AboutWill Expertise Three phase electic motors, controls and related problems or failures, three phase motor installation issues, performance issues, connections, data and duty cycle information. All other electic motors. Specialty motors, repair concerns, performance concerns, obsolete motors and solutions. Other specialty equipment issues. Lost nameplate data and identification, lost connection data.
Also DC motors of all types.
See my profile under Home/electrical at this site
Experience 30 plus years in the electrical motor and apparatus repair industry. VP level management of repair facilities, current owner of my own specialty repair and consulting firm.
Organizations EASA, IBEW [retired], other specialty organizatons, Lubrication, Vibration EDI, Triboelectric Councils
Publications Currently fielding concerns at this site under "Home Electrical"
Education/Credentials 4 year technical, College level specific courses, EASA repair courses, vibration analysis electronic and electrical trade school.
Expert: Will Date: 5/24/2008 Subject: Unbalanced loads on 3-phase motors
Question My question, at least my initial one, relates to unbalanced amperage on my 3
phase motors. The "problem" was first detected when I was trying to size
overload protectors in a magnetic control on a recently acquired piece of
equipment because I kept tripping the overload on T1. Overloads were sized
correctly for "average amperage".
All of my machines are 3 phase with the manufactured leg from a rotary 3
phase converter sized for the largest motor. Each individual machine is used
only by itself, i.e., with no others in operation.
Data (all old Oliver wood working machinery, 1950's and 1970's vintages)
1 hp disk sander, 220V 3 phase
L1, 5.9 full load amps
L2, 2.9 full load amps
L3, 3.2 full load amps (manufactured leg)
Average 4.02 amps, high leg 47% higher
I discovered the imbalance on this piece of equipment trying to size the
overloads. For the record, all are Allen Bradley controls.
5 hp jointer
L1, 7.5 full load amps
L2, 5.9 full load amps (manufactured leg)
L3, 12.5 full load amps
Average 8.7 amps, high leg 101% higher than average
3 hp planner
L1, 10.5 full load amps
L2, 6.6 full load amps
L3, 6.4 full load amps (manufactured leg)
Average 7.8 high leg 34% higher than average
I have lots of questions, and you probably need lots more information but
this gets me started. Obviously I need to size the thermal overloads,
however, AB can help with that. Again, the problem started when I tried to
start up and keep running the disk sander which appears to have the least
imbalance. However, should I be really concerned about the unbalanced
difference in the full load amps across these various machines (motors)?
The output on the manufactured leg is pretty consistent at 250V, the two
single legs are right about 125V@, so the input is pretty consistent.
Just for the record, I am measuring the amperage at the incoming side of the
line, i.e., as it connects to the motor controller. Is this the correct spot. Or
does it really make a difference. I'm obviously not an electrician.
Let's start with this unbalanced issue. Does this concern you. The planer and
the jointer seem to be running and work just fine. However, am I going to
damage something? I'm a little bit hesitant to go any further with the disk
sander in terms of stepping up the overload protection until I feel
comfortable with this imbalance issue.
Any advice/help will be greatly appreciated.
Regards,
Eric
Answer Eric, First of all I have to say I get a lot of good questions as volunteer on this site, and then I get a lot of lazy questions, in other words some people get some idea they can write in for free, get 30 years of experience and expect them to resolve all their issues.
And if I do not solve all their issues I get feedback such as "that was not the answer I was expecting".
In particular to one writer who wanted his generator fixed and or information, and knowing that there is NO information on that generator, I sent him/her to a site that has books and the only book that has connections and manuals, a collection of those type machines, all of $19.00 something, I have no connection with it, get nothing from it, but if he wants info, that is the way to go.
But he/she didn't want to hear that, this person wanted an answer that suited them, cost them nothing, and expected me to tell them to turn a screw one turn and all his problems are over.
Then I get your question, and it goes in the top ten of well done, well thought through, full of effort, full of information, and a lot of work and study on your part.
Nice job.
OK to your question. The balance of three phase current is actually more critical than the empirical value, unless of course the value is twice nameplate.
Here is your problem, a 1 unbalance in voltage applied to an electric motor can create as much as 10 to 50 unbalanced current.
You mention the manufactured leg is constant at 250 volts, and the two single legs are 125 volts.
Explain that to me, are you saying that line to line past your converter you are getting 250, 125, 125 volts?
You need to measure the voltage line to line, unloaded and loaded with whatever past the rotary convertor and tell me what you measure.
1 is about the max anyone wants to see, although utilities have a difficult time getting there, but running a motor over 5
deviation line to line is not recommended.
This type unbalance creates all kinds of problems, as you can see by the running currents.
It creates circulating currents in the motor, heat, loss of horse power all kinds of bad things.
Because this is happening to everything you plug in to the converter I would suspect the converter is to blame, either bad, or a high resistance connection somewhere, but something is horribly wrong, and it is creating a VOLTAGE UNBALANCE which is giving you all these problems.
Once you fix that issue, over current protection will be a snap.
One more note, older motors tend to not "like" over voltage compared to nameplate, while newer motors seem to do OK or even better with say 500 on a 480 rating.
Older motors seem to be able to stand some over voltage and then the opposite is true, lower voltage sometimes makes them run better, say 201 volts on a 220 rated motor.
Your older motors if rated at 220 volts leaves you the rule of thumb, 10 over or under BALANCED> 220 at 10 over is 242 volts, the 250 is probably too high, but check it under load and see what it drops to.
As far as clamping these motors or taking line to line to line voltage readings, do it as close to the MOTOR as you can, and inside the connection box, past the connections is perfect, that tells you if you have something going on with the connection in the box, and all the way back.
NEMA STANDARD Mg 1-14.35 will tell you all about voltage unbalance and that is exactly what your problem is.
Keep me informed on what is going on, service@emrrepair.com is my work e-mail feel free to write there it is easier.
We can then talk about the rotary converters, static converters, and what to do next to fix this problem.
Regardless of what everyone says, a rotary is no better than a static converter, I have hundreds if not thousands in service, and not ONE has come back had a problem, destroyed a motor or any problems of any kind.
static converters that are single in, and three out, also have some added benefits such as built in over current protection, speed control , torque control, ramp, reversing, dynamic braking, all of these parameters are easily set on a keypad, can be accessed remotely in a variety of methods, and are not 500 pound hunks.
A decent rotary set up with the proper capacitors and sized right are good too, but most that you find at converter.com are nothing but tricked out motors and sized for what you need not what they are.
So go measure the voltage and tell me what is going on, you are slowly killing these motors with that kind of amperage unbalance, created by voltage unbalance from somewhere.
