO breaker bigger then 100 amp.
I showed up to do some checking and found out that when the motors were tripping the breakers it was only on start-up. Each motor has its own start-stop. One time they would both start fine, the next one would start while the other wouldn't, and then first one would trip and the second would start and run fine. When they would trip the motor wouldn't even start to budge, it was more like a dead short style trip. Then they will start three or four times in a row and run just fine. Each motor has its own conduit, own conductors, and is about 125 feet away from the panel. I am stumped. The voltage was bit little low, but was within means. The coil voltage is correct on the motor starters. Wire size is correct. Motors are wired correctly. Anybody have any ideas? Thanks in advance.

 

[JFletcher]

A 100-amp breaker is entirely too small for a 30 horsepower 230 volt 3-phase motor, which draws roughly 80 amps depending on construction. The breaker should be sized at least 125 amps, if not closer to the maximum, which would be 200 amps.

 

[GoldDigger]

A 100-amp breaker is entirely too small for a 30 horsepower 230 volt 3-phase motor, which draws roughly 80 amps depending on construction. The breaker should be sized at least 125 amps, if not closer to the maximum, which would be 200 amps.

The concern at the instant of startup is inrush, which can be greater than the running Full Load Amps and can trigger the magnetic (instant) trip instead of the breaker instead thermal trip with its time curve. A higher amp rated breaker will generally have a higher (but not necessarily proportional) magnetic trip.
But a higher than FLA current can also persist for several seconds or more as the motor is accelerating under load, and this can trigger the thermal trip. The guideline of 125 to 200 percent or more of FLA is intended to cover this possibility of thermal trip at starting. By contrast, overloads have a much longer thermal time constant and so can be set much closer to the FLA or RLA value.
In the OP's case, it sounds like the magnetic trip is the issue.

Sent from my XT1585 using Tapatalk

 

[Rsteenson]

A 100-amp breaker is entirely too small for a 30 horsepower 230 volt 3-phase motor, which draws roughly 80 amps depending on construction. The breaker should be sized at least 125 amps, if not closer to the maximum, which would be 200 amps.

My first though exactly. The square d motor cheat sheet says 100 amp recommended for a 230 volt 3 phase and in my experience over the years it hasn't failed me more then a couple of times. The only thing that makes zero sense is that whenever a breaker trips due to inrush current the motor will run briefly and will ramp part of the way. This thing isnt even groaning.... It trips like a dead short.....and also it is intermittent! Sometimes it starts just fine and doesn't even sound like it is struggling.

 

[JFletcher]

Okay, I am confused. Isn't table 430.250 used to determine the ocpd size? If an 30 horsepower 230 VAC motor has a full load amperage of 80 amps, is it a 100 amp breaker, 125%, really on the low end?

What are the voltages at the motors with both of them off, with one running, and with both running? Amperage readings of the motors?

is it possible that the motors are driving fans that are too large for them? If they are belt drive, could changes in ambient temperature cause enough expansion or contraction on otherwise too tight belts to cause an instant trip sometimes and a normal start at others? If they were installed in the middle of summer on a hot day, and it's near freezing now and some areas, would that cause the belts to tighten up considerably?

I see that indeed 100 amps is the largest three-phase breaker they make for the qo panel... Could one handle tie two 2 pole 125 amp breakers together?

Also, I'm assuming with the high leg transformers you have an open delta with two transformers? If so, is it possible the smaller pot is close to overloaded from normal loads, and there would be a voltage dip on it when the motors are started?

please note I'm not questioning you guys troubleshooting or knowledge, rather just trying to further my knowledge on motors which is admittedly weak.

Edited to add... Would it be safe to assume that if the overloads were purchased with the new motors a few months ago, there should be no problems with worn contacts or the like there?

 

[ptonsparky]

Okay, I am confused. Isn't table 430.250 used to determine the ocpd size? If an 30 horsepower 230 VAC motor has a full load amperage of 80 amps, is it a 100 amp breaker, 125%, really on the low end?

What are the voltages at the motors with both of them off, with one running, and with both running? Amperage readings of the motors?

is it possible that the motors are driving fans that are too large for them? If they are belt drive, could changes in ambient temperature cause enough expansion or contraction on otherwise too tight belts to cause an instant trip sometimes and a normal start at others? If they were installed in the middle of summer on a hot day, and it's near freezing now and some areas, would that cause the belts to tighten up considerably?

I see that indeed 100 amps is the largest three-phase breaker they make for the qo panel... Could one handle tie two 2 pole 125 amp breakers together?

Also, I'm assuming with the high leg transformers you have an open delta with two transformers? If so, is it possible the smaller pot is close to overloaded from normal loads, and there would be a voltage dip on it when the motors are started?

please note I'm not questioning you guys troubleshooting or knowledge, rather just trying to further my knowledge on motors which is admittedly weak.

