star to delta

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

Reply to
andrew
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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:-

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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

Reply to
Rob G

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I think you have a slash missing there. Try:

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Reply to
John Rumm

In message , andrew writes

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

Reply to
Tim Lamb

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.

Reply to
cynic

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.

Reply to
Fred

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.

Reply to
Andrew Gabriel

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.
Reply to
Andy Dingley

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

Reply to
andrew

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