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Discussion Starter · #1 ·
Hi guys, just wanted a quick advice from you about this problem.


So at my new job there is a motor that shorted twice that i cant confirm because the other electrician is gone.

And that motor is a 7.5hp 575v 2 speed star delta for a cooling tower.


I could blame the motor but i found 2 weird thing about the control of it


1st there is no timer or mecanical lock between the contactors used for the low and high speed of that motor

2nd thing, for some reason close of that motor they made 1 disconnecting switch for each speed instead of 1 with 6 wire to cut everything together. And if for some reason someone would leave the low speed disconnect switch off while the control is on high speed T1,T2,T3 will stay open which isnt good because in high speed on the diagram that say it should be all together.


What do you think ? Is it possible that one of these reason shorted that new motor again ?
 

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Are you saying that this is (A) a two speed one winding motor, (B) a wye-delta start or (C) a two speed two winding motor? Is this new or existing?

Just a blind guess, two leads are mixed between high/low or start/run. This would cause the motor to "single phase". The O/L should trip on this situation.

Check the wiring from the starter to the motor and meg the motor.
 

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Discussion Starter · #3 ·
Well that motor is 6 wire for low speed i use L1/T1 , L2/T2, L3/T3 and T4,T5,T6 is open ... when in high speed L1/T4, L2/T5, L3/T6 and T1,T2,T3 is jointed together.

The overload was already tripped when i started working on it with 2 time delay fuse blowed up. and on the megger its very clear T5 is open and smell the usual shorted motor.

I double checked the wire to make sure the other guys didnt mix the connection of the wire and the control and protection rating is alright.

That motor worth few thousand dollars thats why i needed some advice on it i dont want to blow up a 3rd one :p

My best 2 guess would be to place a mecanical lock on these contactor and buy a new disconnect switch before trying a new one.
 

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Sounds to me like it isn't a star-delta motor. From your description, I'd say you've got a 2 speed consequent pole motor and it's hooked up wrong. Can you post a photo of the nameplate? What are the speeds and HP ratings?
 

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Sounds to me like it isn't a star-delta motor. From your description, I'd say you've got a 2 speed consequent pole motor and it's hooked up wrong. Can you post a photo of the nameplate? What are the speeds and HP ratings?
That's what I'm thinking too, but also keep in mind that to those unfamiliar with them, it may look like a "wye delta" starter.

A real common problem with two speed motors is that the average electrician, even an industrial electrician, doesn't encounter them very often. So when one comes along and the installer has never seen one, he may not understand that there are different types. So he may have read the rules on protecting "2 speed motors" that say that each set of windings must have its own OCPD, not understanding that this only applies to 2 speed 2 winding motors, not to 2S1W. That would explain the separate disconnects. So if it is a 2S1W consequent pole motor, and someone turned off a disconnect thinking they were only disabling one speed, they were in fact likely causing a severe overload on the remaining circuit. But I'd have to see exactly WHICH of the conductors they were opening to be sure. The other issue is that depending on the nature of the machine, it may be necessary to always start it in low speed and change to high, but if they disabled the low speed, it put too much thermal stress on the motor trying to accelerate it in high all the time.

So yes, a picture of the motor nameplate and maybe a napkin schematic of the power connections would be helpful.
 

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With no interlock and no timer, it's definitely not a wye-delta starter. How many contactors in the starter? A wye-delta starter will have a minimum of 3 contactors, and will for sure have interlocking.

If you've got a 2 speed, consequent pole motor you won't have a timer or interlocking and one of your contactors will have 5 power contacts, and the other will have three. The type of consequent pole motor (CHP, CT or VT) will determine whether it is 5 on the low, 3 on high or the other way around. If you can tell me the HP ratings on the motor, I can tell you which motor it is and how to hook it up. If you send me the photo of the nameplate I can tell you how to set the OL's as well so you don't burn up another motor.
 

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It appears, from your current connections that this is a two speed, single voltage, constant torque motor. If so the line connections are wrong.
Low speed= L1-T1, L2-T2, L3-T3 and T4,T5, T6 open
High speed= L1-T6, L2-T4, L3-T5 and T1, T2, T3 connected

If the phasing and connections are as you say, the motor is doing an instant reversal when transitioning from low to high. This would be a motor killer on something with a large rotating mass.
 

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It appears, from your current connections that this is a two speed, single voltage, constant torque motor. If so the line connections are wrong.
Low speed= L1-T1, L2-T2, L3-T3 and T4,T5, T6 open
High speed= L1-T6, L2-T4, L3-T5 and T1, T2, T3 connected

If the phasing and connections are as you say, the motor is doing an instant reversal when transitioning from low to high. This would be a motor killer on something with a large rotating mass.
A variable torque motor would have the same connections as well but it wouldn't reverse the way he has it connected. Either motor would just run on half the windings and burn those out. :)
 

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Discussion Starter · #9 · (Edited)
Wow i have to check that monday and make sure about that high speed connection is good and make sure both speed go the same way.


If i got anything new i post you the nameplate and a quick design of the control

This application is a water tower and the way the control work doesnt seem to need to be in low speed to start ... we can actualy start it in high speed

Here a quick exemple of the control system

A temp controller in Automatic decide to run in low or high speed depending of the Water temperature.

There is a Selector switch on the panel 3 position Manual and Auto

and the other one is to select the speed for the manual mode ( low or high or nothing)

Also the 3 contactor is : C1= Jump T1,T2,T3 together thats all it does

C2= Low speed and connect L1 to T1 ... L2 to T2 and L3 to T3

C3=High speed and connect L1 to T4... L2 to T5...L3 to T6


When we select low speed only C2 is activated
When we select High speed C1+C3 is activated






Im not at work atm but from that i could remember that motor use 3 contactor
1 for low speed, 1 for high speed + connecting T1,T2,T3 together.
prety sure also the speed was 800 rpm for low and 1725 for high
 

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Yeah, sounds like a Consequent Pole variable torque connection, probably looks something like this.



You may have the order backward in terms of which ones come on for which speed.
 

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Discussion Starter · #12 ·
That's exactly what the motor look like.

And now from what i saw im prety sure the high speed is wired in reverse from what i saw

I will keep doing my test monday and keep all that in mind

oh and yeah i confirmed that my main power is all good and all all the way up to the disconnect switch.

Thanks so much !
 

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Thinking about my earlier post: While your high speed connections are not standard, they would still give an "A-B-C" rotation. 6 (A ph)-4 (B ph)-5 (C ph) phase rotation is the same as 4 (B ph)- 5 (C ph)- 6 (A ph).

If this motor was changed recently there is a possibility someone did reterminate something out of phase possibly causing the motor reversal on speed change.

Other ideas:
Have you checked all of the starter power contacts?

Could a wire, termination, or disconnect be broken or corroded?

Could there be a mechanical problem with the driven equipment?

Are the high and low speed contactors mechanically interlocked? This prevents one from engaging before the other has dropped out.
 

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Discussion Starter · #14 ·
Yeah well i tested all the control with the disconnect switch OFF and was working properly except for the contactors ( there is no mechanical interlock on them)

I tighten up all the connection properly and megged even my main wire to make sure it was all good.


There is 4 thing i really wanna change before a new motor get up there again.

Buy new contactor with the mechanical interlock on them

Buy a new disconnect switch for a 6 wire motor like this one and remove the 2 old disconnect switch installed atm.

Carefully watch that motor nameplate and connection of the old one and see if its alright.

Try a quick rotation test for each speed and make sure both go the same way when the new one is installed
 
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