Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB 1

[123MB]

Hi Guys,

I am in Australia where we use IEC standards and we don't have any requirements for UL listing of protective devices.

We have a class 30 overload application with a soft starter with line contractor. What sort of branch protection should I be using?

The distributor is telling me that we should use a magnetic only (you guys call it an MCP) breaker upstream and let the soft starter do overload protection for the feeder cable, motor cable, and contactor.

What do you think? Will the thermal protection in the soft starter protect the feeder cable and motor cable, even if the soft starter is set up for a class 30 overload?

What about the contactor? can the soft starter be relied upon to do thermal overload protection for it too?

Cheers

 

[jraef]

It's something to ask of the soft starter mfr. many of them require high speed fuses to help protect the SCRs in the soft starter itself, a mag only break will do absolutely nothing in that regard.

Here in North America it could not be done at all. UL and CSA both ceased allowing MCPs to be used on power electronics years ago, even by the starter mfrs. There were just too many problems.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

Thanks for the heads up on that Jeff.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[123MB]

Unfortunately any info I get from the manufacturer I don't really trust since it comes through a distributor, so I am just trying to cross check their advice.

We will have high speed fuses on the starter which will achieve type 2 coordination for the starter, I'm more concerned about the cables and the contactor. [sub][/sub]

 

[Soupercollider]

i don't know about SHOULD use magnetic only, but you certainly can use it. Your cables and contactor should hopefully already be selected based on your Class 30 expectations, so overload protection doesn't need to be too drastic and can be handled by the softstarter (provided you set the FLC and trip times correctly!). All you really need is just short-circuit protection, which the magnetic breaker will take care of. If your breaker has thermal protection you're going to need to have it set much higher than FLC of the motor to allow for that class 30 startup anyway.

In fact this is a perfectly valid setup in IEC 60947.4.1 for a combination starter. I'm pretty sure I saw a starter in there which can was just magnetic + contactor + overload.

 

[LionelHutz]

When you say line contactor, you mean in series with the starter or bypassing the starter?

The soft-starter overload could protect the cables and contactor. The question would be if you sized the cables and contactor so they would be protected by the overload curve you're using.

In North America, UL allows an MCP to be used in a listed combination and CSA allows it in a built-up panel or listed application. For UL, the limitations are not just on power electronics, you can't build any kind of motor control panel unless the MCP and starter are a listed combination.

 

[iceworm]

When you say line contactor, you mean in series with the starter or bypassing the starter?

123 -
Lionel has a good question. Changes the answer significantly.

If there is question in my mind, I'm a fan of laying out the cable damage curve, fuses, and the overload curve on a TCC log-log chart. I don't recall ever seeing a damage curve for a contactor. However, most everything I use is NEMA rated, not IEC.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[123MB]

Hey Guys, sorry have been offline for a few days

When I say 'line contactor' I mean a contactor in series with the starter, on the line side of the soft starter (there will actually be two contactors, for safety)

The issue I have with using the soft starter to protect the cable is that the starter we have does not allow us to select a trip class in accordance with IEC 60947-4-1. The motor thermal protection is very configurable however I can't easily present it in the form of a tripping curve that I could plot the cable damage curve against.

I know most vendors starters (Siemens, Schneider, Rockwell) and most electronic overload relays for DOL starts allow a selection of a tripping class, but this one doesn't.

At it's maximum setting this starter will allow 12xLRA for 120s, which is way way over a Class 30 overload scenario.

The additional complication here is that this is a MCC replacement where the field cabling to the motors will be reused, hence we can't upsize it, and chances are it's already had a hard life as it has been in place for the last ~20 years.

Has anyone visualised a soft starter's trip characteristic against the cable damage curve in the case where the starter is doing overload protection for the cable

And, yes, regarding the contactor the only information I have is a continuous current rating (which is about 1.3x the AC-3 rating) and the short circuit protection that would be required for Type 1 and Type 2 coordination scenarios.

 

[davidbeach]

If the soft starter will prevent thermal overload of the motor, where does the overload of the cable come from? Unless there's more on the cable than the motor, or the cable is inadequate for the motor then preventing overload of the motor prevents overload of the cable.

 

[Soupercollider]

At it's maximum setting this starter will allow 12xLRA for 120s, which is way way over a Class 30 overload scenario.

But you're hopefully going to set it to 6xLRA for 30s (Class 30), or something more specific to the motor specs (locked rotor current and locked rotor time). Assuming that the starter will allow these parameters?

The additional complication here is that this is a MCC replacement where the field cabling to the motors will be reused, hence we can't upsize it, and chances are it's already had a hard life as it has been in place for the last ~20 years.

And hopefully this cable is sized correctly to the motor, if not, then you're going to be in a bit of trouble.

If the soft starter will prevent thermal overload of the motor, where does the overload of the cable come from? Unless there's more on the cable than the motor, or the cable is inadequate for the motor then preventing overload of the motor prevents overload of the cable.

Pretty much this.

 

[jraef]

Just out of curiosity, what's the brand and model of that soft starter? Not having programmable OL protection for the motor that follows a NEMA / IEC motor thermal damage curve would be a non-starter for me. You might want to consider adding an OL relay to one of your line contactors and forget that soft starter being programmed for anything. Anyone that would give you only some vague reference like that is not likely to be accurate either.

In North America, UL allows an MCP to be used in a listed combination...

True, but as I said, no longer for VFDs and Soft Starters, as of I think 2014, maybe early 2015. They stopped allowing NEW submissions like that even longer ago, around 205 or 6 IIRC, but were not forcing "grandfathered" designs to be changed change. That's what ended recently; even older grandfathered designs are no longer acceptable with MCPs, the breakers must be T-M.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[123MB]

I suppose the thing I am struggling with is being able to visualise the soft starter's trip characteristic when it is so configurable. I don't really know how I would do this accurately, but I can get the pencil and paper out and have a go

It would be alot easier, albeit less configurable, for the starter to utilise a standard trip class.

