# 14amp DC motor surge work around?



## LX Kid (Mar 29, 2020)

I'm planning on using a treadmill 100v DC, 14amp, 2.5hp motor for my 6" swing lathe.  I bought a cheap regulated AC power supply and converted it to DC by adding a bridge rectifier and a smaller 200k ohm potentiometer.  It works fine "except" it really draws current when starting and shoots sparks out the back.  If I hand spin the flywheel/fan before turning power on it works great.  Is there a work around to take care of this problem?  Thanks


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## matthewsx (Mar 29, 2020)

Um, I''d have to say probably not.

Why not get the correct power supply? Should be able to use the one from the treadmill, or buy something that doesn't shoot sparks out the back. 

John


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## LX Kid (Mar 29, 2020)

matthewsx said:


> Um, I''d have to say probably not.
> 
> Why not get the correct power supply? Should be able to use the one from the treadmill, or buy something that doesn't shoot sparks out the back.
> John


Only have the motor as it was given to me.


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## matthewsx (Mar 29, 2020)

LX Kid said:


> Only have the motor as it was given to me.



I see treadmills free on Craigslist all the time, alternatively buy something like this.






						AC 110V Motor Speed Controller, 24V-90V Adjustable Single Phase Motor Speed Controller 0.02-400w for 24V, 36V, 90V DC Motors: Amazon.com: Industrial & Scientific
					

AC 110V Motor Speed Controller, 24V-90V Adjustable Single Phase Motor Speed Controller 0.02-400w for 24V, 36V, 90V DC Motors: Amazon.com: Industrial & Scientific



					www.amazon.com
				




I'm not good enough with electronics to comment on the modifications you've done to the power supply you have but I'm pretty confident to say the sparks coming out the back isn't a good thing.

John


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## matthewsx (Mar 29, 2020)

Sorry, that one doesn't handle the amperage of your motor. But something similar, or one of the ones that takes a DC input and will handle 15 amps.


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## matthewsx (Mar 29, 2020)

For the power range you're looking at using your existing motor finding another treadmill will be the most economical choice. It's possible to buy an adjustable speed controller but the price is pretty high. Alternatively just use a 110v AC motor and change the speed with pulleys. I'm working on a 6" Craftsman 101.21200 lathe now that came with a 3/4 hp motor which seems like more than it will need. I do have a 3 phase motor with VFD on my 9" Seneca Falls Star lathe and the variable speed is quite nice but I have more into the motor and VFD than I paid for the lathe. 1/2 hp should be plenty for a 6" lathe, more might cause unforeseen problems.

John


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## Alexander McGilton (Mar 29, 2020)

Some options you could implement though I won't endorse. Place an inductor in series to the motor and a capacitor in parallel. This is an analog approach to  what a soft start motor controller is doing. The inductor limits energy momentary as it accumulates energy, once saturated it for the most part acts as a long wire. A capacitor in parallel will draw power in unison to the motor, slowing down the ramp up as they are both limited in supply of power, once it is saturated it will not consume energy.
Why this is a bad idea is that you need fairly high values to these components to notice any change, and that is where these get dangerous. Inductors and capacitors in AC tend to limit energy transfer, where as in DC they both store and dissipate potentially lethal amounts of energy. This must be mitigated with bleed resistors and flash back diodes that create a closed loops of energy during shut down for safe dissipation. Aside from basic safety and shock hazards, the additional risk is that you will have an uncontrolled motor at shut down, and a power back feed to potential damage other components. 

This is only tangential to my education in the mechanical field, and why I won't give numbers or specifics. I advise that you acquirer a proper dc motor controller or go with a different motor type as said above.


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## tq60 (Mar 29, 2020)

An idea from the clever cheap guy...

Take the rectifier parts and connect to the a.c. line cord to output 120 vac.

Locate a heavy duty light dimmer and use it for providing variable power.

Common dimmer is too small, we have a couple industrial ones that may do the trick.

For 2.5 HP you need 2000 watts (rounding up to next size up) worth of output.

Does your device need that much power?

Sent from my SAMSUNG-SM-G930A using Tapatalk


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## RJSakowski (Mar 29, 2020)

I am confused by what you say you have.  Regulated AC/AC supplies do exist but in my experience, they are used to compensate for variations in line voltage and tend to be transformer based, large, and expensive.  What I see in the picture looks more like a TRIAC dimmer or  motor speed controller as is used for routers, food mixers etc..  As such, you should be able to set the starting voltage to zero or close which should in turn limit the available current.

If you are using a triac based controller, combined with a bridge rectifier, you may want to add a filter capacitor after the bridge.  The waveforms out of a triac controller can be fairly funky with sudden voltage/cirrent rises when the triac starts to conduct in each half cycle.  Inductive loads don't like sudden changes like that.

Motors generate a back voltage proportional to speed which opposes the applied voltage , thus reducing current draw.  When a motor is loaded down,, the speed drops and the back voltage drops , increasing current draw.  When first starting, the speed is zero so there is no back voltage and current draw will be high.  

One reason for a large amount of sparking is a shorted winding or an open winding on the motor armature.  This will be most obvious when the motor is just starting.  Bad contact from the brushes can also be a cause of excessive sparking.  Usually, this is due to worn brushes or a softened brush spring.


