Motor starter help.

[dlt27]

Hi all, thanks again for any input.
I have been asked to look at this starter as motor starts ok, but when switches over to run motor stops and starter just hums.
First of all like I’ve said in previous posts I haven’t had a lot of experience with motors/ starters etc
Am I right in thinking this starter is for a capacitor start - capacitor run motor with no centrifugal switch as it has a timer set to 15 seconds and then 3 contactors?
The motor starts ok and contractor in middle kicks in and contractor on right kicks in but after time delay right contractor drops out, but left one doesn’t kick in.
The part I’m not sure about is that there is a 1M ohm resistor across the 13 and 14 contacts on the left contractor and if I bypass this with some cable it all appears to work fine. I have checked resistor and it reads 1M ohm like it should ( brown, black, green and gold).
Could anybody tell me why the resistor is there in the first place and what it does ( is it to stop a back feed?) and why the starter seems to work fine when I bypass it yet it is reading ok???
Thank you
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Also forgot to mention there is a cable from 14 terminal to A2 of run contractor..

 

[plugsandsparks]

Have you located the cap ? - try testing that first, resistor maybe for safely discharging of cap.

 

[westward10]

Can you show a pic of the motor and it's rating plate. Have you looked at the piece of paper wedged in the back of the enclosure.

 

[davesparks]

Is it definately a single phase starter and motor?
My guess from the picture and description woukd have been that it's a star delta starter.

 

[dlt27]

It’s definitely 240v as no 3 phase on the farm. Capacitor’s are in the motor terminal housing. If it was a dodgy capacitor surely it would be the same even when bypassing the resistor?
I’ve found a drawing now, but struggling to read. If anyone could guide the stages of the starter that would be brilliant. Thank you
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[westward10]

Was going to ask if it was a farm. Sounds like someone has made a two stage starter. What is the motor running is it a heavy load.

 

[dlt27]

Motor plate etc.
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Yes it’s a large motor for the grain. What are the advantages of a 2 stage starter compared to a dol? I presume reduces starting current, but how. Thank you
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Sorry to ask so many questions, but could somebody explain what is the difference between a series parallel motor and a capacitor start/ capacitor run motor as well please? Thank you

 

[Lucien Nunes]

It seems that what you are seeing as a 1M resistor is actually the NTC thermistor (aka Brimistor which was a trademark of STC). Its function is to slightly delay the operation of RN to ensure a full open transition. Bypassing it, as suggested in note 1 for lower power motors, simply causes RN to operate immediately. If the starter works with it bypassed, the Brimistor itself is probably faulty.

If anyone could guide the stages of the starter that would be brilliant.

OK, you asked.

Before we start:

a) Series-parallel starting refers only to the connection of the run / main winding. It is the equivalent of star-delta starting but for single-phase, in that it lowers the voltage across each winding to reduce the starting current until up to speed, without the use of resistors or transformers. The start winding configuration and capacitor arrangement is a separate consideration.

b) There are two aspects to the start-run transition; switching the two run windings A1-A2 and A3-A4 from series to parallel, and disconnecting the start capacitor.

c) There are two possible capacitor configurations; capacitor-start induction-run, in which the capacitor winding is completely disconnected in run, and capacitor-start capacitor-run, in which the winding is disconnected from the start capacitor but remains in circuit via the run capacitor.

d) There are two possible centrifugal switch configurations; either it switches the start capacitor directly (in which case the starter timer is only responsible for the series-parallel transition) or it controls the ST contactor, in which case that contactor controls both the series-parallel connection and the start capacitor.

e) ST4 and RN2 are mutual interlocks that prevent ST and RN operating at once, which would short the main circuit L-N.

Pre-conditions for starting:
a) C2, RN2 and TM2 are all closed, i.e. the starter is not already in either the starting or running state and the timer is reset.
b) If the centrifugal switch is in the control circuit, it must be closed, i.e. the motor is not at speed.

START BUTTON PRESSED
Start button energises ST and TM

ST OPERATES. TM BEGINS TIMING
ST2 prepares a holding circuit for ST and TM, ready for when C2 opens
ST3 energises C
ST4 opens to prevent RN operating
ST main contacts 3-4 and 5-6 connect run winding terminals A2-A3 for series starting
ST main contact 1-2 connects start capacitor circuit (if controlled by starter and not directly by centrifugal switch)

C OPERATES
C2 opens to leave ST held only on ST2
C3 prepares a holding circuit for C, ready for when ST3 opens
C main contacts connect motor to mains

MOTOR RUNS UP TO SPEED
Motor run windings are in series, capacitor winding is in series with start capacitor (if CSIR) or start and run capacitors in parallel (if CSCR)]

TM TIMES OUT (OR CENTRIFUGAL SWITCH OPENS IF IN CONTROL CIRCUIT) WHICHEVER IS FIRST
TM2 or centrifugal switch disconnects ST

ST RELEASES
ST2 opens ST's holding circuit and disconnects TM.
ST3 opens, leaving C held only on C3
ST4 closes to energise RN via Brimistor.
ST main contacts disconnect run winding series connection and start capacitor.

BRIMISTOR HEATS UP
Brimistor resistance falls in a second or so, to the point where it is low enough for RN to operate.

RN OPERATES
RN2 opens to lock out ST
RN3 closes to bypass Brimistor
RN main contacts reconnect run windings in parallel

IF CENTRIFUGAL SWITCH IN MAIN CIRCUIT, IT OPENS
Start capacitor disconnected

Motor is now in running state.

No TL;DR option here. Either you follow it or you don't!
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Re. your specific motor, it's hard to tell from the pics. What I think I am seeing is that the two capacitors are in parallel, making it CSIR, and that there is no centrifugal switch. Therefore, the start winding is controlled by ST and disconnected under timer control at the same time as the series-parallel transition. The maximum timer setting is limited by the start capacitor duty cycle as noted in the instructions.

 

[westward10]

Correct me if I am wrong but the motor plate indicates it has always been connected to a two stage starter, the original probably having a manual start to run handle.

 

[Lucien Nunes]

Agreed, Stimmt, D'accord.

 

[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.