Lathe DC motor power supply

[LatheMan]

Hi, I have a lathe that I am converting to use a 90v DC motor. I already have a speed controller that will generate the correct waveform (to drive the motor at different speeds), but it requires 90v DC inputs.

The motor can pull at max of 10A at 90v, so I need to build a 1000W AC to DC converter. Since using a full wave bridge rectifier will multiply the input voltage by √2 I need to drive it by 60 volts(AC) yielding 83 volts(DC). My question is this, can I use this transformer backwards (assuming it has no other components,diodes,etc) to take 120v mains and step it down to 60vac?

Amazon.com: Step Down Voltage Converter Transformer 1000 Watts. Converts AC 220V-110V-For use overseas in 220V/240V countries(Model VOD 1000): Electronics

Its supposed to convert 120vac to 240vac or vice versa. So it should simply be a large 1:2 transformer, right?

~LatheMan

 

[BrownOut]

NO, it converts 220V to 110V, so you would use it in a forward manner, not backwards. You would connect the 110V to the 220V side and take the output from the 110V side. I can't think of any reason it wouldn't work for you. There might exist a less bulky/less expensive solution, however.

 

[tcmtech]

There are several ways to run a 90 volt DC motor off of standard 120 volt AC power.
The simplest is to use a variac(variable transformer) and a rectifier on the output. That will give you full speed control as well.

Another common way is a basic SCR rectifier bridge that uses phase angle control to get the 90 volts you need. Its basically a higher powered lamp dimmer device.

You can also just use a 120:36 step down transformer set up in a autotransformer configuration to drop the incoming line voltage down part way before rectifying it. being it only has to handle part of the voltage drop you can get by with a much smaller and cheaper transformer. The 120:36 transformer will give you a roughly 92 volt RMS output.

In some cases it completely acceptable to run the motor directly off of the 120 volts AC source with nothing more than a bridge rectifier to convert it to DC. The motor will spin faster but usually if the loads are not high and the amp limit is not passed they will work just fine. I have done it many times. But it does have limits of course!

Basic DC motor drive off of an AC source is fairly simple. You can make it expesnive and complicated but its not really nessisary for an aplication such as yours.

 

[LatheMan]

These things are all over the place, I've seen both types listed with the same pictures. But yeah, the general idea is to use just the transformer part to divide the AC voltage. What would be a cheaper/lighter option? I want to keep ripple voltages as low I can (within reason and expense) because I am not sure how tolerant my driver board is of dirty input. It was intended for use with batteries, so I think if I keep the ripple less than 5v it would be ok.

 

[LatheMan]

My intent with all of this was to slow down the lathe down. It's minimum right now is 120 rpm, which is a little too quick for threading. The machine has no back gear (gear down) either.

Using voltage to control the DC motor causes more arcing on the brushes. Especially in high torque applications like lathing. So I don't want to use that method although I know it works. I already have a good H-Bridge controller that will allow me to spin the motor slowly for threading without cogging or arcing, and it will give me active braking, etc. I just need to supply it with relatively clean DC.

Perhaps the 120:36 in an autotransformer configuration can replace my larger transformer in this power supply?

 

[tcmtech]

Without knowing what your power supply can handle for a maximum input voltage its difficult to say. you could end up with needing a 120:48 volt transformer to get an open circuit DC voltage of around 90 -100 volts at load when the proper size of capacitors are used for the filtering.

I have a Smithy 1340i with the 2 hp heavy duty motor and I have considered changing out the whole power system simply because I dont like its performance. It seems unnecessarily complicated for what purpose it serves. A simple Variac and rectifier would be far more effective in my personal aplication. OR a full VFD system and I replace the Higher speed DC motor with a slower and higher torque three phase motor.

I have the similar problem of not having smooth low speed and higher torque capacity. The machine is built well enough but the electronics and the motor itself are not to my preferences in this working range.

 

[LatheMan]

Ahh, I am somewhat in the same boat as you. My machine (**broken link removed**) normally comes with these chinese copy 3/4hp AC motors. Mine burnt out from doing constant reversal operations for threading. So I figured I would do an upgrade and switch to DC. I have **broken link removed**, and **broken link removed** already.

The controller seems capable of taking up to 190VDC as input, but the motors max field voltage is 90vdc. Not sure how tolerant the motor would be of higher voltages.

I had not considered autotransformers for a power supply. Could I use a smaller transformer in an autotransformer configuration to achieve the same 10A@60AV output?

 

[BrownOut]

After perusing a transformer catalog, I think your first idea may be the best one. I would be tempted to directly rectify the 120V and try to chop it down to the requisite 90V. Tha's how my lathe works, as there is no transformer and it works on 90V. If I were at my shop, I'd see if I could figure out how it's done, but I'm not there. At any rate, that would open another whole can of worms, and I'm sure you just want to get up and running.

Consider splitting your power between your motor and controller and use seperate filters for each to reduce ripple to your controller. Will your controller work @ 80Volts? You're gonna lose some voltage in your rectifier.

 

[LatheMan]

>tcmtech
Ahh, I am somewhat in the same boat as you. My machine (**broken link removed**) normally comes with these chinese copy 3/4hp AC motors. Mine burnt out from doing constant reversal operations for threading. So I figured I would do an upgrade and switch to DC. I have **broken link removed**, and **broken link removed** already.

The controller seems capable of taking up to 190VDC as input, but the motor's max field voltage is 90vdc.

