Interesting Motor rebuild (maybe) 1

[DBCox]

Hello everyone,

I have a question that hopefully you guys can help me with. I am a mechanical engineer and like to dable in some electrical stuff, but am no good at it. Anyway, I need to re-build a small DC motor. Here is the scenerio:

I have a vehicle modified for a wheelchair that uses a gull-wing style door. The fabricators that built it used a window lift motor from a lincoln continental to lift the door open as they are easy to find and relatively cheap. If you are not familar with window lift motors, they are small DC motors (~3" OD and 4-5" long) with a worm drive reduction.

In all reality, this motor is not big enough to lift the door reliably with the current gearing and frequency of use, but I am stuck with all of that due to size, time, and $$ constraints. I replace this motor ~1 time per year as it will usually wear out in 13-14 months to the point it will not lift the door all the way.

The problem is, the motor is no longer available new, only refurbished. I have tried three different brands from cheap to expensive and non of them are capable of lifthing the door all the way. So, it boils down to the remanufactured motor does not have as much torque my old motor that is wearing out quickly. All of the gearing is the same.

I have three options:
1. Add assist to the door (like a gas shock found on rear hatches)
2. Change the gear reduction
3. Rebuild the motor for a higher torque.

Options 1 and 2 are out due to space constraints. I have no idea how to rebuild the motor for a higher output. I am sure it is possible. This is an intermittent use, so heat will not be a huge concern. My biggest concern is current; I only have 20 amps to work with unless I re-run all of the wires...

So, is it possible to rebuild the motor to a higher torque spec?
If so, how is this done?

Thanks!

 

[roydm]

How much trust and travel do you need?
It would definitely help if you could remove the static load with a gas shock.
I recently used some Linak actuators with 130 mm (5") stroke rated 750 Newton.
They are quite cheap, check out

http://www.linak.ca

Good Luck
Roy

 

[DBCox]

Roy,

Thanks for the information. A linear actuator will not work unless I completely rebuild the hinges and surrounding structure, which just isn't feasible for this truck. I could use gas shocks to help it, but again, I would have to fabricate all new brackets, cut covers to make clearance, ect. So, the easiest thing, if possible, is to rebuild the motor for higher torque. I know the motor should be able to supply the torque, given the proper internals because I have been buying these motors for 4-5 years with no problem until now.

Thanks!

 

[Mobius44]

First I recommend you double-check the load hasn't changed, rebuilt motors usually have just as much torque as new ones, so it's strange that none of the motors are working.

If you want to rewind your DC motor thats more than what someone can explain in a text post, so try searching your library or googling for some instructions, or take the motor to a local rewind shop. In simple terms motor output is related to amp-turns, so increasing the number of windings (or turns) provides more torque, as does increasing the amps.

You might add a 6V or 12V battery in series to create 18-24V, the higher voltage will create more amps (and heat!). If you try a higher voltage then make sure to verify the amp draw and motor temp remain safe.

 

[waross]

In the real world, there often isn't room to increase the number of turns on an armature without using a smaller gage wire. If you double the number of turns of the same gage, you also double the resistance. Twice the turns equals half the current. The amp turns remain the same. The amp turns is more a factor of wire gage than the number of turns. Fewer turns of larger wire will increase the current and will increase the amp turns.
However, if this is a permanent magnet motor, the torque will probably be limited by the strength of the magnets. Rewinding or increasing the voltage may be pointless.
Again, change your drive ratio or re-evaluate what you are willing to do to solve this problem..

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Rewinding a motor like that has to be about 3X as hard as any other modification. And is probably 1/4 as likely to exceed.

Motors like the one you describe are engineered to the very limits of materials to keep the cost at the bare minimum. Remember a penny saved can mean several million dollars to a car manufacturer. Even if you somehow increased the motor torque you would next be asking what to make the shaft out of because it would fail, or the bearings will fail, or the worm will strip.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[btrueblood]

Blak Raven,

Two motors?

 

[DBCox]

Thanks for the information guys.

Btrueblood, I did think about two motors and that probably would be easier than a spring/shock assit or any other changes.

However, I am still leaning toward rebuilding the motor. Not because I do not believe you guys when you say it is difficult, but because I have been buying the same (supposedly) motor for years without a problem. The load has not changed and the old motor that is in there still works (just slowly), so I KNOW the new motors do not have the torque that even the old motor has, much less the torque the old one had when I put it in.

What would cause a motor to loose torque over time anyway? Are the magnests failing? Obviously the winding is not changing. My little knowledge of motors leads me to believe it would have to be the magnets, bearings, or brushes.

The reason I keep coming back to rebuilding this is I have used half a dozen motors that are supposed to be identical that have all worked great for about a year each. I first thought I just got a bad motor when I replaced it due to bad luck, but I am on #4 now, and none of them have worked... The new (reman.) motors simply do not have the torque the old ones did. I want to duplicate whatever the manufacturer did to the old motor (either to it or the new one).

I am not going to attempt this on my own. I will certainly take it somewhere, but I am trying to get an idea of what I need to tell them to do/replace.

Thanks!

