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studentjim
12-10-2012, 04:40 PM
I just bought a cnc wood lathe that is quite old. The power supply seems to be well built with a large transformer and capacitor. The driver boards are bipolar with an input of 44.9 volts tested with a voltmeter,the output at the motors is also 44.9 volts. Since the drivers are pretty old and not much info on them (Centroid Micro 1 rev. 891102) I'd like to use the power supply with some newer drivers that I have,but the new drivers are limited to 24 volts so I need to reduce the voltage from 44.9 vdc to 24 vdc. I know that it can be done, but not being an electrical guru, I'm not sure how to go about it. The motors are rated at 4.38 vdc and 4.0 amps dc. Any light on this subject would be greatly appreciated. .............Jim

MaxHeadRoom
12-10-2012, 04:47 PM
You can take turns off accordingly, which is the most efficient way.
If it is a Toroidal transformer it is much easier.
You can reckon on around 2turns/volt.
If you want an exact ratio, you can put on, say 10 turns on and measure.
The EI style transformer is a bit of a pain, but it can be done.
I have done a few in my time and just requires a bit of patience.
Remember that the DC will be 1.414 x the AC secondary, or another way, the AC required is .707x DC.
Max.

studentjim
12-10-2012, 05:25 PM
Thanks Max, I never really thought of doing it that way, but it is an EI style transformer and it may not be worth the trouble.

Barrington
12-10-2012, 08:05 PM
Do you happen to be using a dual input voltage transformer (240/120) which you are using at 120 ?

If so you could use the 240 primary at 120 and get nearly 24 vdc out...

Cheers

.

studentjim
12-11-2012, 09:51 AM
I found a converter that is made in Germany 30 to 80 vdc input, 24 volt dc output available in Canada and USA. The converter is a Feas model DCC9024-2. Just thought I'd post this info perhaps someone encounters a similar problem. Thanks to Maxheadroom and Barrington for you input.

MaxHeadRoom
12-11-2012, 10:29 AM
A couple of things, the convertor is only rated at 5amps, you did not say what the VA of your transformer is?
Your motors are 4amps ea?
At almost $200.00 I would be inclined to get a linear toroidal supply or transformer from Antek or similar.
The ≈ 1 hours work and zero cost, the turns removal sounds a better idea!!.
Max.

ironmonger
12-11-2012, 11:18 PM
You might also look at the secondary of the existing transformer and determine if perhaps there is an un-used center tap available.
If you need to replace the power supplies you might also find out if it would be cheaper to replace the drivers.
Are the 'motors' steppers or servo motors? Sounds like steppers.
I don't do much with steppers, but I believe the name plate ratings on the stepper and the actual supplied voltages don't directly correspond. you may need all that voltage to drive the steppers. If they could have designed it with a 24 volt supply, I don't know why they would have used 45 volts


paul





I just bought a cnc wood lathe that is quite old. The power supply seems to be well built with a large transformer and capacitor. The driver boards are bipolar with an input of 44.9 volts tested with a voltmeter,the output at the motors is also 44.9 volts. Since the drivers are pretty old and not much info on them (Centroid Micro 1 rev. 891102) I'd like to use the power supply with some newer drivers that I have,but the new drivers are limited to 24 volts so I need to reduce the voltage from 44.9 vdc to 24 vdc. I know that it can be done, but not being an electrical guru, I'm not sure how to go about it. The motors are rated at 4.38 vdc and 4.0 amps dc. Any light on this subject would be greatly appreciated. .............Jim

darryl
12-12-2012, 12:13 AM
I was going to suggest looking at the secondary windings as well. If there is a center tap and it's connected to ground, then you are out of luck to re-configure it for a lower voltage. In any case, if the center tap is used there are probably only two rectifiers, not a bridge. If there's a bridge rectifier in the secondary power circuit, then it's going to be fed from a single secondary winding needing only two leads. You could possibly have a configuration where that winding has three leads and a bridge rectifier- if so then there is a chance that a simple mod would give you half the voltage output.

The input voltage, the primary side, could be two windings as well as Barrington suggested. If so, you will probably find them connected in parallel for operation on 110vac, and there will likely be a hookup chart on the transformer somewhere showing how to wire them for 220. That configuration will certainly work on 110 and give you roughly half the output voltage you're getting now. There will be a loss in the maximum power the transformer can deliver, but that may or may not be a problem.

