Star/Delta with reverse

[AdrianUK]

So here's one for the more.... seasoned Engineers here...

Is it possible to create a Star/delta & reversing motor control starter using only THREE contactors?

(I know it can be done with four but I'm sure I've seen a old-school lift controller that did it with just three)

 

[SUNRAY]

So here's one for the more.... seasoned Engineers here...

Is it possible to create a Star/delta & reversing motor control starter using only THREE contactors?

(I know it can be done with four but I'm sure I've seen a old-school lift controller that did it with just three)

Yes it can be done, but it requires (horrible) 2 way contactors. However it leaves the motor live in the same way as regular star delta with 2 contactors.
Lift panels used to use open contactors with very wide opening contacts and some used double acting contactors (basicaly 2 magnets pulling the 'C/O centre off' contacts in opposite directions - effectively 2 contactors in one block), using those it only required 2

 

[bernardgreen]

It also requires something ( time delay / rotation sensor ) to ensure the motor cannot have it's direction changed while still rotating.

Small motors might cope with a sudden change in phase rotation but large motors have been known to jump off their mountings.

 

[AdrianUK]

Yes it can be done, but it requires (horrible) 2 way contactors. However it leaves the motor live in the same way as regular star delta with 2 contactors.
Lift panels used to use open contactors with very wide opening contacts and some used double acting contactors (basicaly 2 magnets pulling the 'C/O centre off' contacts in opposite directions - effectively 2 contactors in one block), using those it only required 2

That makes sense. I saw this on a (still in daily service) 1950s Evans Lift controller. It does indeed use open contactors as well as dashpot overloads.

It was only later that it struck me that I'd only seen three contactors & I was struggling to understand how it acheived star/delta start & reverse - I hadn't considered that one end of the windings would be connected to permanent live.

Thank-you for enlightening me!

 

[AdrianUK]

It also requires something ( time delay / rotation sensor ) to ensure the motor cannot have it's direction changed while still rotating.

Small motors might cope with a sudden change in phase rotation but large motors have been known to jump off their mountings.

This is on a lift controller so it can be assured that the motor is stopped before the direction is changed. The motor is 7 HP (not kW .... its a 1950s motor )

 

[SUNRAY]

So I thought I'd look for the circuit for a reversible star delta with 3 contactors and I've only found one so far, it looks rather butchered to me, presumably adapted from something correct.
Yet another example of c r a p found on t'internet... I've put a big red cross over it as it's wrong:

Don't try this it will very likely cause damage

 

[JohnW2]

So I thought I'd look for the circuit for a reversible star delta with 3 contactors and I've only found one so far, it looks rather butchered to me, presumably adapted from something correct.
Yet another example of c r a p found on t'internet... I've put a big red cross over it as it's wrong:
Don't try this it will very likely cause damage

Am I right in thinking that if there were some 'interlocking' mechanism to ensure that the contactors opened/closed in the right order, that arrangement would be 'OK'?

 

[SUNRAY]

Am I right in thinking that if there were some 'interlocking' mechanism to ensure that the contactors opened/closed in the right order, that arrangement would be 'OK'?

Not quite, the control of the contactors and interlocking was lightly explained in the article.
The star position will start the motor turning and one of the delta contactors will take over, however the other delta contactor by design makes the motor turn the other way when it takes over from star.

 

[JohnW2]

Not quite, the control of the contactors and interlocking was lightly explained in the article.
The star position will start the motor turning and one of the delta contactors will take over, however the other delta contactor by design makes the motor turn the other way when it takes over from star.

Sure, I understand all that - but, if the system ensured that the order (and delays in timing of) opening/closing of the various contactors was 'correct', wouldn't that then be 'OK'?

 

[AdrianUK]

Sure, I understand all that - but, if the system ensured that the order (and delays in timing of) opening/closing of the various contactors was 'correct', wouldn't that then be 'OK'?

Well, no.

The phase relationship between the 'permanent live' side of the motor & the STAR contactor is fixed. Lets assume, for the moment, that closing the STAR contactor makes the motor rotate CW. If, after a delay, the STAR contactor is opened & the delta contactor which is also configured for CW is then closed - all will be OK.... the motor will continue to accelerate upto its full speed.

Now consider the case that the other delta contactor, with two phases transposed (ie CCW rotation) is closed instead. The motor is already rotating (and accelerating) CW...... the instant the contactor closes, the torque on the motor rotor will reverse & demand that the motor changes direction VERY quickly.

 

[JohnW2]

Well, no. .... The phase relationship between the 'permanent live' side of the motor & the STAR contactor is fixed. Lets assume, for the moment, that closing the STAR contactor makes the motor rotate CW. If, after a delay, the STAR contactor is opened & the delta contactor which is also configured for CW is then closed - all will be OK.... the motor will continue to accelerate upto its full speed.

Agreed.

Now consider the case that the other delta contactor, with two phases transposed (ie CCW rotation) is closed instead. The motor is already rotating (and accelerating) CW...... the instant the contactor closes, the torque on the motor rotor will reverse & demand that the motor changes direction VERY quickly.

Again, agreed. However, is the same not the case whenever one attempts to reverse the direction of rotation, whilst the motor is running (whether in star or delta)? One could presumably partially address that issue by having a system such that the star->delta transition was always such that direction of rotation did not change as a result of the transition, but I can't see how one could reverse rotation whilst the motor was running.

Are you saying that the only safe way to reverse the motor would be to stop it and then re-start with the initial star resulting in the direction of rotation that one wanted (with no possibility of switching to 'the wrong' delta)?

 

[SUNRAY]

For a start with this system what current would be drawn/passed by 400V of mains plus 400V generated by the spinning motor in series in that instant it tries to reverse direction?

The motor will have to go through the stopped position and they are not designed to shange speed as quick as the switching would demand.

One of the rides at Chessington had a 200A 3ph supply but the contactors were compressed air controlled for speed to reduce the contacts and rated at 1000A however there were 4 additional windings shorted by contactors for breaking. I believe the original system included a fluid coupled motor-generator to buffer the switching spikes. This system was of course DOL switched but the principle is very similar for star-delta. I also believe it was eventually changed to a pneumaticly driven system.

 

[JohnW2]

For a start with this system what current would be drawn/passed by 400V of mains plus 400V generated by the spinning motor in series in that instant it tries to reverse direction?

As I wrote, I don't see how one can ever safely reverse the direction of a motor whilst it is still 'spinning' appreciably. Am I missing something?

 

[SUNRAY]

As I wrote, I don't see how one can ever safely reverse the direction of a motor whilst it is still 'spinning' appreciably. Am I missing something?

Apologies I thought you were implying the opposite.

 

[JohnW2]

As I wrote, I don't see how one can ever safely reverse the direction of a motor whilst it is still 'spinning' appreciably. Am I missing something?

Apologies I thought you were implying the opposite.

I thought (seemingly wrongly!) that I was clear, when I wrote the above, and my immediately earlier .....:

Again, agreed. However, is the same not the case whenever one attempts to reverse the direction of rotation, whilst the motor is running (whether in star or delta)? .... I can't see how one could reverse rotation whilst the motor was running.
.... [I suspected that] the only safe way to reverse the motor would be to stop it and then re-start with the initial star resulting in the direction of rotation that one wanted (with no possibility of switching to 'the wrong' delta)?