Help with a 3 phase star delta motor

[sheriff78]

I have been called out to replace a DOL starter that somebody fitted last year, It controls a 3 phase motor on an extraction unit. The customer has said that the fan used to start slow and then after 10 secounds it would speed up to full speed. The electrician that changed the starter replaced it with a standard DOL starter that just turns over at a slow speed. I have replaced this with a star/delta starter unit. I was hoping to find some spare cables when i opened up the unit but unfortunaley there wasnt any. So now i am puzzelled as to how it could of ever run at 2 speeds. The motor has got a 3 phase isolator mounted next to it that looks original so how could it of been wired up as star/delta if it only has the one set of cables(red,yellow,Blue)

To get the motor working with the new starter am i right in thinking that i will need 2 sets of cables 1 from the main contactor, and one from the the star/delta contactors. and then remove the links from inside the motor panel.

If this is the case will i need to mount a six pole isolator next to the motor or can i just extend the contol circuit through the isolator so that it cuts off the motor(the motor is mounted outside)

Hopefully this makes sense to somebody.

any help on this would be very appreciated

 

[Marvo]

Usually with a DOL system there's only 3 wires between the motor and the DOL starter (four wire if you include the earth) With a star/delta system you need six wires between the motor and the star/delta control box (seven if you include earth).

I would only spec a star delta system if the motor is larger than 5.5KW 4-pole or if the blade set is plastic and is larger than 630mm diameter.

*Edit* The colours of the wiring can be confusing, rather think in terms of L1 L2 & L3 for supply and U1, U2, V1, V2, W1 and W2 for motor wiring.

 

[b-spark]

The motor has 6 wires in its connection point yes?. (3 winding)?

 

[Marvo]

The motor has 6 wires in its connection point yes?. (3 winding)?

Yes, 3 wndings. Are there 6 wires running from the motor to the old DOL starter?

 

[Hawk81]

Sounds like the motor used to have an inverter instead of a dol starter. This would mean the motor would be wired delta and only use 3 wires ( plus earth). This would mean the motor was 3 phase 230v. If a small motor that is. Or larger motors it could be a 3 phase inverter. This would start the motor slow and ramp up to Full speed.

 

[Marvo]

Sounds like the motor used to have an inverter instead of a dol starter....

Could be, I was thinking along the lines of soft starter maybe.

 

[nickblake]

Is this any help

 

[sheriff78]

yes there are currently 6 wires in the motor, They are linked out at the moment so that there are only 3 terminals. On the back of the plate it has got a diagram to show you how to link it out for star or delta. so it must be happy to be used either way.

My understanding is that when you connect the 6 wires up you remove the links becuase the contactors act as the links when they change over.

When i connect the motor up with 6 wires (star/delta) will this increase the speed of the motor when it changes over and if yes by how much?

also can you get 6 pole isolators for this type of motor or can i extend the control circuit to isolate it outside(the motor is mounted outside the controls are mounted internally)

If the system used to be run through an inverter. would this of increased the speed of the motor? or did the last electrician change it from star to delta because it was blowing the overload every time it started?

i ask this because the customer has told me that the fan used to start slow and then speed up. but now it just does the slow bit. but in my mind the motor will only run at one speed, either the star maximum speed or the delta maximum speed.

sorry to ask so many questions. your help is really appreciated

 

[Hawk81]

If the motor is wired through an inverter it will start slow and speed up. What size of motor is it?

 

[sheriff78]

5kw 3 phase motor

 

[silva.foxx]

Unless I've read all this wrong then changing from DOL to YD won't gain you any speed... unless the fan is struggling to start in which you'd expect the overload to operate.

Why are you replacing the DOL exactly? What's the fault?

Can you show a picture of the motor plate or provide plate details please?

You say in post #8 "...or did the last electrician change it from star to delta because it was blowing the overload every time it started?" so what is the motor connected as now... star or delta?

So is the connection correct for the supply to the motor?

Obviously we do not know the history of the fan control. We do not know for sure whether the supply to the DOL is mains or comes from another source, perhaps an inverter or soft-start tucked away from sight in another panel. Perhaps the motor is/was a two speed motor that had a Dahlander connection

When fitting a six pole isolator for a YD motor ensure an isolator auxiliary is used so when the isolator is switched off it tells the YD control to stop. I've seen a few occasions where the isolator has been used to stop the motor then the motor restarts in delta when the isolator is switched back on! Nothing major on a 5.5kw motor but as you get bigger...



@ MARVO: is that your Boxer in your avatar?

 

[Steve1412]

The customer may have mistaken the time taken for the motor to run upto full speed for it actually having two speeds....we all know that when a big fan starts up(dol) you get the usual rumbling humming noises until it gets to full speed then it usually quitens off a little. They may just think it had two speeds??? As fans starting loads are minimal compared to say silt pumps water pumps etc I would have expected a DOL starter as opposed to Soft start or VSD.

 

[Knobhead]

SilvaFox it’s years since I’ve seen a Dahlander wound motor I would be surprised to use one for a fan

 

[silva.foxx]

Last time I saw one was on some Baltimore Air Coil evaporative condensers on an ammonia refrigeration plant... not too long ago.

We had plenty of two speed motors though, on high shear mixers.

 

[Marvo]

@ MARVO: is that your Boxer in your avatar?

Unfortunately not, he's a handsome chap though for sure, couldn't resist using him for an avatar