2 leg control in soft starters

[jraef]

I am curious as to opinions on the recent trend towards 2 leg (a.k.a 2-phase) control in soft starters. The concept has been around for a while, but recently I have seen it make the leap from under 25HP (18.5kW) to some products being offered at 950HP (710kW)! One manufacturer claims to have solved the negative sequence current issue by virtue of some super secret special 'algorithm' that controls it via current control in the other legs.

 

Anyone done any research on this (besides the manufacturer)?

 

Experiences?

 

Thoughts?

 

My opinion is that the world may not be ready for this concept, but I have been wrong (once) in the past. Is industry ready to accept the risk of motor damage to save a little cost in the soft starter? Maybe they are, these things seem to be catching on. Or is it that the general public is not being made aware of the risks?

 

 

 

Keywords added for pick up by search engines: two phase soft start, two leg soft start, 4-SCR soft start, four SCR soft start, 2 controlled phases

[marke]

Hello jraef

 

I too have seen this increase in two phase controlers and the concept has been around for many years.

In this country, it used to be relatively common to find autotransformer starters that were two phase only, but these were only on small motors.

 

The two phase starter works in the same manner as the three phase controller except that one phase is not controlled. If you apply a standard three phase control pcb with one SCR shorted, you will have problems, but by modifying the controll algorithm slightly, it will work quite well except that there will be an imbalance in current.

 

By altering the firing angles so that the two controlled pahses are not controlled equally, it is possible to balance up the magnitudes of the currents. - looks good on a meter. This only balances the magnitudes and pushes the angles into a greater imbalance. I am not sure if the negative sequence currents improve or get worse by the use of this technique, but there is no doubt that the negative sequence currents are not eliminated as claimed.

 

If you use a controlled environment, you will find that the starting current with the two phase controller is higher than the three phase controller. - that must be a clue!!

 

If the start current is very high, the difference will be less.

 

Small motors have a high volume / area ratio and tend to be more tolerant of short term negative sequence currents than large motors.

I believe that the two phase control does have a place with small machines that have a low inertia and easy start. I would not recommend two phase control for larger machines, or machines requireing extended start times. Motors are more highly stressed with the two phase control than three phase control and if the motor is close to it's thermal capacity during start, the two phase control will shorten it's life.

 

Best regards,

[kens]

Hi jraef, the largest 2 phase soft starters i have experience with are 110 kW rated. These have been chosen mostly from a cost and physical size perspective. I have found that in the larger power sizes ie 45 kW up they need to be carefully applied as the starting torque is not suitable for high inertia loads. As Marke has said the total starting current is also higher along with an imbalance in the uncontrolled leg of around 15- 20%. Having said that these starters can be an excellent solution to many low inertia loads but three phase soft starters will always be a better choice. Unfortunatly many decisions are made on price alone without thought given to the limitations of the technology.

 

Ken

[GGOSS]

I think the technology has it’s place and when promoted/applied correctly it can provide satisfactory results.

 

Unfortunately however there are many suppliers who recommend the use of these products thoughtlessly and that in my opinion can be extremely damaging to soft start business in general. Evidence already exists to support this view.

 

There are many issues surrounding 2 leg soft starters that promoters are not aware of. If ‘some’ of these were brought to the attention of potential customers, the acceptance curve would be a heck of a lot flatter than it has been in recent times.

 

I’m not sure if you wanted motor service life, starter reliability/industrial robustness, personal safety and other related issues raised & discussed in response to your opening question. If you do, I see that this thread will continue for some time and that it will raise some heated debate.

 

I for one would not consider the use of it for motors to 710kW, but am certain there are many out there that would.

 

Regards,

GGOSS

[schow]

Dear all,

 

I found that 2-phase controlled soft starters are getting more and more popular and in much higher kW rating nowadays as well (I thought 2-phase units are used up to 22kW only initially).

 

I have lots of customers start asking for 2-phase soft starters up to 110kW but their only concern is the safety of the units (as you know 1 phase will always on!)

 

Can any one of you advice on how to improve the safety of 2-phase soft starters applications besides applying isolating contactors to cut off the 3-phase mains supply? Sometimes, the motors are located quite far away from the control panels and the technicians, who are doing maintenance on the motors would thought that the power is cut off since the motors already stopped (unless they test the motor terminals first before touching them) but accident will always happen!!!

 

All suggestions and ideas are welcome!

 

Dunke!

 

[jraef]

Thanks guys.

I made a mistake in my original posting. The manufacturer I was referring to stops the 2-leg design at 432A, which they show as 300HP@460V. They have lager units in a similar series, but the larger sizes are full 3 phase control.

