Fractional HP motor wind design 1

[smitty1322]

I am currently working on a design for a customer (construction OEM) that used to buy SKIL handsaws and cut the handles off. They have asked my company to design a 115 VAC, 1 HP complete motor for them.

The problem I have is in the design of the field. We have developed a motor and the customer has tested it. The feedback we received was that the skil motors would "burn up" but when bogged down they would power through. The customer could not burn up our motor but it would stall.

While designing the motor, one difference was obvious. The skil saw had two separate coils on each side, one larger to wrap around the armature and one smaller one inside this larger one. Our field laminations only allow for one coil.

My questions are what this smaller coil actually adds to the performance? Does the smaller coil allow for a stronger field? We have increased the wire size, to try to get more low end torque, as much as space constraints will allow. Any suggestions on what to try next?

 

[teco]

smitty:- please advise speed of the machine.it sounds like you have a stabilised shunt machine.
regds

 

[smitty1322]

Teco,

Here is more information for you. The motor (armature) is going approximately 17K RPM. It goes through a 3.4:1 gear reduction. The laminations have 16 slots and the commutator has 32 bars. All of this is pretty much the same between the hand saw and our armature (rotor).

The problem I run into is the design of the field or stator (I think almost everyone else in this forum is used to the BIG motors and we mainly deal with 12-115V little guys). The hand saw field, as I said, had two separate coils on each side, one small & one big. Now our field lams obviously allow for the bigger coil, but that is all. It seems, by customer feedback, that the smaller coil is filling some void in the magnetic field as it has more low end torque. That, essentially, is my question. What is that smaller coil actually doing? Why does the handsaw motor have so much more stall torque when the armature/rotors are almost exactly the same? Does the smaller coil allow for more points of perpendicularity of the magnetic field?

 

[teco]

smitty:- the answer to your question is yes ,the smaller coil is acting as a inter pole and like a sright series dc motor (traction ) will produce more torque.to produce more accelerating torque try incresing the wire dia on the field coils but keep the number of turns the same. this will increse the field stenth and produce more torque.however you may loose speed.
good luck.
kind regds.

 

[smitty1322]

Teco,

Thank you for your response. Could you elaborate on how exactly the smaller coil acts as an interpole? We have already filled the field lamination with the largest wire size we can wind with. So we can't get any more low end torque than we have right now.

Which brings me to another question. Is there any way, mathematically, to tell whether you would need an interpole? All of our field laminations allow for only one coil. And if you find you need an interpole and you can't physically wind one, as we do with this design, is there anyway to have your cake and eat it too? Some trick of the trade to use when all the nips and tucks with # of turns and wire gauge has been completed? Some special turn ratio?

 

[jbartos]

Suggestion: Visit

http://www.swcoils.com/

for more info.
It appears that interpoles are more appropriate for dc motors.

 

[teco]

smitty 1322:- inter poles are only used when bi direction is required,this also assists the main feild winding and reduces sparking at the commutator on small universal motors.
it appears that your motor (being a series universal motor )is running out of torque under heavy load conditions when the skil motor powers through.
having used the biggest wire size with no effect we can only suggest to either (a) look at changing the output speed, if you reduce the output speed the torque deliverd will go up (kw x 9544 / speed)this will require a gear box ratio change.(b) rewind motor for a higher output power (less turns and larger wire size on arm and feild) however be aware of a higher thermal rise in the armature and feild coils. this is possably what skil do. its quite common practice for short time rated machines.
when the skil motor burns out is it the feild coils or armature or both that burn out ?, on your motor at what point dose the motor stall? are you still using a gear box with the same ratio?
to get back to your questions,no you cant have yor cake and eat it, the lamination stack has to designed to accept an inter pole but as peviously mentioned this is only reqired for bi direction rotation.torque is propotional to voltage sqrd or current sqrd to obtain more torque output either of these have to be alterd.we would reccomend option (b)from above and base your new calcs on 95,000 lines per sqr inch 35 amp-turn per inch.
should you requre a copy of design calcs please forward either fax or email address.
kind regds.

 

[smitty1322]

teco,

the posts have been quite helpful but I don't really have an answer yet. If interpoles are only used in bi-direction, why would they use it in a saw? Saws only turn one way. The only, and I do mean only, difference between our motor (armature and field) and theirs is this lamination design.

This motor is driving a saw. The skil saw when it got bogged down cutting, would just pop up through, climbing out of the cut. Our motor, when bogged down, would just stall. Both motors get bogged down to where they won't cut anymore.

So my question is what exactly does that interpole do to the electrical performance of your motor? Does it increase effeciency, torque, or field strength? Does it decrease amp draw, terminal resistance? If someone could give me a reference guide to building universal motors from the ground up that would be what I am really looking for. Anyone who actually took Motor theory in school, what their text book was? I do appreciate everyone's input, it has made me want to know as much as I can about this problem.

 

[teco]

smitty1322:-
if wou wish to give us a phone call on 006492763639 we would be glad to talk over your problem. bearing in mind we are +12 hours ahead of g.m.t.
kind regds