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Phase converter for a k&t 315 ch?

Jemert

Plastic
Joined
May 6, 2018
I am looking to power up my mill but discovered it has a bigger motor in it than it is supposed to have. The mill is supposed to have a 15 hp motor but instead has a 20 hp. My dilema is that there is a 30 horse phase converter for sale that is a third of the new listed price. I have been told that the phase converter needs to be twice the size of the main motor on the mill, so it would have to be 40 hp phase to start the oversized 20 hp on the mill. Is that accurate advice or do you think i could get by with the 30 hp phase converter?
 
The 2:1 thing is not a hard fast rule in my experience. I have built a few converters and there are a lot of variables that factor in around the construction of the converter and the load on the motor that you are trying to start. Starting a mill motor is generally not going to be under a big load like potentially an air compressor for instance. This can be a help to you. The design of the converter is another thing. I used motors that had limited slip (from synchronous speed), trying to use a 3600 rpm motor if possible. Using balanced capacitance in the run mode also helped. This required experimenting with various loads on the converter. If you are buying a commercial unit I would pick their brains to find out as much as possible about their construction design and what they would recommend. There are literally thousands of threads about converter construction and lots of info is yours for the digging through.
 
I think you’ll be fine with the 30hp. I use a 20hp converter to run my 15hp Blanchard grinder without any issues. It fires up within a few seconds.
 
A phase converter is a large subject, there are lots of possibilities.

Consider no phase converter:

* A 3 phase motor will run on single phase if it is already spinning.
Some people rig up a pulley that they can wrap a rope around to spin
the motor just like starting a lawn mower. If they spin the motor
fast enough and they throw on the power at the same time it will speed
up to normal operating speed. This will give about 2/3 of rated horsepower.

* You can also rig up a so called "pony motor". This is a small
single phase motor that spins the main motor so the power can be
applied. It acts like the starter in your car. Easier than the
pull rope method described above.

Various type of phase converters:

* Static phase converter. This is the cheapest type of converter.
This is just a capacitor in a box with a relay that disconnects the
capacitor after the motor starts. This relay is either a timer set
for 1/4 to 1 seconds. Or the relay detects the initial current surge
and closes connecting the capacitor. When the current drops to normal
levels the relay opens. With this system the motor will give about
2/3 of the rated horsepower.

* Dynamic phase converter. These look like a motor and a box bolted together.
They are somewhat like a static converter and an ordinary 3 phase motor combined
in one unit. They often have additional capacitors to improve the quality
of the 3 phase power. The main motor will produce more than 2/3 of it's
rated power but not full power unless carefully tuned. This tuning process
is very difficult and requires special equipment.

* VFD (variable frequency drive). These are the best of all. They give
very good 3 phase output, the motor will give full rated horsepower.
They provide soft start and variable speed. They will protect the motor
in an overload condition. Today they are generally cheaper than a dynamic
phase converter.

Hope this helps.
 
Thank you guys so much for the helpfull information on the converter sizing. I missed out on the deal with this particular phase converter but i will use your helpfull advice with purchase of one when the time is right. I guess i need to do a little more homework on this subject.
Thanks again!
 
Another point with VFD's is that you don't necessarily need one as large
as the motor size. You need VFD large enough to spin the motor up.
Remember it does not take 20 HP to spin a 20 HP motor up to running speed.
Since a VFD has slow start capability there is no initial surge like
throwing the switch in a traditional setup. It may only take 1 HP
to spin your 20 HP motor up. Obviously you cannot get more that 1 HP
out of your 20 HP motor in that case.

If you don't need 20 HP out of your mill you could buy a smaller VFD.
Depending on how you use your mill you may only need 5 HP or less.
Also there is no problem if you happen to overload the VFD by using
more than 5 HP of power, it will just shut down. You then just reset
it any no harm done. Not like in the old days where you would blow
a fuse.
 
Another point with VFD's is that you don't necessarily need one as large
as the motor size. You need VFD large enough to spin the motor up.
Remember it does not take 20 HP to spin a 20 HP motor up to running speed.
Since a VFD has slow start capability there is no initial surge like
throwing the switch in a traditional setup. It may only take 1 HP
to spin your 20 HP motor up. Obviously you cannot get more that 1 HP
out of your 20 HP motor in that case.

If you don't need 20 HP out of your mill you could buy a smaller VFD.
Depending on how you use your mill you may only need 5 HP or less.
Also there is no problem if you happen to overload the VFD by using
more than 5 HP of power, it will just shut down. You then just reset
it any no harm done. Not like in the old days where you would blow
a fuse.

Using a significantly smaller VFD is not likely to work, since VFDs have circuitry to monitor the feedback from the motor, in terms of current absorbed, phase shift, etc. A big motor has characteristics significantly different from a smaller one and, most likely, the VFD would interpret the feedback from a larger motor as a fault situation.

Paolo
 
Using a significantly smaller VFD is not likely to work, since VFDs have circuitry to monitor the feedback from the motor, in terms of current absorbed, phase shift, etc. A big motor has characteristics significantly different from a smaller one and, most likely, the VFD would interpret the feedback from a larger motor as a fault situation.

Paolo

That is certainly possible. However, the typical VFD is designed as a general
purpose drive. As such it does not know a lot about the motor that it is driving.
It cannot make assumptions about the electrical characteristics of the motor.
A general purpose VFD has to be able to drive all kinds of motors: a motor
that matches the HP rating, a motor much smaller than the HP rating, a modern
motor, and an antique motor. All of these examples will have very different
characteristics. A general purpose VFD motor has to drive them all.

The case where the VFD is driving a motor larger than it's rated HP will
probably not be that different. All that the VFD needs to do to protect
itself is to insure that the current flow does not exceed the rating of
it's electronics. It really does not need to worry about the specified
HP of the motor, only the actual HP load at the moment.

That said, you may be right, it may not work. VFD's are not that expensive
these days and getting cheaper as time goes on. If it were me I would
get a 5 HP VFD and give it a try. If it does not work the VFD could be
used elsewhere.
 
All the VFDs I have used so far have a configuration menu and each of the many parameters has a default value. As far as I recall, the rated power of the driven motor and the rated RPM are among them. I don't think the VFD will accept a value greater than its rating.
The result, I believe, will be the VFD interpreting the excessive current absorbed when starting even without anything connected as a locked rotor condition or, anyhow, an overload.

As far as I know, a VFD rated for 5 HP motors using single phase input (most of the large VFDs expect 3PH input, some of then could be used with single phase, derating them at ~67% their 3PH rating, others will throw a fault for missing input phase) are still at least $200 or more.

Paolo
 
For what it's worth, I have a home brew 5HP rotary converter which has never had any problem starting either of 2 7.5HP motors I have had on a shaper. Of course, the shaper has a clutch so not much starting load.
 








 
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