Two motors starting by one star delta circuit 3

[emadmoad]

situation is as following :
I have one generator wich supports all the plant loads 24 hours.
one of the loads is a concrete mixer which is driven using 2 motors ,37KW each.
These 2 motors are started using one star delta circuit(3 contactors) and it is made by the manufacturer.
the plant is working OK for 3 years untill now , but nearly every day at night and only at night upon starting these 2 motors the plant lights are completely shut down and I think the generator is interrupted for 1 or 2 seconds then it works normally !!!!(but it does not stop)
MY question is : IS THIS INTERRUPTION A SORT OF VOLTAGE DROP DUE TO MOTORS STARTING ???
HOW CAN I ELLIMINATE IT ?
ANOTHER QUESTION : IF I MADE EACH MOTOR STARTS USING ONE SEPARATED STAR DELTA ( 3 CONTACTORS STAR DELTA FOR EACH MOTOR) DOES THIS REDUCE THE STARTING CURRENT LESS THAN IF THE TWO MOTORS ARE STARTED USING ONE STAR DELTA CIRCUIT ??
thanks

 

[jraef]

First off, it is a BAD idea to start multiple motors off of one start-delta starter, especially open transition as yours is. Even with one motor, there is a slight slowdown in the motor during transition, because for a very brief moment, the motor is disconnected from the line and momentarilly becomes a generator. There is also a phase shift in the power when it reconnects in Delta, and combined with that regeneration at a slightly different frequency, there is always a somewhat severe "transition spike" that occurrs, often followed by a severe zero-cross. Having two motors doing that simultaneously would exacerbate that situation and could be really damaging to all of our electrical systems!

In all likelyhood, that severe voltage disturbance is making your lighting system re-start (assuming it is HID lighting). This is a common problem with start-delta starters. If you separate them, the only way to make it less severe would be to stagger their starting, and if one motor does not provide enough torque to accelerate it, that may create a different problem.

I would suggest using a solid state starter to ramp them to full speed together. No transition, no spikes, much less voltage disturbance.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[Marke]

Hello emadmoad

I agree with jraef. There is an FAQ on the switching transient associated with star/delta starting. The transient can be several times greater than DOL start currents and can cause more problems than useing a DOL starter. Watch the operation of the starter, and you will almost certainly find that the problem is caused by the open transition switching.
I have experienced the switching transient shutting down gnerators, breaking shafts, extinguishing discharge lighting etc. You should replace the star/delta with some form of closed transition starter or a continuously variable starter such as a soft starter.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[aolalde]

emadmoad

Your circuit performance will improve notoriously if you separate the starter for those two motors and delay the start of the second motor for several seconds.

 

[emadmoad]

Thank u all
I'd like to make some points clear:
1) The 2 motors must work together as both are rotating one concrete mixer, so I cannot delay one for few seconds.
2) the star delta circuit was made by the manufacturer of the plant and it is working since 3 years ago
and as you know the star delta circuit reduces the starting current and torque .....
So don't you think that if I changed to DOL it would be more trouble to the generator ??

 

[Marke]

Hello emadmoad

When you use a star delta starter, you will reduce the start current while in star but if you swtich to delta at less than full speed, the start current will jump up to the same current as DOL, plus there is a severe transient at the point you change from star to delta. This transient creates major problems on the supply if the supply is weak.
Elimination of the tranient will probably solve your problems. The transient will be eliminated if you use a soft starter rather than a star delta. This will also reduce the start current. You can use one soft starter to control two motors.
The soft starter reduces the start current and torque and does not produce any steps or switching transients, This makes it superior to the star delta starter.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

the star delta circuit was made by the manufacturer of the plant and it is working since 3 years ago
and as you know the star delta circuit reduces the starting current and torque .....

Just because the manufacturer of the concrete mixing plant made it, doesn't mean it was the best thing for your system. Most OEMs will use start-delta for two reasons only:

1) They know they cannot use DOL (otherwise they would, because of reason #2)

2) It is the cheapest thing they can get away with.

There is absolutely no technical advantage to using star-delta as a method of reduced current starting. None. It's only value is in being the cheapest method for whomever must supply the components. Solid state is usually the best method as far as the end user is concerned. Autotransformer starters are even better than star-delta, but they have their own set of problems and limitations and are more expensive than solid state anyway.

I don't see where anyone here suggested that you start DOL, we assumed that was already impossible, hence the star-delta starter. What we said was either stagger start separate start-delta starters if you can, or change to a solid state soft starter. Since it appears that you are not comfortable with stagger starting, I see that you have only one logical choice.

Actually two, albeit the 2nd one is not really logical. You could leave everything the way it is and continue suffering the consequences and increasing the damage to your equipment.


"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[itsmoked]

emadmoad; I agree with all the previous replies and want to repeat jraef's point that original equipment designs are occasionally wrong!! I have seen many cases where things were done incorrectly by those who should know. Cars often have wrong designs yet that wrong design may be repeated a million times a year in production. Often the original design was based only on COST and nothing else.

Any way, this is what we are talking about in soft starters:

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

http://www.ab.com/en/epub/catalogs/12768/229240/229262/3073021/1179233/

 

[Marke]

Hello emadmoad

If you have a good look at the information on star delta starters you will understand why you are having problems.
have a look at faq237-613 and

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

As jraef has stated, star delta starters are the lowest cost reduced voltage starter and used to be the most commonly used reduced voltage starter. This was because there were regulations in many countries that you had to use a reduced voltage starter and the star delta starter qualified as such. In practice however, the star delta starter generally does more damage than a DOL starter, both electrically and mechanically due to the very high switching transient and the inability to reach full speed before stepping to full voltage.
Best regards,

Mark Empson

http://www.lmphotonics.com