# 120 AC to 120 DC

#### leod503

Jan 22, 2016
16
Before building my first vfd, I would like to build a easy and inexpensive ac to dc converter. It is actually a 130 v 3.8 hp commercial treadmill motor. This is simple enough for me to build, just a full bridge and smoothing cap, right? Does a motor need a smoothing cap? I can't seem to find a quick answer to that. A simple diagram with the actual parts I need to order would be great. I am a good machinist and auto tech, electronics, not so much. A simple speed controller diagram with component list would be nice too. Lot of them I can find online, none near the voltage and amperage to run a nearly 4 horsepower motor. I need to power a milling machine with a variable speed motor ( changing belts and pulleys really bites)without spending a lot of money on another VFD and three phase motor. Thanks for your help, and patience with a beginner. Remember, keep it simple k.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,540
Welcome to electronicspoint.

Here are a few simple answers - probably not very stisfying to you:

Does a motor need a smoothing cap?
No.

A simple diagram with the actual parts I need to order would be great.
A simple diagram for what? A vfd? That's a complex piece of electronics and quite a few safety issues need to be considered. Judging your level of experience from the questions you posed, I recommend you buy an off-the-shelf vfd. Don't risk anything by building one yourself.

Regards,
Harald

#### rickselectricalprojects

Feb 1, 2015
118
If you rectify 120VAC then you will get around 170VDC.
Building a VFD is pretty hard and if you are somewhat new to electronics (like me) then you should probably build something not as complex and that isn't powered of mains voltage. Also building a large speed controller would also be pretty difficult.

#### AnalogKid

Jun 10, 2015
2,662
3.8 horsepower is over 2500 watts. Between the current level and the thermal management issues, there is nothing simple about it.

ak

#### Minder

Apr 24, 2015
3,247
I assume this is a DC motor, if so you do not need a VFD as this produces variable AC frequency, a current common way of controlling a DC motor is either by a SCR bridge circuit that does not require a capacitor,
The other is PWM which does require a suitable separate DC power supply, the SCR bridge outputs a little less that 120 as no capacitor is used, the PWM supply on 120 equals around 165vdc.
Which is no problem for a 130vdc motor as the average current and rpm is controlled by the PWM signal.
Out of the two, the SCR bridge is the easier to build or the KB versions can be picked up on ebay.
M.

#### leod503

Jan 22, 2016
16
I have a kb electronics speed controller on my dc motor running a power steering pump I am using as a coolant pump with a spade bit to drill deep holes, but that is just for a 1/2 hp motor.It is 180 v running of 240 v. I see that it is a rather simple circuit. The VFD would be a project for sometime in the future, or not at all. I don't need to run any three phase motors right now, I need a speed controller for the dc motor. I have seen numerous schematics, but usually for a toy motor. I need specs for a large one. The motor claims 3.8 hp,continous duty, but since it just about half the size of my 1/2 hp GE industrial motor, I suspect that is Chinese hp if you take my meaning. It does say 2830 watts however. I assume I would have to run a couple circuits parallel to handle that much current. So, that being said, can you point me towards a dc motor speed controller diagram with specs so I don't have to look up a catalog full of components and adapt a diagram. I can do it, I am just real busy right now and could use a shortcut.

#### leod503

Jan 22, 2016
16
Welcome to electronicspoint.

Here are a few simple answers - probably not very stisfying to you:

No.

A simple diagram for what? A vfd? That's a complex piece of electronics and quite a few safety issues need to be considered. Judging your level of experience from the questions you posed, I recommend you buy an off-the-shelf vfd. Don't risk anything by building one yourself.

Regards,
Harald
Actually, the terse smoothing cap answer is exactly what I needed to know. Thank you. The vfd answer does not surprise me at all as it took me a couple hours just to feel comfortable programming the three I have. The dc motor speed controller is what I was actually wanting the info on.

