# Wiring help for a DC motor

#### JonathanEngr

Joined Oct 21, 2011
7
I have been browsing through the forums trying to find an answer to my question but haven't had much luck. Here's my dilemma:

I have an old conveyor system that I'm trying to get operational again (more as a gadget... not industrial use, etc. Eventually want to use it as an off-feed table for my table saw). No motor or wiring came with the conveyor, but I know it had a 1/17th HP 100 rpm gearmotor on it before. I want to be able to vary the speed from 0 to max rpms, and was told that DC motors are superior for this application. Simple enough for some, but not for me.

I thought I'd be able to go find a DC motor that had the ac/dc converter built-in, as well as a variable speed dial <grin>. What I've found has been a nightmare.

It seems I can buy a 90V or 130V DC motor (they're all over eBay) and do a "simple" ac to dc conversion using a bridge rectifier and capacitor to smooth the current. Great. The only problem is... I have no idea how to size and/or choose a rectifier or capacitor for such a setup. Electricity in general gives me indigestion. Could someone help me with this step? Is there a better way to do it?

Also, I thought of going with a 12V motor at first because I could supposedly use a washer motor, etc. Besides, 12V is friendly since we all have cars However, finding a speed control for the amps it would draw is difficult (for me). Also, can someone verify this for me... I have NEVER understood this. Let's say my motor will require 600 watts... if my motor is 12V, wouldn't that be a 50 amp current? And if I used a 90V motor, my amps would be 7ish? As a result, would that require much larger wires for the 12V setup, or does the wire size depend *on the wattage*? I have never understood that.

It would be awesome if someone could say "you need this part #, this gauge wire from here to here, from there to there" and so-on, but I don't expect that. I'm actually a very handy guy, but electricity is my achille's heel.

Again, I just want to wire up a motor to the belt so it will run, and have the ability to vary the speed. As this is a tinkering project, I don't want to buy the whiz-bang super motor and control 9000zx. A decent, used eBay motor, (hopefully) wire I have here, and the rest of the controls I"ll need to buy. Speaking of which, how will I choose the variable speed control? Most of them I've seen say things like "12V-48V variable speed control rated at 5 amps" and so-on. To me it seems it would be able to carry more amps at 12v and less at 48...., but again--I just don't do electrical very well!

Sorry for the long post, and thank you so much in advance for any help anyone can send my way!

#### JonathanEngr

Joined Oct 21, 2011
7
Yup--checked Graingers at the rec'd of my Dad. You're right... they are *not* inexpensive. A lot more than I would like to spend.

As for the off-feed table, I will use it when sawing boards, etc--no need for it to "pull" the board. Just something to move the excess out of the way.

As for questions, most of them involve wiring from ac to dc voltage and speed control (listed in my first post). Thanks for your reply!

#### #12

Joined Nov 30, 2010
18,217
That variable frequency drive looks like the Whiz-bang 9000ZX!
More likely that a retail PWM module would work. I've seen them on this forum, but don't know where to find them. Somebody please post the source.

Simpler answers: The wire size depends on the amps. Period.
A controller rated for 15 amps at 12 volts will not produce 30 amps at 6 volts. The amp rating is absolute.
Yes, 600 watts at 12 volts is 50 amps and 600 watts at 90 volts is 7ish amps. Much smaller wire is required at 7ish amps.

PWM means pulse width modulation. It applies full voltage in little tiny bites. That way, you have excellent torque at very low speeds. For higher speeds, it applies bigger bites until it is "on" all the time.

I'll go look for those PWM modules now.

