Rotary resistance starter advice

[joel89]

hi all, just looking at upgrading a george ellison old fillled rotary resistance starter on our forging hammer (the starter is pre-war 1924) the contacts are worn and the mechanical lever springs are also worn been temp fixing it for a year or 2 now, the motor is a 3ph 415v slip ring motor (big old) thing, just wondering what would be the best thing to use a VFD, Soft start, or contactors? anyone else done anything similar on things as old?

cheers

 

[Marvo]

This is a very specialized area, I've seen medium voltage slip ring motors on mining machinery for rock crushing converted from oil filled rheostats to motor drives but I wasn't involved directly in the project apart from some of the cabling. Interesting stuff though they weren't standard VFD's, they had regeneration built into them. I'm sure there are suitable drives available for your application but like I mentioned it probably would be a design for a company that specializes in this area.

 

[Darkwood]

This is a very specialized area, I've seen medium voltage slip ring motors on mining machinery for rock crushing converted from oil filled rheostats to motor drives but I wasn't involved directly in the project apart from some of the cabling. Interesting stuff though they weren't standard VFD's, they had regeneration built into them. I'm sure there are suitable drives available for your application but like I mentioned it probably would be a design for a company that specializes in this area.

What are your thoughts on shorting the slip-ring out and just running off a VSD as a standard induction motor? - I suspect though with the age of the Kit the windings won't be up to the modern day methods of using a VSD.

 

[Barry White]

The manufacturer of the original control gear is still trading why not contact them they may even have a repair kit for the original or give you advice on what to use.
http://www.ellison.co.uk/ellison pictures.htm

 

[davesparks]

An small scale engineering workshop would be able to make up the parts you need normally.

 

[Marvo]

What are your thoughts on shorting the slip-ring out and just running off a VSD as a standard induction motor? - I suspect though with the age of the Kit the windings won't be up to the modern day methods of using a VSD.

The starting currents can get out of hand, depends on the supply and whether it can handle it as well.

I'm not sure how large the motor is we're talking about here but those liquid rheostats on 1000 plus kilowatt motors give me the heebie jeebies, I've seen the result after they've gone bang and burst into flames because of a fault and it's not a pretty sight and I certainly wouldn't have wanted to be within 30 meters when it happened.

 

[Lucien Nunes]

Could you post motor specs and perhaps some pics? What kind of starting duty is it up against, a heavy but free-turning flywheel? I often work on old motors and we have some rotor resistance starters of similar age still in use. Multi-step rotor resistance starting is a very effective method that is difficult to beat on a slipring motor under these conditions, it keeps the current down, avoids voltage spikes on the stator and generally looks after the motor and the supply fuses. In any event we tend to avoid PWM starting and VFDs due to the risk of insulation failure, although in some cases that risk may be purely speculative. A multi-contactor solution might work but let's take a look at the beast...

 

[Marvo]

As far as insulation goes it might state on the motor if it's grade 2 or grade 3 but either way I'd expect a VFD to considerably reduce the life expectancy of the windings on an old motor.