Trying to run a treadmill motor without the controller

[Ricka]

Hi, all; I see that there some pretty knowlegeable people here so maybe someone can advise me:

I'm trying to run a dc permanent magnet 100v treadmill motor from house current via a bridge rectifier, not through the usual pwm controller. I'm using the choke that came with the treadmill and tried a 1200 mfd cap across the output of the rectifier to smooth the current, but I'm still getting too much sparking and a little smoke at the commutator. The rectifier shows 108v output, which I wouldn't think would be too excessive (I only intend to run the motor for about 15 seconds at a time). I also tried a good sized resistor to cut the voltage a little, but no difference. I already fried one motor. It lasted awhile (it's to raise and lower a milling machine table) but obviously something wasn't right. Any thoughts? Thank you very much.

Rick A

 

[NascentOxygen]

Hi Ricka, http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

You are rectifying 110V AC? This has a peak value of 155V, and if the choke is a good choice it averages out to 99V. The capacitor probably won't contribute much in the way of smoothing here. (Unless you have the cap on the wrong side, then you might have nearer to 150V out.) This might explain why the meter would show 104V.

The inductor may be the cause of the sparking, because each time the current in an armature coil breaks there will be a high emf. A capacitor should ameliorate this, if it has a suitable value. Can you try a 0.22 μF polyester cap instead of (or as well as) that electrolytic?

Also try omitting the inductor, i.e., short it out. The inductance of the motor might be sufficient.

Perhaps the motor is not rated for 100V?

 

[jim hardy]

i have run DC treadmill motors from bridge rectified household power (120V in US) with no ill effects.

it sounds to me like like you have AC across the motor, perhaps from a defective or mis-connected bridge?

what is rating of your bridge compared to your motor's starting current?


old jim

 

[Ricka]

NO:

We actually have 117v house current here, hence 108vdc. I was having the trouble I've described before I added the choke, so that may not be the problem, but I can try a cap as you suggest.

OJ: Could be a bad rectifier, I guess. It's rated at 35a as I recall; I used six of them in parallel to rectify my AC buzzbox welder and they work OK for that. The motor is rated at 100v, 21a according to the sticker on it, pretty standard treamill motor.

Thank you for the suggestions; tomorrow I'll follow up on them.

Rick A

 

[NascentOxygen]

As you plan to use it, will you need to run the motor at full speed? If not, a series resistor could be tried. I know you said you used "a good sized resistor" but that sounds a bit vague. So under load the motor could draw up to 20A at 100V? At the moment are you running it with a load, or on no load, and seeing excessive sparking? What current is it drawing as you are currently using it?

If you are measuring 108V at the motor, I don't think there could be a faulty rectifier.

 

[jim hardy]

i'd think a 35 amp rectifier would do the job.

simple things first - make sure neither of the motor's leads is connected to frame and earth, for that would bypass half your bridge. Draw it out- housepower has neutral tied to Earth at panel, so if motor is earthed internally then you have a sneak circuit around half your bridge.

You could try running it from your rectified buzzbox output, that'd give you transformer isolation.

Another simple test, if it's mechanically feasible - spin that motor with an electric drill and see whether it generates DC voltage , a permanent magnet machine should make voltage directly proportional to speed. Perhaps you could light a car headlamp with it, that'd check its ability to make current at the low speed you can get from an electric drill.
If it sparks when spun i'd be suspicious of shorted turns in the armature.
Does it smell burnt?

Nascent's questions are right on track for good troubleshooting.

 

[Ricka]

Some more good suggestions. I tried a different rectifier today, no help.

I'm running it no load, but this is the second motor I've tried here. I used the first on the mill for quite awhile, raising and lowering the table OK, although it seemed to spark too much. Finally it started smoking and gave up the ghost. Raising the table was a considerable load, but I don't think that's what cooked the motor. If anyone's wondering, it takes 20 turns of the crank to raise the table one inch, so it gets tiresome cranking it by hand!