Edited to add... Would it be safe to assume that if the overloads were purchased with the new motors a few months ago, there should be no problems with worn contacts or the like there?

These are are most likely a typical grain fan. That size would most likely be centrifugal but could be axial flow. No belts. They have a motor starter included and installed on the units as a package. He needs to provide the short circuit protection (and a disconnect next to the fan).

 

[tom baker]

my sq d motor data calculator calls for a 110 CB TM breaker

 

[ptonsparky]

The centrifugal can take.a.long time.to start.

 

[Jraef]

If it trips instantly, it's not a thermal issue, it's that the mag trips of the breaker are interpreting the inrush as a short circuit. That's exactly why the NEC allows you to go up to 250% of the motor FLC for a T-M breaker. As a gross general rule, T-M breakers that have fixed mag trips will be set for 7-8 times the breaker rating. But the QO breakers are only 4 times the breaker rating. So on a 100A breaker, the mag trip is factory set for 400A. On a motor rated for 88A (from the NEC chart), that's only 450% and usually Locked Rotor Current is 600%. Modern Energy Efficient motors can be even worse! So sometimes it might be less, sometimes more, it will depend on the motor characteristics and at what point of the incoming sine wave the contactor closes, hence the seeming random nature of it; you cannot predict that, which is why the NEC allows the breaker sizing for a motor circuit to be high enough to make it not matter. You could have used up to a 225A breaker for that motor, I would not have gone with anything smaller than 200A.

 

[ptonsparky]

The centrifugal can take.a.long time.to start.

 

[LarryFine]

I'm in agreement with the 200a-breaker guys.

 

[Rsteenson]

If it trips instantly, it's not a thermal issue, it's that the mag trips of the breaker are interpreting the inrush as a short circuit. That's exactly why the NEC allows you to go up to 250% of the motor FLC for a T-M breaker. As a gross general rule, T-M breakers that have fixed mag trips will be set for 7-8 times the breaker rating. But the QO breakers are only 4 times the breaker rating. So on a 100A breaker, the mag trip is factory set for 400A. On a motor rated for 88A (from the NEC chart), that's only 450% and usually Locked Rotor Current is 600%. Modern Energy Efficient motors can be even worse! So sometimes it might be less, sometimes more, it will depend on the motor characteristics and at what point of the incoming sine wave the contactor closes, hence the seeming random nature of it; you cannot predict that, which is why the NEC allows the breaker sizing for a motor circuit to be high enough to make it not matter. You could have used up to a 225A breaker for that motor, I would not have gone with anything smaller than 200A.

What is "T-M breaker"? I was seeing 440 amps on startup current (when it actually turned), so that would make sense for the QO breaker not being sufficient on the magnetic trip. The incoming sine wave theory makes sense as well considering the intermittent starting and tripping. I thought long and hard about the only other farm location in the area I know of that has a high leg system and 30 hp bin fans. I stopped in there last night and you wont guess what I saw The customers past electrician had installed the same QO 200 amp MB panel as I did (this was 10-12 years old). He had a QO 3p sub-feed lug kit installed in the panel feeding a 100 amp disconnect next to it with 100 amp time delay fuses in it!
SO......If I were to put this sub feed lug kit in for each of my motor feeds, do you think I need to put supplemental OCP in next to the panel even though I have a 200 amp MB protecting both of them as the code allows anyways? Not an ideal install this way, but there is no easy way to mount fusible disconnects by this panel. If I only wouldve known that QO doesnt make a 125 amp 3p breaker this wouldv'e never happened. Live and learn I guess. Thanks!

 

[Rsteenson]

I'm in agreement with the 200a-breaker guys.

I agree. If I would've known that there was no 3p breakers bigger then a 100 amp I would've never installed this panel.

 

[Rsteenson]

The centrifugal can take.a.long time.to start.

They can, no doubt. This one wasn't even the worst I have seen when it would actually turn over.

 

[Rsteenson]

Okay, I am confused. Isn't table 430.250 used to determine the ocpd size? If an 30 horsepower 230 VAC motor has a full load amperage of 80 amps, is it a 100 amp breaker, 125%, really on the low end?

What are the voltages at the motors with both of them off, with one running, and with both running? Amperage readings of the motors?

is it possible that the motors are driving fans that are too large for them? If they are belt drive, could changes in ambient temperature cause enough expansion or contraction on otherwise too tight belts to cause an instant trip sometimes and a normal start at others? If they were installed in the middle of summer on a hot day, and it's near freezing now and some areas, would that cause the belts to tighten up considerably?

I see that indeed 100 amps is the largest three-phase breaker they make for the qo panel... Could one handle tie two 2 pole 125 amp breakers together?

Also, I'm assuming with the high leg transformers you have an open delta with two transformers? If so, is it possible the smaller pot is close to overloaded from normal loads, and there would be a voltage dip on it when the motors are started?

please note I'm not questioning you guys troubleshooting or knowledge, rather just trying to further my knowledge on motors which is admittedly weak.



