415/720 volt motor connection

[Chocolate]

Hello all,
I have been asked to reconnect a motor which has a manual star delta starting knob for clockwise/clockwise operation. The motor is 5.5kw 415 delta/ 720 star 10.3 amp. I'm a bit baffled regarding the 720 star connection for starting as the supply is 415 volts, should I just connect it dol in delta configuration.
Any feedback appreciated.
Thanks

 

[telectrix]

the 720V is the upper voltage at which the motor will run efficiently in star. at 414V you want it to run in delta, but you can start it in star @ 415V. it will just be slower in star. and speed up when switched to delta.

 

[Chocolate]

Thanks tele I can get it done now. Was just confused.

 

[Darkwood]

How was the motor connected before, if the existing starter is a manual Star/Delta then you will have 6 wires at the motor I assume?

You cannot just connect it up how you see fit, it's starting options may relate to the machines design and needed to prevent nuisance tripping or fusing.

As Tel' expresses a motor winding rated for 720v will be undervoltaged when connected to 400v so limiting current inrush, once up to a certain point in it starting cycle then the delta is operated bringing the correct voltage rating across the correct winding arrangments.

If you are unsure in this side of the business, I would suggest you get some background experience first there is a lot more to it than just guessing how it was connected and getting it wrong can be very costly if you haven't accounted for the how the machine operates and the starting characteristics of the motor... it maybe just a simple reconnection but be careful you don't become complacent about dipping your toes into this side of the industry.

My personal opinion here is to leave alone based on the question you asked as it shows you shouldn't be getting involved, others may disagree with me on that but I see the fallout on a weekly basis from people trying to do this kind of stuff with lack of knowledge and experience.

 

[Chocolate]

Thanks Darkwood, the motor has the six leads going back to the manual starter which is a rotary type for the cw/ccw star/delta start. Apparently it is the original motor which has just been rewound.

 

[telectrix]

you may need some expert help in connecting up the starter as darkwood said. get it wrong and it could end up in tears.

 

[Chocolate]

Thanks guys. I'm pretty sure I'll be ok although the rotary switches are a pain compared to a auto star delta starter.

 

[Darkwood]

If you have the original connection plans your ok, if not then this isnt as simple as you may think, also the windings may have been wound and terminated in a different order, this isnt uncommon so you need to know how to rectify such an issue, motors in reverse can damage some machinery so knowledge is essential.

 

[necam]

It is not unknown for motors to change direction when changing from star to delta if not connected properly.
Great care must be taken to identify the correct connections

 

[Darkwood]

It is not unknown for motors to change direction when changing from star to delta if not connected properly.
Great care must be taken to identify the correct connections

Please elaborate, how would incorrect connections change motor direction when the star to delta change is made?

 

[necam]

The phasing should be the same in both star and delta. If connections are incorrect the motor will still start ok in star, but change direction on delta changeover.

 

[davesparks]

The phasing should be the same in both star and delta. If connections are incorrect the motor will still start ok in star, but change direction on delta changeover.

if it is electrically possible to change the rotation at the changeover to delta (which I can't see being possible but motors aren't my field) then it surely is not mechanically possible.
If the load requires the motor to be started in star rather than delta to get it moving from a stand still then there's no way that the delta connection will be able to stop the rotation and start it again in the opposite direction.
I can only imagine that the result would be some horrible noises, and hopefully something tripping out.

 

[necam]

Agreed. Was just highlighting to the original poster the importance of making correct connections. Not something that should be attempted by someone who doesn't know what they're doing.
The motor wouldn't run in the opposite direction, but it is possible to reverse two of the phases, with not very pleasant results.

 

[davesparks]

Agreed. Was just highlighting to the original poster the importance of making correct connections. Not something that should be attempted by someone who doesn't know what they're doing.
The motor wouldn't run in the opposite direction, but it is possible to reverse two of the phases, with not very pleasant results.

I'm struggling to see what would actually happen. I've tried drawing it out and come up with three options.
1, the standard delta connections, clockwise rotation
2, both ends of each winding connected to the same phase, motor just coasts to a stop
3, a twisted version of the standard delta diagram but still with clockwise rotation.

I can't work out how the unpleasant results are going to occur, could you draw a diagram?

 

[Darkwood]

@Nicam.. I think with all respect here, you do not understand how Star/Delta works, yes you can get the motor outputs wrong but it wouldn't try reverse direction on the changeover due to how it all works, at worse you would get a struggling motor with a misaligned phase sequence but not a reversal, your input phase orientation to the star delta starter denote the direction regardless if you screw the motor connections up.