How do oil immersed starters work

[AlexGoode]

Went to a job this week where we replaced a slip ring motor. We went back as it was not running upto full speed which we found was due to the oil immersed starter unit that the rotary wires onto. Someone on site pushed what seemed to be some sort of a cog with in the starter unit and this got the motor going bit I am struggling to understand how the unit actually works and what is inside if it, whether there are any parts to maintenance. Can someone help with this. Thank you

 

[Lucien Nunes]

The starter contains resistances and contacts to switch them in and out of circuit. In a typical starter, rotating the control shaft moves a drum which closes the contacts in a series of steps to cut out the rotor resistance progressively as the motor runs up to speed. In some types there are also contacts that switch the stator circuit. The oil serves the dual function of protecting and insulating the contacts and cooling the resistances.

It sounds like the drum was not moving to its final position with all the resistance out of circuit, which as well as reducing motor performance can cause the starter to overheat. It sounds like it would be worth giving the unit a general check-over regarding the condition of the contacts and mechanics.

What size is the motor and what was wrong with it?
NB the title says 'heaters' which has probably been autocorrected from 'starters.'

 

[AlexGoode]

Thanks for the in depth reply, yes it should say starter and not heater. I think it was a 250kw motor that went in to replace a motor that burnt out. Is there anything other than the small motor (guessing it is for a pump for the oil) and the bar and contacts ontop of the tank to maintain and inspect? There is also a square unit above the tank which had 3 single cables wired into to it from the main tank, woild it be some sort of resistor bar setup?

 

[Lucien Nunes]

There are various different constructions and I wouldn't like to say what the correct maintenance scope would be or what the separate box is without seeing the unit in detail. This is also a bit bigger than the units I've actually worked on myself. If you had any pics I'd be interested to look.

 

[AlexGoode]

Think its sorted, thanks for the help