wye delta closed transition starting 10

[butter]

hi, sirs.

i've never had an idea about wye delta closed transition starting, even though i've worked as an electrical engineer for 20 years.
these days i need to motor controllers using closed transition type. so, through handbook, i've known of what that means.
i, however, didn't find out information for resister value and control circuit. and there is no one here to help me.

while i find of a closed transition method of wye delta starting by internt, i konw of you as an expert.

i'd like to know the formula and the specification of resisters using a closed transition starting, ohms and wattage .....and protection for resisters...
the induction pumps are a three phase, 380v, 60hz 15kw, 22kw and 45kw.

please reply to me asap.

thanks,

namki lee

 

To tell the truth, I don't know too except the information of catalog. But I'm afraid that is annoying you because of maintenance problem.
Instead of it, what about using the other voltage reduction starting controller, such as auto transformer, reactor, soft starter etc.?
As for efficiency and cost, is there any difference?

 

[jbartos]

Suggestion: Visit

http://www.squared.com

for closed transition wye-delta starter
Specifically for:
CLASS 8630--Wye-Delta Starter

Wye-delta starters can only be used on wye-delta motors which have six leads that allow for motor winding to be connected in either a wye or delta configuration. During start up, the windings are connected in the wye resulting in 58% of line voltage applied across two windings. This reduces both inrush and starting torque to 33% of the delta connected values. After a set time delay, the motor leads are switched to the delta connection.

The wye-delta starter is available in both open and closed transition configurations. Closed transition starters are supplied with an additional contactor and resistor bank used to keep the motor windings energized for a few cycles until the transition from wye to delta is complete.

 

[butter]

thank you, jbartos.

do you happen to know the formular to choose the value of resister such as ohm and watt of resister?

do we need the any protection for resister?

 

[infomaniac]

Butter,

I suggest you use a softstarter instead. It is smaller, less expensive, and you do not have any transition spikes like open and closed transition W-D starters do. The softstarter is also much more flexible in that it allows you to adjust the reduced voltage settings by simply turning a potentiometer or changing a digital program parameter if it is a more advanced softstarter. Wye-delta softstarters are on their way out.

 

[Marke]

Wye-delta starters (or star-delta starters) operate by initially connecting the motor in star, and then after a predetermined period of time, reconnecting the motor in delta.
The objective of this type of starter, is that it is a reduced voltage starter and will reduce the start current and the start torque.
The start current in star configuration, is one third of the dleta current, and the torque is one third of the delta torque.
If the reduced torque is sufficient to accelerate the machine to full speed, there may be some advantage from the use of this starter provided the transients are eliminated. In reality, very few situations actually reach full speed before the transition to delta and so no benefit can be attained. It is very common for machines to rech about half speed before the motor runs out of torque in star, and at half speed, the delta current is almost LRC so there is no advantage.
Standard starters are open transition, and so there is a large transient current and torque developed at changeover, causing more damage than DOL or Full Voltage starting.
There is a scheme called the Wanchop method, for closing the transition on this type of starter. This entails the connection of an auxiliary delta conatcor accross the main delta contactor with resistors in series with each phase. The sequence is to connect the motor in Wye (star) and then close the aux delta contactor. Then open the Wye (Star) contactor and close the main delta contactor.
The theory is this. When you open the wye contactor, current continues to flow through the motor winding via the aux delta contactor and resistors. This keeps the stator field synchronised to the supply and prevents the motor from acting as a generator. To be effective, the current through the resistors must be signioficant and be of the same order of magnitude as was flowing in wye connection. The resistors must be low value and high wattage.
For a brief moment, the resisors are in circuit while the whe contactor is closed, so the resistors are effectively wye connected in parallel with the wye connected motor. This results in a high current draw from the supply and a high current draw through the resistors. They must be able to sustain this!!

The bottom line is: to be effective, the resistors must be sufficiently low in value that a sgnificant current continues to flow through the motor windings while the resistors are in circuit, they must be able to operate in wye (star) connection across the supply so they must have a high power rating, and the motor must get to full speed in wye connection before the changeover, or you would be better off with a full voltage (DOL) starter.

In practice, I have seen amny installations using a small control relay as the aux delta contactor and 10W radio type resistors. This may serve to get round the regulations, but in reality achieves nothing.

To make it work, you are essentially using the components of a primary resistance starter inside a wye delta starter. You would be better off just to use the primary resistance starter (provided it was set up correctly) or a DOL starter.
A correctly selected and setup soft starter is definitely and better option!!
Best regards, Mark Empson

http://www.lmphotonics.com

 

[butter]

Marke,
As for the value of resisters, can we use the same resisters which is used in the primary resistence starters as the resisters of wye delta closed connection starters?

And could you exactly put forward to me the resisters' ohms and watts values of starters for motors? (380v, 3phase, 60hz, 15kw, 22kw, 45kw)

 

[Marke]

Yes, I would suggest that you need to select resistors that would give a start current of about 200% - 300% as a primary resistance starter. The continuous power rating can be less than a primary resistance starter, but the transient power rating would need to be the same.

Best regards, Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion: The manufacturer of wye-delta closed transition starter is supposed to provide resistors for your motor size. Ultimately, their value may be proprietary.
Even though soft starters are challenging the wye-delta starting arrangements, there appears to be some disadvantages of soft starters in terms of harmonics that deteriorates power quality and somewhat impacts the ordinary motor too, which will be seen in time. When properly applied, the wye-delta starter should be a good solution, which incidentally their record proves. They are essentially used for the reduction of the motor starting current, which they abruptly and simply accomplish so that average electrician or technician can follow , especially if they are manual and inexpensive, reliable type whith the operator having a function of the time relay. The soft starter malfunction may result in discarding the whole starter. However, as the time elapses, they are gaining the popularity and market in spite of the manual wye-delta starters being very simple, reliable and hopefully inexpensive.

 

[Marke]

Hello jbartos
Yes, one would normally expect the supplier of the starter to provide the resistors, but many provide inappropriate sizes!!
In regards to the wye delta vers soft starter, very few machines reach full speed in star, so the start current (ignoring the switching transient) is close to DOL start current, so ther is no benefit from the use of the wye delta. Additionally, the transient torque and current can be in the order of 20x the rated values, so there is much more damage to the machine and supply than using a DOL starter. The only benefit of the wye delta starter (other than selling more contactors) is that it is a cheap way to meet the regulations in the 50Hz world were a reduced voltage starter is required. Soft starters will usually reduce the start current more that wye delta, and believe it or not, they can last a lot longer. I know of many soft starters that have operated faultlessly for more than 20 years!! I also know of many wye delta starters with a contactor life of less than three years, so ther is no easy answer!!
Harmonics are only an issue during start, except where an energy saving algorithm is used. Harmonicw from variable speed controllers are more severe and are continuous causing far more supply problems than soft starters.

Best regards, Mark Empson

http://www.lmphotonics.com

 

[Marke]

butter
I would suggest that the resistance value I would use you could work out the following way:
R = V/1.73 /FLC
V = line voltage, FLC = Rated motor current.

P = VxVx4xFLCxFLC for 5 seconds
allows for some safty margin for repeated starts etc.

Someone else may care to comment also.
Best regards, Mark Empson

http://www.lmphotonics.com

 

[Marke]

butter
OOPS the power should read
P = V x 2 x FLC for 5 seconds!!

Best regards,
Mark Empson

http://www.lmphotonics.com

 

[Marke]

There are many missconceptions when it comes to motor control, but the transient is not one of them.
The problem is two fold. One, you must get to at least 90% speed before you step to full voltage (with any starter) or you will draw almost Locked Rotor current making the reduced voltage starter totally superfluous. The problem with wye delta staters is that the torque in wye (star) is one third of the full voltage torque, and if this is insufficient to accelerate the load to full speed, you can not change the voltage or current to increase the torque. The output torque in wye connection is a function of the rotor design and there is a very large variation between motors (LRT ranges between 60% and 350% but typically 100% - 240%) The load torque depends on the driven load, and sometimes that can not be reduced.
With the wye delta starter, there is an open transition between the wye connection and the delta connection. During this period, the motor generates voltage that is dependant on the motor speed and is not synchronised to the supply. When the delta contactor is closed, it is purely chance what the phase relationship between the generated voltage and the supply voltage is. If the two voltages are exactly in phase, there is no problem, however if they are exactly 180 degrees out of phase, then the effect of cloing the deltat contactor is the equivilent of closing with twice line voltage until the magnetic field is repolarised and synchronised to the supply. This causes a severe current transien and torque transient. These transients do cause major damage on the supply and driven load. This transient is not shown in the link you gave. the illustration on

