Mind the short circuit!

[Realstylz]

Anybody ever taken note of the risk in trying to change the direction of run of a motor running in star-delta mode?

 

[Engineer54]

What risk, when you know what you're doing there shouldn't be any risk at all!!

 

[Pete999]

No risk whatsoever, switch off, test, lockoff, prove dead and swap a phase conductor, hello

 

[Darkwood]

What has the star-delta got to do with it ..... you rotation phase change any motor with a decent speed and power rating and the consequences could be dire! ....


So what happened or is this just your hypothetical comment!

 

[guest119]

Were you working Live?

Boydy

 

[Knobhead]

Come on, what were you doing?

 

[Marvo]

Kinda scary, hopefully you can clarify exactly what you have in mind.

 

[Lucien Nunes]

I'll take a guess at what he means. I've seen a couple of numpties reverse two leads at the motor, instead of the supply. The motor will then start correctly in the opposite direction in star, then when the starter changes to delta two motor phases get paralleled and the other short-circuited. I drew a sketch of this mistake on a recent thread, IIRC.

One example was an organ blower. There was a temp supply while the building was being refitted, which was the worst lashup you can imagine. The organ was due to be recorded for a CD, and the organist went in to test it before a final tuning session. He sat down to play but the organ was short of wind. Being a clever chap he put two and two together and suspected the blower might have been reconnected backwards, when it will still blow but not at full power. He went to the blower room, his suspicion was confirmed, so he rang the contractors who sent a 'sparky' over to 'correct' it. The following day he went straight to the blower room, hit the local start button, saw the shaft begin turning in the right direction and immediately went back to the organ console. Before he had played more than a few bars, the fire alarm went off.

Turns out the stator leads were crossed, the motor had carried on singlephasing because there was no singlephasing protector nor even a thermal overload on the old starter, and the shorted winding had decided to give out some smoke. I got a panic call saying 'can you fix it, we need to tune and start recording today'. When I saw the lashup, I refused to touch it, but having tested the motor and found it still apparently OK, I explained that if the twit who had crossed the leads came back and tried the simple trick of connecting A1, A2, B1 etc in the motor to the same terminals in the starter, he would find it ran correctly. He did, and they got their recording session in the end.

E2A: Here's the sketch. You'll spot that the phase sequence has been reversed at one end of the stator, instead of the supply. Works fine in star, then all hell breaks loose in delta...

 

[Darkwood]

The OP was ambiguous but you might have a point Lucien.... but could also mean various scenario's but as OP hasn't responded again we are all clutching at straws.

 

[Knobhead]

Beats me how anyone can mess this up. The starter terminals are usually clearly marked, reversal being achieved by reversing the line connections.

I’ll be honest I’ve had very few dealings with Y∆. Everywhere having adequate supplies to start drives DOL. Y∆ due to the reduced starting torque at just 33% wasn’t up to what we needed.
There is also an added disadvantage, at the change over point the current can peak to 10X FLC due to the rotor dropping out of synchrony.

 

[Realstylz]

Yeah! Think of doing so to the cables from the motor to the contactors as you can also consider for DOL motors

 

[Realstylz]

Thanks dude! A little of experience can count on that one. Most people tend to overlook that aspect of the restriction of leads you can swap if it comes to Y-delta.

 

[Realstylz]

You got it, Sir! Just wanted to raise the alarm on my ignorant fellas whom I knew would just rush to dismiss my post.

 

[Realstylz]

Glad you gave it a second thought!

 

[guest119]

HU?????????

 

[D Skelton]

Most people tend to overlook that aspect of the restriction of leads you can swap if it comes to Y-delta.

What restriction? The conductors can be swapped at the MCC after the isolator but before the main contactor or at the motor, the same as if the motor had been wired to start DOL. The only difference being that with YD you need to know what you're doing if you want to change rotation at the motor seing as you have 4 out of 6 conductors to swap instead of 2 out of 3.

 

[Darkwood]

You got it, Sir! Just wanted to raise the alarm on my ignorant fellas whom I knew would just rush to dismiss my post.

So this is all a pointless hypothetical conversation about something we all take as 2nd nature, anyone making this mistake is either drunk or not competent to work on such set-ups ....so what exactly was the purpose of the thread if it was hypothetical you should have said rather than getting everyone figure out if you had made a big mistake and wanted guidance.

There's nothing wrong with discussing theory just let us know we are doing so please.

 

[Engineer54]

You got it, Sir! Just wanted to raise the alarm on my ignorant fellas whom I knew would just rush to dismiss my post.

I think them ignorant fellas, (myself included) are making much more sense than you are, at this point in time!! lol!!

 

[PEG]

Hi realstylz,are you sure you have hit the right forum? I'm sure i have seen you in concert supporting "plug-braking" at the M.E.N arena....hell of a gig...:lips: