ReplyOhms law doesn't work for induction. You would need to know PF.Originally Posted by mjm88
Professional Member
i was looking at an airhandler today at a powerplant that was tripping the breaker. After jumping through all the hoops to get the clearance to work on the thing. i started checking for shorts. i found the fuse block for the 3 phase blower motor burnt up. I went to check the windings of the motor to see if they were grounded and they were not. but i was getting 2.4 ohms across all the windings in the motor. if ohms law is correct and i have a 460 volt motor rated at 6.2 amps should i be reading 74.19 ohms? if so then would it be safe to say the windings are shorted? i feel like this is a dumb question and i shouuld know this by now but i just cant get this through my head today.
Professional Member*
Ohms law doesn't work for induction. You would need to know PF.
Professional Member
power factor is 86.5
Regular Guest
2.4 ohms sounds right for that size motor.
Regular Guest
just do a normal check of system you might want to meg wires back to mcc panel. check incoming power at your starter, check your contacts, voltage drop across etc. but burned fuse block sounds like loose connections.assuming heaters,fuses,wire gauge are all sized right. then fire the unit up and take a current draw, see what you got and go from their.
Guest
the ohms reading are right, you need a megger and test the windings to ground.
AOP Committee/Professional Member*
I think you are looking at efficiency, and not power factor.
Power factor is a value between 0 and 1, and is a ratio of active power (in watts) divided by apparent power (in volt-amperes). A purely resistive load has a power factor of 1, or "unity." Motors are a combination of resistance AND induction. When you have induction, you have a counter-force that opposes current flow in the windings of the inductor.
An ohms law calculation based on winding resistance and rated amps is meaningless, because counter EMF will limit current in the windings just as effectively as a higher resistance would do.
[Avatar photo from a Florida training accident. Everyone walked away.]
2 Tim 3:16-17
RSES CMS, HVAC Electrical Specialist
Member, IAEI
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Professional Member
hey guys i appreciate the help! i got it going this morning. the fuse block was burnt up because of coal dust shorting between phases. replaced fuse block and fuses and she took off. put on a new belt and amped it out and all is well. i was just confused and a little intimidated by the size of the equipment. and not being able to bring power back to the system very easily without signing on and off clearances and all that good stuff that just takes lots of time
Professional Member*
I work at a power plant also. The saftey cleances take a lot of time but its billable time and there for cya and your safety now the mines are a different story MSHA regulations are just laughable. My first day at the power plant the "tech" and I were replacing a compressor HE condemed. I ohlmed the winding found that they were good. He said they were weak since they were equal also he said he had put a HARD START kit on it and It wouldnt work a few days before. I dont how he become a techncian the unit has a 3 phase compressor. Im still waiting to see the ladder schmatic for that lol.
Professional Member
i've been working there for a couple weeks now. taking over for another tech that was working there. its just frustrating not being able to turn power on and off as you please. especially when you have a short somewhere.
AOP Committee/Professional Member*
Coal dust? Wow.
Hope you wear a mask!
[Avatar photo from a Florida training accident. Everyone walked away.]
2 Tim 3:16-17
RSES CMS, HVAC Electrical Specialist
Member, IAEI
AOP Forum Rules:
Professional Member*
if the area was humid or dust on the windinds might of read shorted....all 3 should read the same between or to ground.where the covers on the contactors closure arc will trip the breakers as the arcs catch each other mean while your looking at the load....check the contactors for shorts between the L1,2,3 and T 1,2,3 sides and to ground.i.ve read fuses in humid mechanical rooms closed on a digital meter gone out into the street and the fuse is blown(open)...
"when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY
Professional Member
i think that is a smoken gun woe.
Professional Member*
if the area was humid or dust on the windinds might of read shorted....all 3 should read the same between or to ground.where the covers on the contactors closure arc will trip the breakers as the arcs catch each other mean while your looking at the load....check the contactors for shorts between the L1,2,3 and T 1,2,3 sides and to ground.i.ve read fuses in humid mechanical rooms closed on a digital meter gone out into the street and the fuse is blown(open)...
"when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY
Regular Guest
Is it a chilled water AHU?
Few week ago I went to a service called, I found one of the fuse blown, the costumer told me that it wasn't the first time this happened,
The issue had been happening almost FOR a year, anyway a long story short the chilled water valve was wired wrong. It was a N.O valve so every night once the fan stopped by schedule or system reached set-point the chilled water kept flowing through the coil so the blower motor was sweating like crazy!! I End up installing a aux relay to be able to close it after every time that reach set point.
Regular Guest
Other things to note in large industrial plants is that they don't always use WYE transformers alot of cases they use a Delta/Center leg ground transformer. L1 and L3 will have the motors thermal overloads which will open and save the motor so if L2 was the ground it could have blown the fuses.
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