Star / Delta O/L’s - Which is Correct

A is correct. The overload should always disconnect all power from the motor. B cannot do this.

Sorry PWS I didn't make it clear that the O/L relay would be in the control so as to drop the line contactor out. Not breaking the circuit it's self.

Yes but - as in the other thread - it's use is more about accelerating a large load i.e. big machines.

So which acceleration phase takes more 'stall current seconds' is the one with 'extra protection.

Both are over and above protection (not shown) ahead of the primary switch.

Or both are correct - depending.

Now you're absolutely certain that you're not being indecisive about this?

Heeeeheee - 17 posts on and has "it depends" changed?

Nothing really changed other than my mind, following some of the reasons put forward I'd now opt for "B" if forced in to it.

I still don't like either of them!

And I don't like the idea of 4 x 866A's as a bridge, but it's handy to understand the principals - well I think so - because you never know when it might come in handy somewhere totally different.

(follow this link in that post)

Both are correct. In A the O/L has to be set at the line current, while in B it is to be set at phase current. In B, it is a smaller relay, and, sometimes, cheaper (direct vs CT-relay). Most Indian and European starters are wired like B.

+1 GA.

As he said, both work, both are "correct" but the reason you don't see "A" everywhere is strictly economic. If it is a 50HP motor, it probably makes no difference, the cost of a 50HP rated OLR is probably almost the same as that of a 30HP OLR (58% of the size). But if it is a 100HP motor, the OLR in "A" would have to be a more expensive version that can carry line current. Then if it's a 500HP motor, it's back to being a toss-up because either way, you have to use CTs for the OL, and what's the difference in the cost of 1000:5 CTs vs the cost of 600:5 CTs? Next to nothing.

Personally, I like to use "A" because I tend to hate Star-Delta anyway and if someone insists, doing it with method "A" leaves open the possibility of using any other starting method in the future once the user figures out that Star-Delta is costing him money in torque damage. but that's just me...

A and B has same function. In both case the main Contactor only open. Doesnt matter whether o/L is located. It is important which contactor is connected through.

B is correct. In this connection O/L relay is set at phase value of current of the motor giving protection to motor in star as well as delta connection. The relay would get true thermal image of motor.

Dear Happy Singh

Your reasoning is good and most acceptable.

R.Panneer Selvam

thanks

GA from me.

This is the real answer for the B design. Better protection for star running in case it got stuck in that stage.

I don't suppose there is or ever will be a right answer. I dislike star delta but if forced I'd go for A.
As I said in my OP I'm used to plants where you could start 400HP motors DOL without trouble. The only time I came in to contact with YΔ is with items of kit bought with the electrics as part of the package.

Perhaps the 'right answer' is it's about the only way you could do it, before 'modern electronics'.

as being lucky to work in mills where we use what we got. 'b' would be the most familar one for myself, we would use that set-up w/ ct's for high inrush currents; i.e. chippers, hogs and other cutting loads - and 'a' w/ motors of low inrush currents; i.e. conveyors or transportation belts.

METHOD B IS THE CORRECT PROCEDURE.

THIS ALSO REMOVES ALL the POWER AS WITH THE OPERATION OF THE OVERLOAD THE MAIN CONTACTOR DROPS OUT.

THE RATING OF THE O/L RELAY SHOULD BE 0.58Ir ,and is universally adopted by SIEMENS/SCHNEIDER/ABB etc .

B is always preferred and use in practice. As Mr. Happy Singh already mentioned it is very improtant to note that the motor will get true thermal image for effective protection against overload( resulted in overheating of windings).

Both are correct but when you are using star/delta starter to run the motor against the load ,for example high starting torque required and stalling current for a longer time like ID Blowers i prefer to use B,OTHERWISE due to overlaod main contactor will dropout.universally major players like L&t/SIEMENS/SCHNEIDER/GE PREFERS METHOD -B and economical. Method -A is preferable for lighter /smoother loads.i hate star delta starters for large machines better to use,soft starters.

Cheers, msamad, as I said there're not something I've had a lot to do with and I don't like them. Following the episode with the 800HP starter in an earlier thread http://cr4.globalspec.com/thread/60397 I've gone right off them!

The only reason I asked the question was out of curiosity. The reason for my preference for method A was simply ease of understanding the O/L setting, but you've made me change my mind. Not that I intend to install any of the horrible things unless I'm forced. Now I'm retired I can't be forced anyway

BOTH ARE WRONG sorry guys i have to shout here. In delta connection there is a necessity to connect motor rotar wingings in proper phase sequence.

Oh - here we go again

Why don't you go and back door a thread - learn what you need to know, then never come back and say "oh now I get it" to the person you are yelling is "wrong!"

Not that this thread needs to drone on further with more indeterminate outcomes, but I have to differ on a point in msamad's post.

"Now consider the case A:

Here the O/L protection is installed in the line. The setting of the relay is set for 95A protection. This setting is perfectly works for the delta operation but it is much higher for star operation. Because the line current is 55A (for star), but setting is 95A. Since the motor runs very short time in star operation, we can ignore the overload protection for this short time period. But what happens when the star-delta timer mal-functions (keeps motor to operate in star for longer period) and the motor is overloaded in this period? The motor will be damaged.'"

Where is the damage going to come from? The motor winding current is rated for 95A. If the line current is 55A and the timer malfunctions, leaving it in Star indefinately, we can assume that the motor torque will be insufficient to fully accelerate the load and it goes into a stall condition. In that condition, the motor winding current would theoretically rise to Locked Rotor (once the rotor stopped moving, which actually would not happen, but let's look at worst case scenario). LRC would be 600% of FLC, but because it is stuck in Star, the maximum developed current will be 33% of that, so 200% FLC. The OL trip curve will of course take longer to trip at 200% FLC, but that STILL is the same thermal damage curve of the motor windings, it STILL should be adequately protecting that motor. i.e. any damage is no different than if the OL were in the B location. It's all relative.

So once again, no different.

And by the way, anyone who uses Star-Delta without an incomplete sequence trip should have their head examined. But of course, that adds COST, which is the ONLY reason anyone uses Star-Delta in the first place, so it never happens...