I have an auger that moves wood chips it is 3ph and has a star-delta
starter. It pulls about 20A as star and 35A delta when loaded.

I know that it will attempt to reach synchronous revs (2900) so, given the
torque needed to move the woodchips is the same, is the only difference
that the rotor slips more when in star configuration? Is it more likely to
overheat if left running in star configuration?

AJH

On 6 Apr, 22:05, andrew wrote:
I have an auger that moves wood chips it is 3ph and has a star-delta
starter. It pulls about 20A as star and 35A delta when loaded.

I know that it will attempt to reach synchronous revs (2900) so, given the
torque needed to move the woodchips is the same, is the only difference
that the rotor slips more when in star configuration? Is it more likely to
overheat if left running in star configuration?

AJH

Andrew
I thought I knew possibly enough about S-D starting to pass comment,
but decided I would google just to check.

I found this page:- http://www.mandc.co.za/pdfsStar_Delt...x plained.pdf

which may or not help you, but has enough information on it in my
opinion to show that OK you will take far less current but you will
also have seriously less torque, which could well be fatal in that
there is a far greater danger of motor stall and hence over-current
trips operating - or not operating, with the corresponding smoke
getting out and failure of the motor ever to run again.

What I had forgotten from university days was the importance of
achieving near synchronicity when in star to avoid a current surge
when switching over. This does rather imply that the motor has to
have adequate power in star connection for the load, but then the
delta connection gives the load overhead to avoid slip and the
potential of the rotor dropping out of lock - ie stalling.

Rob

Rob G wrote:

I thought I knew possibly enough about S-D starting to pass comment,
but decided I would google just to check.

I found this page:- http://www.mandc.co.za/pdfsStar_Delt...x plained.pdf

I think you have a slash missing there. Try:

http://www.mandc.co.za/pdfs/Star_Del...xplai ned.pdf

--
Cheers,

John.

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In message , andrew
writes
I have an auger that moves wood chips it is 3ph and has a star-delta
starter. It pulls about 20A as star and 35A delta when loaded.

I know that it will attempt to reach synchronous revs (2900) so, given the
torque needed to move the woodchips is the same, is the only difference
that the rotor slips more when in star configuration? Is it more likely to
overheat if left running in star configuration?

Ducking the question as others have answered, I have lots of working 3ph
motors in the 1 to 3hp range recovered from various bits of scrapped
agricultural equipment. Mainly 4 pole if you are trying to reduce
speed/current.

regards

--
Tim Lamb

On 6 Apr, 22:05, andrew wrote:
I have an auger that moves wood chips it is 3ph and has a star-delta
starter. It pulls about 20A as star and 35A delta when loaded.

I know that it will attempt to reach synchronous revs (2900) so, given the
torque needed to move the woodchips is the same, is the only difference
that the rotor slips more when in star configuration? Is it more likely to
overheat if left running in star configuration?

AJH

Synchonous speed depends on the number of pole pairs the motor is
built with. To a close approximation a two pole aims for 3000 RPM,
four pole for 1500 RPM, 6 pole 1000 RPM and so on. The more poles the
heavier the motor.

You have not specified the power rating of the motor but something
pulling 35A from 400v three phase under full load is a serious bit of
kit.

Without going into great detail the whole idea of using a star/delta
starter is to control the current demand surge on starting and also to
reduce starting shock torque on the load which with high inertia loads
can damage drives etc. In star the current is reduced but so is the
available "oomph!" so if you remain in star you have a serious
shortage of available power to drive the load. Under load the slip
will increase with risk of stalling but also if the motor relies on a
shaft fan for cooling it will suffer greatly as the air movement over
the cooling fins depends on the cube of the speed.

"cynic" wrote in message
...
On 6 Apr, 22:05, andrew wrote:
I have an auger that moves wood chips it is 3ph and has a star-delta
starter. It pulls about 20A as star and 35A delta when loaded.

I know that it will attempt to reach synchronous revs (2900) so, given
the
torque needed to move the woodchips is the same, is the only difference
that the rotor slips more when in star configuration? Is it more likely
to
overheat if left running in star configuration?

AJH

Synchonous speed depends on the number of pole pairs the motor is
built with. To a close approximation a two pole aims for 3000 RPM,
four pole for 1500 RPM, 6 pole 1000 RPM and so on. The more poles the
heavier the motor.

You have not specified the power rating of the motor but something
pulling 35A from 400v three phase under full load is a serious bit of
kit.

Without going into great detail the whole idea of using a star/delta
starter is to control the current demand surge on starting and also to
reduce starting shock torque on the load which with high inertia loads
can damage drives etc. In star the current is reduced but so is the
available "oomph!" so if you remain in star you have a serious
shortage of available power to drive the load. Under load the slip
will increase with risk of stalling but also if the motor relies on a
shaft fan for cooling it will suffer greatly as the air movement over
the cooling fins depends on the cube of the speed.

When you say serious, given torque is proportional to current in the rotor,
it implies torque will be (root) 3 less, or just over 1/2 torque in delta.
That may be sufficient for the OP? Obviously stalling the motor should be
avoided or certainly must not be prolonged.

In article ,
"Fred" writes:

When you say serious, given torque is proportional to current in the rotor,
it implies torque will be (root) 3 less, or just over 1/2 torque in delta.
That may be sufficient for the OP? Obviously stalling the motor should be
avoided or certainly must not be prolonged.

Star-delta starters I've dealt with (not many) had overload
thermal trips that IME will trip in star mode if the motor is
too heavily loaded to gain a reasonable speed in star mode
(or in that case, bearing gone too stiff).

The earlier comment about being almost up to speed before
switching to delta certainly didn't apply to the ones I
was working with (driving giant centrifugal fans).
Star mode probably got them up to half speed, maybe a bit
more, before they switched to delta.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On 6 Apr, 22:05, andrew wrote:
I have an auger that moves wood chips it is 3ph and has a star-delta
starter. It pulls about 20A as star and 35A delta when loaded.

Star-delta is good for huge motors at the end of wet string, great for
fans and brilliant for compressors with unloader valves. Otherwise
it's an unhelpful complexity.

All that star-delta switching does is to reduce the effective rating
of your motor (both torque and current) by 2/3rd when starting as a
star. _IF_ this reduced torque is acceptable when starting (i.e.
you're not starting against a load) then you can avoid a peak current
spike at startup and let the load come up to speed more gradually.
Great for fans and sawbenches, only useful for compressors if you can
vent an unloader valve for this period and a damn nuisannce if you're
trying to release a stuck auger.

There's also the possible benefits of star-delta starting. These
aren't in any savings on the motor, just in the capacity needed for
the supply wiring. So if your supply is _already_ capable of supplying
35A, you've not much to gain. This is also why star-delta isn't
usually seen on small 3 phase motors, only the biggies.

Tim Lamb wrote:

I know that it will attempt to reach synchronous revs (2900) so, given the
torque needed to move the woodchips is the same, is the only difference
that the rotor slips more when in star configuration? Is it more likely to
overheat if left running in star configuration?

Ducking the question as others have answered

True there were a lot of answers :-)

I think Rob G and Andrew G told me what I wanted to know which agreed with
what I thought. I should have remembered this from physics but it was 40
years ago.

Given that this is an existing machine, German and recent, I guess they had
a reason for the star-delta switch.

Also given the torque needed to drive a full auger is constant and that the
power and from Fred's calculation torque (I suspect at a given slip) is
halved when in star then the conclusion is that slip is greater, hence
current goes up, risking overheating.

1'll tell them not to do it...

AJH