 

What I find incredible is that they apparently have NO shorted SCR detection circuit! That would be critical IMHO because in that design even one shorted SCR will present a power path through the motor winding. Without a detection circuit, you wouldn't know until the motor burned up! Maybe they intend for these devices to ALWAYS be used downstream from a line isolation contactor, but they do NOT state that explicitly nor does their manual show that as even an option. I would imagine that the reason is because the only advantage to a 2-leg format is cost and panel space savings, but if you must add the isolation contactor it negates that advantage. Just a guess though, I can't get into their heads on this.

[marke]

The safety/reliability aspect is another issue that I did not mention in my previous post.

 

I believe that :

1. An isolation contactor must be used on the input side of the soft starter.
2. The isolation contactor must be operated at every start so that the soft starter only has voltage applied while the motor is running.
3. There must be some form of shorted SCR protection that opens the line contactor on fault.
4. The voltage rating of the SCRs should be higher than normally used for three phase control.

If the soft starter is left on line while the motor is not running, there is full voltage on the motor windings relative to earth. If there is any condensation occuring during the off time with the full voltage applied to the winding relative to earth, there is a far greater chance of an earth fault developing.

If the starter is left on line while not operated, there is a far greater chance of an accident with an operator assuming that the motor is isolated.

With the motor left on line, the full phase to phase transient voltage occurs across one SCR as opposed to across two SCRs in the case of three phase control.

 

DO NOT USE TWO PHASE CONTROL WITHOUT A LINE CONTACTOR that opens when the motor is not running!!

 

Best regards,

[schow]

Hi Mark,

 

Thanks for your comments, it seems that 2-phase contorlled ssts are facing some safety problems (besides all the possible damages could be done on the motors) but I was just wondering since it is having these safety issues, how can it complied to CE or UL?

 

Most of the 2-phase ssts manufacturers do not have units c/w isolation contactor to cut off mains supply when ssts are idle but almost all of them claimed that the units are complied to IEC/EN60942-4-2, is this directive does not consider safety at all?

 

I was told that even for 3-phase controlled ssts. the outputs are not totally galvanic isolated from 3-phase inputs. Thus, is input isolation contactor a "must" for 3-phase controlled units as well?

 

Cheers!

[marke]

Hello schow

 

Yes, I agree, there are potential safety risks that are associated with the use of 2 phase controllers, however the manufacturers have been granted the required approvals.

This could of course be due to the regulations not being updated to the two phase control technology, or it could be due to the use and certification of these devices as components, not complete starters.

 

I would be particularly concerned where two phase starters are installed without a contactor and relying only on the SCRs disconnecting the motor under fault.

In some countries, the regulations require that the starter disconnects all live conductors from the motor. his can not be done with a two phase controller, neither can it be done with an SCR Diode based design, or with an inside delta connection. I do not believe that relying on the SCRs to "disconnect" the motor with a three phase controller is sufficient either, but I do not make the rules.

 

Best regards,

[kens]

Hi all, I feel that the safety issue is one of the greater problems with the use of 2 phase SS. The fact that there is voltage present at the windings when the motor is not running will catch people out. However I believe that any SS should not be considered an isolation device. Good practices of lockable isolation points and Test,Test,Test are essential when dealing with any electrical device. I would recommend installing a line contactor on any SS for both the safety aspect and also to protect the SCRs against line transients when off. In areas of poor power quality a line contactor can avoid hefty repair bills. Replacing SCRs can sometimes cost as much as a new softstarter and the cost of a line contactor is nothing compared to someones life.

Personally I never touch motor terminals unless i have confirmed isolation and then tested for voltage. I did learn this lesson the hard way!

 

Ken

[schow]

Dear all,

 

Another problem I faced with 2-phase ssts is that it has difficulty to start high inertia applications such as heavy centrifugal machines. None of the SCRs was damaged during the start but the motor just could not be started. Is this the common problem for 2-phase ssts? Is there any chance I can start the same machine with a 3-phase sst?

 

dunke!

[marke]

Hello schow

 

I would expect that the motor would still start provided that the start current was high enough.

The issue with starting high inertia loads with two phase control, is that the negative sequence currents will reduce the start torque for a given start current. This will increase the slip losses in the rotor causing extra rotor heating as well.

As the start voltage is increased, the imbalance reduces and of course when full voltage is reached, the imbalance dissapears altogether.

 

If you are trying to start a high inertia load at say 400% current, then I would expect a longer start time than with a three phase controller, and in some cases, there may not be sufficient torque to get to full speed.

