Hello group, long time reader first time writer.


Situation:
- Two 600VAC motors (yes, Canada) 200Hp, each driving common shaft.

Starter #1
- size 5 isolation contactor, soft start, size five ss bypass contactor
- each starter is a cubic, self enclosed, top-fed / bottom-load siemens contactor

Starter #2
- size five contactor
- this starter is more bulky, vertical lift, top-fed / top-load, three separate contact housings, furnas contactor

Typical startup
- isolation contactor pulls in, softstart ramps, bypass contactor pulls in on SS up to voltage, delay, starter #2 pulls in

1. I do not understand why starter #2 is a different style. I am unable to obtain original manufacturing specs and I am searching for a reason why not to replace starter #2 contactor with same style as starter #1.

2. Necessity powers my questioning as my starter #2 coil hums louder than most humans and is on its last legs. Furnas has discontinued this coil.

Possible solutions:
a) replace 600v coil with 120v coil which we seem to have in abundance.
b) replace bulbous contactor with other style, standardize inventory.
c) run screaming into the night.

Why would the factory use two different styles of contactors? Should not what is good for starter #1 be good for starter #2?
There are a few more cases of this example at this facility but with size 6 contactors.

Thanks in advance for criticisms / comments

- Monte Stevens

Hello Monte
Welcome to the forum

I would suggest that there is no need to make both starters the same if the same start order is always used.
Reason:
Both motors drive the same shaft, therefore motor is up to speed before second motor is started. If the second motor is up to speed before it is started, there will be no severe overload current during start. It is sufficient to just across the line (or Direct On Line) start this motor. It could also be argued that as the contactor for this motor does not carry an overload current, it could be rated at AC1 rather than AC3, reducing the contactor size and rating.
If you preferred to have both starters identical, there is no problem, just no advantage in including the soft starter at extra expense.

If you alternated the lead motor, (which one starts first) then there would be a requirement for reduced voltage starting on both machines and therefore making both starters the same.
Why would you alternate the lead motor? There are considerable stresses placed on the motor during start, and if you always have ths same motor taking the start load, then this motor is going to age quicker and probably fail earlier. Keeping the starts equal between the two machines, would a) extend the life of the machine and b) enable more frequent starts.

Incidently, the bypass contactor can be rated AC1 as opposed to AC3 for the line contactor, reducing the size and cost of this contactor.

Best regards,