Went to eBay. Typed in PWM. First thing on the page: 500 volt, 8 amp PWM controller, $34. It's Chyneez. Possibly junk, but an example. There are better PWM controllers available. Last edited: Thread Starter #### JonathanEngr Joined Oct 21, 2011 7 Terrific! Well, I finally have the answer to amps and wire size. That has bugged me for years. Okay--so you think I'll be better off with a 90v or 130 v DC motor? I noticed that there are PWM controllers out there for DC motors, but none for AC motors... I assume AC motors require something different? The reason I leaned towards DC is someone told me that AC motors can get overheated and/or damaged if they are not "made" for variable speed. Any truth to this? Here is the link to the PWM controller I was looking at for the 12V motor. Would this work on a 90V motor with lower current? http://www.virtualvillage.com/12v-3...dium=shcomp&utm_campaign=google_shopping_feed Initially this was the motor I was looking at. 6 amp draw at regular operation, but *60 amp* peak draw. https://www.surpluscenter.com/item.asp?item=5-1649&catname= Does this make sense? That would be what--720 watts needed? This is what threw me for a loop... could NOT find a 12V 60 amp PWM module. This is where someone stepped in and told me to get a 90V or 130V DC motor and use a bridge rectifier and capacitor to operate the motor. It went from somewhat straightforward to extreme difficulty (for me!). Is this rectifier/capacitor setup a real solution? Is it fairly straight-forward? #### #12 Joined Nov 30, 2010 18,217 Bigger volts is better. The Variable Frequency Drives are made for AC motors. I don't know if they can make EVERY AC motor work well. A 12 volt PWM controller will not work with 90 volts. The 12 volt rating is the maximum voltage it can survive. You can't find a 60 amp PWM controller because it's hard to get 60 amps through those tiny transistor legs. After all, residential wiring requires size 6 gauge wire to carry 60 amps. Consider that! and try got a higher voltage motor. The rectifier/capacitor setup is trivial to us. Transformer, rectifier, capacitor. I = radical2 capacitance frequency current. You try to keep the non-DC component below 10% of the DC rating. for a 90 volt motor at 7 amps, 7 = 1.414 x C x 120Hz x 9V 4700uf (a standard size) should do it. The first one I find costs$23
The transformer is expensive, too.

This is beginning to make the 9000ZX Variable Frequency Drive look reasonable!

#### JonathanEngr

Joined Oct 21, 2011
7
So--I'd need a 4700 uf capacitor... what about the rectifier? I understand that I would need a bridge rectifier? How are they sized? On amp rating? For speed control, couldn't I use a fan speed control on the AC side before converting it to DC via the rectifier?

#### shortbus

Joined Sep 30, 2009
8,429
What about a geared AC motor with a Variable-frequency drive
Now I may be wrong on this but never seen a VFD for single phase or 120V. They can have a single phase 240V input but all of them I know of are three phase out put.

#### crutschow

Joined Mar 14, 2008
26,735
So--I'd need a 4700 uf capacitor... what about the rectifier? I understand that I would need a bridge rectifier? How are they sized? On amp rating? For speed control, couldn't I use a fan speed control on the AC side before converting it to DC via the rectifier?
Bridge rectifiers are rated by both voltage and current. You need at least a 200V rating for 110VAC and current rating at least 25% higher than the maximum motor current. They will also need to be mounted on a heat sink.

A fan speed control would not work well to control the voltage. You could use a Variac transformer at the bridge input, but they tend to be big and relatively expensive. You might find a cheap one on ebay, though.

#### EVETS

Joined Jan 29, 2011
21
You can get a relatively inexpensive 90vdc drive from Grainger or other places I'm sure. I just bought 1 from them for work for around $200 for a 1/4 hp motor. 120vac in - variable speed out with a 10k ohm resistor. I'd look on Ebay or search 90vdc drive on the internet. Hope it helps. #### BillB3857 Joined Feb 28, 2009 2,542 If you are going to use a DC motor (90V, either PM or wound field) check out these drives. I don't work for them but have used them. Reasonably priced units. http://www.kbelectronics.com/ Thread Starter #### JonathanEngr Joined Oct 21, 2011 7 Bill--Funny! That's the drive I posted right before your post. I'm looking at the KBIC-120. #### BillB3857 Joined Feb 28, 2009 2,542 Bill--Funny! That's the drive I posted right before your post. I'm looking at the KBIC-120. I didn't know that Galco was distributing KB products. I always went directly to KB. Thread Starter #### JonathanEngr Joined Oct 21, 2011 7 Well, I went to KB but they didn't show prices (maybe I didn't dig deep enough). I stumbled on Galco after Googling "KBIC-120" looking for prices. You can find them on eBay for less than$50. Definitely cheap enough!