If you are able to adjust the start current, then by increasing the start current, you should be able to start most loads, abeit in some cases, at a higher start current than with a three phase controller.

 

Have a good day,

[GGOSS]

It seems that we are all pretty much on the same page here.

 

The following may appear as ramblings from a mad man but hopefully it contains something that might be useful to someone.

 

When 1 and 2 leg soft starters first became available on the Australian market in early/mid '90's, most promoters of the technology did not hide the fact that these products did not control all three phases to the motor. Most of us at that stage were also not hesitant to advise potential customers of the products limitations and that they were in fact intended for use with an upstream DOL starter.

 

In more recent times however there has been a notable decline in the technical capability of those promoting soft starter products….and not just in Australia.

 

One saying I have that tends to rub many people up the wrong way is that 'you don't know what you don't know'. For example a newby enters a training room and is advised by the trainer that it is an acceptable practice to install 1 or 2 leg soft starters without isolation contactors. If that newby simply accepts that to be true, there is a good chance that he will proceed to educate his customers in the same fashion. For me a stand out student is someone who receives information, thinks about it beyond the training room and comes back with lots of 'what if' questions.

 

Back to something I stated previously. I suspect the acceptance curve of these products would be a heck of a lot flatter if customers were aware of all the issues surrounding them.

 

Are we failing as educators or are we putting aside our own integrity and credibility in pursuit of another order? Either way, it’s not pretty.

 

What is very apparent is that there are far too many promoters and users of the technology, who simply don't know what they don't know.

 

Regards,

GGOSS

 

[schow]

Hi GGOSS,

 

I totally agreed with you that there are some one out there "do not know what they don't know" but unfortunately, even the manufacturers themselves did not specify that an isolation contactor is a "must" to be used with the 2-phase controlled ssts.

 

I believe this is simply due to the cost issues, to add in an isolation contactor before the sst might need some surrounding control devices such as relays, timers etc... but this will make the sst uncompetitive if compared to conventional starting methods like star-delta or auto-trans. The price of the sst itself already at least twice the cost of star-delta not to say the extra costs incurred from the other control devices for isolation contactor.

 

Perhaps this is a bit out of the topic but I am afraid it is a fact that lots of users would still consider cost first rather than the benefits could be gained from soft starters compared to conventional starting methods!

 

Please correct me if I am wrong.

 

danke!

 

[jraef]

No, I think you are both right on on this. There is a definite lack of edification, maybe even purposeful (but then again, I am want to fall into conspiracy theories anyway) on the need for isolation contactors. I believe that it has everything to do with the fact that the ONLY reason to implement 2-leg control is to save cost and maybe panel space over full 3-leg control. If however you are REQUIRED to add a line isolation contactor, the cost / size difference is now rendered negligible, so the reasoning goes out the window. In other words, it is in THEIR BEST INTEREST to pass lightly over that subject, so I believe we will never see them promote the concept. The "don't know what they don't know" crowd is in fact their target market then!

 

Further comments on the isolation contactor issue however.

In any SCR based AC controller, there is always the possibility of leakage through the SCRs. Digital meters will read line potential on the output terminals of a soft starter with no motor connected. This has in the past been used as an argument in favor of line isolation contactors, but IMHO, no motor work should be done whatsoever without an air-gap disconnect means being opened and locked open UNDER ANY CIRCUMSTANCES! Here in the US we have an organizations called OSHA (Occupational Safety and Health Admin.) who dictates this very explicitly. Most other countries have similar rules. So a line isolation contactor does not technically qualify as a safety disconnect anyway. It is redundant to one. The only valid reason then for needing an isolation contactor in front of a soft starter is the issue of protection of the SCRs from spikes and surges, and that is legitimate in some locations. In my 20+ years in this business on the West Coast of the US where we do not have a lot of lightning however, I have replaced only maybe a dozen SCR sets due to surge damage, out of thousands of soft starters sold and installed here. In Florida on the other hand, the lightning capitol of North America, they are an absolute necessity.

 

That makes an indirect argument for it being OK to use 2-leg control of course, except that in practice, most people who even NEED to use isolation contactors will opt to take the risk instead because they don't want to spend the extra money. Foolish though it might be, it is nonetheless a reality. That makes the additional risk of losing 1 SCR on a non-isolated 2-leg controller even more ominous. Besides, with the units I described not having shorted SCR detection, it's a moot point anyway unless as Marke said, the contactor is ALWAYS opened when not running, which requires additional circuitry, cost etc.

