1. Simple Question

smaller motors can either be connected in star or delta right, but how do you decide? I was taught that it depends on what it would be used for. Like star will give 220 across the windings and have less torque, Delta will have 380 across windings and give more torque. So delta if motor is going to work hard and star otherwise???? Is this way of thinking correct?
Other side is that it depends on the supply voltage.
3phase 220v or 3phase 380v, dictates the connection.
So please can any one clear this up for me.
O and if i have a drive that is supplied by 220 and in turn supplies a 3phase motor how should that motor be connected and y??
thank you

2. Hi Gei. This isn't a simple question, it's half a dozen complex questions.

Originally Posted by Gei
smaller motors can either be connected in star or delta right, but how do you decide?
Rotation speed will be different depending whether you connect in star or delta. Some motors are not rated to be connected in either, check the voltage options on the motor tag plate first.

Originally Posted by Gei
I was taught that it depends on what it would be used for. Like star will give 220 across the windings and have less torque, Delta will have 380 across windings and give more torque.
Not necessarily. I prefer to use the term 'shaft power' than torque which can get very complicated. The shaft power is the power required to drive the load at the running rpm of the motor. As the shaft power increases so does the load current drawn through the motor. If you exceed the maximum design run current of the motor it overheats and either damages the bearing lubricant or burns out the windings.

Originally Posted by Gei
So delta if motor is going to work hard and star otherwise???? Is this way of thinking correct?
Not necessarily.

Originally Posted by Gei
Other side is that it depends on the supply voltage.
3phase 220v or 3phase 380v, dictates the connection.
You get a three phase star wiring scheme with a neutral and a three phase wiring scheme without a neutral. Usually a standard 3-phase induction motor will wire delta without a neutral. Again it depends on the specific motor and application.

Originally Posted by Gei
So please can any one clear this up for me.
O and if i have a drive that is supplied by 220 and in turn supplies a 3phase motor how should that motor be connected and y??
thank you
I'm assuming you're talking about a VFD. There are a few VFD's which can be supplied single phase 220v and output 380v 3-phase. I've never seen one of these larger than 2.2 kw. The limiting factor is the 220 volt supply requirement becomes very large and can no longer be supplied from a standard 16Amp socket outlet. The connection to the motor would usually be a standard 3 wire delta connection and earth. Consult the literature to avoid singed eyebrows. :-)

3. Originally Posted by AndyD

Rotation speed will be different depending whether you connect in star or delta. Some motors are not rated to be connected in either, check the voltage options on the motor tag plate first.

I assume Gei is talking 3 phase only.

Surely the speed of the motor is determined by the number of poles and the frequency of the supply source e.g. a 4 pole 380/220 volt motor in SA turns at +- 1440 RPM and cannot be changed unless you run it through a VDF which can change the supply frequency and hence the speed.

Further I have found that you find many motors especially from China that are wired 220 / 380 volt connection which means that if you have the 6 wires on the terminal block , you have to change the connection to star to take care of the 380 volt supply .
BUT as you say...............check the plate on the motor first to determine the required voltage/connection.

In order to save some electricity in my factory I am running quite a few 380 volt motors in star ( light duty applications only ) and have found this quite satisfactory, just be careful to ensure that the power output in star is sufficient to do the job without over heating and a possible burnout with "spitzen sparkin" and much smoke .

Incidentaly, you do know that electricity IS actually smoke. Whenever something goes wrong with any electrical apparatus you normally see smoke coming out ! That is the electricity leaking out

4. Originally Posted by Martinco
Incidentaly, you do know that electricity IS actually smoke. Whenever something goes wrong with any electrical apparatus you normally see smoke coming out ! That is the electricity leaking out
Actually, I think that the smoke you see is actually a user notice that the appliance generates to tell you that your appliance warranty is now expired. It has nothing to do with the malfunction of the appliance - that is a seperate event that takes place, coincidentally, just after the user notice is issued.

5. Originally Posted by daveob
Actually, I think that the smoke you see is actually a user notice that the appliance generates to tell you that your appliance warranty is now expired. It has nothing to do with the malfunction of the appliance - that is a seperate event that takes place, coincidentally, just after the user notice is issued.
Man........you got it !

6. Originally Posted by Martinco

I assume Gei is talking 3 phase only.

Surely the speed of the motor is determined by the number of poles and the frequency of the supply source e.g. a 4 pole 380/220 volt motor in SA turns at +- 1440 RPM and cannot be changed unless you run it through a VDF which can change the supply frequency and hence the speed.
On paper or in a zero load scenario you're right, the rotation speeds should be identical in star or delta supply formation. The output torque in star is often less than a quarter of the same motor wired in delta which in a load situation results in lower rotation speed and possibly stall condition.

7. in s.a. generally motors up to about 4 kw are designed to run in star
and above 4 kw in delta.
but as mentioned the rating plate must be checked.if it says 380 volt star and 230 volt delta then it is must be connected star in s.a. and delta in a country with 230 volt 3 phase . these windings are designed to handle 230 volt maximum.these are normally american motors.
regarding the single phase vfd,s i have only come across the models that supply 230 volt 3 phase output. its probably easier to manufacture. the motors should be connected in delta. in star there would only be about 130 volts across each coil.

regarding the 3 phases and neutral connection to a motor,i have never seen it. what application would it be used for?
with 3 phase heating elements in star connection you could run it without a neutral ,but if 1 element blows the other 2 would only have 190 volts across each one. with the neutral connected to the star point it would stay 230 volts even if 2 elements are faulty.

8. U are correct. but I think what matters most is the current, especially the starting current, thus why we have star/delta starter. As for u, u are much concerned on the torque, it is wise to connect your motor in delta as torque is directly propotional to current(in delta current is 1.73 times the current in y). And for the supply, the supply are always connected in y. So u will only have 380V on the side of the motor. And as u said is for the small motors, the starting current wont be that much.

9. Thanks for all the replies especially bergie . Yours cleared it up for me.