Anyway, will this do what I need?

#### strantor

Joined Oct 3, 2010
5,505
This is beginning to make the 9000ZX Variable Frequency Drive look reasonable!
Now I may be wrong on this but never seen a VFD for single phase or 120V. They can have a single phase 240V input but all of them I know of are three phase out put.
http://www.factorymation.com/s.nl/it.A/id.191/.f?sc=2&category=30

I've never used this brand of drive before, but it claims 120V single phase input, 0-230V output for 99\$ (I'm assuming you need 240V input to get 230V output). I think its very possible that the 9000ZX is your cheapest option. motors aren't cheap, nor are DC drives.

This would require your motor to be 3phase; I haven't seen that information come out yet, is it 3phase?

What you mentioned about motors that aren't meant you be run at variable speed, being forced to run a variable speed does hold some water. Here in industry, we *should* only use inverter duty rated motors with inverters. I have never tried to use a non-inverter duty rated motor with an inverter, but I have heard that as long as you don't run them at too low speed, it *shouldn't* overheat. Running a regular motor with a VFD (aka inverter, drive, speed controller, variable speed drive, etc. = the thing I linked to) *shouldn't* outright kill it, but rather shorten the life of it. That's why we don't do it here in industry, because we run motors 24/7 and we want them to last as long as possible. For you and your tinkering, I don't think you would put enough hours on it to matter. I would think that as long as you keep an eye on the motor and make sure it doesn't overheat, it will probably outlive you. But then I don't know you or how much you will use the thing. there is plenty of information out there about it if you're up for a read.

BTW inverters are the AC equivalent of you PWM controller. They basically "construct" an AC wave out of alternating positive and negative pulses of varying "duty cycle". - plenty of keywords there to follow if you're interested.

#### hspalm

Joined Feb 17, 2010
201
Now I may be wrong on this but never seen a VFD for single phase or 120V. They can have a single phase 240V input but all of them I know of are three phase out put.

I don't know about the 120v AC american standard. But most VFDs have inputs for both single phase and three-phase input at line voltage. The line voltage input is then rectified and switched at different frequencies for rotational speed control. So any voltage high enough for the motor is fine, AC or DC, it's rectified before switching either way. Most industrial motors are 3-phase wired, the ones used in single phase applications are three phase motors with either separate single phase windings or simply a capacitor between two windings to replicate the third phase. These motors are not expensive compared to DC motors at same voltage/performance!

#### strantor

Joined Oct 3, 2010
5,505
I have never seen higher HP (>5hp?) drives that will take a single phase input. maybe my eyes just aren't open. Its always the low power ones that can do that. What I've seen when you try to put single phase into the larger ones (or any drive that wasn't meant for single phase) is that you get some kind of "Incoming Phase Loss" or "line voltage Dropout" alarm. if you want to put DC into the drive, you should get one that has Common DC Bus terminals, so you can bypass the rectifier section. I would imagine (jst guessing) that putting DC into the recitfier would overstress one set of diodes, while leaving the other set unused.

#### shortbus

Joined Sep 30, 2009
8,429
@Strantor and hspalm - should have worded my response better. I meant "120V or single phase output."

#### strantor

Joined Oct 3, 2010
5,505
@Strantor and hspalm - should have worded my response better. I meant "120V or single phase output."
yeah, I was agreeing with you. I quoted you, then went on to say basically the same thing to strengthen the point.