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Capacitor start for jointer?

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Forum topic by ChrisCarr posted 05-14-2011 05:31 AM 1683 views 0 times favorited 11 replies Add to Favorites Watch
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ChrisCarr

196 posts in 1651 days


05-14-2011 05:31 AM

I am in the process of rewiring a 3 phase induction motor to single phase for my jointer. Do most of you guys have capacitors on your jointer’s motor? I know a jointer isn’t a tool that needs high start torque. Just wondering.

- Chris


11 replies so far

View Loren's profile

Loren

7826 posts in 2401 days


#1 posted 05-14-2011 06:07 AM

Rewiring a 3-phase motor for single phase? You mean you have a phase
converter or you are re-winding the motor?

Capacitors, as I understand it, are essentially batteries that hold an additional
charge and help the motor get started against the forces of gravity and inertia.

Most modern machine motors are either capacitor start or universal. Some older
machines with integrated direct-drive motors don’t use capacitors. I have
a few old motors lacking capacitors and they work ok in start-up.

-- http://lawoodworking.com

View Minorhero's profile

Minorhero

291 posts in 1358 days


#2 posted 05-14-2011 06:23 AM

Most modern motors are capacitor motors. Personally I think they are easier to work on because you don’t have to worry about the position of brushes when you take them apart.

However, it is really a non-issue. Capacitor start or induction, all that matters as far as the blades are concerned is RPM and HP and there is no difference there between different types of motors.

Are you using a VFD, static or RPC?

View ChrisCarr's profile

ChrisCarr

196 posts in 1651 days


#3 posted 05-14-2011 03:45 PM

I am re-winding the motor, it is 2 poles (3450 rpm). I have to add start windings so it gets a split-phase shift and starts, the capacitor isn’t essential it just gives a stronger startup, but my table saw motor uses capacitors for start + run.

A converter seems to be just a way to avoid re-winding and it seems more logical in the long-run to re-wind and not have to worry about another piece of equipment just to run my one motor.

Brushes arn’t used in induction motors. Only repulsion motors use brushes and they arn’t true induction motors like our tools mostly use.

Universal motors use brushes because they can only run on dc (direct current) power, so with brushes and a commutator they can run on AC (alternating currrent). Universal motors give much higher start torque than induction but arn’t as powerful in all or last as long.

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Loren

7826 posts in 2401 days


#4 posted 05-14-2011 05:38 PM

I know rewinding a 3-phase motor can be done. It’s allegedly costly
enough to have it done at a motor shop that most people either
get another motor or get a phase converter. If you know how to
do it yourself maybe it’s just a matter of you investing some of
your time.

I’m curious to know how the project goes for you.

-- http://lawoodworking.com

View ChrisCarr's profile

ChrisCarr

196 posts in 1651 days


#5 posted 05-15-2011 03:40 PM

To rewind a motor for single phase you have to know how an induction motor works. The basic theory is what you should learn first. I have no experience what so ever, but motors are the simplest thing once you understand how they work. You don’t need to know a lot about it, just how the windings are oriented in 3 phase and single phase and of how an induction motor work .

I don’t know why a motor shop would charge a lot. but then again they do have to press-out the iron core, unlaminate the core, unwind, rewind, etc. A motor has a load of turns.

I am going to document how i do this.

View EEngineer's profile

EEngineer

906 posts in 2366 days


#6 posted 05-15-2011 05:20 PM

I’m friggin’ dying here too. Sooo much misinformation in just a few posts.

Motors are pretty simple once you understand how they work. However, I haven’t seen that anybody here actually understands how they work.

Start here

More here

and that still won’t get you anywhere near being able to design a rewind on an electric motor. Designing motor windings is a lot more than just running wire. Rewinding a motor is a difficult, long process. That’s why motor shops exist.

I won’t even touch the description of universal motors and why brushes/commutation are needed; suffice it to say it is wrong. As for capacitors acting like batteries to hold additional charge? In an AC circuit? No.

Let me suggest a much simpler course. It is entirely possible to run a 3-phase motor on single phase. Just wire two of the phases to your single-phase source. You will have to start it. A rope around the pulley is enough to get it started.

In fact many of the simplest rotary phase converters used a three phase motor with a rope pull on a pulley to get it started. Since every motor is also a generator, the 3-phase motor running on single-phase provides three-phase power for any other motors hooked up to it.

Use capacitors to provide some phase shift and apply it to the 3rd phase that was left unconnected in the example above to provide self-starting. Use low grade “starting” caps and a time-delay relay to drop them out after the motor has enough time to come up to speed OR use higher grade “run” caps to provide a third phase continuously while running. Basically, your three-phase motor already has the “split-phase” winding you talk about adding for single-phase operation; you just need to use capacitance to provide a phase shift instead of higher winding resistance.

There are entire sites devoted to using three-phase motors on single-phase. Google is your friend.

-- "Find out what you cannot do and then go do it!"

View ChrisCarr's profile

ChrisCarr

196 posts in 1651 days


#7 posted 05-15-2011 09:12 PM

Eengineer, I get the feeling you think I am jumping into this project ignorant. I have done reading on these subjects the last several months. I am no professional or even experienced at this but from the information I have read it seems simple to atleast me.

I am choosing to rewind instead of use a phase converter because Its a flat out better solution. Rotary phase converters require an idler just to balance things out and that is not reasonable if you only need to run one 3 phase motor. Static converters, burn out the motor sooner or later and decrease power so i won’t use those. VFD’s are expensive and do much more than just what I need so its like buying a cabinet saw when all you need is a bench saw for the job.

Rewinding is the only practical solution.

View EEngineer's profile

EEngineer

906 posts in 2366 days


#8 posted 05-15-2011 10:32 PM

Well, yeah, your comments show a less than perfect understanding of how motors work. But, hey, have at it. I’m a big fan of “learn by doing”.

Static converters are a little more stress on the motor than 3-phase but I have talked to people who have used them 15 years on the same motor. Induction motors are tough beasts, you are not going to burn one out just because it is on a static converter. If you spend the time to balance the current in all three windings by sizing the capacitors right, the stress is little different than with three-phase. I have also talked to people who have run their three-phase motors on single-phase for years; much more stressful, still living. Yes, you don’t get as much HP out of the motor but you probably don’t have the room for windings the size you need to get the same HP single-phase anyway. One of the reasons they use 3-phase is that the resulting motor for the same HP is smaller. The reason is that they need windings of smaller diameter for the same HP. You do realize you will need heavier gauge wire for the same HP single-phase, right? VFD’s get smaller and cheaper every year but, yeah, who needs variable speed on their jointer or table saw?

Keep us posted with lotsa pictures. I know I’ll be following this…

-- "Find out what you cannot do and then go do it!"

View Florida_Jim's profile

Florida_Jim

66 posts in 1631 days


#9 posted 05-15-2011 10:44 PM

I don’t mean to be too critical. I’ve retired from the utilities business. I’ve worked with motors for thirty years.
I don’t think your idea is practical.
First, you need a form to wind your coils on. Motors are designed to operate on a certain number of coils, a certain number of windings, and right size wire on those coils. Deviations from the design will cause overheating, due to improper current draws. Also you would need fiber insulating spacers to install your coils.
Most importantly you need to completely dip you rewound stator in an insulating varnish, which by the way is not cheap. Finally after several dips, you need a large oven to back the stator.
Without complete rewinding the motor, I don’t understand how you’re going to rewind start windings into an existing motor frame. Even if you could, you’d have to have a method to disconnect the start windings after the motor is up to speed.
Most single phase motors either utilize a centrifugal switch, or a potential relay and a capacitor for boost to get the motor started. Then drop the start windings out of the circuit, when the motor is running.
I’ve dealt with a lot of smaller submersible pumps that were actually 3 phase motors, which were “derated” for single phase operation. The controls were actually a static phase generator, with start and run capacitors, and a potential relay.
As for rotary converters they can be pretty efficient. I’ve built many for commercial applications for sewage lift stations that large pumps, but only had single phase power available.
The key is to use capacitors to balance between phases.
I have a 5 HP converter in my shop. I use it to run a Bridgeport mill, a metal lathe, and a surface grinder.
Here’s my “RPC”.
Inside showing the relays, start, and balance capacitors
The outside of the panel
5hp Idler motor

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getlostinwood

224 posts in 1355 days


#10 posted 05-15-2011 11:02 PM

I just went through the same idea in the end it was way easier to buy a TEMCO 50. wired up in place of the switch, now life is great and done in a two hours.
http://www.factorymation.com/s.nl/it.A/id.196/.f?sc=2&category=32

Minorhero saved me dollars and pointed out the sense to going this route. there is a difference between DIY & DIYsmart. look into setting up a vfd. Feel free to PM me and I’ll give you my number if you want personal experience info

-- The basis for optimism is shear terror

View ChrisCarr's profile

ChrisCarr

196 posts in 1651 days


#11 posted 05-18-2011 01:35 AM

I just started slicing wire out. All 6 poles (4 slots on the laminated core = 1 pole) are 22 gauge ALUMINUM wire. I am replacing it with 22 gauge copper magnet wire. I think I will make my running windings/poles 26 gauge. I don’t plan on “baking” my windings, just shellac binding them.

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