" 3 phase" for the masses, motor trouble

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Old 04-25-05, 01:07 AM
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" 3 phase" for the masses, motor trouble

I get called in to a water plant occasionally to work on some of the lift pumps. I've got a 75 hp motor there that likes to burn out every 2 years or so. In the past the underground wirings been replaced after it blew out. I replaced the starter, note it's the only full voltage starter still at the site, other 2 are transformer starts. I've installed a phase monitor because I thought that in the past a fuse could have blown out on start up and the motor would be injured by single phasing before the ol would catch it. I used to have this same problem at the pumps at the town pond that pumps the water to the water plant. They had a weird transformer hookup that utilized 2 transformers and only 2 hot legs of the primary high voltage feed. 1 phase to ground would usually read around 115 v to ground, 1 phase around 500 v, and 1 phase over 1000v, and believe me a lot of digital meters don't do over 1000v. Actual phase to phase voltage is 480-500v. After 1 of the transformers blew at the pond (1 had to convince the power co it was blown, they didn't believe it until I laid my hand on it), they installed a 3 phase y sytem and everything was cherry. So I've still got this problem at the plant. To make a long story tedious does anyone out there think this high voltage may be causing the blowouts? I currently have 500v on one leg, 800v on one leg (to ground), and over 1200v on the third leg to ground. Effective voltage is still 500v between phases. I'm thinking the field insulation is breaking down, not phase to phase, but phase to ground. Note this is not the 575 3 phase undgrounded textile mill power, I don't have a problem with that. It's a different animal all together. 2 primary leads feeding 2 transformers. 2 secondary leads from the transformers, 1 from each one. and an additional secondary tap from each transformer tied together to produce the third secondary leg. Comments?????
 
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Old 04-25-05, 01:53 AM
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I've worked on lots of three phase stuff, but I've never been subjected to the screwy setup you describe. It wouldn't surprise me that a motor could develop a short phase to ground and start pulling some current on that path and start smoking the windings.

I was on a ship a while back that used 3000 hp 600 volt GE motors for propulsion. They had to be real careful to keep the windings clean. The real, real, fine copper dust flying off the brushes would coat the windings and get into the atmosphere surrounding the motor and you would get an occasional flashover. I know that there were some motor burnout problems on that ship in the past.

You are probably talking about an induction motor, however. Could be a simular problem, though. What kind of atmoshere is the motor subject to? At the voltages you are talking about, a coating could be building up on the motor windings and flashing to ground causing the motor to self destruct. About the only thing I could suggest is to keep careful megger records on a weekly or monthly basis to see if a conduction path is forming.

At the voltages you are measuring you can indeed have a problem if the motor windings aren't keep clean and dry.
 
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Old 04-25-05, 03:42 AM
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Motor is outside, setting on a clarifier well, Its damp, but not dirty or dusty. Occasional lightning strikes take out the telephone remote contacts on the starters.
 
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Old 04-25-05, 08:28 AM
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Jedi9,

Monitoring the motor with a clamp on ammeter, an infrared thermometer and a megger as jughead has suggested will go a long way to troubleshooting the motor burn out problem.
If you are a private contractor the benefits of this would be an easy sell.

If you are getting 1200 volts to ground then clean windings are a must.
 
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Old 04-25-05, 09:37 AM
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I would suggest that if the motor doesn't run continuously then have some sort of a heat lamp on the motor windings. They do that sort of thing on ships when they are dockside to keep the motor windings from getting too moist during periods of time when the motor will not be run. Using something like 'Electro Clean' on the windings once in a while would be a good maintenance proceedure as well. Everything must be clean and dry if you are going to use that kind of phase to ground voltage. I don't know what it is costing to keep replacing the burned motors, but it might be cheaper in the long run to just fix the voltage problem.
 
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Old 04-25-05, 01:15 PM
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What you are describing sounds like an open delta transformer connection. The primary of the transformers are connected like a "V" with a hot phase at each end and the common point being grounded. The secondary can be connected as a 3 or 4 wire delta. The fact that you are getting relatively constant readings phase to phase, and incredibly weird ones phase to ground lead me to believe that it is probably hooked up as an ungrounded 3-wire secondary, and the voltages you are reading to ground are probably capacitive coupling.

The advantages of this kind of connection are not having to pay for a third primary phase brought into your property. The disadvantage is that this bank has extremely poor voltage regulation, particularly if you are at the end of the line where the primary voltage is not balanced.

We had this same problem with a well. As explained to me by a Franklin Pump representative, a relatively small voltage imbalance (say 5%) can cause a much larger current imbalance in the windings (say 15 to 20%). (Cut me some slack on the numbers here....I don't have the specific ratios, but you get the point).

As explained by the Franklin guy, these currents circulate within the winding of the motor itself and aren't actually seen by the O.L.'s; so often the motor burns out and you think the O.L.'s don't work. He went so far as to tell me that some manufacturers will actually void a warranty if they find out you are using an open delta on their equipment. We took his advice, pulled in the 3rd primary phase, constructed a full 3-transformer bank and never had another problem.

Franklin has an excellent hot line number and their staff is very knowledgable, even if the pump you're having problems with isn't a Franklin (as was the case with ours). Good luck!
 
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Old 04-26-05, 12:44 AM
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Jughead, there are a total of 3 motors, 2 of which alternate, the two that alternate are switched once a day so that each motor is used off & on 2 days out of 3. There are heaters in the pump section but these have been long ago disconnected.
WFO, excellant post, thanks much, this is exactly what I have and I have experienced current imbalance in excess of 10% which I don't like. There is a third primary line there, just no transformer. If you don't mind I would like to C&P your post to show to town officials. When we ended up getting the three transformers at the town pond our trouble there disappeared.
Also I more thing, I'm not comfortable with 1200V on a conductor rated for 600V when the conduit going up to the service disconnect is full of water all the way to the top of the box connector (about 2.5 - 3 foot off the ground) Don't ask me how, wire is buried in a trench withut conduit. With a Y connection, i can get that voltage down to 277 and I have no problem with the conductor in water.
 
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Old 04-26-05, 01:00 AM
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Thanks to all who replied and thanks for your comments
 
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