 

As to the 'why can they get away with it" issue, I can offer this, cut and pasted directly from their FAQs on this product line (the series reference is obscured on purpose).

 

QUESTION:

Is it permissible that one phase is always enabled for the ***** soft starters?

 

ANSWER:

In compliance with section 7 of DIN VDE 0100-460, a switch must be used for operational switching but may not be qualified. The installations permissible for operational switching are listed in section 537.5 of DIN VDE 0100-537. Among other things, the following can be used: Switches Semiconductor devices Contactors Furthermore, it is stated in section 7.1.2 of DIN VDE 0100-460 that not all of the active conductors must be switched. In section 9.2.2 of DIN EN 60204-1 (VDE 0113 part 1), it is stated that semiconductors can also be used. However, there is no definition about whether all conductors must be switched. This means two-pole switching-off by semiconductors is sufficient.

So they are saying, in essence, that "Since nobody says we can't, there is no reason not to." I don't know about anyone else here, but that argument NEVER worked with my parents!

[GGOSS]

Jraef & Schow,

 

I don't disagree at all with what you have both said, however I still firmly believe that it is up to us to advise potential customers of the issues prior to taking an order. This might be an old school attitude but it allows me to sleep well at nights, and it keeps my customers coming back.

 

The aim of any soft start manufacturer is to identify ways of rminimising the price difference between its SSRVS products and electromechanical alternatives. However in their attemps to do so, they have produced products that are (in my opinion) less industrially robust than ever before.

 

An electromechanical RVS customer who transitions to SS expect that the products will provide certain advantages AS WELL AS that they be as reliable and provide a similar service life to the solutions they have transitioned from.

 

In an attempt to reduce costs, SS manufacturers have opted to remove SCR's, compromise ratings (start current, start time, starts/hour, ambient temp) and add internal bypass contactors that are not capable of sustaining the 'run time' overloads that are common and experienced in a number of industrial applications. Many soft starters on the market at the momment struggle to meet Class 10 overload characteristics, but they are offered with internal motor protection that gives adjustability to Class 30 and beyond. These observations are widespread and do not reflect on any one SS manufacturer.

 

The abovementioned will no doubt result in some short term gains, but what does it mean for the SS industry in the longer term?

 

I have seen far too many customers migrate to SS, experience difficulties and then migrate back to electromechanical RVS options. Sometimes this occurs over a short period of time, in other cases the customer may persist for many years before changing back.

 

Surely there are other avenues for SSRVS manufacturers to reduce product cost!

 

Regards,

GGOSS

[GGOSS]

Almost forgot,

 

I know of several people that have got themselves 'hooked up' when working on motors controlled from 1 and 2 leg soft starters. There's no doubt they made silly mistakes, mistkes that could have proven fatal. If however they had made the same mistakes whilst working on a motor controled by a 3 leg soft starter, they would not have been zapped at all, isolation contactor or not.

 

Regards,

GGOSS

[jraef]

Almost forgot,

 

I know of several people that have got themselves 'hooked up' when working on motors controlled from 1 and 2 leg soft starters. There's no doubt they made silly mistakes, mistkes that could have proven fatal. If however they had made the same mistakes whilst working on a motor controled by a 3 leg soft starter, they would not have been zapped at all, isolation contactor or not.

 

Regards,

GGOSS

 

Darwinism should eventually take care of them, but unfortunately there are always new replacements that will step in to challenge designers, and the tendancy to hire cheaper labor will likely increase that Darwinian Elimination process...

[jraef]

...

 

Surely there are other avenues for SSRVS manufacturers to reduce product cost!

 

 

Well when you analyze the variable costs of producing a soft starter, it breaks down to SCRs, heat sink, PCBs and amortized engineering expense (assume cost of sales and support is constant across design platforms). I doubt that much more can be done to reduce the PCB costs, they are already dirt cheap. That leaves SCRs and heat sinks, which is exactly what we are observing in the market as cost cutting measures.

 

Ironically, probably the best theoretical avenue to cost reduction would be greater promotion and market acceptance. Increased demand would spawn higher production rates for the SCRs and eventually lower component unit costs. An example of that would be VFDs and IGBTs.

 

Unfortunately the current trend towards risky designs may actually DECREASE acceptance in the market and reduce demand, which will ultimately result in fewer SCR sales and higher component unit costs. Short term gains that I fear may kill the entire industry. I for one agree with you that the responsible thing is to make sure the consuming public at large is better informed. One of my reasons for posting this was to encourage just this kind of discussion and hopefully Google and the like will pick it up when users are looking for answers.

