Sizing bypass contacts for Soft Start 3

[WoodMonger]

We recently had an old analog soft start fail which we replaced with a new Baldor digital soft start. This is driving a 100hp fan motor for a dust collection system.

The original setup had no input contactor on the line-in side (just a nearby disconnect) and also had no bypass contactor to take the load of the soft starter once up and running.

This has worked fine for the last 12 years but it just doesn't seem to be the optimal setup. There is a safety issue in that there is 480v showing on the output side of the starter (in stopped state) unless the disconnect is switched off and then there is the issue of several hundred watts of heat being generated inside the enclosure by the SCR's on the soft start.

First order of business was to get the unit up and running again which we have done. Now I'd like to go back and correct the setup with a bypass contactor and possibly an input contactor. The problem is that a magnetic contactor sized for this motor costs nearly as much as the soft starter - and I need two of them!

My question then is do I need to install contactors rated for the FLA of the motor in question or can they be downsized due to the fact that they are not being used to switch the load?

Thanks in advance,
Conrad

 

[boaterbill]

When the soft-start has ramped the motor to full speed, switching to the bypass contactor will still require the contactor to be able to handle the FLA of the motor. Some bypass contactors are rated to start and stop the motor without the soft-start and are rated for full starting current.

Hope this helps

 

[Marke]

Hello WoodMonger

The Line contactor is subjected to the starting current of the motor, so must be rated for starting that size of motor. In European terms, that means it should be AC3 rated.
The bypass contactor only closes once the motor is up to full speed and as such is not subjected to start currents and so it is common to use a contactor based on its thermal current rating only. (European AC1) This does reduce the size of the bypass contactor. If you wish to use the bypass contactor as a means of starting the motor if the soft starter fails, then it must be rated for start duty.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

Marke is dead on as usual, but since you are likely in N. America (you used HP instead of kW), the In-Line contactor must be fully rated for FVNR starting of the motor, the bypass contactor can be sized by using the "thermal current" or "resistive current" rating of the contactor. i.e. FLA only.

If on the other hand, you want the bypass contactor to serve as an emergency re-start contactor, then you must also size it as an FVNR contactor AND provide an overload relay. But being that you chose a Baldor digital soft starter, hopefully you bought a separate OL relay anyway because that one is NOT UL listed to provide OL protection, even though the function is built-in. It only monitors 2 of the 3 phases for OL current and that is not acceptable in the US (and probably Canada as well). Baldor is usually good at telling you this if you ask and/or read their manual, but not all of their distributors are educated enough to understand what that means. So just be aware.

But let me say a word about In-Line contactors. I am not a big fan of wasting money and in the majority of cases, an In-Line isolation contactor on a soft starter is a waste. ALL SOFT STARTERS WILL SHOW LINE VOLTAGE ON THE OUTPUT WHEN MEASURED WITH A DMM. It is an inherent design issue when using SCRs, a combination of leakage through the SCRs themselves and the RC snubber networks (if provided). However if you were to use an old analog meter with an iron vane movement (as opposed to a digital meter), the slight burden of the meter would drive that reading down to nearly zero volts; the current is usually measured in mA or even mico-amps (if there is no snubber). So it is mostly a fluke (pun intended) of DMMs that is allowing you to see the leakage potential, it is not all that dangerous. Besides, nobody should be opening that enclosure or working on that motor without the disconnect being opened anyway, right? RIGHT??

So my bottom line opinion, unless you are in an area subject to frequent lightning strikes (which can damage the SCRs), I wouldn't waste money on the in-line contactor. And by the way, I work for a contactor manufacturer so I would theoretically want to promote more contactor use.

