Two speed one winding motor troubleshooting advice

[MTW]

Someone in another forum questioned the conductor size (#2) on the shorting circuit that goes to the 3rd contact, connected to 1, 2 &3. the contactor is supposed to be sized for a minimum of 50% of the current rating of the other contactors, because it is the Y point for half of the circuit (in high speed, it is two parallel Y winding sets, one set is already connected, the contactor connects the others). So with the FLA rating of the motor requiring 500MCM cable, the shorting circuit conductors should have been a minimum of 3/0, maybe 4/0 with VD. My thought is that someone erroneously sized those conductors for the Low Speed current, but in Low, those conductors don't even carry current. So what I think is happening is that the under sized conductors are creating a VD in that half of the 2Y windings, evidenced by the fact that the two sets of windings are not showing the same current, and they should be.

This is my take as well. Conductors a should be sized for half of the FLA, as well as the contactor for the second half of the winding.

As evidenced by the OP first post.

High speed shorted connection current to problem fan motor:
T1 - 161.9A
T2 - 154A
T3 - 156.6A
High speed shorted connection current to other normally operating fan motor:
T1 - 135.5A
T2 - 136.6A
T3 - 139.4A

The Low speed and High speed shorted conductors are a sun resistant direct burial 90C 600V, three conductor and ground 2AWG jacketed cable. When this motor is operating in high speed the three conductor cable is showing very warm with our IR camera, naturally since it has about 30A higher flow than the 2AWG is rated for. I have found some cracked insulation on the individual 2AWG conductors in the motor junction box and sealed these up with silicone high voltage tape and super 33.

Some major voltage drop going on here, causing the other half of the winding to pick up the slack. Roasting cables and bearings in the process. Should be pretty simple to compare wire sizes and contactors between fans.

Also is good to note that there are three types of 2S1W motor types, and the one in use here is the variable torque. Low speed horsepower is one quarter of the Hi speed horsepower. The other types can be found here. Also know as Dahlander or Consequent-Pole type.

NEMA starter application data

MTW

 

[Besoeker]

Some major voltage drop going on here

Would that be an issue with the motor uncoupled i.e.the no-load condition?
And would you really expect this motor to have been cabled differently to the others used on the same application?

 

[Sahib]

Meggar the winding at high speed position. Compare it with a similar but good motor with similar operating conditions. If reading low, winding fault.

 

[Besoeker]

Meggar the winding at high speed position.

I'd be amazed if that wasn't done at the motor shop.

That said, my money is still on it being a motor issue. If the rotor contacted the stator there could be shorted turns causing the higher currents being measured.

 

[Russs57]

We have some information to go with. Motor draws more amps at location than at motor shop. Motor has been acting up for sometime.

Hence my suggestion to eliminate all starters and the smaller wiring used to short motor leads in high speed. If motor amps match results from shop when set on the floor next to starter, with all starters bypassed, then we move forward more easily.

 

[Ingenieur]

Too small for me to read, enlarged your drawing to 250% but still a little hard to decipher some things because of poor resulution at that magnification.

But looks to me like when connected in delta each winding sees 240 volts across it. When in the parallel wye - same windings each see 277 volts across them. Wouldn't that give you close to same overall output, sort of like difference in applying 208 or 240 to a 208-240 volt motor, one will draw a little more current but speed and horsepower will otherwise be nearly the same.

in delta 240/coil but 1/2 the current (coils in series, 2Z)
in wye 277/coil but 2x the current (coils in parallel, Z/2)

it depends whether configured for constant or variable torque
what I showed is constant (at each speed)

 

[Besoeker]

in delta 240/coil but 1/2 the current (coils in series, 2Z)
in wye 277/coil but 2x the current (coils in parallel, Z/2)

it depends whether configured for constant or variable torque
what I showed is constant (at each speed)

This indicates otherwise:

High: 460VAC / 250HP / 290FLA / 1.15SF / 1790RPM / 2 SPD 1 WDG Variable Torque
Low: 460VAC / 62HP / 100FLA / 1.15SF / 890RPM / 2 SPD 1 WDG Variable Torque

 

[Ingenieur]

This indicates otherwise:

High: 460VAC / 250HP / 290FLA / 1.15SF / 1790RPM / 2 SPD 1 WDG Variable Torque
Low: 460VAC / 62HP / 100FLA / 1.15SF / 890RPM / 2 SPD 1 WDG Variable Torque

but is it variable pitch?

 

[Besoeker]

but is it variable pitch?

I don't think the motor is. Anything else is irrelevant if the problem observed is with the motor uncoupled. I thought had already been clarified.

