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Soft starting old 36 kW motor at 230V 3~ Contactors burned.

Dag S K

Aluminum
Joined
Aug 22, 2007
Location
Norway
My father in law has an old sawmill. It was refurbed in 1980 with an automatic Y-D starter, now the main contactor is defect and spares are difficult to get.
This old motor has an extremely high start current, I have not been able to measure, but it is protected with 3 fuse elements of 120 Amp, 90, amps fuse elements blows now and then, but usually they are OK. (just to identify start current in Delta)

This saw mill is the motor with a gear and a heavy saw blade. That is the starting load.

How could we do this better in a safe way.

The power supplied are stable 230V 3 phase from a delta wound transformer with no grounded point.(50 Hz)

Modern contactors are much better covered and will not fit in the old cabinet. At this stage I see no good solution within reasonable costs for that 80 years old man still playing with his sawmill.

dsk
 
I think every solution will be larger than the cabinet will hold..

Soft start operates by reducing voltage to the motor, in general. Or by changing the motor to draw less current.

If the motor is wound delta, and the connections are all available, then it is possible to start the motor as a "wye" connection, which has the same effect as reducing the voltage to 57% of normal. When it has started and is speeding up so that current has reduced, the connection is changed to the normal delta.

Otherwise, resistance or impedance can be put in line during starting, so as to reduce voltage to the motor. This will reduce the start current. Either a resistor or inductor can be put in series, resistors are cheaper.

The voltage can be reduced by an autotransformer, again typically to somewhere around 57%, during starting, and when the current has dropped, the transformer is cut out of the circuit. This has the advantage of reducing the input current more than the other methods, due to the transformer ratio.

All methods can be expensive, require extra contactors, and take up extra space. With a high inertia load such as a sawmill, the start time can be longer as the saw speeds up more slowly. So the parts used (resistors, transformers) must be able to stand the current longer than for other types of load.

The best method as far as space and expense is probably the "wye start, delta run" method, because it only requires extra contactors. But this only works with a delta connected motor that has the connections available. And you may not have room to fit more contactors in the existing cabinet.

(Of course, a VFD will provide a soft start, but this is a very expensive way to get soft start at that power level)
 
The existing method is using a wye-delta starter.

In Southern California there is a company that can install new contact points on your point plates. Can't recall their name. If the defective parts are just contacts then there might be a similar place to send them where you live.

I suggest you read about wye-delta starting and then examine what you have. Check that it is being done correctly.
 
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Perhaps a photo of controls would help. Most likely the controls are IEC and were not sized for the application. As JST pointed out, you have few options that are not expensive. The current control lasted nearly 40 years. It's unlikely the mill will be used by your father another 40 years. Just replace what you have with current equivalent controls.

Tom
 
The existing method is using a wye-delta starter.

In Southern California there is a company that can install new contact points if you send the, your burnt-in ones. Can't recall their name. If the defective parts are just contacts then there might be a similar place to send them where you live.

I suggest you read about wye-delat starting and then examine what you have. Check that it is being done correctly.

I can replace contacts. I have done thousands for locomotive braking and reversing switches. I am out of that business due to reorganization in GE. I only have silver, no tungsten silver or cadmium oxide. If Tom thinks silver with 3% copper will work, I can do them.

Bill
 
I can replace contacts. I have done thousands for locomotive braking and reversing switches. I am out of that business due to reorganization in GE. I only have silver, no tungsten silver or cadmium oxide. If Tom thinks silver with 3% copper will work, I can do them.

Bill

Thanks for the thumbs up, but I have no experience with silver/copper. Most likely what he has is silver cad.

Tom
 
As a very VERY rough guess, if 90A fuses "occasionally blow", it's likely a 15HP 230V motor. In Wye-Delta, the starting current is 33% of normal, so if 90A = 33%, then normal starting current is 270A, which is then 600% of 45A, and 45A FLC is probably a 15HP 230V motor, might be a 20HP (54A).

You can get a decent soft starter with an integral bypass from Allen Bradley, an SMC-3, part #150-C60NBD (rated for 60A) that is roughly 3" wide x 8" high x 5" deep.
Link
rexel-SMC-3-85-1-jpg-300Wx300H.jpg


To JST's point since you ALREADY have a Wye-Delta motor and therefor you have 6 leads going to the motor, you could also use a soft starter that is sized for 58% of the FLC and wire it up "inside the delta". This is something that is challenging even for experienced electricians and not something I ever recommend for novices, but it IS possible if the size (and cost) makes the full rated unit untenable. In that case you would buy a 30A unit, 150-C30NBD, and is is physically smaller; roughly 2"W x 6"H x 4"D and RELIGIOUSLY follow ALL wiring instructions, after having read and FULLY understood them, at least 3 times, before attempting.
 
A Siemens Soft starter new is about 850 Euro at 106 Amps
Looking on Ebay you may find a used soft starter or VFD for cheap

Peter
The cheaper Siemens soft starters only control 2 of the 3 phases, the middle phase is a piece of bus bar. This is terrible on the motor because it inherently unbalances the voltage and current, which heats the motor very very fast, plus any slight glitch in the unit can allow current to flow to the motor uncontrolled, meaning you can't shut it down without killing the breaker. Although it can work for light duty things like pumps and fans that accelerate in a second or two, I would never use a 2-phase soft starter on something like a saw.
 
Add a load side inductor to take the spikes out. A full filter can do even better, but may not be required.
 
Add a load side inductor to take the spikes out. A full filter can do even better, but may not be required.

Yes.

However, a inverter, filters and the like are surely not going to meet the O.P.'s
criteria ?

1. Low cost
2. Fit in existing box.
 
Yes.

However, a inverter, filters and the like are surely not going to meet the O.P.'s
criteria ?

1. Low cost
2. Fit in existing box.



Just about nothing except "use as-is" will satisfy that second criterion... If that is a requirement, it is a real case of "make this better, but do not change anything".

Because it is already Y-D, then the rest of the possibilities involve larger items.... resistors, transformers, and the like.
 
I am happy for all suggestions. The idea of actually repair the contactor it selves is worth testing. I am mot familiar with that but I guess we could give it a try. They live about 2 hour away from me so this project might take some time, but it would have been nice to get it working when the snow is gone. So it other words during Mai. We have got about 4 times the normal amount of snow this winter :-)

dsk
 








 
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