3 Phase Motor

B-MAN

Last night I got a call from a customer about one of there machines blowing fuses (100 amp). When I arrived I noticed that the motor was sitting crooked. The tensioner assembly broke loose and in turn the bearings went out. I replaced the motor which is a 20 HP 3 phase 240/480 volt. I hooked it up the same way that the existing motor was "star" 1,7 - 2,8 - 3,9 - 4,5,6.
When the motor is started it it draws 70 amps and should kick into high speed and drop down in current, but it doesn't. The motor seems to stay in the starting mode and won't kick up in speed. What's going on, please advise.

Sparky Joe

What do you mean by the motor "kicking in to high speed"?
Motors have one speed unless you have some sort of speed controller installed.

The first thing I would look at is what broke the 'tensioner' in the first place.....?
Point being that things usually happen for a reason, and your description of the problem doesn't indicate that the motor 'died of natural causes'

frenchsparky

What you should do with this if this is a belt driven usage.

for me what i will do is unhook the belt in first place and turn the shaft on the motor itself first to feel the bearing is binded in first place if not bind at all or feel rough then start the motor without the belt on to see if the motor can spin up easly without any effort if so.

then check out the voltage when the motor is running unloaded the voltage should be pretty much the same + or -5% max and the amparage should be the same along too


what is your voltage supply is at 208 or 240 or 480 ??

the way you mention the connecton on the motor that i am sure that on 240 volts

for 20 hp Motor if run propely it will read about 53-55 amp range @230 volts

what kind of starter you have there ??

DOL [ driect on line ]
S.S. [ soft start ]
autotransformer starter
other ?? { VFD or other }


btw the 3 phase motor dont have any starting windings they are self starting not like single phase motors are


the other thing i am sure you may know this or not is the speed the same as old motor it ran before ?

and there is a trick i done that once a while but i save it for last restort if the current is really unbalnaced is i read the current on each legs at the motor itself if want to know i will expain it later


Merci, Marc

CE1

You have a overload somewhere or have lost a phase. Remove the load from the motor if you can and check the amps on all three phases. A 20 Hp motor connected to 230 VAC should draw about 45 amps full load and about 15-20 amps with no load.

Carry On!

B-MAN

Turns out that when the motor was rebuilt they put a Wye diagram sticker in the j-box indicating that it was a Wye wound motor. I took the leads apart and tested them and found out that it was a Delta wound motor, WTF. Hooked it up as a delta 2,8,4 - 3,5,9 - 1,7,6 and it fired right up to speed with no problem. I thought there would have been damage from the improper connections but there doesn't seem to be.

frenchsparky

good you found it if it ran on wye connected you will really reduced the voltage by 1.73 [ sq root of 3 ] and that compounded by increseing current as you can see it.

if that happend very brieif it should not do any side effects at all but for long term it will overheat the winding pretty easly

Merci , Marc