Power rating of star connected motors 1

[gridlink]

I am a little confused about the ratings of a standard 3phase motor when it is connected in star rather than delta. We are working on a project where we want to insert a capacitor beween two of the phases to provide a circulating field so that the motor will run from single phase supply. The only? way so far seen is if the motor is wired in star.

My belief? is that the power of the motor will be reduced (E*I) if connected this way. is this true or is the power the same despite the differing connections.

many thanks

Gridlink

 

[cbarn24050]

Hi, I would forget about your plan altogether, If you want to run a 3 phase motor from single phase supply use an inverter.

 

[aolalde]

The different connections of a motor are to make it work at different voltages.
For example a 220 Volts Delta connected motor will work properly at 380 volts if it is wye connected. When the voltage does not match the connection the motor will be saturated if the voltage is high or weakened if the voltage is low. Motor power capacity changes with the squared ratio of voltage change.

Three phase motor windings are designed to work while excited with a balanced three phase voltage and they are too sensible to voltage imbalance.

What you are trying is to convert a 3 ph motor in a single phase motor. For that you have to redesign totally the windings, starting devices, expected power output and speed.

 

[gridlink]

Thanks guys for the thoughts. I may have muddied the water a bit by introducing the 1-3 phase stuff. If I can get a comment simply of the power issues if i wire (say) a 20 HP 3 phase motor in Delta and then in Star Both using a 460 volt 3 phase supply. Do they have the same power?

 

[GGOSS]

Hello gridlink,

You're spot on. By connecting in star you are in fact reducing the voltage accross each motor winding. The hp available to you is 1/3 that shown on the nameplate.

The above assumes the motor is designed for 460V delta connection.

Regards,
GGOSS

 

[cbarn24050]

Hi, star/delta connection is not a user option. You must use the correct connection as staed on the namplate for your line voltage.

 

[jbartos]

Suggestion: The following was posted in this Forum in the past, and pertains to this thread:
Can anyone please give me the formula for calculating the line currents taken from the starter for a 3-phase motor if the same motor is connected in
(a) star/wye connection
///Iy=Pwatt,y/(sqrt3 x Vline-line x eff x pf)\\(b) delta connection
///Id=Pwatt,d/(sqrt3 x Vline-line x eff x pf)\\The common formula is P = Root 3 x V x I x P.F. x Efficiency
But is that for a star or delta connection?
///Yes, if properly applied. Specifically,
y-motor connection:
Pwatt,y=sqrt3 x Vline-line x Iline,y x pf x eff
d-motor connection:
Pwatt,d=sqrt3 x Vline-line x Iline,d x pf x eff
The difference is in Iline,y and Iline,d
Iline,y=(1/3) x Iline,d
since
the voltage across the y-connected motor winding will be Vline-neutral=(1/sqrt3) x Vline-line
and the current for d-connected motor winding will be
Id=Iline,d/sqrt3
Considering that the motor winding impedance Z is constant from y-connection to d-connection, then:
Ilind,d=sqrt3 x Id
Iline,y=Vline-neutral/Z
Id=Vline-line/Z=sqrt3 x Vline-neutral/Z=sqrt3 x Iline,y
hence,

Iline,d=sqrt3 x Id=sqrt3 x sqrt3 x Iline,y = 3 x Iline,y

Therefore,
Pwatt,d=3 x Pwatt,y
\\
There are tables of typical motor currents available.
///Usually, a manufacturer catalog provides tables with specific motor currents. The generalization is usually obtained by motors application engineers or designers.\\Are those currents for star or delta?
///This has to be properly stated in terms of the motor winding to avoid any ambiguity\\ e.g. 15 kW 400V 28A,
///This can be for d-connected motor or y-connected motor. The motor connection must be known, if needed, to avoid ambiguity.\\ 1.1kW 400V 2.5A
///The motor connection must be known, if needed, to avoid ambiguity.\\I have an ABB motor with 2.2 kW 380-440V star/wye 4.9A on the nameplate.
///This motor appear to be suitable for the motor terminal voltage range 380V to 440V and it is star/wye connected.\\But the test sheet on the ABB web page says 400V delta 4.85A 2.2 kW; 400V star 6.3A 3kW
///This is an inconsistent rating for the motor since the delta connection at 400V shows the smaller current, 4.85A with respect to the y-connection at 400V current, 6.3A.\\The nameplate says star - the test sheet says delta. How do I connect it?
///If the test sheet does not accompany the motor and overrides the nameplate data, then the motor nameplate information governs.\\I contacted ABB but am waiting for a reply.
///The confirmation from ABB is recommended because of the posted ambiguity.\\I have seen statements that "small" motors are connected in star.
///There are many statements as this one. The y-connected motor with three leads in the terminal box is someone simpler to produce, has smaller voltage stresses etc.\\One said less than 4kW, another said less than 11kW.
///This can be.\\Does this vary with manufacturers?
///Probably.\\What happens if a small motor is connected in delta?
///If provisions are made in the motor terminal box, it is understood that the manufacturer provided the provisions for y-connection and delta-connection. However, the y-connected motor should then have shaft load reduced to 1/3 of delta connection. However, the motor voltage rating is important. It is not possible to connect y-connection Vline-neutral voltage, e.g. designed for 220V, motor to 380V Vline-line motor since the motor winding would burn. Remember that the motor winding impedance is constant equal to Z.\\

 

[jbartos]

Comments on the original posting marked ///\\\:
We are working on a project where we want to insert a capacitor between two of the phases to provide a circulating field so that the motor will run from single phase supply. The only? way so far seen is if the motor is wired in star.
///Not necessarily. It may be implemented with delta wound motors too.\\
My belief? is that the power of the motor will be reduced (E*I) if connected this way.
///Yes, not only power but also efficiency and power factor.\\ is this true or is the power the same despite the differing connections.
///It depends. The motor nameplate power rating per the nameplate winding connection is the motor rated HP no matter whether it comes with the delta winding connection or the wye winding connection.\\

 

[gridlink]

Thanks guys, your comments have cleared up the situation well for me.