Motor starter help.

[Lucien Nunes]

@dlt27 How did you get on? Did you find out anything about the NTC or leave it bypassed?

 

[dlt27]

Where could I find a brimistor to replace the existing one. I’m struggling to find one.
Many Thanks
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This is the what I thought was a 1M ohm resistor that I bypassed. Could it be a brimistor? Thank you
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[Lucien Nunes]

That, as you say, is a regular 1MΩ 1W carbon film resistor. But 1MΩ is too high to serve any useful purpose in the starter that I can think of, so I am not sure why it is there. However, looking at where the heat shrink sleeves meet the resistor body, there appear to be solid wires poking out with round flat bits on the end or possibly loops, and traces of a black substance. That would have been the Brimistor, a black rod of NTC material between those two ends. They do sometimes crack apart with age.

First, do you need it? Note 1 on the drawing states it is not needed for units below a certain current rating that I can't read, which probably have different contactors fitted. See whether the present application is above or below that figure.

Next thought, Brook Crompton still exist, I'd give their technical dept a call. They might be able to supply one from stock, or advise a modern equivalent NTC to replace it. Brimistors were just one popular manufacturer's range of NTC's, it doesn't have to be a Brimistor.

NTC's are still available and used in various electronic applications. There are a few dozen standard types, that have different resistances, current ratings etc. Much the same as the Brimistors of old, which were sold in a range of ratings more usually targeted at jobs like limiting inrush in AC/DC valve radios and TVs. If we have to, we can select something suitable ourselves.

 

[dlt27]

Thanks again Lucien Nunes I will hopefully try Brooks Crompton on Monday..

 

[marconi]

I think the 1 MOhm 1W resistor is used to provide mechanical rigidity in the flexible link so that the Brimistor can be placed across it, thereby reducing flexing stress which might damage the fragile end connections of the Brimistor. Without the resistor all the flexing stress is applied to the Brimistor leads. 1MOhm because only a tiny current flows through it so does not change the circuit's operation.

 

[Lucien Nunes]

That sounds reasonable, I have done similar.TBH I had expected the original arrangement to have been a bit more sophisticated like a slice of tagstrip, as the bare ends of the Brimistor can't have been left flapping around. I assumed this was a retro-bodge but perhaps not.

 

[dlt27]

Thanks again. Could anybody send a link for a thermistor/ brimistor that I could use to replace the faulty one..

 

[marconi]

I did some rough calculations which indicate you need a brimstor or ntc thermistor with a cold resistance of about 2500 Ohms. I struggled to find a surge current limiter that might substitute because the max cold resistance I have found so far is 120 Ohms. It would be clumsy to put 20 of these in series. maybe my sums, oiled by half a bottle of Muscadet are wrong.

I did find some old brimistors for sale with a resistance of 3000 ohms.
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Brimistor CZ1 3000 Ohm 20°C STC Thermistor (Rare) | eBay - https://www.ebay.co.uk/itm/Brimistor-CZ1-3000-Ohm-20-C-STC-Thermistor-Rare-/112790433533

THis one drops its R to circa 300 Ohms or lower when hot. Of course it is eventually short circuited but one should not assume that.

 

[dlt27]

Thank you marconi

 

[Lucien Nunes]

My hunch is that the delay on a CZ1 will be too long. I'm not near my STC data book but I think they were popular for 100mA series heater chains, in which they give a gentle 10-20 second warmup. The original part in the starter looks like it had much lower thermal mass, so if the resistance and hence dissipation on 230V is similar, the delay would have been much shorter.

My other hunch is that when a call to Brook Crompton will either get one in the post for a few bob or they'll say 'we always tell people to bypass them, they were never really needed.'

@dlt27, were you able to confirm the motor FLC as per note 1?