View Full Version : Alternative star-delta starter arrangement.

March 15th, 2004, 08:12 AM
Hi experts.

This isnt PLC related, but you guys are the best to ask anyway, so here goes...

You probably know the typical layout of a star-delta starter:
A short circuit protection before all contactors.
Then an overload protection after the main contactor (so that the current is monitored correctly during the star phase).

This layout is a little cumbersome. We would prefer to have two Circuit breakers, one before the main contactor, and one before the delta-contactor (see the attached diagram, it explains it much better).
The advantage is that we can have the normal arrangement with the supply bus connected to the circuit braker, in turn connected to the contactor(s), and again connected to the terminals etc.
We avoid to split up in a special YD motorstarter combination with separate short circuit and overload protections.
And there isnt much money to be saved using the "normal" layout.

My collegue asked me and newer having seen such an idea before I couldnt say that it is NOT a good idea. Are there any arguments against such a layout ?


Jim Dungar
March 15th, 2004, 10:24 AM
Why are you mounting 2 circuit breakers? Is your equipment intended for North America?

In the typical European Star-Delta starters that I am familiar with the protection is:
One circuit breaker on lines L1, L2 and L3 in front of your contactors K1 and K3.
Overload protection is in motor leads U2, V2, and W2 and before your contactors K3 and K2.

In typical North American Wye-Delta starters that I am familiar with the protection is:
One circuit breaker on lines L1, L2 and L3 in front of both main contactors (your K1 and K3).
Overload protection is after contactor 1M (your K1) and in front of motor leads T1, T2, and T3 (your U1, V1, and W1).

March 15th, 2004, 11:51 AM

I can see advantege to this connection but it minor and it waste of time and money.
The advantege is you have protaction on your motor while the motor is starting (I assume your CB set for nominal current/1.73).
BUT it happend for only 2-5 secound during the starting so it not so importent.
From other side I dont see why not.
The bottom line is.You save some wireing work and pay in additional CB.
Pay attention You will confuse 99% of the electricianes who will meet you starter.

March 15th, 2004, 12:00 PM
Is it only installing technics question. Coul it be difficult put 2 pcs 16 mm2 wires to main contactor? Are those CBs directly connected to busbar ?

March 15th, 2004, 02:35 PM
Why not like this? I normally use either Sprechur + Schuh motor circuit breakers (KTA7), contactors, YD bridging bar set and mechanical interlock. No timers, driven through PLC. AB should have then as they own S+S. Makes life very quick and easy when used with S+S busbar system as well. Automatic type 2 co-ordination is the other bonus.


March 16th, 2004, 02:23 AM
Thanks guys, for your interest in this topic.

Jim, seppo,
Image how our panels look like, a busbar with taps to a bank of circuit breakers, from the circuit breakers connections to a bank of contactors, from the contactors connections to the terminal strip.
If one motor require a softstarter or small vfd, then that simply replaces the contactor - keeping the layout simple and neat.
But a start-delta starter requires us to break the layout, if the overload is after the main contactor. Thats why.

Bob, Arik,
I must protect also in the star phase as the motor that triggers the idae is for a fan with a throttle regulation. In other words, the startup can last for a long time, and protection is necessary if the throttle isnt closed at startup.

I would consider the single circuit breaker layout (like Bobs attachment) in combination with thermistor protection.

March 16th, 2004, 04:41 AM
It is generally considered that if a motor on star delta is not up to speed in 20-30 seconds you are using the wrong type of starter. Perhaps you should look at auto transformer.

If not up to speed in this time, there would be an enormous amount of stress being placed on the motor. I remember that from my apprentice years in the (very) early sixties.


March 16th, 2004, 05:23 AM
some fans have a very long startup time. And it is quite common to use Y-D starters for these.

Anyhow, we have just decided to use a layout with a single circuit breaker, combined with thermistor protection.

March 16th, 2004, 07:04 AM
some fans have a very long startup time. And it is quite common to use Y-D starters for these.

Fans may need a long startup, common to use wye/delta...NO. There is no application that calls for a wye delta start to last more than a few seconds. A wye/delta is not a soft start, its a mean to diffuse the inrush current on motors that have high amperage if applied DOL. Prolonging the delta connection can be highly detrimental to the motor.

If you need to "ramp" the fan you should use an inverter, the cost is comparable to using 2 or 3 starters/contactors in a wye/delta configuration.

I see nothing wrong with the separate CB's as long as you interlock the starters.

March 16th, 2004, 08:47 AM
"A wye/delta is not a soft start". Well I never said that.

"Fans may need a long startup, common to use wye/delta...NO."
edit: I should have said that they have a long startup time BECAUSE the use of Y-D starters.
Maybe I have overstated it by saying it is "common" to use star-delta for large blowers. I said that because I have been involved with one project with a 2.5 MW blower supplied at 380V (ammeter scale was in kA !), and many other projects of medium sized blowers (in the 100 kW range), all with Y-D starters.
The startup time for these can be relatively long (it also depends on the application). Needless to say, motor and load has to be matched so the blower can accellerate fast enough when in star connection.

The blower in question is only 37kW, so you are probably right that it doesnt need a long startup time.

March 16th, 2004, 09:15 AM
Is it possible close the flow by valve and open it after fan have full speed ? This is the way for very big fans as 2 MW.

March 16th, 2004, 10:32 AM

I must protect also in the star phase as the motor that triggers the idae is for a fan with a throttle regulation.

I don't understand how your proposed circuit gives any more overload protection than a standard circuit. The standard circuit has the overload in series with one end of each winding. The overload is sized for the FLA of the winding, not the line FLA. This will give equal protection in both wye and delta.

The second overload in your circuit will see no current when in star and the same current as the first overload when in delta. What is the purpose of the second overload?

March 17th, 2004, 02:02 AM
Yes it is standard to close the throttle before startup.
(If the thottle isnt closed at startup, the fan will never get up to speed and it will trip when changing to delta).
But even with this it takes a long time to start up.

the arrangement isnt supposed to provide better protection.
It is supposed to keep the layout arrangement of the panel AND keep the overload protection like a single overload relay in the motor leads (measuring each phase).
The second CB doesnt provide overload protection, but you must have a shortcircuit protection. For simplicity it is just the same as the first CB.

March 17th, 2004, 06:25 AM
Jesper, have you flow curve from fan-manufacturer ?
There you can see if the motor is correct for it.
Is the flow what it have to be ?

Can You start it directly to delta ? It will start faster !

March 17th, 2004, 06:52 AM
you are stating the obvious :rolleyes:

By the way, I think that the fan manufacturer matches the fan to the motor (standard motor), not the other way around. In that way he can control exactly that the fan can start up in Y with just enough torque to spare.

March 17th, 2004, 02:48 PM

From another view I think there is safety problem with your circuit
You need to connect the brekers together in a way when you shut down
one the second will shut too.

March 18th, 2004, 02:05 AM
thats a very good point !
Someone could mistakenly think that the motor terminals are safe when only one of the CBs is switched OFF.

Well that decides it. I cannot recommend the idea any more.

Thomas Sullens
March 18th, 2004, 07:26 AM
I agree with ArikBy there could be a safety issue. If you change the way it`s been done for ever. Some of us old timers are slow to learn.
By the way I would think a Y delta starter would be considered a soft start sense it only developes 57% starting torque. Because of the low staring torque we`ve had trouble starting small chipper motors 250 Hp and less also bandmills. Sometimes it takes 30 seconds sometimes 200 the reason for the long start is that it trips the main breaker. As you know in the field you have to live with what ever the engineers design and send with the equipment.