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Electric Motors/troubleshooting 3phase motor


I have an industrial 1940's-1950's 3 phase 24" planer. When we initially
set it up with an Arco rotophase converter to generate the 3 leg. This leg would be L3-T3 blue wire. this is located in a separate starter switch. This magnetic switch is has taps which has T1,T2,T3 and corresponding L1,L2,L3. This motor a1.5HP is single speed and drives the feed rollers. The other motor a 5Hp drives the cutter blades. and has a separate switch.
 When hooked up the bigger motor works, and there is power to the smaller motor in the separate box. The drive motor hums and does heat up I have stopped the switch before anything happens. There is no binding on the motor with the pulley spinning free. I suspect the motor may need new brushes or its not hooked up correctly. Broken wire? some where in the switch? the power source is there for the third leg. Any solutions?

Some phase convertors only provide a third leg to obtain rotation then drop out,  the rotary type should provide a constant and somewhat balanced third leg constantly,   
Jonathon  there is always a solution

However the sizing is often misleading,   for example     they will say    [just using numbers as examples not detailed relevant amps to HP etc]

so they say unit A is     10 HP with a 1.25 service factor,      which would mean up to 12.5 HP could be the max load,    

Then they will say unit B 12.5 HP capacity with 0.0 service factor,   and often charge more for this unit as it is rated 12.5 HP     

The rip is they are the same identical machines,    

If you go here

notice on the right  middle     it says  send in form for multiple motors,

I don't know what unit roto unit you have and am not accusing roto of this misleading sizing    but it happens with convertors all the time,  

Motors do not like unbalanced voltage     three phase motors always vary   by manufacture by age  by speed by efficiency    

If a three phase induction motor sees unblanced voltage  say  230 239 229   it is the average of all the voltages   then the motor can demand often up to 10 times CURRENT unbalance for every 1% voltage unbalance      and that is a huge problem

And that varys  some motors are more sensitive to voltage unbalance than others,   

The older the motor usually the more forgiving but not absolutely       

If the roto unit is sized too small     that could cause this,   if anything in the wiring has a high resistance or not tight connection  that is a possible problem   

It sounds like the rotophase is just too small for the inrush of both motors,   

You need to measure the voltage at the motor line to line to line     if there is a huge unbalance  it must be corrected,    if the voltage is way low or way high   that needs correction,    

but you have to know what the motor is seeing to make conclusions      

So you first need a volt meter,    then a clamp on amp meter,      

Ideal is 460 -460-460  or whatever it is producing,    but one or two volts is not usually a problem  but 10 volts might be      

Then if the roto phase is too small in capacity   if both motors try to start at the same time,  the roto must be sized large enough to start both     if they dont start at the same time sometimes a smaller roto will get by  but not often,  

Im a bit confused which is the main and which is the smaller and which is the older,    

But it sounds like the rotophase is too small    in HP capacity     

First check your voltages line to line to line      if all those are OK   then look to the size of the rotophase,      old motors often need more inrush current to start      

If you isolate the motors will they start one by one,?

I need the roto data  and the motor nameplate data of both motors,     you check the voltages and get in touch with roto either by form or call them   

YOUR better bet in my opinion  is a FREQUENCY DRIVE  that takes single phase and converts to three phase  I have installed thousands with no problem   

VFDS only go so big that convert single to three phase,     but I believe they have up to at least 10 hp  in single to three phase,

I would look at either one large VFD or two sized to the motors     one for 5HP and one for the 1.5      it gets complex when you try to provide fabricated three phase to muliple motors,    It sounds like you have a constant three phase producing roto unit,  not a start only three phase unit,  and rotary units are pretty tough if sized right    but if they are too small even though the HP rating is on the money it is the starting KVA that is lacking that is why the motor sits in locked rotor

By the way the highest current a motor can demand is locked rotor    locked rotor is just at stand still    no matter what the load is or no load   the magnetic power needed to turn the rotor has nothing to do with load or no load,     now if the load is heavy and drags the starting motor from locked to speed for too long    that is a problem   different but a problem,        

So id you will check all voltages   correct any unbalances      check with roto by providing the motor data,     and how they interface,        then we will meet in the middle and decide the nest course of action,    

Dont forget to run each motor individually with nothing but ONE motor at a time      if they each start   and then wont when they are both fed,   it is 99% too small of a roto    

If the motors are dual voltage 9-12 leads make sure you have the connections correct for each motor,   it is easy to wire a delta when it should be wye    and no way for me to tell what they are  there should be a connection diagram on each if not,   when we check all else  I can teach you how to use an ohm meter to determine if the motor is wound wye or deltaa internally          

Here it is in case you have no connection plate,  dual voltage   motors,   

Wye wound  will ohm  out     1-4   7-8-9   then 2-5    7-8-9   then 3-6  7-8-9

Low voltage probably 230 or 220 or 240   would be line to 1-7  line to 2-8  line to 3-9  connect 4-5-6 in a bundle,   

High wye is the same as high delta   so note that

Wye or delta wound    line to 1   line to 2  line to 3    4-7 connect tape off   5-8 same  6-9 same

A delta wound motor will ohm out  1-4-9      3-6-8     2-5-7

low voltage delta connection would be line to 1-6-7   line to 2-4-8  line to 3-5-9

high voltage is the same for both wye and delta  

Make sure you have those right if you have more than three leads       

Check your voltages,  run each independent of each other    contact roto with the nameplate data     then send me the roto model and any specs  and the specs and connections of each motor a clear photo of the nameplate will work  and a clear photo of the connection diagram will work          then we will compare what roto says about your two multiple motors   and what I see,       in there somewhere is the solution          

A delta wound dual voltage will ohm out  

---------- FOLLOW-UP ----------

QUESTION: thank you for your answer. I don't think the roto phase is the problem. it does have the capacity to handle 3-4 hp motors. and provide constant. voltage ie 3rd leg. there are three commercial machines hooked into the roto phase separately. The planer had no problem I its operation. until the starter switch for the roller feeders kept on dropping its connection. it is a magnetic switch with heaters. when this box was changed out with an another box, then the drive motor  quit or just sat there and hummed. Is it possible that iam missing a connection sequence ?

Jonathon,  sure it could be in your connections, but you seem to sharp for that to happen.

The key to ANY induction motor is the voltage AT THE MOTOR    Now how all this sequences I cannot be sure of,  but I am a firm believer in starting with the simple stuff   that is often overlooked,          why was that box replaced?

What I am hearing is all was fine until that box was replaced,      this happens all the time   a component in a system fails,   there is no root cause evaluation and the same problem is repeated.

Can you jump around the "system"  and run that motor right across the roto phase?    Without a bunch of time and trouble?

If so do so,   that will take the motor out of the equation,    

Any chance somewhere in the box the motor is seeing a single phased condition?

I would verify the motor,   just in case something internal or a dual voltage connection in the connection box did not come loose,   look for something simple,          and give me the voltages at that particular machine,  line to line to line,   

It can be tricky trying to run three phase motors through a convertor,   you get back emf and all kinds of other issues,   that are not present when dedicated three phase is present,    

So two things,  voltage at the motor,        then verify the motor will operate by itself less overloads,          in there you will find the issue

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Three phase/ AC DC single phase motors, controls, any problems or failures, motor installation, performance issues, connections. All other electric motors/gearboxes/apparatus. Specialty repair concerns, obsolete motors and solutions. Other mechanical or specialty equipment. See my profile under Home/electrical at this site


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