 

[itsmoked]

Got a picture of this motor and the application point?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I suspect that your motors are permanent magnet motors and that the magnets are losing some of their strength over time. You can't fix that with a rewind. That would also explain the poor performance of the remanufactured motors. Weaker magnets would not be an issue unless maximum torque was demanded as in your application. Before rewinding, try running 15 or 18 volts to the motors and see if the torque improves. If a higher voltage helps, then a rewind may help. It is an easy test and if you prove me wrong, so be it, you will be the winner.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DBCox]

itsmoked and waross,

Thank you for the information. A picture of the motor can attached. Unfortunatly, I cannot get a picture of the application right now as it is behind several covers and in a tight spot. The next time I take it all down to replace, I try to get some pics.

Waross,

I believe it must be the magnets. I have not set anything up to run 15-18 volts to it, BUT, it runs a lot better with the truck running, which brings the voltage up from ~12.8 to 14.4 VDC... Sounds like a re-wind may be the ticket, now its a matter of finding someone who knows how to do it in my area...

 

[waross]

Have you checked your voltage drops? measure the voltage at the motor and at the battery with the motor running. Check the voltage from the positive battery terminal to the positive motor terminal. Likewise from the negative battery terminal to the negative motor terminal. If you are seeing more than about 0.2 volts drop in the wiring, try larger supply conductors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

That's with the motor running... With only 12V nominal wire drop has a huge impact on available torque.

Thanks for the picture blakrapter.

There are an awful lot of those same type motors around. I have two for a project myself. Perhaps you could just use a different one. Surplus you are talking ~$50. I cannot see you getting someone to rebuild any motor for less than about $100.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BobM3]

Waross -

I don't follow your reasoning that doubling the number of turns using the same wire gage would get you the same amp-turns. Wouldn't that only be true at stall (where current is only limited by resistance)? If the armature is spinning then back emf is the main impediment to current so, for instance, doubling the resistance (and number of turns) does not halve the current.

Blakraptor -

You might try a close fitting steel tube around the motor housing. If the existing motor body is thin then it might be limiting the flux density. More iron would get the flux density (and torque for a given current) higher.

 

[rtronics]

Question: Does rewinding a PM motor also include flashing the magnets to restore initial magnetism?
Regards,
Ray.

 

[waross]

My comments regarding the number of amp-turns was directed at magnets in general.
Consider a coil wound with 100 turns of wire. The average resistance is 1 ohm per turn. the applied voltage is 100 volts.
100 turns = 100 ohms. 100 ohms at 100 volts = 1 amp.
1 amp times 100 turns = 100 amp turns.

200 turns = 200 ohms. 200 ohms at 100 volts = 0.5 amp.
0.5 amp times 200 turns = 100 amp turns.

As the number of turns increases, the inductance tends to increase, the losses decrease and the amp turns tend to remain the same.

For more amp turns use more voltage or use a conductor with a lower average resistance per turn. That usually means a larger wire, or changing to silver or to a super conductor.
Larger wire is more economically feasible.

At stall or close to it, back EMF is negligible. The lower resistance will permit more torque which will result in greater speed which will increase the back EMF.

The initial current through a series motor is determined by the motor resistance. The torque is dependent on the current but may be limited by magnetic saturation.

The initial current through a Permanent Magnet motor is determined by the motor resistance. The torque is dependent on the current but it may be limited by the strength of the permanent magnets or by magnetic saturation.

In this instance I suspect that the strength of the magnets may be limiting the torque but I am not sure. The torque may also be limited by magnetic saturation of the rotor. If the motor develops more torque at a higher voltage, then a rewind may be successful.

This is a special case where space is limited on a slightly less than marginal design. I would look for another option than rewinding the motor, but it's not my motor. If a higher voltage test shows that the torque is not being limited by the magnet strength or saturation, and the owner wants to spend the money on a rewind, that is his choice and I can't say that it is a wrong choice.

Question: Does rewinding a PM motor also include flashing the magnets to restore initial magnetism?

I usually give the magnets to a small child to play with and throw the rest away. It is not usually worth the time to service small motors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DBCox]

Hmm, thanks for the suggestions guys. I will see what it'll cost to re-wind it. I will also try the steel tube trick and see what it does.

The biggest problem with going with a different motor/gearing/etc is that I will have to take a lot of time and some $$ to do. I have a new vehicle on the way so I cannot see dumping a lot of $$ and time into this one. Not to mention, when that truck doesn't go due to repairs, retrofits, whatever, I cannot go anywhere. So, from a time standpoint, I am MUCH better off to get a replacement motor with the same bolt pattern, gears, etc than changing everything up for a different motor. I guess that I could just keep taking the motor back and trying new ones. I will probably eventually get lucky with 1.

 

[nevjersey]

On the idea of sticking with a motor or 2,
you might get in touch with Jim Husted
of Hi-Torque Electric. Maybe you could
find a way to run the motor at 24V?
Jim would know...

His current fame is for big motors,
such as the siamese twins that
drive the quickest electric car.

http://www.hitorqueelectric.com/

White Zombie is here:

http://www.plasmaboyracing.com/whitezombie.php

(I'm not affiliated with either,
but Jim often answers my emails with
helpful advice

 

[DBCox]

Thanks for the information guys. I decided to order a gas shock to assist the door. After looking at all of the options, I finally decided that would be my cheapest and most reliable option. I will end up having to cut holes in 2 covers and build special brackets, but I think I can squeeze it in there.

The shock has come in, but I haven't had a chance to install it yet because it has been in the shop with the driving control down... If I can get those fixed, then I'll move to repairing/modifying the door actuator.

 

[itsmoked]

Thanks for the comeback blakrapter.

Oh and the interesting links nevjersey.

Keith Cress
kcress -

http://www.flaminsystems.com