Using only 2 CT's for motor OL protection
- Thread starter Pitt123
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nollij
Member
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- Washington
Overloads on two of the three lines will protect against any overload condition.
Old Motor Starters (think the 50s/60s) only used Overloads on two lines. Every Motor Starter bought today will have overload sensing on all three lines.
I can not find an NEC requirement to use an overload on every line.
Old Motor Starters (think the 50s/60s) only used Overloads on two lines. Every Motor Starter bought today will have overload sensing on all three lines.
I can not find an NEC requirement to use an overload on every line.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Depends on the meaning of "you" in your sentence.
In general, "you" cannot. Article 430 Section (and Table) 37 requires that ALL ungrounded motor conductors in a 3 phase circuit be protected by thermal overload protection devices*.
*Exception: An overload unit in each phase shall not be required
where overload protection is provided by other approved means.
Therein lies a somewhat controversial issue. UL will allow "you", if you are a motor controller manufacturer, to build a motor controller using sensing on 2 if the 3 legs, as long as they (UL) have approved your OL sensing scheme as appropriately protecting all 3 conductors. This is not as confusing as it may appear. Basically if you make AC motor controllers, VFDs and Soft Starters is where we see this mostly, and if your ENTIRE motor controller is getting UL listed, they will require that your OL protection scheme, however you chose to do it, is separately tested, evaluated and listed before they allow the listing of the entire unit. That is how VFDs get listed for motor thermal OL protection when in fact some designs are NOT directly sensing the current on each individual leg, they are sensing the DC bus current and CALCULATING the motor phase currents. There are also some 2-leg Soft Starters out there that do a similar thing to what you asked about. But unless "you" have very deep pockets to pay for the roughly $20k it costs to get this through UL, then "you" cannot hope to get away with it.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
The "old overloads" issue changed in 1978 I think, somewhere around there anyway. Must be 3 now.
Technically speaking how would this protect against an overload somehow occuring on one of the 3 phases? Due to the fact that there will be increased current in the other two phases, or is this not a plausable situation?
Do you need a minimum of 2 to protect for single phasing condition on one of the three legs? Is this the only thing that technically mandates the (2) CT's
hurk27
Senior Member
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- Portage, Indiana NEC: 2008
in motor over loading, if an over load occurs on one phase it will also occurs on at least one other phase, only a fault can occur only on one phase, which motor overloads is not design to protect for that.
But wont an overload on one phase cause the current to spit between the other two phases and thus maybe not be so great on the other two phases to be detected as an OL. For example in a wye configuration.
Or will a true "overload" condition cause the slip to change in all three phases and thus and overload current as a result of the motor being overloaded will therefore effect all phases equally.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
But think it through for a bit...
How can you have a THERMAL OVERLOAD on only one phase in a 3 phase motor without it involving a fault?
In other words what hurk27 was trying to tell you is that you are chasing a problem that basically can't happen in the real world.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
So getting back to the 2 CT issue, you were allowed to use 2 CTs in the past for that very reason. Then people started doing things like hooking up the 2 current sensors in an open delta (just like getting 3 phase voltages from 2 transformers) to do current measurement and then started using that for motor OL as well. But that then opened up the possibility of motor damage from unbalanced current conditions. So to eliminate that possibility, they (the powers that be) just went ahead and required thermal protection in all 3 phases. The inherent risks of allowing 2 OLs to be misused was not worth the minor amount of savings it gained in the industry. Or so the story went when it was explained to me in 1979 when it affected me and we had to go through and change out all of our 2-OL starters. (Even though they were grandfathered in, I worked at a steel mill and our plant EE was on the code committee, so he felt he had to lead by example at the time)
Jraef
How do you read section 432 (A) (1)?
I read it as a thermal overload device such as a 3 thermal heater relay units .
If the vendor of the combination starter has drawings showing two of the heaters being used to sense motor overload that meets the requirement.
Does 430.37 refer to sensors or protection devices?
Some manufacturers of HVAC equipment use two pole contactors to control three phase motor on there equipment. I don't like it but there is a section of the code that allows it. I was reading section 430.37 as making sure people didn't use a 2 pole relay to stop a 3 phase motor.
Two motor overload sensors can adequately protect a motor from overheaing. I have use two overload heaters to stop a motor and the third as an alarm. The heater size on the alarm unit is one or two sizes less than the other two. It pass the 05 code. I don't think the inspector would have caught it but we brought it up and explained the purpose.
