Power requirements

[topchippyles]

2 minutes ago, eggsarascal said:

A 3 phase motor starts in star then switches over to Delta, you are pissing up the wrong tree, Les.

Your just being a prick now eggs. I am not telling the guy how to wire his setup  if it is an older type motor then that is what is needed, 

Edited June 27, 2020 by topchippyles

[eggsarascal]

1 minute ago, topchippyles said:

Your just being a prick now eggs.

You want a row message me, let's leave it off these pages.

[topchippyles]

5 minutes ago, eggsarascal said:

You want a row message me, let's leave it off these pages.

Is this close enough for you. You can argue over the most stupid things eggs. If i had not done it and paid out a vast some then i would not comment.

 

Rule of Thumb

Electric Motor Size		Minimum Generator Req'd (KVA) by Starting Method
HP	KW	D.O.L to KVA Size (Note 1)
15	11	37.5
20	15	50
25	19.6	60.5

[openspaceman]

6 minutes ago, eggsarascal said:

A 3 phase motor starts in star then switches over to Delta,

Yes for big motors; when the motor starts from stationary each voltage cycle there is a sudden rush of current, and this creates a massive torque, once it is moving the squirrel cage armature starts generating a back electro motive force which opposes the supply voltage and reduces the current. With star delta it's effectively wired as a 240V motor as it comes up to speed as otherwise the inrush current on each cycle would likely overload the supply, once it's rotating sufficiently fast for the back emf to reduce the current even at the higher voltage it is switched to act as a 440V motor.

 

I agree that the rule of thumb is you need to supply 3 times the current to start up a motor because of this inrush. There are other ways around this by controlling the current  or the frequency

 

The Power=Volts x Amps is true but because a motor is an inductive load the current and voltage are out of phase so even when it is running at rated speed it will be drawing more amps  than the power suggests, which is why the motor will be rated at 15kW but the generator supplying it will need to supply the 15kW when the voltage is out of phase with the  voltage, so it will need more than 15kVA to run at rated power. Hence  the windings of the generator will need to be able to withstand the supply voltage and the higher current, which is why a generator is rated in kVoltsAmps rather then kWatt.

[topchippyles]

14 minutes ago, openspaceman said:

Yes for big motors; when the motor starts from stationary each voltage cycle there is a sudden rush of current, and this creates a massive torque, once it is moving the squirrel cage armature starts generating a back electro motive force which opposes the supply voltage and reduces the current. With star delta it's effectively wired as a 240V motor as it comes up to speed as otherwise the inrush current on each cycle would likely overload the supply, once it's rotating sufficiently fast for the back emf to reduce the current even at the higher voltage it is switched to act as a 440V motor.

 

I agree that the rule of thumb is you need to supply 3 times the current to start up a motor because of this inrush. There are other ways around this by controlling the current  or the frequency

 

The Power=Volts x Amps is true but because a motor is an inductive load the current and voltage are out of phase so even when it is running at rated speed it will be drawing more amps  than the power suggests, which is why the motor will be rated at 15kW but the generator supplying it will need to supply the 15kW when the voltage is out of phase with the  voltage, so it will need more than 15kVA to run at rated power. Hence  the windings of the generator will need to be able to withstand the supply voltage and the higher current, which is why a generator is rated in kVoltsAmps rather then kWatt.

Spot on and the fact i put deta/star was just referring to the motor type not the way its wired up or starting current. Some people just have nothing better to do on a saturday afternoon. 

[Justme]

Yep dealer is right.

 

Motors take a lot extra to start up & get to speed.

 

Rating is running not startup.

[spudulike]

What you could do is stick in an underrated power system and spend the savings on a fire extinguisher and a pair of trainers!

I love it when guys ask a question and argue when they don't get the answer they desire...no disrespect to the OP whom I hope is looking for a logical explanation and think he has it now!

 

[topchippyles]

24 minutes ago, spudulike said:

What you could do is stick in an underrated power system and spend the savings on a fire extinguisher and a pair of trainers!

I love it when guys ask a question and argue when they don't get the answer they desire...no disrespect to the OP whom I hope is looking for a logical explanation and think he has it now!

 

I put eggs in the same  (He be along now you watch) ?

 

 

Edited June 27, 2020 by topchippyles

[oldoaktree]

I believe I have the answer to my opening question. Thanks 

[eggsarascal]

19 hours ago, openspaceman said:

Yes for big motors; when the motor starts from stationary each voltage cycle there is a sudden rush of current, and this creates a massive torque, once it is moving the squirrel cage armature starts generating a back electro motive force which opposes the supply voltage and reduces the current. With star delta it's effectively wired as a 240V motor as it comes up to speed as otherwise the inrush current on each cycle would likely overload the supply, once it's rotating sufficiently fast for the back emf to reduce the current even at the higher voltage it is switched to act as a 440V motor.

 

I agree that the rule of thumb is you need to supply 3 times the current to start up a motor because of this inrush. There are other ways around this by controlling the current  or the frequency

 

The Power=Volts x Amps is true but because a motor is an inductive load the current and voltage are out of phase so even when it is running at rated speed it will be drawing more amps  than the power suggests, which is why the motor will be rated at 15kW but the generator supplying it will need to supply the 15kW when the voltage is out of phase with the  voltage, so it will need more than 15kVA to run at rated power. Hence  the windings of the generator will need to be able to withstand the supply voltage and the higher current, which is why a generator is rated in kVoltsAmps rather then kWatt.

Soft starter, no need for a massive generator.