Star delta motor stuck in star consequences

[ewandingwall]

Hi Guys,

Is my understanding of why it is bad for a star/delta motor to be stuck in star correct?

1) Star delta starting is used as a method of starting larger motors at a reduced voltage of root 3 , which reduces the large starting current drawn by the motor during start up, therefore protecting the stator windings from burnout due to overheating. As the voltage across the motor windings are reduced by root 3 during starting in Star, this causes the line current drawn to be reduced to one third of the delta current.

As the torque will be reduced according to the square of the applied voltage; if the voltage is reduced by a factor of root 3, then the starting torque in star will be reduced by the square of the factor,therefore by a factor of 3, which reduces the starting torque to one third of the delta torque.

The power output of the rotor is also reduced to one third of a delta connected motor.

The design requirements of the motor is allow it to be started in star BUT run in delta, to match the required torque and power requirement of the driven load.

Therefore by running the motor in star means running the motor at one third of the torque and power output, meaning that the rotor will not match the required torque and power demand by the load, therefore the motor will stall and overheat.

Can you please pick out any technical inaccuracies in my answer.



Thanks,

Ewan

 

[R-fur]

One of the main reasons for star delta starters is to reduce the starting current that would occur if the motor was switched direct to delta. DNO's often put limits on starting currents.

 

[brianmoooore]

A motor permanently connected in star needs a higher supply voltage than one connected in delta. If it gets that higher voltage then it's fine.

 

[Petej999]

Hi Guys,

Is my understanding of why it is bad for a star/delta motor to be stuck in star correct?

1) Star delta starting is used as a method of starting larger motors at a reduced voltage of root 3 , which reduces the large starting current drawn by the motor during start up, therefore protecting the stator windings from burnout due to overheating. As the voltage across the motor windings are reduced by root 3 during starting in Star, this causes the line current drawn to be reduced to one third of the delta current.

As the torque will be reduced according to the square of the applied voltage; if the voltage is reduced by a factor of root 3, then the starting torque in star will be reduced by the square of the factor,therefore by a factor of 3, which reduces the starting torque to one third of the delta torque.

The power output of the rotor is also reduced to one third of a delta connected motor.

The design requirements of the motor is allow it to be started in star BUT run in delta, to match the required torque and power requirement of the driven load.

Therefore by running the motor in star means running the motor at one third of the torque and power output, meaning that the rotor will not match the required torque and power demand by the load, therefore the motor will stall and overheat.

Can you please pick out any technical inaccuracies in my answer.

No such animal as a Star Delta Motor, star delta is a method of starting a 3 phase Motor , sorry meant for the OP

Thanks,

Ewan

 

[Petej999]

A motor permanently connected in star needs a higher supply voltage than one connected in delta. If it gets that higher voltage then it's fine.

Explain please

 

[brianmoooore]

Dual voltage motor.. .. .

 

[ewandingwall]

Dual voltage motor.. .. .

Dual voltage motor connected to a star delta starting circuit. Is that better professor moore?

 

[Petej999]

Lost me I'm afraid,,,,,,,,,,,,,,,,,,,

 

[timhoward]

My two cents:

Star delta starting is used as a method of starting larger motors at a reduced voltage of root 3 , which reduces the large starting current drawn by the motor during start up, therefore protecting the stator windings from burnout due to overheating. As the voltage across the motor windings are reduced by root 3 during starting in Star, this causes the line current drawn to be reduced to one third of the delta current.

Generally agreed, section crossed out is not a given. As stated above it reduces startup/inrush currents. It isn't a given the windings would overheat. It may cause problems with protective devices operating though.

As the torque will be reduced according to the square of the applied voltage; if the voltage is reduced by a factor of root 3, then the starting torque in star will be reduced by the square of the factor,therefore by a factor of 3, which reduces the starting torque to one third of the delta torque.

Agreed, the lower torque is the price paid for the lower current.

Therefore by running the motor in star means running the motor at one third of the torque and power output, meaning that the rotor will not match the required torque and power demand by the load, therefore the motor will stall and overheat.

That very much depends what the motor is doing and I don't think you can generalise to this degree. You can certainly say you have reduced torque while running in star configuration.
I think you have the right idea about how it is working, but are being a little dramatic about the consequences!

 

[ewandingwall]

Thanks for the detailed reply. Why would it give issues with protective devices opening? Thanks

 

[timhoward]

Thanks for the detailed reply. Why would it give issues with protective devices opening? Thanks

I just meant that if the motor started in delta with a high torque load the inrush current would be large. This might exceed the time/current graph of the protective device and cause a
trip/blow.

 

[westward10]

If the motor has a load applied to it and has been designed to run in delta with this load then running in star with a reduced torque for a period of time may not be a wise thing. If the motor is cooled by veins and fan then this effect is also reduced.

 

[Lucien Nunes]

@timhoward gives some good explanations there and I don't have much to add. I have seen 400/230V motors running permanently in star due to mis-connection, where the problem was not immediately noticed because the load did not demand full torque or exact speed from the motor at any time. One example was an organ blower with a 5hp 4-pole motor that was suspected of delivering slightly low pressure. It was wired in star and slipping too much, achieving something like 1350 rpm instead of 1425, and it wasn't quite keeping up . The motor itself was completely unharmed.

I would amplify that burnout of the windings with DOL starting is most unlikely. Even large motors can often be started DOL but with unpleasantly high starting currents. A squirrel-cage motor can easily take 5x FLC at standstill, so the above blower example would take upwards of 40A, which would cause the lights sharing the submain to dip visibly (you can just about see the dip with star-delta starting).

The actual starting torques and currents at different voltages depend somewhat on the design and size of the motor, not just on the theoretical ratios. At a standstill the slip frequency is equal to the supply frequency and the rotor cage inductance plays a major part in determining the torque, since the current is seriously out of phase with the stator flux wave. Hence the advantage of rotor-resistance starting over star delta, as this improves the rotor pf as well as limiting the current.

 

[ewandingwall]

@timhoward gives some good explanations there and I don't have much to add. I have seen 400/230V motors running permanently in star due to mis-connection, where the problem was not immediately noticed because the load did not demand full torque or exact speed from the motor at any time. One example was an organ blower with a 5hp 4-pole motor that was suspected of delivering slightly low pressure. It was wired in star and slipping too much, achieving something like 1350 rpm instead of 1425, and it wasn't quite keeping up . The motor itself was completely unharmed.

I would amplify that burnout of the windings with DOL starting is most unlikely. Even large motors can often be started DOL but with unpleasantly high starting currents. A squirrel-cage motor can easily take 5x FLC at standstill, so the above blower example would take upwards of 40A, which would cause the lights sharing the submain to dip visibly (you can just about see the dip with star-delta starting).

The actual starting torques and currents at different voltages depend somewhat on the design and size of the motor, not just on the theoretical ratios. At a standstill the slip frequency is equal to the supply frequency and the rotor cage inductance plays a major part in determining the torque, since the current is seriously out of phase with the stator flux wave. Hence the advantage of rotor-resistance starting over star delta, as this improves the rotor pf as well as limiting the current.

That is an excellent explanation thank you

 

[DPG]

That is an excellent explanation thank you

He knows his stuff to be fair. Or he bluffs very well.

'She was only an organ blower's daughter, but she knew an awful lot about pumping'