I guess it'd run off the welder, albeit slowly at 30-40 volts. We used to use an electric motor to run the traction motors on a diesel locomotive when we set up a portable lathe to turn the wheels. I'll try the electric drill trick, might tell me something. Don't really smell burning, although I'm only running it momentarily, not long enough to heat it up 'til things start melting.

I'll let you know what I find out tomorrow. Thanks again.

Rick A

 

[jim hardy]

check your welder it is probably more like 70 volts open circuit and drops to 35-ish when loaded. Welders are built that way, high voltage to strike the arc but intentionally "just awful" voltage regulation so voltage drops to barely enough to maintain arc when it's going good. With appropriate caution and care, you can use your rectified buzzbox output on low current setting as a quick charger for large truck batteries.

anyhow - something is not right and here in US it is a common enough mistake to attempt using a bridge with unfortunate combination of housepower and an earthed load. The result is always a fried bridge and smoke. So it's something to check for , is all i was thinking. Probably you knew that already.

old jim

 

[Ricka]

Didn't get a chance to do more than spin the motor with a drill today, but it put out current as predicted. I'd think the welder open curcuit voltage wouldn't matter, it's still below the motor's rating. Hope to try that tomorrow. Thanks.



Rick A

 

[jim hardy]

one last thing

the treadmill motors i tinkered with were fairly high speed, like a few thousand RPM.

One of those would have to be geared or belted down to drive a handwheel at reasonable speed...
horsepower is (Torque X RPM X 2pi)/33,000
a 2hp(?) treadmill motor rated at 5000 RPM is capable of not very many ft-lbs of torque,
it needs a gearbox to deliver 2hp at 50 RPM and 100X more torque...
without one it'd be overloaded and soon burn up.

let us be careful that we both have similar mental images in place... it's those simple assumptions that foul us up!

i hope you have the reduction gear train from the treadmill as well as its motor.

old jim

 

[Ricka]

Jim:

Yeah, they turn at about 6600 rpm. Knowing that, I built a worm and pinion reduction gearbox, plus there's about a 3:1 reduction from the treadmill belt and pulleys. It worked well until the first motor fried. It turned the handcrank a lot faster than I could by hand. It takes 240 turns to move the table a foot. The only limitation I ran up against is, it'll raise the table with vise OK, but with vise and rotary table or dividing head it's a little too much.

Rick A

 

[jim hardy]

aha

so if you can measure the current you'll know how hard the motor is working...

its rated current should be about

(rated hp /746) gives rated watts

(rated watts / rated volts) gives rated amps, approximatelya clamp-on meter around AC wire going to bridge would be easiest.

 

[Ricka]

Jim:

Yeah, I can do that; have a clamp on meter. Although, don't know if the classic hp formula applies to a treadmill motor- they seem to rate them HIGHLY optimistically!

Rick A

 

[jim hardy]

yes that's common practice in audio industry to rate a piece of gear for its peak instantaneous power capability.

on AC motors i am used to a "cont" in the duty spot meaning continuous duty.

treadmills might be not capable of continuous operation at max rating, i don't remember specifics of the nameplate on mine, but they should run until armature wires overheat. Your 15 seconds sounds okay provided there's a couple minutes between run cycles to cool down.

still i'd be curious to know how much current the motor is drawing.
An overloaded permanent magnet motor will be susceptible to something called "armature reaction" which is the high armature current distorting the internal magnetic field, and that'd cause sparking and reduced torque which would aggravate the situation further. "Interpoles" were invented around 1890 to get around that problem but the only treadmill motor i ever took apart didnt have them.

So - measure that current and if it's much more than result of the simple calc above, increase your geardown ratio to get current down closer to max rated... shoot for <1.5X, that'll produce 2.25X rated internal heating; and surely they left that much margin in the field magnets.

old jim

 

[Ricka]

Jim:

That sounds like a logical explanation for my problem- asking too much of the motor. In theory, you'd think lifting the table on a 9" X 42" mill at the rate I do wouldn't tax a motor like that. Before I tried the treadmill motor, I had a .5 hp AC wash machine motor, which lifted the table OK, but a starter winding gave out. I'm guessing it wasn't made to start under load. Worked well for quite awhile, though.

