Power requirements

[gensetsteve]

On 27/06/2020 at 17:01, 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.

The above is correct . In theory the start up current is actually 6 times but the power factor helps out a little so three times is normally sufficient. A motor that is star delta starting will help things further rather than direct online . 

I see an engine powered pto years ago like a power pack would that be a better idea less thermal and friction losses 

[topchippyles]

15 minutes ago, gensetsteve said:

The above is correct . In theory the start up current is actually 6 times but the power factor helps out a little so three times is normally sufficient. A motor that is star delta starting will help things further rather than direct online . 

I see an engine powered pto years ago like a power pack would that be a better idea less thermal and friction losses 

Eggs (New john holmes) will be along to tell us we are all wrong about this subject ?

[topchippyles]

On 27/06/2020 at 16:02, eggsarascal said:

It's Watts x amps = voltage, I think. Someone better at maths than me should be able to give you a clue.

You should be on count down eggs lad 

[eggsarascal]

31 minutes ago, topchippyles said:

You should be on count down eggs lad 

And you should be back in your box, Given time you'll do a funny.

[topchippyles]

2 minutes ago, eggsarascal said:

And you should be back in your box, Given time you'll do a funny.

Ok john (egg) holmes (sex god) 

[eggsarascal]

1 minute ago, topchippyles said:

Ok john (egg) holmes (sex god) 

Still can't get the hang of being funny, can ya ?

[topchippyles]

6 minutes ago, eggsarascal said:

Still can't get the hang of being funny, can ya ?

sex-god. Noun. (plural sex gods) An incredibly sexy man. A man with astounding sexual prowess.  ?