Motor starting

[fattony]

Having a few troubles at work with a motor which is DOL and has a start up current of 48A the DNO say the max we can have is 25A so Im looking for a way of soft starting it, maybe with an inverter.

Has anyone had experience with this situation before?

 

[ericmark]

I have used softstarts with and without bypass contactor. Inverters are similar but really for when you also want to control speed or run three phase motors off single phase supplies. There is of course the old star / delta or if already running in star auto transformer or resistor start.
Much depends on the start torque required. Pumps we used auto transformers but crushing plant we used resistor start in some cases up to 6 stages on resistor start.
Softstart and Inverters can produce some odd wave form changes and EMC problems.
The motor also can be modified SEW produce motors with inverters, brakes, and ASii control all built into the motor.
As with most things there are many options and the costs of electronic control has lowered so where a complex Ward-Leonard system would have been used now a simple inverter is cheaper option.
Although cranes still use Ward-Leonard system because of the reliability problems using electronic methods.
Eric

 

[fattony]

wow thanks for the reply, its a 3 phase motor (compressor) I will have a google about

 

[ericmark]

With a compressor you need to include the de-load valve in circuit. There are different methods of de-load. I would not think an inverter will work very well but from memory softstarts have a supply to by-pass contactor which could also be used to work the de-load valve.
Is it piston or hydrovane and if hrdrovane is any de-load already built in or will it need adding?
With de-load system resistor and auto transformer start can also be used.
Cost I am out of touch with.

 

[fattony]

mmm i need to find out some more info about the motor I think

 

[ericmark]

What type of compressor?
With old piston type there are two ways to de-load.
1) Hold inlet valve open
2) Hold inlet valve closed
So either it sucks and blows to atmosphere so de-loaded or if it can't draw any air in it has nothing to compress again de-loaded.
The main thing you need to find out is how you can de-load compressor and everything else will revolve around that.

 

[fattony]

Ok my boss has emailed some specs over to me he has had a bit more dealing with trying to source something and this has been reccommended to him.

LOVATO ADX0017B SOFT STARTER

Its priced at around £1100, is there anything similar we could use but cheaper, as to compressor specs I'd have to wait until monday when im back on site

 

[ericmark]

On this type of thing I would be going to local supplier to find what is now on the market.
Although it says it has an integrated bypass relay what it does not say is if there is a connection to this to trigger the de-load valve.
Also be aware the supply with need protecting with semi-conductor fuses a motor overload on it's own in not good enough.
I have tried before to use web sites but so much discount is offered on electrical goods that only real way is to contact a supplier.
Anyway until you know how de-load is done you can't really get much further.
Most compressors have a two stage set-up and once the pressure reaches the set limit it first de-loads compressor then if it remains de-loaded for set time it switches off motor.
With these integrated systems it is not always easy to add further control and I had a nightmare with three compressors I looked after where they would not start and stop in sequence and I had to use a PLC to control them in the end.
Also of course leaks and usage will have a bearing on how controlled although the last compressor should shut itself down often leaks stop that happening. Although one place I worked we we forever being called out when someone forgot to turn it off and it would auto start every couple of hours.
Have you thought of asking the compressor manufactures they may have a ready built kit?
Or at least may recommend an alternative starter.
Eric

 

[ricicle]

I am a little confused as to the DNO's response. What is the FLC of the motor, what size are the supply fuses and what is the maximum demand of the installation?

 

[fattony]

Im very confused at the DNO response as well makes you wonder if they are just trying to push us into a corner of upgrading their supply.

The full load current is 9A and the manufactur said it has a start up of 48A and its DOL, the DNO are saying the biggest start up current they could support is 25A DOL unless the customer wishes to have the HV cables on the telegraph poles replaced at his cost

Every we speak to about it say they've never heard of anything like it.

 

[ricicle]

what size are the supply fuses and what is the maximum demand of the installation?

Why did you contact the DNO in the first place. I doubt if it would be the HV side that needed upgrading anyway.
A standard 100A 3ph supply should be able to carry that start up current for a brief period anyway even if there was other loads operating ( I havn't got my T/C characteristic graph for 88's or 1361's nearby)

 

[fattony]

We did contact the DNO in the 1st place as we calculated we needed a 125A 3 phase supply, the problem was the quote of £12,000 to provide this, as they'd have to upgrade the HV cables. But they took about 6 weeks to actually quote, in this time the customer had ordered his compressor.

We changed a few bits of equipment (car ramps) and we have now calculated that 100A 3 phase,is ssuficient, what the DNO's concern is that the compressor starts up and someone down the road complain's that his lights are dimming caused by this compressor.

We know the size of the supply is large enough its just the DNO saying the largest start up current we're allowed is 25A 3 phase, unless the customer pays £12,000 to get the supply taken from a different telegraph pole, which would also increase the supply to 125A, sorry dont have all the figures for max demand to hand