Edited to add... Would it be safe to assume that if the overloads were purchased with the new motors a few months ago, there should be no problems with worn contacts or the like there?

1. 235 volts p-p at idle. 227-230 with both motors running. The voltage is a bit low from the utility at this rural farm site. Motors are drawing nameplate amps @ 77.

2. This fan manufacturer is one of the largest and best quality in the business so I would trust that their engineering is correct. Also, I have wired a hundred fans of this size on this size bin with no issues, the only difference is 99% of them are 480 volt. They are direct driven centrifugal fans.

3. I like this idea!

4. Power company was out already and said their end is good.

5. I understand completely. Motors and their millions of different applications always throw different problems at us. This is the best place to gain knowledge IMO.

6. All components are brand new. If it was only one of the two motors I would probably have tried changing starters already just to rule it out. It has happened before.

Thanks!

 

[ptonsparky]

What is "T-M breaker"? I was seeing 440 amps on startup current (when it actually turned), so that would make sense for the QO breaker not being sufficient on the magnetic trip. The incoming sine wave theory makes sense as well considering the intermittent starting and tripping. I thought long and hard about the only other farm location in the area I know of that has a high leg system and 30 hp bin fans. I stopped in there last night and you wont guess what I saw The customers past electrician had installed the same QO 200 amp MB panel as I did (this was 10-12 years old). He had a QO 3p sub-feed lug kit installed in the panel feeding a 100 amp disconnect next to it with 100 amp time delay fuses in it!
SO......If I were to put this sub feed lug kit in for each of my motor feeds, do you think I need to put supplemental OCP in next to the panel even though I have a 200 amp MB protecting both of them as the code allows anyways? Not an ideal install this way, but there is no easy way to mount fusible disconnects by this panel. If I only wouldve known that QO doesnt make a 125 amp 3p breaker this wouldv'e never happened. Live and learn I guess. Thanks!

The sub feeds should work. I would be inclined to a larger fused disconnect at the fans with maybe 125 amp fuses. Problem I see now is the 160 amp continuous on the 200. Should work but... Torque all your connections and make sure you end up with no hot spots especially on the main. Hi leg deltas can run higher amperage on one leg. Rotate the phases on each of your fans until you find the combination that has the least difference for each one.

TM is thermal magnetic.

 

[ptonsparky]

Your feeder taps would be considered outside so a run to the disconnect would not need to be fused at the panel. It is outside, right?

You may be lucky that your fans are within sight of the controllers and motors but without a means to lock open the disconnecting means, you’re starting to get into liability issues. Farm wiring used to be an anything goes but insurance companies are getting tired of paying and try to find other pockets.

 

[Cow]

We do work for a couple of grain facilities.

The bin fans are typically some of the hardest starting longest ramp time motor loads on site.

I would highly recommend installing the maximum size OCPD allowed by code, while also monitoring the voltage/current at startup at the motor to make sure you don't have a utility company or wiring issue creating excessive voltage drop at the motor.

 

[sgtsparky]

soft start may allow you to use the 100A

soft start may allow you to use the 100A

At my plant we use soft starts to reduce starting currents, in some cases transformem based some VFD styles. I usually use UGLY's for sizing and it shows a 125A as the OCPD. this will be more expensive than tapping the buss and putting larger breakers in but it could also provide speed control for the fan.

 

[Jraef]

What is "T-M breaker"?

T-M = Thermal - Magnetic, basically a standard off the shelf circuit breaker, what the NEC refers to as an "Inverse Time" circuit breaker.

I was seeing 440 amps on startup current (when it actually turned), so that would make sense for the QO breaker not being sufficient on the magnetic trip. The incoming sine wave theory makes sense as well considering the intermittent starting and tripping. I thought long and hard about the only other farm location in the area I know of that has a high leg system and 30 hp bin fans. I stopped in there last night and you wont guess what I saw The customers past electrician had installed the same QO 200 amp MB panel as I did (this was 10-12 years old). He had a QO 3p sub-feed lug kit installed in the panel feeding a 100 amp disconnect next to it with 100 amp time delay fuses in it!
SO......If I were to put this sub feed lug kit in for each of my motor feeds, do you think I need to put supplemental OCP in next to the panel even though I have a 200 amp MB protecting both of them as the code allows anyways? Not an ideal install this way, but there is no easy way to mount fusible disconnects by this panel. If I only wouldve known that QO doesnt make a 125 amp 3p breaker this wouldv'e never happened. Live and learn I guess. Thanks!

You are required to have an individual branch circuit OCPD for your motor, you cannot use the Main CB for the entire panel as the Branch OCPD if that's what you were thinking (I kind of got lost in your description a little).

The fused disconnect at the other site works probably because fuses will have a different protective characteristic curve and the time-delay aspect is likely allowing them to hold in for the motor. Still not ideal, I would have used bigger fuses too, but in that other case it apparently worked.