 

[Ingenieur]

the op measured voltages at the motor terminals

all problem motor readings
Output high speed @ bottom of contactor: 480
High speed voltage at leads in junction box: 476
High speed voltage to ground at leads in motor junction box: ~274

measured fla v drop 4/480 or 0.8%
500 mcm drop: sqrt3 x 110/1000 x 0.05 x 330 = 3+ ~measured

shorted ~ sqtt3 x 110/1000 x 0.2 x 155 = 6

but on the shorted coil v drop is a combo of 500 mcm and #2
~ 110/1000 x 0.05 x 330 + 110/1000 x 0.2 x 255 = 5.3 or 1%

imo the v drop does not seem like an issue also all units use the same size conductor

there will be a slight imbalance
one coil will have a few v more

I'm leaning towards an onsite mech issue, fan drive, bearings, etc
this may have contributed to the failure that led to the rebuild

 

[Ingenieur]

I don't think the motor is. Anything else is irrelevant if the problem observed is with the motor uncoupled. I thought had already been clarified.

unloaded readings mean nothing

it's not the motor
and imo not the wiring
what does that leave?

the lo speed i is < rated
the high speed > rating

mech issues will be amplified by the square of the speed

 

[Besoeker]

the op measured voltages at the motor terminals

all problem motor readings
Output high speed @ bottom of contactor: 480
High speed voltage at leads in junction box: 476
High speed voltage to ground at leads in motor junction box: ~274

measured fla v drop 4/480 or 0.8%
500 mcm drop: sqrt3 x 110/1000 x 0.05 x 320 = 3, ~measured

shorted ~ sqtt3 x 110/1000 x 0.2 x 155 = 6

but on the shorted coil v drop is a combo of 500 mcm and #2
~ 110/1000 x 0.05 x 320 + 110/1000 x 0.2 x 255 = 5.2 or 1%

imo the v drop does seem like an issue also all units use the same size conductor

there will be a slight imbalance
one coil will have a few v more

I'm leaning towards an onsite mech issue, fan drive, bearings, etc
this may have contributed to the failure that led to the rebuild

The disparity in current occurs uncoupled. Off-load.
Can we go from there ?

 

[Besoeker]

unloaded readings mean nothing

it's not the motor
and imo not the wiring
what does that leave?

the lo speed i is < rated
the high speed > rating

mech issues will be amplified by the square of the speed

OK. What mechanical issues did you have in mind that would result in a 45A difference and that same difference when loaded?

 

[Ingenieur]

The disparity in current occurs uncoupled. Off-load.
Can we go from there ?

those reading are meaningless
lo speed i < rated
hi speed > rated

v drop was suspected to be an issue
imo it is not and I showed how I arrived at that conclusion

 

[Ingenieur]

OK. What mechanical issues did you have in mind that would result in a 45A difference and that same difference when loaded?

the unloaded readings mean nothing
even more so when a fresh motor is compared to a worn one

 

[Besoeker]

those reading are meaningless

Wouldn't you not expect the no load current to be the same in two motors of the same design?

 

[Ingenieur]

Wouldn't you expect the no load current to be the same in two motors of the same design?

if of the same age/wear, within 10%
in this case no

the bearings could be loose on the old motors

for all we know the rebuilt motor could be reading the same as prior to the rebuild
they noticed the hi fla and mistakingly thought it was the motor so had it rebuilt when it was actually a mech problem
he stated this has been an ongoing problem for years, before and after the rebuild

how did they notice this issue? it wasn't tripping?
it should be close to doing so, operating ~ 115% x fla
his 'good' motors are running at 95%, too hi imo
what are the ol's set at? sounds >1.15 to keep it running
like they knew they had an issue
most mfgs want no more than 100%, some 90%

the 'good' motors >95%
the bad ~115%
happening for years
v at motor > 470 and drop <1.5% worse case

imo not a wiring or motor issue
imo a load or motor sizing issue, or both

 

[Besoeker]

imo a load or motor sizing issue, or both

I respectfully disagree.
Back round we go. Uncoupled means no load so we can disregard loading.
It is the same rating as the other motors but takes 45A more current so we can disregard sizing also.
What does that leave you with?

 

[Ingenieur]

another consideration
fla to both coils ~ 327
i from shorted coil 158
this means i from other coil ~ 327 - 158 = 169
balance 164 +/- 5-6 A or +/- 3.5%

v drop/coil to wye center
lo : 330 x 0.05 x 110/1000 = 1.8 (based on supply to wye center, shorted at motor)
so ~ 275 across lo coil
hi: and from before 5.2 across hi since it has the #2 in the path to the wye center

i delta (277-1.8)/(277-5.2) = 1.5% max assuming coil z is identical

 

[Ingenieur]

I respectfully disagree.
Back round we go. Uncoupled means no load so we can disregard loading.
It is the same rating as the other motors but takes 45A more current so we can disregard sizing also.
What does that leave you with?

no load reading are meaningless

lo i z< rated
hi > rated

the SAME issue has existed for YEARS before AND after rebuild
I suspect since installed
subject motor ~ 115%
the other motors >95%
yet noting trips, ol's likely dialed up to max

not the motor
seems to not be supply/voltage
that leaves mechanicals

motor tested fine at shop
issue in field, not motor, not wiring, leaves mechanicals