I recently changed out the starter on a friend's Dodge PU and am toying with the idea of trying the old one (just the solenoid was bad) on the mill. If I can rewind a microwave transformer to get the 12v, I may be in business. How does that idea strike you?

Rick A

 

[jim hardy]

Car starters make incredible torque, so much that you must never energize one on the workbench without clamping it down. It'll throw itself across the bench if your current source is big enough .

Your idea ought to work fine
but you might shop Ebay for a 200 amp bridge,
or place about 0.01 ohms in series with each 35 amp bridge to force current sharing.
An automotive 30 amp blade fuse should be about 0.002 ohms, so a fuseholder plus a few feet of #12 wire in series with each bridge ought to make them share current.

There' s plenty of hobbyist sites with good directions for re-winding microwave oven transformers , which are called MOT's,
but i must say two things:

1. Remove the High Voltage winding VERY FIRST THING.
NEVER ENERGIZE A MOT WITH ITS HV WINDING STILL THERE , for it is lethal.
The HV winding is spot welded to the metal transformer core and that is a grave danger. You do NOT have the protection of transformer isolation until you completely remove the HV winding and replace it with an insulated LV winding..

2. Don't cut the metal core, you can never get it back with zero air gap.

You will find both precautions on any decent hobbyist site and i cannot overstate the importance of #1. I cringe at the kids who post their MOT experiments on youtube, and several have got killed or maimed..

Doubtless you are aware but there may be less experienced readers about.
Stay safe my friends!

old jim

 

[Ricka]

Yeah, even these treadmill motors will give a pretty good twist; I clamp them in the vise. Wasn't aware of the idea of resistances on the rectifiers. I'd read about the problem of unequal load sharing but haven't observed any problem with my welder yet.

I've fooled with MOTS enough to know what they can do at 2000v. Have to remove the hv winding anyway to fit the 12v winding in- it'll be pretty big wire. FWIW, a dandy use for a MOT is a homemade soldering gun. You use the small winding in the middle, makes about 5v, and ground one end to the work and the other to a carbon electrode. It makes instant heat and gets hotter than an ordinary soldering gun.

Thanks for the ideas. I'll let you know how it unfolds.

Rick A

 

[jim hardy]

Thanks to you as well !

that sounds like it might make a spot welder with suitable clamp...

do you use a plain carbon torch electrode for soldering?

old jim

 

[Ricka]

You can make a spot welder out of one:
http://www.5bears.com/welder.htm
I looked for the website that told me how to make the soldering gun, but no luck. I used a piece of 1/4" carbon arc gouging rod- it's cheap at a welding supply store. I just extended the small middle winding, put an alligator clip on one for the ground and attached the other end to the carbon rod, about 3-4" long, and encased the rod in a piece of snug-fitting rubber air hose to make a handle. The rod gets pretty hot. I also mounted an on/off switch on the MOT. I also tapered the end of the rod to a dull point. Pretty simple and works well, only down side is having to attach the ground clamp, usually to the other end of the wire I'm soldering on.

 

[Ricka]

Old Jim:

Could you also answer another question? I've been looking on E-bay for a rectifier big enough to run this starter motor, and most of the bigger ones are three phase. The internal wiring looks similar; do you know if one would work with single phase? Thanks.

Rick A

 

[jim hardy]

it'll work fine just derate it by 1/3 ie a 100 amp is good for more like 67...

be sure it's not a "half bridge" which has rectifiers pointing only one way, you'd need two of those.

digikey has Microsemi APT100 halfbridge - 100 amp 200 volt, only $12 or so if you could fit in enough wire for centertap winding...

 

[Ricka]

Thank you, Jim; wouldn't have thought of that. I also got to thinking how big the transformer is in a good sized battery charger and it would seem that one that would power a starter motor would have to be bigger yet, much bigger than a MOT. Maybe it wouldn't get too hot during the short bursts of run time I need. Nothing's simple when you're trying to make something out of nothing!