 

PS

I'd write a white paper on it, but one never knows where one's next paycheck is going to come from!

[marke]

Years ago in New Zealand, we used to have our local electrical wiring rules and these rules said that all motors greater than 750 watts must have motor protection and said motor protection must disconnect all live conductors from the supply. There was a similar rule in Australia, but that one said "must disconnect at least two live conductors from the supply".

In the early days of soft starters, there was a move by some to use the SCRs to provide the "disconnect". I went to the regulating body and asked the question 'was it satisfactory to use the SCR to provide the diconnection?' and the answer was a definite NO. This mad life a little interesting because the obvious next question was in regard to the numerous speed controller in the industry that provided the motor protection but did not incorporate an isolation contactor. Eventually, the interpretation was relaxed to allow for the non use of a contactor and the use of solid state devices as a "fault disconnect" but not as an isolation device.

 

My understanding is that the requirement in most countries, is for there to be an isolator (usually lockable or within sight of the equipment) and a fault disconnect. Two separate devices. A contactor can not be used as both, however it does appear to be acceptable to use a breaker to provode both functions.

 

Defacto use of the output stage of VSDs to provide the "fault disconnect" has created the general acceptance that there is no need for a contactor if the solid state devices are capable of turning OFF. While I am not a fan of this practice, it has become the accepted method used in the field.

 

I am a very strong fan of the use of a line contactor with drives and soft starters such that the contactor opens under fault conditions. This will provide a much higher level of safety than is currently achieved. Additionally, if the contactor is controlled such that the solid state equipment is disconnected when not running, you will greatly reduce the potential for failure due to voltage transients on an unloaded supply.

 

I also believe, that where a VSD or a soft starter is employed without a contactor, the disconnect device used as an isolator must by AC3 rated and by suitable for load break switching. Ever tried opening an off load isolator to stop a motor because the SCRs have shorted??

 

Food for thought.

 

Best regards,

[schow]

Dear all,

 

6 months ago, we had installed a 7.5kW 2-leg sst for a 5.5kW motor, pump application (without input isolation contactor fitted)

 

Yesterday, I received this complaint, which the customer complaint that the 5.5kW motor was burn and they claimed that the 2-leg sst has heated up the motor and eventually damaged it because there is always 1-phase alive even after the motor stopped.

 

I doubt it since the motor should not be heated up without any current flowing thru the winding even 1 phase is still live.

 

Do any one of you have any good technical write out for me to explain to my client telling them the motor damage should not been caused by the 2-leg sst?

 

Danke!

[jraef]

Are you absolutely sure of that? If even 1 SCR shorts on a 2 leg soft starter, you WILL damage the motor. If one SCR shorts on a 3 leg version, there is no return path for current. But on a 2 leg design the return path is always present, that hot leg.

[schow]

Hi jraef,

 

Thanks for your useful info but nothing wrong with the sst, we took it back and it run fine with other motor, I am a bit surprise also why the failed motor did not damage the sst.

 

Any idea or advice?

[jraef]

The only possible association could be the fact that using 2 leg control causes an unbalanced current to be pulled by the motor during ramping. Unbalanced currents do two things:

 

1) It increases the negative sequence current (DC component) in the motor, which results in negative torque expressed by the rotor. This is why, as mentioned in earlier posts, a motor being ramped by a 2 leg controller needs more current per unit of accelerating torque than a 3 leg controller; it essentially has to fight the negative torque as well as accelerate the load. Siemens has addressed this issue in their Sirius line of 2 leg controllers by something they call "Polarity Balancing" which reduces (not eliminates) the negative torque aspect of this. No other 2 leg controller has this capability as far as I know, so they will be subject to this phenomenon.

 

2) The current in the uncontrolled leg is going to be 10 to 20% higher than in the controlled leg no matter what. Even Siemens' super duper feature does nothing to help with this. Keep in mind that this extra current in the one leg will not be adding useful work, it will just be there warming things up a bit more.

 

More current always means more heat, so a motor being started with 2 leg control is going to heat up more in each start cycle than one with 3 leg control. Generally if the ramp time is short, this is somewhat irrelevant. But if it is a high inertia or high friction load that takes a long time to accelerate, and/or the starting duty cycle is high so the extra heat does not have time to dissipate, this can severely shorten the life of the motor.

 

 

[GGOSS]

I think you'll find the technology behind the Siemens and AuCom 2 leg soft starters is very similar. In fact scope traces may leaving people wandering if there has been a technology exchange somewhere along the line.

 

Regards,

GGOSS