 

[waross]

Anecdote with a point.
I saw a 250 HP rated starter used as a bypass contactor on a 400 HP motor. I was not directly involved with the original installation but I was involved with the repairs. The electrical foreman queried the use of an under sized starter. The engineering PHd responsible for the design was not very nice. He stated that they had tested the starter in question and found that it could carry the full load current without over heating. He mentioned his PHd and finished by telling the old foreman that "You should know your place and stay in your place!"
One test that had been neglected was interrupting full load current. The starter was not able to contain the arc developed when breaking full load current. Worse, the contactor design allowed the arcs to issue out of the front of the contactor. The arcs joined phase to phase and
"Crow-barred" the motor. (Instant low impedance fault across the motor terminals.) The motor became an induction generator feeding into a fault. The motor slowed down rapidly. In fact the motor slowed down faster than the fan that it was direct driving.
The short list of damages;
400 HP motor, bent shaft.
Expensive special design inverter destroyed.
Large fan, bent shaft.
Coupling, destroyed and pieces of shrapnel in all parts of a fair sized fan room.
Lots of smoke in the control cabinet.
Morals.
Be guided by the manufacturers ratings.
Listen to the old guy once in awhile.
respectfully

 

[jraef]

waross,
While your story does illustrate a valuable point, MOST good soft starters are designed to use the SCRs to turn off the load, not the bypass contactor. The difference is in that the good ones keep the gate circuit on-line all the time so that when the contactor opens, the power shifts back to the SCRs, then the SCR gates are turned off. Some of the cheap junk soft starters use marginal gate firing circuitry and so to keep them from cooking themselves, they disable the gating once the bypass is closed. Your example perfectly illustrates the danger of that scheme.

I should note that some newer digital designs also turn off the gating circuit now, but when you remove the run command they instantly re-establish it, open the open contactor, then still use the SCRs to stop the motor. The digital technology allows for that virtually instantaneous gate control resortation.

 

[WoodMonger]

Thanks for all of the replies!

The Baldor starter was selected because of it's availability - a good part of our production was down and we needed a replacement fast.

What I think we'll do is add an appropriately sized bypass contactor both to reduce heat in the enclosure and to give us a backup in the event the starter fails. We'll forgo the in-line contactor as it doesn't seem to be a necessity.

Thanks again,
Conrad

 

[GGOSS]

The use of a line/isolation contactor also acts to increase stsarter reliability particulartly in areas where power quality is questionable.

SCR's are most vulnerable to damage from voltage transients when they are connected to supply but NOT gated on. The addition of a line contactor (preferably controlled via a relay on the soft starter) acts to extend start life and minimise downtime.

Back to the leakage issue, whilst there is a solid connection between the soft starter and the motor terminal, each terminal is in fact a virtual neutral and therefore any measurment for voltage between these points and earth will display close to zero. Whilst it is theoretically possible to ouch one of the terminals without getting a zap, I certainly wouldn't recommend. it.

If however a loose connection or open circuit develops between the output terminal of the soft starter and the motor terminal, the voltage at those terminals increases towards line voltage.

In Australia many soft starter are used without line or isolation contactors even though our wiring rules (unless recently changed) state that any motor above 7.5kW must be fitted with overload protection, and in the even of a fault the motor shall be automatically disconnected from the supply. SCR's do not disconnect the motor!

Alternative options ( to the isolation contactor) may be a circuit breaker fitted with a shunt trip, or preferably a circuit breaket fitted with an undervoltage trip or no volt release. The latter being more fail safe.

BTW, further analysis of SCR failures experienced within can be attributed to transient over-voltage conditions.

 

[fornhamspark]

Hi all,
Have read the posts but no body has gone on to the subject of the line contactor being used for emergency stop .

What would you size the bypass contator for AC3 Or AC1 if the line contactor was dropped out while running at full speed. Surely the bypass contactor would have to be rated for Ac3.

Iam refering to the point raised by (waross)

 

[Marke]

Hello fornhamspark

If a line contactor is used and appropriately rated, there should be no problems using an AC1 contactor as a bypass contactor. If the line contactor opens while the motor is running at load, it will break the connection, not the bypass contactor.
The greatest danger is where there is no line contactor and there is a severe fault. At that stage, it is up to the bypass contactor and the SCRs to interrupt the fault current.
As Jraef previously described, the SCRs are able to carry the load and then interrupt it when the bypass contactor opens.