#### leod503

Jan 22, 2016
16
If you rectify 120VAC then you will get around 170VDC.
Building a VFD is pretty hard and if you are somewhat new to electronics (like me) then you should probably build something not as complex and that isn't powered of mains voltage. Also building a large speed controller would also be pretty difficult.
Hmmm, my 1 hp kb controller actually looks like a fairly small circuit about 2 1/2" square. Looks like about 5 scr's on the heat sink, a couple power resistors, a couple caps and about six or so trim pots for setting the variables such as accel/deccel.

#### Minder

Apr 24, 2015
3,247
What I have done in the past for larger motors, is pick up one of the smaller low power KB SCR drives and removed the 3 rectifiers and 2 SCR's and mounted higher current versions off board on a larger heatsink.
The KB has a shut down input for reversing so that the drive comes up in acceleration rather than reverse at full voltage (I1 & I2)
M.

#### leod503

Jan 22, 2016
16
What I have done in the past for larger motors, is pick up one of the smaller low power KB SCR drives and removed the 3 rectifiers and 2 SCR's and mounted higher current versions off board on a larger heatsink.
The KB has a shut down input for reversing so that the drive comes up in acceleration rather than reverse at full voltage (I1 & I2)
M.
That certainly gives me some ideas, thank you.

#### Fish4Fun

##### So long, and Thanks for all the Fish!
Aug 27, 2013
471
I actually have one of those treadmill motors I bought with the intention of turning into a variable speed spindle...then I bought a 2.2kW Spindle/VFD.....so just haven't got around to that project yet...lol....my thought on it was pretty straight forward....I was planning on scouring EBAY for some high voltage Mosfets or IGBTs capable of switching @ 20kHz to 30kHz without getting too lossy and then using simple PWM from line rectified AC....obviously the circuit would need to be on it's own breaker, and likely have to parallel up 10+ mosfets or a couple of IGBTs and use some heavy-duty heat sinks....switching 20A+ @ 170Vdc is going to generate some heat....Something like the SIHG25N40D might only take three or four in parallel, (and it has very respectable Tr/Tf....) But 20A with an RDSon of 0.170 is 68W of heat (best case) that has to be removed from the TO247 packaging....with four of them (assuming equal current distribution...big assumption!) thats still over 4W / Mosfet before switching losses.....hrmmm.....And don't skimp on the rectifier! I was absolutely going to put a large capacitor bank on the rectifier....(found a deal on ebay and ordered 100 x 470uF 250V snap-in capacitors around the same time I ordered the treadmill motor...not sure how many I was thinking I might use, but I know how many I bought ;-) )

Using an SCR based rectifier is likely the simpler/cheaper way to go, but if you are planning on using the motor in a CNC machine you better make sure you put a robust filter on the front end, using thyristors to switch that much current without a well designed line filter is going to wreck havoc on your mains supply....not to mention the EMI it will put out....not all that different from a thyristor based welder....

Good Luck!

Fish

#### leod503

Jan 22, 2016
16
I actually have one of those treadmill motors I bought with the intention of turning into a variable speed spindle...then I bought a 2.2kW Spindle/VFD.....so just haven't got around to that project yet...lol....my thought on it was pretty straight forward....I was planning on scouring EBAY for some high voltage Mosfets or IGBTs capable of switching @ 20kHz to 30kHz without getting too lossy and then using simple PWM from line rectified AC....obviously the circuit would need to be on it's own breaker, and likely have to parallel up 10+ mosfets or a couple of IGBTs and use some heavy-duty heat sinks....switching 20A+ @ 170Vdc is going to generate some heat....Something like the SIHG25N40D might only take three or four in parallel, (and it has very respectable Tr/Tf....) But 20A with an RDSon of 0.170 is 68W of heat (best case) that has to be removed from the TO247 packaging....with four of them (assuming equal current distribution...big assumption!) thats still over 4W / Mosfet before switching losses.....hrmmm.....And don't skimp on the rectifier! I was absolutely going to put a large capacitor bank on the rectifier....(found a deal on ebay and ordered 100 x 470uF 250V snap-in capacitors around the same time I ordered the treadmill motor...not sure how many I was thinking I might use, but I know how many I bought ;-) )

Using an SCR based rectifier is likely the simpler/cheaper way to go, but if you are planning on using the motor in a CNC machine you better make sure you put a robust filter on the front end, using thyristors to switch that much current without a well designed line filter is going to wreck havoc on your mains supply....not to mention the EMI it will put out....not all that different from a thyristor based welder....

