Treadmill Motor

Lots of great ideas about re-purpose of treadmill motors on this forum. I've found they can be from 90 to 130VDC and more that 2½HP. The last treadmill I took the motor from had a transformer choke wired to it and maybe the others did too but I missed them.

Question:

If I re-purpose the motor do I also need the choke even if I used the homemade SCR drive described in other threads?

Point of Interest:

The treadmills with incline have a boss 120V gear-motor with pinion, small rack and one I found even had a photoelectric position sensor on it. I'm sure to find a use for that. Does anyone know how to find out what the pinion pressure angle so I can get a longer rack?

You all would be amazed at the treadmills that are scrapped. One was still in the original box, not even assembled yet. I love rich people.

I have been told that including the choke makes the motor run more smoothly when powered via a basic diode bridge "DC" power source or SCR controller.

Yup. A bridge rectifier typically outputs a lot of ripple so a filter capacitor or a choke is usually used to smooth out the ripple for a more constant DC voltage and a more constant running motor.

I tried the scr/bridge rectifier route but the brushes arced badly. Didn’t try a choke or cap.. I found a treadmill with a controller that has terminals that output 80/160 dcv so I am waiting for a $20 dc controller to arrive to see if that cleans up my act.
Dick

I have 2 treadmills that I am currently disassembling 1 is an older unit with a 1750 RPM 120/240 volt cap start cap run 1 1/2 Hp with a variable pitch drive pulley the other has the 130vdc 2hp magnet motor in it I don't know how much any of the electronics in it will be of any use as it sat out in the weather for several years but both machines have quite a bit of useable steel in them as well as the rollers and bearings so like butchering a hog I will try to save everything but the squeal.
the 1 1/2 Hp AC motor runs as I've already tested it too bad it wasn't a 5 or 6 hp as I need 1 of those to build a belt grinder which will wind up being a combination centerless grinder / flat face sander and pipe notching machine but I have a couple of 8 hp 3 ph motors that I was planning on making a 16 hp rotary phase converter out of

It isn't just the scr's/ regulators or whatever is upstream that put out ripple -
what really arcs and fries the commutator is the back-EMF from the turning rotor coils themselves,
who give a nasty HV return spike when the brushes releases,
thus necessitating the caps from each terminal to ground & also between the terminals.

Then quite a few manufacturer's don't even install them, as they're more into selling than the longevity of their products.

AFAIK a single "ferrite donut" is merely trying to protect the motor controller from that HV hash & RFI compliance,
whilst letting the motor fry itself to to death, to the lively cheer from the motor producers & service staff.

Personally, I cram in as big caps, as close I can as possible, on any DC motor I use -
but then I'm only just another DIY cheapskate.
Caps: <10 muF total non-polarized small ceramics and biggie plastic, max VDC at least twice the applied.

Cheap idiot test to see if your snubbing caps are adequate:
1) Run motor naked (without suppression) and listen to an AM radio tuned between stations. Behold the arcing!
2) Install caps and watch if/ when the arcs fade out - listen for less noise on the radio.
(If you're into scientific method, measure comparatively the radio's speaker output with an AC voltmeter!)
3) Try adding, or even bigger caps.
4) Rinse and repeat the steps above 'til satisfaction, or you reach "the diminishing returns point".

2 cents

Tricky stuff. A choke keeps the current running at the moments the SCR or the bridge diodes are closed. But where does this current came from? If there is no suitable source, very high voltages surges will occur and your electronics will die. So in a well designed circuit a flywheel diode is a must. For 50 or 60 Hz supply voltage I would recommend to use the choke for a more smooth and silent running. Connect a diode between the choke input connection (at the SCR/bridge side) and the mains return, with the cathode to the positive pole and anode to the negative pole. So for the normal motor voltage the diode is not conducting. The result is that the current is kept alive through the diode when the bridge or SCR is closed. Of course the diode must be specified for the peak voltage of the grid and the maximum motor current. Take ample margins!