>BrownOut
Yeah, after trying to find a suitable transformer through the usual suppliers (digikey, etc) I found the all wanted 75$ or more for a lump of iron and copper.

The only other option is doing the autotransformer configuration mentioned by tcmtech, which would allow me to use a smaller transformer before the rectifier, but I am having a hard time finding a diagram and formulas for sizes of transformers in that configuration.

 

[LatheMan]

>BrownOut

The controller is designed to run on a number of voltages. It is simply a PWM chopper with some current sensing and over current protection. My understanding is that the rectifier will loose around 2 volts due to the diodes, but that the full wave bridge will increase the voltage by 1.414... which is why I would need 60vac in.

120vac --> 2:1 transformer --> 60vac --> Full Wave Bridge = (60*1.414)-2 --> 83V dirty DC --> capacitors --> H-Bridge controller --> DC motor

 

[Hero999]

I'd recommend a 120V to 90V transformer with a rectifier on the output. A halogen lamp dimmer can be connected in series with the primary to vary the speed of the motor.

 

[LatheMan]

The halogen lamp dimmers are SCR or Thyristor control. I basically already have one of those. Where would I get cheap 120 to 90vac transformer? Almost all that I can find are well over 100$.

 

[LatheMan]

>[BrownOut] Directly rectifying the 120 AC would yield 168V dirty DC. This was my first consideration since 90 is so close to 120. What kind of circuit could chop 168v down to 90v and still deal with 10 amps?

This option would be best because it would eliminate the transformer.

 

[BrownOut]

If you're good at designing electronic circuits, you might design a PWM/Filter circuit. The tricky part would be sensing the voltage and adjusting the pulse width. I'd expect to do alot of tweaking to tune it to your particular system. Hmmm...? Might be more trouble than it's worth. I'll see if I have a schematic, but can't gaurentee.

 

[Hero999]

It depends on what you mean by cheap and the power level you require?

My local supplier sells transformers with twin 45V secondaries which can be connected in series to make 90V.

**broken link removed**

Another thing: make sure your dimmer can handle inductive loads, most halogen dimmers will be fine but some will be killed if you connect a large transformer. Either way, it's possible to reduce the possibility of damage by connecting a snubber network across the transformer or dimmer.

 

[LatheMan]

Thanks everyone for your posts and considerations.

Let me recap my issue because I feel we are getting off track.

I do not need a motor controller. **broken link removed**. It's just a chopper, its input voltage is its output.

I do not want to drive my motor with SCR or Voltage based controls. All of these have major draw backs. Either they will damage the motor with harmonics, or have poor low speed torque. I've read enough posts of others attempting to drive variable speed motors like this and not getting good performance. Read PWM and Heating to understand why.

I need a clean 90VDC power supply rated at 10amps.

Possible solutions:
Transform 120AC->60AC->Rectify to 85V->Filter
Rectify 120AC->120DC->Filter->Regulate down to 90V
Dimmer 120AC->90VAC->Rectify->Filter

Hero999, are you suggesting I use the dimmer to get 90v and then rectify and filter it?

 

[shortbus=]

I think you'll find that your DC motor is going to like instant direction reversals even less than your AC motor did.

I put one of these on one of my lathes; Burden Sales Surplus Center Item Detail It works great.

One other thing to think about- when you use a speed controller on a motor to slow it down by a large amount, it causes the motor temperature to go up! Not from an electrical problem but from the cooling fan not turning fast enough to keep it cool.

As far as getting it as slow as a backgear, you might consider a smaller pulley on the motor. I added a two step pulley on my 12x40 lathe, and it makes all the difference in the world.

Try here for reasonable transformers; https://www.antekinc.com/

Just my $0.02

 

[tcmtech]

I am not flowing what type of controller you have. I have never seen a lathe that was 90 volt battery powered.


When controlling the speed of any DC motor if the control system is designed properly there is no difference in its output capacity or motor efficiency regardless of method. Its the badly designed or incorrectly designed stuff that causes the problems.

As far as I know the only way to burn up a motor is to exceed its amp rating for a long enough period of time for the windings to cook or run it without any cooling system if its designed to use one and cook it that way.

 

[tcmtech]

As far as getting it as slow as a backgear, you might consider a smaller pulley on the motor. I added a two step pulley on my 12x40 lathe, and it makes all the difference in the world.

Thats mainly why I want to change my lathe over to a VFD drive. Right now the DC motor it uses is rated at 2 Hp @ 5000 RPM. With the VFD I can use a stock three phase motor that is rated 2 Hp @ 1175 RPM. If I need more speed I just have to run it on a higher frequency. At 250 Hz my 1175 RPM will be basically the same as the old 5000 RPM DC motor but will do the low speed lugging work far better..

 

[BrownOut]

Thanks everyone for your posts and considerations.



I need a clean 90VDC power supply rated at 10amps.

Possible solutions:
Transform 120AC->60AC->Rectify to 85V->Filter
Rectify 120AC->120DC->Filter->Regulate down to 90V
Dimmer 120AC->90VAC->Rectify->Filter

Hero999, are you suggesting I use the dimmer to get 90v and then rectify and filter it?

Option 3, using a lamp dimmer ahead of the rectifier won't really work. The lamp dimmer changes the RMS voltage by changing the wave shape, so it won't really work for rectifying. You're first idea is still the best way to get going, though it's going to be pretty heavy. Sometimes, you just have to compromise.