Monitoring 2 phases of a motor can adequitely protect a motor from Overload ( not the motor or conductors from short circuit).
Overload on a TOCC plot shows up way to the left of the fuse, or circuit breaker or cable damage curves.
The solid state units have an alarm built in so I don't expect to have to do it anymore.
How do you read section 432 (A) (1)?
I read it as a thermal overload device such as a 3 thermal heater relay units .
If the vendor of the combination starter has drawings showing two of the heaters being used to sense motor overload that meets the requirement.
Does 430.37 refer to sensors or protection devices?
Some manufacturers of HVAC equipment use two pole contactors to control three phase motor on there equipment. I don't like it but there is a section of the code that allows it. I was reading section 430.37 as making sure people didn't use a 2 pole relay to stop a 3 phase motor.
Two motor overload sensors can adequately protect a motor from overheaing. I have use two overload heaters to stop a motor and the third as an alarm. The heater size on the alarm unit is one or two sizes less than the other two. It pass the 05 code. I don't think the inspector would have caught it but we brought it up and explained the purpose.
Monitoring 2 phases of a motor can adequitely protect a motor from Overload ( not the motor or conductors from short circuit).
Overload on a TOCC plot shows up way to the left of the fuse, or circuit breaker or cable damage curves.
The solid state units have an alarm built in so I don't expect to have to do it anymore.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
BJ,
I can appreciate your position on this and I don't totally disagree with your premise, it's just that I think you are discussing the technical issues, I was discussing the CODE issues.
Assuming you meant 430.32 (A) (1) because there is no 432...
As I was taught and have interpreted it, 32 establishes that there SHALL BE overloads, 37 was added to tell us how many.
As with many issues like this, if an equipment manufacturer has a piece of equipment NRTL listed in this way, then as the installer, your job is done. You don't need to 2nd guess them. But they PAID BIG BUCKS to have that done* and if they did it with 2 OLs to save $2.32 per controller, it's because they sell a LOT of controllers. For us in the field however, we cannot do this on our own without violating 430.37. By the way, I don't know of any manufacturer of standard electro-mechanical 3 phase starters that is still using 2 OL protection, it just isn't worth it for them.
430.37 specifically says "...overload units such as trip coils or relays". This is not the part that references contactors etc., that is a separate issue. This entire section is about motor protection. Don't get me started about HVAC manufacturers, but let's just say "minimally acceptable" is the primary design criteria for them. What they are doing is banking on 430.38, which describes HOW an OL device will stop a motor. "...shall simultaneously open a sufficient number of ungrounded conductors to interrupt current flow to the motor" is a statement of minimally adequate design, and they jump all over it.
I never said it would not work, but it is specifically forbidden in the NFPA 70 Code, section 430.37 and they had their reasons when they did it in the late 70s. Every inspector is the Authority Having Jurisdiction however; "the Demigod of his own little world"; and if he decides you have made a valid argument, then he can accept it, even if another inspector would not. It is still technically a code violation so just because one inspector accepted it, that doesn't mean anyone can do it and get another inspector to think the same way. Many have tried... and failed, in things like this as many members of this forum will attest to.
* I have an example but it's a tangent so I'll put it in a separate post to avoid this one being so much longer.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
An example of just how expensive this can be for companies.
Quite some time ago Baldor, who at one time owned one of the original soft starter designs, decided to get out of the business of manufacturing soft starters in this country and chose to brand-label a product from a UK manufacturer. Apparently some marketing genius at Baldor who made that decision didn't watch his Ps and Qs and failed to notice that the UL listing that import had was NOT as a "motor starter" but just as a "Soft Start Controller"; the subtle difference being that a controller is like a contactor, it is not responsible for protecting the motor, just turning it on and off. The import unit did have Motor Thermal OL protection built-in, but they only monitored 2 of the 3 legs and they did not pass the separate UL test for that as I mentioned earlier. So poor Baldor had to punt and they were required to add external old-fashioned 3 pole thermal OL relays to each and every unit they sold in the US. In addition, the digital display only reports OL faults caused by their internal (non-UL) OL protection, so if the external OL trips and stops the motor, nobody knows why. For all intents and purposes, this virtually killed Baldor's soft starter business. I think it may still be this way but I haven't checked their product in a few years, I never use them.