Rick A

 

[jim hardy]

Thanks for the kind words.


i believe your MOT will be okay for the short duty cycles you plan.
Fitting in large enough wire will be biggest trouble, i think. Varnished Magnet wire will be a help there..insulation is thin but more delicate, protect it with some fiberglass tape to blunt sharp corners of the core. Scotch 77 i think is one variety .

MOT's have something called "Magnetic Shunts" which bypass magnetic flux around the secondary when load current gets high. Purpose is to make them self-current-limiting like a welder. Those might help protect your rectifiers and give the starter motor a more gentle start characteristic, so i wouldn't remove them before the trial run.
But you doubtless know more about MOT's than i do - you've already built a soldering machine out of one ! Sorry - just trying to be thorough, it's the old worry wart in me.


Good Luck ! old jim

 

[Ricka]

Jim:

Don 't assume I know more than you! All I know about things like this is what I've learned on the web (like all Will Rogers knew was what he read in the papers). I used a MOT to make the choke for the DC conversion of my welder. I used bare copper cable and wrapped electric tape around it- adequate for the low welding voltage but still thin. I live in Hilo, Hi and probably can't buy heavy varnished wire (no industry here). I didn't know the purpose of the shunts and would have left them out but for your suggestion. I imagine being a worry wart has kept you out of trouble at times. I wouldn't describe myself that way, but I will admit to being considerably more careful than when I was young.

I've ordered the rectifiers on E-bay and hope to be putting all this together when they arrive. Another approach occurred to me: I have a couple of 12v cordless drill batteries, wonder if one would hold enough charge to raise the mill table. I don't use it often (just a hobby machinist) and could live with recharging between uses. Sound logical?

Thanks,
Rick A

 

[jim hardy]

drill batteries... interesting...

if they won't, i'd wager a lawn tractor battery would.. $25 at Walmart. Fishing department carries plastic battery box for them. Maybe even a lawnmower starter motor from salvage yard, they're a lot smaller than a car starter.

 

[Ricka]

A lawnmower battery crossed my mind, being small but holding enough charge to run a starter. Of course, I'd have to BUY it, which I always try to avoid! I think I have a mower starter kicking around, too. Today I was rewiring my starter to run in reverse IAW this: http://www.diygokarts.com/images/starterpics/150/Starter Modification.pdf
I'm constantly in awe of all the info available on the web.

Rick A

 

[jim hardy]

i don't at all undertand that three brush starter...

but to reverse a DC motor you need only reverse the relation between armature and field, ie reverse direction of current flow through EITHER ONE of them but not both. Doubtless you've already observed reversing battery to it doesn't reverse direction, which it would for a permanent magnet motor.

Most starters are "Series wound" which means the current that flows through armature also flows through field, so field is made of very large wire as shown in that link.


some 1960-ish Chrysler starters had a second field that's wound with very small wire, called a "shunt field". Its purpose is to limit starter speed should Bendix drive fail to engage. If you find one in your Dodge reverse it, too.

Indeed, Web seems to help fulfill some ingrained human desire to help. Makes one think of C S Lewis.
May it gradually turn us away from mischief...

old jim

 

[Ricka]

Yeah, I can't believe the starter was made with three brushes; I'd guess someone removed it, but who would go to the trouble of drilling out rivets, rather than just leave out the brush?

I separated the fields from the brushes as he described but haven't got the motor to run that way, with one field lead and one brush lead going to + and the other two going to -.
I figure if I run current through the fields they ought to act like permanent magnets and I only have to swap the brush leads for reverse. So why isn't it working? Haven't had much time to devote to it lately, but I'm guardedly optimistic.

Rick A

 

[Ricka]

Jim:

Been awhile, but I finally solved my problem. A guy was giving away a couple of mobility scooters on CL and I took the 24v DC motor out of one and powered it with a rewound MOT and couple of 35 amp bridge rectifiers in parallel and an old treadmill choke. It seems to work fine, but I don't understand why I had so much trouble trying to run a treadmill motor with essentially the same setup. When mechanical things act up, the trouble can be seen; not so easy with electrical things! Anyway, many thanks for your advice and patience.

Rick A