Quote:

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Originally Posted by Crusty

Yup. A bridge rectifier typically outputs a lot of ripple so a filter capacitor or a choke is usually used to smooth out the ripple for a more constant DC voltage and a more constant running motor.

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Crusty, is the transformer choke wired in series with one of the DC legs or what? I'm an ME and some of this electrical stuff is mystifying.

Quote:

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Originally Posted by Carnel

Tricky stuff. A choke keeps the current running at the moments the SCR or the bridge diodes are closed. But where does this current came from? If there is no suitable source, very high voltages surges will occur and your electronics will die. So in a well designed circuit a flywheel diode is a must. For 50 or 60 Hz supply voltage I would recommend to use the choke for a more smooth and silent running. Connect a diode between the choke input connection (at the SCR/bridge side) and the mains return, with the cathode to the positive pole and anode to the negative pole. So for the normal motor voltage the diode is not conducting. The result is that the current is kept alive through the diode when the bridge or SCR is closed. Of course the diode must be specified for the peak voltage of the grid and the maximum motor current. Take ample margins!

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Carnel, would you be so kind as to provide a schematic?

Others have commented on the current smoothing effects of a series choke and parallel capacitors for voltage smoothing and arc suppression so i will not go into that again.
i just want to point out an aspect that I have never seen mentioned and which depends on the use to which the motor will be put. Many applications do not require perfectly smooth velocity over a revolution, these include tools like a drill press and lathe for example. However, if you want to drive a grinding wheel and achieve great results then you need the motor to spin as close to constant speed during a revolution as possible. If you use an SCR or thyristor controller then at less than full speed you will have a very spikey supply which requires a lot of smoothing. On the other hand if you use a PWM controller then the higher carrier frequency will be much easier to filter and you will need a much smaller choke with minimum iron in it. Of course a PWM controller is also fine for a drill or lathe.
So why are SCR controllers used at these power levels? It is simpler older technology and until relatively recently was cheaper, but nowadays PWM controllers are cheap enough so I see no reason to not use one.

Yup the choke is in series. When current first passes through it some of that energy goes into building a magnetic field in the choke core until it can build no more. When the voltage to the choke is reduced, the magnetic field in the core collapses, in turn generating current in the windings. So, in a circuit it has an overall effect of smoothing out varying incoming voltage (if the choke is chosen specifically for the electrical operating conditions).

A transformer from a microwave oven usually has enough meat in the core to make useful inductors (chokes). I'm adding DC to my Horror Freight transformer welder and I'm using as many turns of welding cable as I can get into the core to smooth the voltage ripple. Since they're wound for reasonably high current at 120V input they can also be used to make a custom transformer by winding a new secondary. Determine how many turns are in the primary then divide by 120 to arrive at the turns per volt for that transformer, then use that figure to determine how many turns are needed for your desired output voltage, then wind a secondary with that number of turns of magnet wire selected to handle your desired output current. If it fits you're crusin' on easy street.

Thanks for that Tony, I have just acquired a treadmill for parts and your clear explanation will help me a lot, I was beginning to get a bit confused with all the recent discussions.

nhengineer, for a working schematic the detailed motor and choke specifications are needed. And also with these, after building a prototype, a test is a must with enough safety measures as to avoid frying the motor, choke and electronics. Tony's suggestion of using a Pulse Width Modulated (PWM) regulator is a good choice. The motor is 2 1/2 HP (net mechanical output?) which equals about 1850 Watt. At 130V the current will be approximately 18A. For PWM control you need a power supply delivering a smooth 130V DC voltage at this current without generating a lot of grid pollution. So a well designed switch mode power supply is needed. The best thing is to achieve the supply and regulator which was used in the original treadmill.... Engineering is not always simple ;-)).

