# Siemens 3RW44 Soft-Start and 55KW motor problem.



## egdavies (Mar 22, 2019)

I’Hi all,

This is my first post so please bear with me.

I'm working on a biogas powered generator that consists of an asynchronous 55kw 3ph 400v motor coupled to a v8 gas engine that supplies powder to the grid. It has been in use for just over two years. To start, the engine starter is engaged and then a Siemens soft-start (in delta) operates over 10 seconds to bring the speed up to 1480 rpm. (50Hz). When at this speed fuel is introduced into the engine and the motor starts producing power instead of using it.
The problem started with an engine over speed fault which I traced to a broken flexible coupling.
After this was replaced the soft-start fails to start the motor, it just hums and then times out, although when the coupling is disconnected the motor does turn slowly but does not pick up speed.
I suspected the Siemens 3RW44 25-1BC44 soft-start so replaced it with another new unit, different make but same specification, and this causes the main 100A mcb to trip despite changing parameters as suggested by the supplier . So I tried it on another unloaded 30kw motor and the main mcb trips the same. This all happens too fast to see fault codes. The original soft start works with the 30kw motor.
I have tested the motor as best as I can. All winding have the same resistance, 0.17ohm on my meter, 0.35 for 30kw motor and over 200Mohm on the insulation test so seems OK to me.

To sum up:

1. Original Soft-start & 55kw motor --> motor hums or turns very slowly with on load (voltage across each windings about 137v)
2. Original Soft-start & 30kw motor --> OK
3. New soft-start & 55KW motor --> trips fuse
4. New soft-start & 30kw motor --> trips fuse

My conclusion is a faulty 55kw motor and a faulty second Soft-Start, but I can’t believe the motor is faulty.

My questions:
Is it possible for the Siemens 3RW44 to be faulty on the larger motor and OK on the smaller motor?
Are there any other tests I can do on the motor?
Is there any likelihood that an electrical fault caused the coupling to fail?

I’d much appreciate any help.

Thanks


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## gpop (May 14, 2018)

I would have to prove that all 3 legs are really there (amps). 

Its easier with a scope meter to see what really is happening during the start up (power quality meter). But the general rule of thumb is that a humming motor is single phasing. 
Now you have a interesting set up because when its coupled the engine starting motor is acting as a bump start for the electric motor. This would give a 3 phase motor missing a leg direction and if the breaker could hold the load it would probably spin up on 2 legs. 

The only other thing i can think of with out seeing a lot more details is that you have 6 leads back to the soft start and you reversed one set (makes real weird noises if you reverse one coil pack). Maybe some one tried to reverse the motor and only switched one set when there were 2 per phase.


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## JRaef (Mar 23, 2009)

Most likely the 55kW motor has a shorted winding, but the original Siemens 3RW44 is set up for Current Limit, and the "similar" soft starter that you tried to replace it with did not, or is was one of the cheap "2 phase" versions that people to selling now. 



So when the 55kW motor is connected to the Siemens starter, the soft start feeds into the shorted winding, but limits the current to a level that is not causing the breaker to trip until the soft starter trips itself off on Failure to Start or something. Then when you connect the other soft starter that is not (or cannot) do Current Limit, the current jumps to the trip point immediately and tripped the breaker. That then probably caused a shorted SCR in that 2nd starter, so it tripped again on the 30kW motor because you have now damaged it. However when you connect the original Siemens starter to the 30kW motor, there is no more short so it functions normally.


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