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## graham-xrf (Mar 29, 2020)

The Inductor-Capacitor low-pass filter limiting the transient is the elegant way to do it, but the downside is, as has been pointed out by @Alexander McGilton, the components store large amounts of energy in the inductor magnetic field. On switch-off, it tries to come out, generating unwelcome voltages in the process, capable of spot-melting and sticking, ultimately burning out contacts.

Another way is to use low value wire-on-ceramic resistors, and a cheap relay timer, and a motor contactor. The timer is configured to ON-DELAY for a time enough to take out the surge, and allow the motor to spin up some. Once the armature is spinning, it will be generating it's back-EMF, which subtracts from the applied voltage, leaving only enough to keep the off-load motor spinning via the resistor, which limits the current anyway.

Then, the delay timer runs out, and operates the motor contactor to short out the resistor, and apply the full voltage.
It was my version of a cheap soft-start, and it worked well. I used 47-ohm resistors in one, and 15-ohm resistors in another, but the best value for you has to be worked out for your motor.

Remember that if you rectify 110VAC, you get a voltage 1.414 x 110 = 155.5 volts.
Connecting that on a 2kW motor which is at a standstill leaves only the (very low) resistance of the armature copper. That is pretty much a short-circuit until the motor gets going. A few shots of that would not have been kind to the commutator and brushes.

Another thing to consider is using an AC universal brush motor on DC when it was likely designed for AC may have problems. Like any motor with brushes, the field coils voltage stays in step with the armature voltage, so the motor turns the same direction, despite it has AC applied. It will turn OK with DC also, but the difference is that when using AC, the inductance of the motor windings goes some way to limiting the current at 60Hz, on every cycle. When you hit it with DC, in the milliseconds before it spins up, the inductance gets only one shot at limiting the transient, and the current goes up to whatever the (low) DC resistance lets it level out at. This is why I used an external resistor.

The same thing happens with AC on a cheap site isolation transformer with not enough copper and iron in it. It has a switch-on surge that tries to be x10 to x20 the current it normally has when the alternating field is established. Putting power into a magnetic thing which has only got metal in there without any field is like offering a near short-circuit.

I would be wary of using a dimmer-style AC power control on anything except a resistive load. It relies in the AC output to swich off the triac every cycle. If you rectify, and then add inductive load, and perhaps capacitor, you can seriously modify the waveform it relies on. That arrangement may also generate the father and mother of RF interference on your mains waveform. There is a reason that proper semiconductor power controllers have input filtering, and power factor correction, and much higher frequency switching. The components like inductors and capacitors can be so much smaller.

I am all for re-purposed kit. I do it myself, but normally, I keep the stuff using the kind of power it was designed for.

@RJSakowski has very good advice about semiconductor power controllers. The kind you would use for a dimmer has a simplistic phase control to alter when the triac switches. One can use solid state relays, for AC or DC, and I love them. There is a common kind which only switches on the AC as the voltage passes through zero, and another kind that switches the instant the gate is opened. I have discovered that if one controls the switch-on (AC) to happen when the voltage is still high, but on it's way down, at about 80% of the peak. then the current trying to surge is faced with a voltage that is heading for zero, and is going to get there in about 3 milliseconds. You can almost eliminate the switch-on surge.


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## Z2V (Mar 29, 2020)

MC-60 treadmill motor control boards are all over eBay in the $40 - $60 range.


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## LX Kid (Mar 29, 2020)

I added a choke to the negative side of the motor and things got a whole lot better.  I then changed the motor to a 18 amp motor and looks like it will work fine.   I also believe that I need to find a 150k ohm pot for better linear motor control.  Guess I'll order one and find out. "For the fun of it all!"

.


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## Nigel123 (Mar 30, 2020)

I hooked up a treadmill motor to a 6 inch lathe and used a variac transformer also called an auto transformer with a bridge rectifier
It works good variable speed and constant load no load speed
What ever your set up is it has to keep the voltage constant and the current changes for different loads
Maybe with a smaller lathe the torque will not be an issue


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## markba633csi (Mar 30, 2020)

Glad you got it working- check the motor brushes are not too worn from all the arcing/sparking- if you can remove them that is
-M


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## LX Kid (Mar 30, 2020)

The weight of the flywheel and the Chuck combined should give it a good inertia and constant speed.


Nigel123 said:


> I hooked up a treadmill motor to a 6 inch lathe and used a variac transformer also called an auto transformer with a bridge rectifier
> It works good variable speed and constant load no load speed
> What ever your set up is it has to keep the voltage constant and the current changes for different loads
> Maybe with a smaller lathe the torque will not be an issue



That's a good idea "but" I'm trying to reduce the footprint size of the Atlas/Craftsman 618 lathe.  It's going to be sold anyway when restoration is completed.   Might be better to scrap the DC motor idea and go with the  AC 1/3 hp motor to keep things original.


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## Nigel123 (Mar 30, 2020)

You have the parts so maybe give it a try like you say the set up may give you constant speed


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## LX Kid (Apr 5, 2020)

I was able to find a treadmill power supply and used a 5K pot and it works quite well.  I don't have any problems with either power supply as they both work good.  I haven't put either one to use on my lathe and just have been tinkering around with both.


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## hman (Apr 6, 2020)

Just a note of caution ... on the MC60 the three leads to the 5K pot are at nearly 115 volts AC above ground.  Be sure to insulate the pot terminals!


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