It would also make me feel alot more secure about someone messing with the settings later on and compromising the cable protection. I know alot of field commissioning/programming types who would modify soft starter trip characteristics which would affect the cable protection without knowing it.

Anyway I think the main thing I wanted some assurance on was the use of the soft starter to protect the cable when the soft starter's trip characteristic is not one of the standard trip classes.

Even if the motor FLA matches the cable ampacity if the starter's settings are wrong then the cable may not be protected in certain areas on the curve. I dont think this is possible if you can only select up to a class 30 overload, but I haven't proved this myself.

 

[123MB]

The soft starter is an AuCom EMX3 manufactured in New Zealand, if you really want to make my day you can download the manual here and see what you think.

http://www.hub.aucom.com/manual/1056-710-04840-00-emx3-user-manual-en

Thanks all for your time I really appreciate it. I have been on this forum a long time and I have read alot of posts from this thread's respondents and I definitely am very grateful for your advice

 

[123MB]

And this

http://www.hub.aucom.com/application-notes/1086-710-07124-00-emx3-motor-overload-protection-en

 

[jraef]

That starter provides Class 10 OL protection, which means it is going to be a full I[sup]2[/sup]t thermal trip curve as it should be. There is no instruction as to whether or not it can be adapted to Class 30 exactly, or how to do it if it can, but you can tweak the trip characteristics in IB, IC, and ID parameters to align to the motor thermal damage curve if you know it. So do you know for sure that your motor is rated for Class 30 protection? That would be rare.

If so, and you call Aucom to get guidance on how to set up that protection, keep in mind though that in doing so, you may exceed the thermal ratings of the soft starter itself, meaning you may need to have over sized it to use it at Class 30. At a miminum, Class 30 implies that the motor will be able to handle Locked Rotor Current of 600% for 30 seconds. The basic sizing of that soft starter will be LRC for 10 seconds, so without contacting them for what adaptation it might be capable of, you may be looking at sizing the soft starter at 3x the motor FLA to attain this capacity.

One of our esteemed members of this forum, MarkE, is the founder and inverter of the Aucom soft starter. Too bad he has been quiet here lately, he could likely help. But I know he would be upset to hear that nobody at Aucom stepped up to help you, have you called them directly?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

Look for Mark here:

http://www.lmpforum.com/forum

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[123MB]

Bump

Thanks all for your responses so far

Thanks jraef. I don't know that my motor is rated for Class 30 OL, no. I will be getting the motor data tomorrow and I will have a look at it's design characteristics and go from there. I have tried to contact Aucom directly but I am always forwarded on to our local distributor, who are not Aucom.

Update on this project is as follows...

Background that was missing before is that this project is replacement of DOL conveyor starters with soft starters - 30kW 55A motors direct coupled to grain conveyors.

The starters are AuCom EMX3 already oversized - nominal current 105A and thermally they have a nominal FLC of 60A at 50 degrees C at a duty of 4.5xFLC for 30s with 570s off time (105A:AC53b 4.5-30:570)

The technical risk I am trying to manage, and it comes up all the time in DOL to RVSS projects, is the potential for the soft starter to not be able to produce enough starting torque to get the loaded conveyor away.

I am not versed at all with conveyor calculations and I am not confident (without a very clear how-to) of doing conveyor calculations to determine the minimum breakaway torque required and relating this back to the motor's reduced voltage torque characteristic. Also the project does not have the budget for me to school myself on these calcs. Dont get me wrong I would love to learn how to do these calcs accurately but it just doesn't fit into the bounds of this project budget or timing.

So the current limit on this starter is 600% of FLA, but the starter has an instantaneous trip of 6x nominal rating = 600A. It seems the only thing I can do is to design this system to be able to do as close to a DOL start as I can - as I know the conveyors get away fine at the moment with a DOL start.

So, if I wanted to get the starter to do 700% or 800% FLA on a current limit start, 800% would be 440A which is under the starter's instantaneous trip level of 600A. However, this would require me to 'fake' the motor FLA setting in the soft starter, which would adversely affect the motor and cable protection. This could be fixed by installing an external overload relay.

So really the question I have to answer is can I justify the installation of an external overload relay on this starter.

 

[123MB]

Oh also, I tried to join the forum at lmpforum.com but I never got the automated email reply for verification of my email address. I was looking around at the forum there and it seems like alot of the questions are right up my alley so I am dissapointed I was not able to post there, especially if the inventor of the starter in question is the admin there! so I am really not happy about that.

 

[jraef]

Hey, sometimes he is on vacation...

ANYWAY, even if you could set the starter for 700 or 800% current limit, it's pointless. Your MOTOR is what determines the maximum torque it can supply. The soft starter can only REDUCE that torque. Locked rotor torque on a standard Design B motor will be about 160% of FLT, which from a dead stop will equate to around 600% of FLC. You cannot make the motor draw more than that no matter what controls it. So if you are concerned as to not having enough torque, you have the wrong motor, soft starter or not.

PEAK torque from an AC induction motor takes place at the Breakdiwn Torque point, which typically occurs at around 80% speed shen started DOL. the only way to take advantage of that peak torque capability is to use a good Vector Drive, which can "force" the motor into delivering that briefly at any time.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

Hi Jeff.
A question;
Breakdown torque at 80% speed implies 20% slip, or a rotor frequency of 12 Hz.
Can you program most drives to begin ramping from 12 Hz, and if you do, will you be able to accelerate at near breakdown torque? I realise that the drive may have to be oversized.
Thanks Jeff.

Bill
--------------------
"Why not the best?"
Jimmy Carter