Location of overload in motor starter circuit 2

[jimmy2times]

We have a motor starter on ship that has the overload wired before the contactor in the power circuit. This doesn't seem right and follows the natural convention ive followed over years to put overload after contactor as per OEM circuit arrangement for coordinated components. Im from IEC part of world so familiar with type 2 coordination iaw IEC 60947-4-2

It then got me thinking what is the issue if overload preceeds contactor in the circuit. Is it just natural convention that has developed over years or it is because this is how the components are tested and that is the arrangement in which they are validated for e.g type 2 coordination.

I was thinking through short circuit and overload conditions and how each component in the string would react and i could not see an issue with overload preceeding contactor.

I was thinking maybe contactor welding and requirement for minimum impedance in circuit for clearance of fuse. But that was clutching at straws.

This is circa 3kw low voltage rating rather than bigger motors.

Would be grateful of reasoning if other than the natural convention that was always adopted.

Thanks.

 

[itsmoked]

Realize you're talking overload NOT short circuit. You aren't going to have an overload before the contactor. Maybe a short but not an 'overload' which is defined as an overloaded(overworked) motor. Shorts are all handled by the breaker protection which IS ahead of the contactor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jimmy2times]

Thanks keith,

I appreciate your answer but no motor current is going to flow until the main poles of the contactor are closed so what difference does it matter if the overload is connected before it or after it.

If it is electrically connected before contactor and prior to a start request there is a short circuit somewhere within the starter circuit (which is after the fuses and up to within the contactor) the fuses will clear it. Overload would sense it but obviously not react in time ahead of fuses.

With same arrangement If the motor is started (and contactor closes) the if there is motor overload (not short circuit level) then the overload elements react to that overcurrent and the contactor should drop out (assuming overload contactors wired in control coil circuit).

So what is reason that overload MUST sit after the contactor?

 

[waross]

The reason is for ease of connection and to avoid mistakes.
North American starters have always been pre-wired with the control power going to the field devices, then to the sealing contacts and the coil and then to the overload contacts.
There was little chance of the field connections inadvertently disabling or shorting across the overload contacts.
That said, it is permissible to re-wire a starter to accommodate special control circuits.

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

 

[jraef]

It's about the history and development of motor starters. In the "olden days", motor starters were very much larger than they are now, and original designs (invented by Harry Cutler in 1900) were almost all vertical lift, gravity dropout (many still are). Material science of the times did not favor the reliable use of springs and levers so that contactors could be mounted in any position, making vertical mounting essential to their function.

So if the OL relay was ahead of the contactor, it would also be on top of the starter. That means that the heat coming from the contactor would affect the tripping of the OL relay. In the original NEMA OL relay, there was no automatic compensation for ambient heat, so everything affected the heater element selection (more so than it does now). Even after the "Bell Crank" armature design in the 1920s allowed for horizontal mounting of the starter, the wiring convention was established and to move the contacts would have run a risk of errors. Remember, in those early days electricians were cheap and engineers were expensive. So the basic core design of NEMA starters, fostered in the rapid change environment of the Automotive industry, was based on almost rote mechanical replacement tasks they were allowed to do, basically without much thought (because Henry Ford did not think that electricians were capable of it, another long story). Minor changes in how those tasks were preformed could therefore mean costly mistakes. Take as evidence the fact that to this day, NEMA motor starters are marked with the numbers "1, 2, 3" for control circuit connections. That's because it was even too confusing to those early electricians to be expected to figure out how one brand connected differently from another brand, so NEMA simplified the wiring scheme to a simple 3-step process. "Don't think, just do".

We have, of course moved well beyond that and not only are electricians much smarter than Ford ever thought them capable of, but product designs are such that it no longer matters where the OL relay was. When IEC motor starters began infiltrating the US market 30 years ago, electricians got their first taste of having the OL relay mounted in a non-NEMA format. I worked for a German control equipment mfr in the early 1980s and we were doing that, so I can attest to the fact that this cause a lot of confusion, some of which still lingers.

We humans are, after all, creatures of habit.

"Will work for (the memory of) salami"

 

[flexoprinting]

Hah!

I thank the OP for the post, I often wondered that myself. Jeff you are awesome, loved the History lesson and reasoning for earlier designs.

Chuck

 

[jimmy2times]

Jraef many thanks for that comprehensive reply. I had come around to the thinking that it was a historical development as i mentioned in original post.

That said i still didn't like what i saw and it looked unnatural. A colleague mentioned that there could be many more instances on the ship were this has happened (we happen to look at this one starter as the motor is been upgraded). The original design of the motor control circuits was motor embedded thermistor protection only and according to ship staff over years the starters have been changed to replace thermistor relay with overload relay. So my colleagues concern was how many more instances has this happened and do we need to work our way systematically through the ship inspecting all motor starters for cases where the overload preceeds the contactor and rectify the wiring.

I started to think about it and could see no reason why we would have to do this with the motor control components we have (although as i said i would not have authorised these changes and i think i would continue to always design starters with overload after contactor regardless, for reasons mentioned by others in this post).

You have given me some confidence to stand with the decision not to go through the exercise of checking this detail across the ship (although there were lots of other dodgy things done in this starter modification, so we will look
at a few other starters on board and check the integrity workmanship carried out in those instances!)

 

[jraef]

Just to add more credence to the issue of how unimportant the actual location of the OL relay is, on many solid state soft starters the OL relay works from CTs in the power circuit and because the current must go through the CTs so that the soft starter does not trip on phase current loss when the bypass is energized and nothing flows through the soft starter any more. So in effect the OL relay is ahead of both starter methods. Really, it just doesn't matter and longer.



"Will work for (the memory of) salami"

 

[jimmy2times]

thanks jraef. yes I noted in IEC 60947-4-1 (extract attached). That even there it shows arrangements with a combined thermal magnetic device preceding the contactor. but the case of fuse is clearly shown with overload after contractor and that is reflected in OEM literature in my experience. Hence I wondered whether it was a specific tests relevant to fuse combination (as we have on ship) that required overload to be after contactor. The text of the standard doesn't seem to make any reference to the location of the overload.

I will work on basis that I don't need to take further action unless somebody comes up with an alternate case.

thanks for your help, much appreciated

 

[DFordPE]

If it is an IEC motor circuit protector, it will typically be wired ahead of the contactor. These devices replace both the fuses/CB and the OLR.