How can I troubleshoot motor burnout caused by a missing neutral bridge?

[roro36]

Hi,

We installed a chiller unit that had a working compressor. We turned it on, but nothing worked, after some fault finding, we found in the main panel, also newly installed, the neutral bridge was missing. This created problems as the neutrals on the chiller then created false highs on some phases and shorts to others... So a usually have 400V p-p, now 1 would read 100V, another 380V and another almost 600V.

We found the missing neutral, added it, but now the compressor motor has burnt out. Could the missing break have caused this?

Aside to that we found the overload was set to 9A when the motor was rated to 7A.

Any thoughts?

 

[Mordred]

Sounds like the stator windings are fried. Measure the resistance on each winding they should all be the same. Also make sure none of the winding are shorted to ground. Depending on the size of the motor the stator could be rewound. On small motors this however is not cost effective.

Depending on stock and the winding tests if proven bad you may resolve the problem by changing out the stator. If this motor is controlled from a vfd or soft starter also check for damage of the drives.

 

[roro36]

Yeah... I know the motor is stuffed, what I'm trying to figure out is the cause? Could the fact that no neutral was present, but that through some elements a neutral was connected which would have pulled the neutral high, and thus throw off the values between phases, have caused the burnout?

 

[Aussielec]

I can't see how a broken neutral will affect your phase to phase voltage. It will though affect your phase to neutral voltage since your single phase equipment has lost all reference to the star point of the transformer.

The problem being is with no neutral you will get a high phase to neutral voltage on the lightest phase whilst a lower voltage on your heavier loaded phase.

The chiller motor itself is probably alright since it only cares about line voltage. But It looks you might of fried some of the single phase control gear for the compressor sorry to say.

Also if there's NO power to the chiller then obviously there's a problem on your control side somewhere, so is there actually a voltage present on the chiller terminals or not?

 

[Mordred]

Yes common symptoms are imbalanced loads between windings which also are indicative of harmonics and single phasing.

Lol Aussie' s answer came while I was typing and explains it rather well

 

[roro36]

Ok, so if 2 phases are connected through 2 separate resistive elements to neutral, i.e. 2 relays, but now the neutral had no star point reference, the neutrals on the other side of the relays ate joined, but reading high and this can give the false reading between phases?

Maybe I'm remembering wrong, but I'm sure the p-p voltage was off as well as the p-n voltage.

So if all the circuitry is fine, everything works, but the motor is fried definitely, could this have been caused by a absent neutral connection back at the main panel?

 

[Mordred]

If I'm visualizing correctly you had the neutral wire connected to the motor. That same wire not connected at the supply point. If that's a correct visualization then that means essentailly no neutral at the motor.

 

[Mordred]

Think of it this way the neutral conductor needs to handle the currents of all 3 phase legs. If a connection is lost between a phase leg and the neutral then the current will find another least impendance path to ground.

 

[Aussielec]

Alright let's take a step back here for a second.

Couple of questions.

Where did you take these voltage readings at?
Did the voltages return to normal once you reattached the neutral?
Is your chiller motor single or three phase?
Have you tested the compressor to see that it's actually damaged?

Again I'm trying to figure out how your line voltages were so unstable when you lost the neutral.

 

[Mordred]

Also did any of your leg fuses pop? (3 phase)

 

[Aussielec]

Ok, so if 2 phases are connected through 2 separate resistive elements to neutral, i.e. 2 relays, but now the neutral had no star point reference, the neutrals on the other side of the relays ate joined, but reading high and this can give the false reading between phases?

It shouldn't. If you connect two heater elements both with equal resistances between the two phases they will be series and the voltage across each would be half your line voltage. So 400/2 which will give you 200 volts across each element. If however you add another element of equal resistance and take the feed from the third remaining phase the voltage would be in your case 400/√ 3 which would give you 230 across the elements. All this assuming no neutral is present. But this is a balanced load so no need for a neutral.

However if the resistances are all different then there will be unequal voltage drop across the elements if there is no neutral present. This is why I think you may have fried some of the control gear for the compressor.

 

[Mordred]

Thanks for answering that first part. Even though I'm not the OP lol. I' m going a lot on memory here as my current field is in the two way radio industry. Prior to that 15 years Industrial electronics. There is enough crossover to electrical that I can assist to a point

 

[roro36]

Hi,

I have attached a simple diagram. The control gear is all correct. It was the neutral at the back. The motor and contactors would not come on until we gave a neutral. To check that we had this problem connecting one of the neutral lines in the control panel to Earth solved all the problems and it ran. So we went looking for an absent neutral and found it.

So what I gather is that the motor burnt out for other reasons and not because of the absent neutral.

 

[Mordred]

Other causes could be numerous. I hope you find it, the phase to phase voltages you measured I've seen caused by single phasing. One common cause is when one leg fuse pops. Motor will keep running but at the wrong voltage/currents per legs. The 9 amp setting indicates that the motor may have been nuisance tripping and rather than find the cause someone upped the setting from your 7 amp FLA.

 

[Mordred]

Here is a handy drives manual from Allen Bradley, I often find this reference useful

 

[jim hardy]

The motor and contactors would not come on until we gave a neutral

This is why I think you may have fried some of the control gear for the compressor.

So what I gather is that the motor burnt out for other reasons and not because of the absent neutral.

I'm with roro and aussie -
Any way that motor could have got single phased? Are its contactors mechanical or solid state?

 

[wirenut]

A couple of questions:
1) what does relay 1 control?
2) what does relay 2 control?
3) what turns on/off the 3 phase to the motor?
4) are relay 1 or 2 part of a motor starter for the motor?
5) if a starter (contactor and overload block) is used to start/stop the motor, did you check/reset the overloads after the neutral was fixed?
6) did you test the relay coils to see if they burned out?
7)what is the coil voltage of the starter if present?
Based on your diagram i cannot tell if there is a proper magnetic starter on this chiller (i assumed there was when you said the fla was set @9A instead of 7A)
Also mag starters (in the USA) run one side of the coil supply (neutral on 120vac/277vac or a phase if 208/220/240/480vac) through the overload relay. On overload this turns off the coil power to the contactor section and shuts down the motor.