In situations where no line contactor is used, there could be an argument to use a AC3 rated contactor as a bypass contactor to give greater fault interrupt current capacity, however the issue with AC1 rated contactors and AC3 rated contactors is typically overload current related rather than opening/interrupting capacity. For example, look at

http://www.benshaw.com/fvc/Upload/fvc-366.PDF

page 1-5 which shows a RSC150 contactor having an AC1 rating of 210 Amps, an AC3 rating of 150A and an interrupt capacity of 10KA. If we compare this with the RSC220 which has an AC3 rating equal to the AC1 of the RSC150, it has the same rated interrupt capacity.

In the case that Waross described, a 250HP contactor was used to bypass a 400HP installation. I would suggest that the thermal rating was exceeded in this case and that the contacts were overheated before opening and that was a major contributing factor.
Look at the RSC 600 and you see that the AC3 rating is 630A and the AC1 is 660Amps. There is not much difference in some cases.

It is a commonly held belief that the AC1 and AC3 ratings are related to interrupt capacity rather than thermal capacity. I am of the understanding that the AC1 AC3 ratings are primarily about the contact overload capacity and that in most cases, the interrupt capacity is not an issue provided that the contacts are not overheated.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[ScottyUK]

In the case of a severe fault I hope that it would be cleared by a fuse or breaker and not a contactor. Contactors trying to clear major faults tend to disintegrate. I clearly remember one event where an MCCB failed to open and the contactor in the MCC tried to open before the definite-minimum-time ACB incomer upstream of the MCCB cleared the fault. The MCC cubicle was destroyed. I might have a photo or two lying around at work.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[jraef]

I think several of us directly stated that the line contactor must be fully rated (Marke said AC3, an IEC term; I said FVNR, a NEMA term).

As ScottyUK pointed out, a contactor is NOT intended to be a fault interrupting device, but it does need to have a "withstand" rating (also now known in the US as a SCCR rating), i.e. the 10kA that Marke mentioned. But this rating is as a stand-alone device. Often you can get a higher overall series tested withstand rating by pairing the contactor with either a fuse or circuit breaker, but technically only if the contactor has been tested in in that series configuration. That means if you want to use an MCCB, you will likely need to use a contactor of the same make as the MCCB; most manufacturers are not inclined to test competitive products.

But in that scenario, you will also often find that both the AC1 and AC3 ratings of contactors are listed behind the same circuit breaker or fuse, so the point of how you size the contactor as far as withstand ability would be moot.

 

[LionelHutz]

Marke is correct, the withstand fault rating and the current rating are not really related. He's also correct that the case Waross describes sounds like that bypass was sized to be even under the AC1 rating.

A bypass contactor sized to the AC1 rating is capable of opening and closing the AC1 current rating. Therefore, it can handle opening the rated motor current.

During a fault, the fuses or breaker should open first so the fault withstand of the contactor tells you the circuit it can be used on.

I guess this shows that the starter should just be purchased as a bypassed unit. 3 of those Benshaw RSC220 contactors are used as part of a 838A starter and the starter has a 65kA withstand rating.

 

[waross]

Hi folks:
The starter I remember was a type of European design common years ago. The design sometimes failed opening less than full load current on a day with high humidity.
Some of you may remember the design, there were six windows on the front face of the contactor. De-ion plates were stacked on edge in each window. When interupting heavy currents the arc would issue forth from the edges of the plates. Sometimes the arcs would join up and take out the circuit fuses.
We had a customer with a bridge crane beside a river. We sent a service-man out several times a month to replace fuses. (Always on a cool damp morning.) We eventually replaced every contactor in the box with a respected brand of North American made starter of the same ratings. We never had another flashover.
Several years later the 400HP motor was running (for a short time) on a 250 HP contactor of the same type. The arcs on the load side joined up this time and "crow-barred" the motor, hence the bent shafts.
I realise that this type of failure is not a factor with many modern starter designs but I came away with the lesson;
"I may not know all the factors involved in testing and approving equipment so be very careful using equipment outside its normal ratings."
Actually the starter in question was a line starter, not a bypass application, but I believe the lesson is still valid.
Respectfully