Quote:

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Originally Posted by tonyfoale

Others have commented on the current smoothing effects of a series choke and parallel capacitors for voltage smoothing and arc suppression so i will not go into that again.
i just want to point out an aspect that I have never seen mentioned and which depends on the use to which the motor will be put. Many applications do not require perfectly smooth velocity over a revolution, these include tools like a drill press and lathe for example. However, if you want to drive a grinding wheel and achieve great results then you need the motor to spin as close to constant speed during a revolution as possible. If you use an SCR or thyristor controller then at less than full speed you will have a very spikey supply which requires a lot of smoothing. On the other hand if you use a PWM controller then the higher carrier frequency will be much easier to filter and you will need a much smaller choke with minimum iron in it. Of course a PWM controller is also fine for a drill or lathe.
So why are SCR controllers used at these power levels? It is simpler older technology and until relatively recently was cheaper, but nowadays PWM controllers are cheap enough so I see no reason to not use one.

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Attachment 30702

This PMW baby eBay (90VDC output) is around $7. Do I still need a choke for a lathe drive?

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Originally Posted by Carnel

The best thing is to achieve the supply and regulator which was used in the original treadmill...

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That is the easiest but I would dispute that it is the best. Treadmill controllers have various features included which are there for safety in that application. Features like slow runup and slowdown to prevent shooting the runner off the end. There are other features as well, suited to that application. These features make the original controller somewhat less than the best for workshop tools.

Quote:

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Originally Posted by nhengineer

Attachment 30702

This PMW baby eBay (90VDC output) is around $7. Do I still need a choke for a lathe drive?

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Is 1kW enough for your application? That type of controller will work fine on a lathe without a choke, you could use one but NOT the same type as you would use for an SCR control. The core in those would be too lossy for the 15kHz carrier frequency and would absorb power and heat up.

In previous questions on this theme I have pointed out another potential problem with using variable speed motors on workshop machinery. it is a question of simple physics.
If you run at reduced speed with some form of mechanical gearing then the available spindle torque will increase in proportion to the motor:spindle speed ratio. On most machine tools you generally need more torque at slower speeds because you are dealing with larger or harder material (why else would you be running slower). If you have a single gear ratio with a variable speed motor then the spindle torque will remain approximately constant over the speed range. In other words at slower motor and spindle speeds the torque will not be multiplied as it is with mechanical speed change methods.

The answer to this is to use a well oversize motor. I replaced a 1 hp motor on my mill with a 5 hp, and a 2 hp on the lathe with a 5.5 hp. Even with that I could detect much lower torque at very low speeds on the lathe. That is no problem for me because the nature of my work does not require that characteristic. My advice before involving yourself in the effort of fitting up a motor and control system is to carefully review your working requirements regarding torque and speed. A 1 kW motor on a lathe does not give you much torque margin for low speeds. A motor rated 1 kW at say 3000 rpm will only be able to deliver around 100 W at 300 rpm. If your work does not require high torque at low speed then you should be OK, otherwise you may be disappointed. A lathe is probably the hardest of all common workshop machines to get working well with these motors, unless it is a very small lathe.

Hey Tony,

Yup, speed reducer = torque multiplier. I should have though of that but I'm working on design of a small lathe for which 2.5HP (± a bit) would be more than adequate. I'm thinking of a scaled up by a small amount Gingery design. I have a mega, WW-I vintage Monarch (11" swing X 6' bed) but the headstock slop doesn't allow for any kind of accuracy. I'd like to rebuild, hoping I live long enough, because just about everything is worn out. I need a small, semi-precision lathe to make some stuff for its big brother so big brother can make big stuff.

Quote:

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Originally Posted by nhengineer

Hey Tony,

Yup, speed reducer = torque multiplier. I should have though of that but I'm working on design of a small lathe for which 2.5HP (± a bit) would be more than adequate.

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If you use a 2.5 hp motor you will need at least a 2 kW controller. the one that you previously linked to was a 1 kW unit.

Quote:

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Originally Posted by tonyfoale

If you use a 2.5 hp motor you will need at least a 2 kW controller. the one that you previously linked to was a 1 kW unit.

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Thank you Tony!