Hi guys I’ve got a weird one that I’m hoping you industrial guys may be able to help with. I’ve had this fault on a motor that drives a blower unit via belt. I first arrived and saw that the motor would not ramp up to speed and end up tripping. I then replaced the belts on the motor and tested again the motor ramped up fine. I tested a few more times and I would then again have this intermittent fault where the motor would not run up to speed and trip. The motor was tested electrically and is sound. All connections have been checked and are all good. The set up has two blowers standby and duty so before replacing the belt I tested on the other motor circuit to test if it’s the soft start or not and had the same problem so now I’ve put the problem down to the motor. Do you guys have any other ideas of what the issue could be or if I’ve missed anything?
Soft starter
- Thread starter Jc1996
- Start date
Help Support Talk Electrician Forum:
Are we talking about a basic star/delta starting system or something more sophisticated?
- Joined
- Aug 3, 2009
- Messages
- 1,797
- Reaction score
- 267
Is the motor “wind milling” while sitting off so the starter has to overcome the windmill effect (if opposite to motor driven direction)
is there a fault code?
No fault code no just takes out the main breaker.
I’m not sure I 100% sure if what you’re saying but the motor does not have to overcome the opposite rotational force no. I’m just extremely baffled it must be something mechanical if when I connected the motor into the standby starter it did the same thing. Thanks for your help guys, just can’t get my head round this as if the main breaker goes out after a few seconds it’s as though the motor is struggling. However would this make the soft start output lower voltages?
I’m not sure I 100% sure if what you’re saying but the motor does not have to overcome the opposite rotational force no. I’m just extremely baffled it must be something mechanical if when I connected the motor into the standby starter it did the same thing. Thanks for your help guys, just can’t get my head round this as if the main breaker goes out after a few seconds it’s as though the motor is struggling. However would this make the soft start output lower voltages?
The belt and pulley rotate freely
- Joined
- Aug 3, 2009
- Messages
- 1,797
- Reaction score
- 267
Have any plant conditions changed between them working and not working?
I was going to say does the motor run up ok with the belts disconnected but I reread your post and you said the stand by motor and blower does the same. Hence I am heading down the plant conditions.
Pipe blockage, damper shut in, maybe?
I was going to say does the motor run up ok with the belts disconnected but I reread your post and you said the stand by motor and blower does the same. Hence I am heading down the plant conditions.
Pipe blockage, damper shut in, maybe?
sorry, but was it the same motor on both starters or two different motors on different starters
what size is the motor, and what electrical readings did you get when you tested the motor?
last question does the motor run OK without any load on it ?
Sharpend
"It Just Is"
Is the rotor turning freely? Is is still centred or is there a wobble?
Off the top of my head approx 15kW. Motor moves freely. For the winding resistance I’m getting very low ohms 0.0ohms and IR >999meg to phases to earth and tested at the motor in between all 3 windings >999meg
What are the individual winding resistances on the motor, including the supply cables as measured at the soft start end?
Wiring connected motor end, disconnected soft start end,
Can you do LCR readings for this as well as high accuracy low resistance measurements?
0.00 Ohms is meaningless.
You need to measure the winding resistance to check for damage in the form of turn-to-turn shorts or open turns.
This is a start towards checking motor winding imbalance.
All readings will need to be within 10% of each other.
The absolute value is of less importance.
To help any more, greater information on the application will be required.
Wiring connected motor end, disconnected soft start end,
Can you do LCR readings for this as well as high accuracy low resistance measurements?
0.00 Ohms is meaningless.
You need to measure the winding resistance to check for damage in the form of turn-to-turn shorts or open turns.
This is a start towards checking motor winding imbalance.
All readings will need to be within 10% of each other.
The absolute value is of less importance.
To help any more, greater information on the application will be required.
I am sure the windings are balanced. I checked on my MFT and all values were identical. So what measurements are you suggesting? Measuring the resistance of the cable from the output of the soft start to the motor? Thanks @Sidewinder
It’s intermittent though. It doesn’t always happen so it’s hard to measure anything electrically when it does happen as it happens so quick. No I haven’t I haven’t got a tachometer in my tool box unfortunately.
Two motors using the same soft start? Or is it one for each motor? Confused as you say you tested the other motor and had the same problem bit think it’s a motor issue? Are the motors sharing the same fan or do they have separate fans in the blower? It’s possible the soft start is failing but check the fan blades and bearings are rotating smoothly and not tight etc. if its an erratic issue possibly not the motors. As mentioned above check each motor winding separately and isolated from the controls etc if you were getting 0.0ohms it means you are not testing the windings correctly or your meter is not suitable for the task in question.
Forget the MFT to start with. It's the wrong tool for this job, in fact, absolutely useless for measuring a motor winding resistance.
You need a digital multimeter, such as a Fluke 87 or similar.
Better still an LCR meter.
Disconnect the cables from either the motor end or the soft-start end.
If you disconnect at the soft-start end, you are also checking the integrity of the cable to the motor and checking for a high-resistance joint in that part of the circuit also.
It enables you to "see" what the soft-start "sees".
Measure between the phases with a proper test meter get the resistance of the cable and the winding.
L1-L2 L2-L3, L1-L3.
Reverse polarity doesn't matter too much at this stage.
You are looking for a delta, in this case the delta needs to be <10% across the readings.
If you see nothing in the resistance then do the inductance, impedance, capacitance & resistance with an LCR meter such as something like this:
https://uk.rs-online.com/web/p/lcr-meters/2035762Again, between phases, tabulate the results and then compare for <10% delta.
You might want to do reverse polarity on the LCR test also because of the way these interact with the windings.
If the motor passes muster on that, and IR (your MFT @ 500V will be OK for insulation resistance), it's probably OK, so you have a soft-start fault.
I note that there is mention of two motors, are there two soft-starts?
How is the unit wired?
Do you have a schematic?
What is the drive from the motor to the fan?
Vee belt?
Are there any blockages in the duct work?
Could be either side of the fan.
Put a clamp meter or PQA on the incoming supply to the soft start and check firstly the inrush current and see what that equates to.
You say, "main breaker", is this for the whole building, or the supply breaker for the fan circuit?
What is the motor rating and how many poles?
What is the fault code on the soft-start?
What is the make, model, and rating of the soft-start?
What is the circuit supply breaker?
What has changed, has it worked fine for years, or is it new?
No it’s just one. They are identical blowers. The fan rotated freely. There is no fault code on the Soft start due to the breaker tripping before anything else. ( I don’t think selectivity has been corrected accounted for as we have 63amp BS88 fused at the fused disconnect then 50 amp type C upstream.)
Can I ask why a MFT on low ohms is not sufficient?
And it’s a V belt I haven’t gone down to stripping anything down yet as it seems to be working fine now. Just wanted to understand the theory to why it was happening.
And it’s a V belt I haven’t gone down to stripping anything down yet as it seems to be working fine now. Just wanted to understand the theory to why it was happening.
that sounds like the motor
if you have two soft starts and two fans with the same motor and it trips, I would change the motor
Similar threads