Good Luck!

Fish

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#### Minder

Apr 24, 2015
3,247
No you cannot run them in parllel, there will always be a mismatch.
You would be better off looking at higher current off board devices to replace the existing.
M.

#### leod503

Jan 22, 2016
16
No you cannot run them in parllel, there will always be a mismatch.
You would be better off looking at higher current off board devices to replace the existing.
M.
Ok, that does sound like my best bet. I will pick up a 12 amp controller, then see what digi key price and availability is. Unless you can recommend a better source. They are who I get my tesla supplies from. so just swapping out the 3 rectifiers an 2 scr on a nice large, possibly air cooled, heat sink will work? any details I should know concerning the scr signal input. Your mention of the reversing I could use some clarification on, you mention l1 &l2. I assume you are talking about back emf protection built in to it so it doesn't get fried if reversed before coming to a full stop?

#### leod503

Jan 22, 2016
16
Ok, that does sound like my best bet. I will pick up a 12 amp controller, then see what digi key price and availability is. Unless you can recommend a better source. They are who I get my tesla supplies from. so just swapping out the 3 rectifiers an 2 scr on a nice large, possibly air cooled, heat sink will work? any details I should know concerning the scr signal input. Your mention of the reversing I could use some clarification on, you mention l1 &l2. I assume you are talking about back emf protection built in to it so it doesn't get fried if reversed before coming to a full stop?
I don't suppose you just happen to have some part numbers and or specs handy? Would digi-key part # 497-6158-6-nd silicon controlled rectifiers and GBPC2506A-ND rectifiers be good choices? Not sure I am reading all my specs right.

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#### rickselectricalprojects

Feb 1, 2015
118
Hmmm, my 1 hp kb controller actually looks like a fairly small circuit about 2 1/2" square. Looks like about 5 scr's on the heat sink, a couple power resistors, a couple caps and about six or so trim pots for setting the variables such as accel/deccel.
A 1 HP motor is equal to about 750 watts and 3.8 HP motor is equal to about 2500w.

#### Minder

Apr 24, 2015
3,247
I don't suppose you just happen to have some part numbers and or specs handy? Would digi-key part # 497-6158-6-nd silicon controlled rectifiers and GBPC2506A-ND rectifiers be good choices? Not sure I am reading all my specs right.
You cannot use a bridge rectifier, two singles and two SCR's form the bridge, something in the order of these would be more like it.
VS-40TPS08APBF-ND & VS-40EPS12PBF-ND
M.

#### leod503

Jan 22, 2016
16
You cannot use a bridge rectifier, two singles and two SCR's form the bridge, something in the order of these would be more like it.
VS-40TPS08APBF-ND & VS-40EPS12PBF-ND
M.
That's what I needed, thank you. Once I get the one I am going to work on and start familiarizing myself with it I will have a better idea what I am doing.

#### Sinewave

Feb 15, 2013
129

Can I add my suggestion, which is that if you want to run a mill or a lathe for that matter via a variable speed motor rather than changing gears etc, then you're going to have some torque issues unless this is some serious motor, which would be quite big and very expensive. A single phase motor on machining equipment will run like a horse with a wooden leg or as a good friend of mine says, like a dog 'pooing' (or another adjective) razor blades.

You would be best with a three phase motor and if you're on a single phase supply, you can of course use single phase to three phase converter.

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