3 phase

[matt1e]

If you have any experience of this then you will know that a motor designed to operate at 400 volts in delta can be started in star which gives approx 58% voltage across the windings but also means approx 58% of the maximum torque as well.

I have and I do

If you have a motor designed to run at 695 volts star then unless it is virtually running free it will not start at approx 33% (230v) of it's rated voltage when started in star. The motor will probably stall and the overload will operate if correctly set.
Believe me, I have experienced it !

?????????????

You are making the same mistake as many and misunderstanding motor rating plates
a motor designed as you say to run at 695 star is equally designed to run at 400v delta
delta = voltage rating of 1 coil
star = 2 delta coils in series (hence higher voltage rating)
so if a 3ph motor is rated for example 230/400 then it would be connected to a 3ph 400v supply in star config or a 3ph 230v supply in delta config
a motor rated 400v/695 would be connected to a 3ph 400v supply in delta, or if it was a biggun, for reasons you have already given

The motor will probably stall and the overload will operate if correctly set.

either connected to a variable speed drive or connected star to start and delta to run to enable it to soft start.
many times I have had to strip out motors to take for rewinds because they have been 400/695 rated and connected in star to a VFD

 

[ricicle]

many times I have had to strip out motors to take for rewinds because they have been 400/695 rated and connected in star to a VFD

Which is exactly the reason why a motor rated at 695V star cannot be connected to a star delta starter. Each winding will only have 230V across it during the star part of starting !!

 

[ColJack]

voltage is relavent depending on what you measure it to..

230V phase to neutral ( or star point ) is the same as 400V phase to phase...

star is not 2 delta coils in series.... although I see the comparrison you are trying to make.. it's LIKE 2 coils in series, giving you half the voltage across each coil ( the reality is more complicated and likely above the heads of the average DIYer, so you were trying to simplify it )

in star configuration you get 230V across the coil, in delta configuration you get 400V across the coil. ( for those interested, it's single phase voltage X (sqrt 3) )

230V 3Ph would be 133V single phase..

 

[matt1e]

many times I have had to strip out motors to take for rewinds because they have been 400/695 rated and connected in star to a VFD

Which is exactly the reason why a motor rated at 695V star cannot be connected to a star delta starter. Each winding will only have 230V across it during the star part of starting !!

your not getting it are you? thats the whole point of a star delta starter
reduced voltage = reduced current

 

[ColJack]

Which is exactly the reason why a motor rated at 695V star cannot be connected to a star delta starter. Each winding will only have 230V across it during the star part of starting !!

it's exactly this reason that they CAN and ARE OFTEN wired star/delta starting..

you get 60% of the voltage across the coil and this in turn reduces the starting current.. it also reduces the starting torque so using star/delta starting on motors that are intended to operate at above 65% of it's rated torque , is a bad idea.. it will likely stall the motor and damage it's windings..

 

[matt1e]

voltage is relavent depending on what you measure it to..

230V phase to neutral ( or star point ) is the same as 400V phase to phase...

star is not 2 delta coils in series.... although I see the comparrison you are trying to make.. it's LIKE 2 coils in series, giving you half the voltage across each coil ( the reality is more complicated and likely above the heads of the average DIYer, so you were trying to simplify it )

yes agreed but you know what I meant

in star configuration you get 230V across the coil, in delta configuration you get 400V across the coil( for those interested, it's single phase voltage X (sqrt 2) )

for those interested, it's single phase voltage X (sqrt 3)
but I know what you meant

you get 60% of the voltage across the coil and this in turn reduces the starting current.. it also reduces the starting torque so using star/delta starting on motors that are intended to operate at above 65% of it's rated torque , is a bad idea.. it will likely stall the motor and damage it's windings..

very good point, not all dual voltage motors are designed for star/delta starting but TBH my original post was to point out to ridicle that he was wrong to say ericmarks statement was incorrect I wish I had just let him come back on and explain cos i'm into valuable drinking time now
ps its 67% to be pedantic
cheers

 

[ColJack]

for those interested, it's single phase voltage X (sqrt 3)
but I know what you meant

that's what I said ...
ok so I hit the wrong button..

 

[matt1e]

no probs mate I believe ya
off to pub now for some refreshment
cheers

 

[ericmark]

However larger motors as 400 volt delta and 695 volt star being arranged that way to allow star / delta starting

A motor of that configuration could not be connected for star/delta starting !

Why not? That is the standard method for star / delta start by putting 400 volt on a 695 volt winding it draws less current and allows it to start spinning once near full speed then we change to 400 volt configuration.

The other methods are resistor and auto-transformer start with both those methods you can use more than two stages.

A 695 volts winding will be running on 57.55% of it's rated voltage when connected to 400 volt which is normally OK. But pumps for example where load can't be removed in same way as many other motors will often have resistor or auto transformer start.

However it is all immaterial to the question all I was pointing out was there are two common configurations on 400 volt motors.

As to burning out when they stick on start only if overloads have been incorrectly selected or placed and I have seen that.

I do remember many years ago seeing a motor connected to a flywheel with some capacitors and once started it generated a false three phase and then all other motors could be started. This was back when I was an apprentice in late 1960's or early 1970's and I can't remember how it was done now. But at that time we still had DC in some area's and the motors were heavier built I would not want to try it with a modern motor.

But some old methods are returning I found some tower cranes with the ward leonard system which I had thought was no longer used. Only other place I had seen it was on a derrick on the Falklands. Does reduce inrush with sudden loads but very waist full.

 

[conny]

tower cranes with the ward leonard system which I had thought was no longer used.

Do some work in the rarely used/disused parts of Liverpool docks or on an old tug and you will find these systems still in use! Because they are only used very infrequently it is cheaper to leave them in place and give them a quick once over before using them. Lucky if they get used once every 3 or 4 years to be honest.

 

[ColJack]

a 400/695 motor does not HAVE 695 windings.. the windings are for 400V..
the 695 is the star configuration where it's still 400V across each winding.. it's just 695V phase to phase.
the 400 is for the delta configuration where it's 400V across each winding, and 400V phase to phase..

 

[ericmark]

a 400/695 motor does not HAVE 695 windings.. the windings are for 400V..
the 695 is the star configuration where it's still 400V across each winding.. it's just 695V phase to phase.
the 400 is for the delta configuration where it's 400V across each winding, and 400V phase to phase..

OK point taken but it is being rather pedantic only was making point two common configurations. And as far as I can see he does not need three phase anyway!