http://www.lmphotonics.com/m_start.htm

does give some indication of this transient. The closed transition wye delta starter does not generate any more torque, or accelerate the motor to a higher speed, it simply (used correctly) eliminates the transient - commonly refered to as the auto-reclose effect.
NB the transient is not a function of the motor design, autoreclose transients occur with all induction motors.
Best regards,
Mark Empson

http://www.lmphotonics.com

 

[TheDOG]

Good explanation Mark. I'll throw in my $0.02...
Another point is that the wye-delta is usually a timed based transition. Therefore, the speed at which the transition occurs will vary depending on the loading of the motor, mechanical wear and even supply voltage fluctuations. It is therefore impossible to ensure that the transition occurs at a speed that minimises the reclose affect.

 

[tmahan]

I am curious on the statments regarding harmonics and softstarters. I can see that during the ramp up or down that the firing of the SCRs would result in some distortion. I don't think though that this would be a significant deterrent to using a softstarter since the ramp time are usually less than 10 seconds for most cases. Once the SCR's are gated to the on position no further distortion would result correct??

 

[Marke]

Hello tmahan.
That is correct. There is a short burst during start and also during stop if soft stop is employed. Provided the energy saving function is defeated, there are no harmonics during run. VSDs produce harmonics continuously and cause much more problems on the supply. In my expereince, I have certainly experienced problems from VSDs causing transformer and switchgear problems due to the waveform, but I have not experienced harmonic related problems from soft starters appart from soft starters employing SCR/Diode construction which cause even order harmonics during start.
Best regards, Mark Empson

http://www.lmphotonics.com

 

[jraef]

For interesting and authoritative reading on the transition spike issue, I offer this article by Richard Nailen. The website is from a Fire Pump Controller manufacturer but it is a good paper, and Mr. Nailen has a great reputation.

http://www.mastercontrols.com/EngInfo/Articles/Nailen/TA_RNail.htm

I'll chime in on the Soft Starter harmonics issue as well. Phase control firing of SCRs does produce low order harmonics, detectable up to the 11th, in varying amounts as far as THD is concerned. The distortion is greatest when the firing angle is lowest, then steadily decreases as the angle increases. If you were to keep the output at a low angle, say 10% voltage, continuously, you may be able to detect as much as 40% THD. Of course, your motor would not be doing anything usefull at 10% voltage, so it is not really an issue. When operating at full output (SCRs gated full on), Soft Starters without bypass contactors contribute less that 0.01% THD. If a bypass contactor is used the THD contribution is non-existant. Only when the SCR firing angle is phased back continuously is there cause for concern. As Marke pointed out, this WILL be an issue if an "Enegy Saver" feature is active because it is phasing back the SCRs continuously. This makes it an undesireable feature IMHO, but let's not get off on that tangent again.

Another new "Feature" to watch out for on this subject is "Phase Voltage Rebalancing" being touted by Allen Bradley and some others. This works by lowering the output voltages to the "least common denominator" so to speak. If phase A is 3% low, it phases back B and C by 3% as well. This means that again, the SCRs are being phased back continuously and harmonics are again a problem. It is features like this that give Solid State Starters a bad name. Subvert the dominant paradigm... Think first, then act!

 

[Marke]

Very good article!!
Best regards, Mark Empson

http://www.lmphotonics.com

 

[jraef]

I appologize for my last statement (It is features like this...) and would retract it if I could. It was biased and unnecessary.

Hey look, I violated my own sig line! Subvert the dominant paradigm... Think first, then act!