Quite some time ago Baldor, who at one time owned one of the original soft starter designs, decided to get out of the business of manufacturing soft starters in this country and chose to brand-label a product from a UK manufacturer. Apparently some marketing genius at Baldor who made that decision didn't watch his Ps and Qs and failed to notice that the UL listing that import had was NOT as a "motor starter" but just as a "Soft Start Controller"; the subtle difference being that a controller is like a contactor, it is not responsible for protecting the motor, just turning it on and off. The import unit did have Motor Thermal OL protection built-in, but they only monitored 2 of the 3 legs and they did not pass the separate UL test for that as I mentioned earlier. So poor Baldor had to punt and they were required to add external old-fashioned 3 pole thermal OL relays to each and every unit they sold in the US. In addition, the digital display only reports OL faults caused by their internal (non-UL) OL protection, so if the external OL trips and stops the motor, nobody knows why. For all intents and purposes, this virtually killed Baldor's soft starter business. I think it may still be this way but I haven't checked their product in a few years, I never use them.
My experience was with paper mills which are a group like some others know more than the rest of the world. They would buy the starter wired wirh the third OL independantly wired so they could connect them to the DCS or local PLC.
I never questioned them on it because I had seen it done for years in power plants. Even though 90.2 exempts power plants they were usually more conservative than the NEC requirements.
The question is moot now, the solid state starters are smart enough to call you at home if you get an overload.
I never questioned them on it because I had seen it done for years in power plants. Even though 90.2 exempts power plants they were usually more conservative than the NEC requirements.
The question is moot now, the solid state starters are smart enough to call you at home if you get an overload.
There are a group of sophisticated heavy industrial engineers in the pulp & paper and petrochemical sectors that do tend to know more than the rest of the world. Look at where most technical industrial knowledge is coming from - generally it's published by the IEEE. The Industry Applications Society within the IEEE has the Petroleum and Chemical Committee and the Pulp & Paper committee among others. These two committees and their associated conferences and publications are a major source of technical information for the entire electrical industry. The most well known and widest reaching project right now is some research and testing on arc behaviors that is going to impact the next revision of IEEE 1584, the arc flash calculation standard - the next revision will likely be released in early 2012. The other important group is the IEEE Power and Energy Society, which deals with electric utility concerns and also writes the ANSI standards for transformers and other major equipment.
The nice thing about a lot of the folks involved in the PCIC and Pulp & Paper committee are that they seek real technical information, they don't just read the NEC and other codes and listen to manufacturers and accept what they hear. They tend to dig deeper than most people I run across in commercial and many of the other industrial sectors.
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Thanks for the replies guys. I understand now what is required by codes and standards that being all three phase must be monitored.
I guess from a technical standpoint any overload condition would effect more then one phase because you either have the slip effecting all three phases or you have a single phase condition which would involve and overload on two of the phases. This would be the only way to get overload currents from true overload conditions.
Even with a wye connected load it is not possible to have the slip only effect one of the phases thus causing an overload current on the one phase?
In order to have only an overload on one of the phases then if must be a short circuit in which cases the fuses or breaker used for short circuit protection would come into play.
I guess from a technical standpoint any overload condition would effect more then one phase because you either have the slip effecting all three phases or you have a single phase condition which would involve and overload on two of the phases. This would be the only way to get overload currents from true overload conditions.
Even with a wye connected load it is not possible to have the slip only effect one of the phases thus causing an overload current on the one phase?
In order to have only an overload on one of the phases then if must be a short circuit in which cases the fuses or breaker used for short circuit protection would come into play.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Bingo...:grin:
- Location
- Wisconsin
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- PE (Retired) - Power Systems
Actually there are several other methods that will cause severely excess current to flow in one leg.
Summary from a paper presented to the IEEE Textile Industry Conference in 1969.
If the motor is served by a Delta-Wye transformer which loses one of the primary legs, the result on the secondary could be two legs at 1.15I and one leg at 2.3I. Another case is a combination of large and small motors (10:1 ratio) on one feeder: the small motor may act like a phase-converter and try to supply the full load current of the larger motor.
Single phase operation caused by an open circuit in a delta-wye transformer primary was also discussed in a article published in the August 1959 edition of "PRODUCT ENGINEERING".
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