If the secondary is already wired using the center tap, then it's a dicey thing to remove windings to leave a lower voltage. Both sides of the center tap would have to have the same number of windings removed so it remains balanced. Sometimes the windings are on top of one another, which would make this impossible to achieve. However, if this is a single secondary winding, then yes you do just remove enough to lower the secondary voltage.

There is another option, depending- if the windings are visible and accessible, there is the possibility that you could tap into them to gain access to a lower voltage instead of having to remove windings. It's also unlikely, but possible.

The Artful Bodger
12-12-2012, 03:19 AM
Did someone mention this? If you reduce the transformer secondary voltage by removing windings you are also reducing the power the transformer can handle. Same thing if you change the primary voltage (or winding) to get the desired lower secondary voltage.

If you want the same power at half (approx) the voltage you need to wind half the number of turns at twice the current carrying capacity. Either rewind with heavier wire or unwind the existing winding and rewind it with two wires in parallel.

EVguru
12-12-2012, 05:24 AM
You do appreciate that dropping the stepper motor supply voltage is going to lower the achievable speeds (without dropping steps) by a big chunk?

You might end up with a machine that is painfully slow.

studentjim
12-12-2012, 09:43 AM
Thanks for all the suggestions, but not being electronically gifted, I've decided to go with something a little more modern. Since the driver boards are are quite old (circa 1993) who knows what problems might be lurking within. I do have some new stepper drivers that are limited to 24 volts so to make things simpler I will just buy a 24 volt power supply. I am able to buy a good working power supply locally that has two 24 volt outputs @ 12.5 amps per output for $50.00 so that's the direction I've taken. Again thanks to everyone for your suggestions. ......Jim

MaxHeadRoom
12-12-2012, 11:13 AM
Did someone mention this? If you reduce the transformer secondary voltage by removing windings you are also reducing the power the transformer can handle. Same thing if you change the primary voltage (or winding) to get the desired lower secondary voltage.

If you want the same power at half (approx) the voltage you need to wind half the number of turns at twice the current carrying capacity. Either rewind with heavier wire or unwind the existing winding and rewind it with two wires in parallel.

The VA of the transformer will not change, lowering the voltage will increase the secondary current capacity accordingly, but this depends on the wire gauge of the secondary as to whether it can handle this current.
The problem with using a 240v primary setting on 120v is the efficiency (Q) of the transformer is reduced (turns/volt is increased)and consequentially reduces the available VA.
Max.

The Artful Bodger
12-12-2012, 01:23 PM
The VA of the transformer will not change, lowering the voltage will increase the secondary current capacity accordingly, but this depends on the wire gauge of the secondary as to whether it can handle this current..

Which is why I said the current handling capacity of the secondary would need to be increased...

ironmonger
12-12-2012, 02:10 PM
As others have mentioned, you may have problems driving the steppers at 24 volts. Just because the power supply and the driver board are happy at reduced voltage doesn't necessarily mean the steppers will be.
If you cut the voltage in half, the resistance of the circuit still remains the same. the current will be halved and so will the power output.
Like I said... let us know

paul




Thanks for all the suggestions, but not being electronically gifted, I've decided to go with something a little more modern. Since the driver boards are are quite old (circa 1993) who knows what problems might be lurking within. I do have some new stepper drivers that are limited to 24 volts so to make things simpler I will just buy a 24 volt power supply. I am able to buy a good working power supply locally that has two 24 volt outputs @ 12.5 amps per output for $50.00 so that's the direction I've taken. Again thanks to everyone for your suggestions. ......Jim

Evan
12-12-2012, 07:56 PM
If you cut the voltage in half, the resistance of the circuit still remains the same. the current will be halved and so will the power output.

Volts x Amps = Watts. Drop the voltage in half also cuts the amps in half. That drops the power to one fourth.

ironmonger
12-12-2012, 09:26 PM
Evan, you are absolutely correct. Sometimes my fingers get ahead of my brain.

I guess the upshot of all this is if the existing boards are working, just run it even if they are old. Hell, I'm old and most of my junk is working... :-)

If the boards die, buy something like this:
http://www.cnc4pc.com/Store/osc/product_info.php?cPath=75_74&products_id=359

My guess would be that at 1/4 the power those steppers won't work very well. Even if you retrofit new steppers and drivers along with a new power supply, the power rating of the existing motors was chosen for a reason. Stick with that...


paul



Volts x Amps = Watts. Drop the voltage in half also cuts the amps in half. That drops the power to one fourth.

kf2qd
12-12-2012, 10:07 PM
The motors are rather old, and the drivers are rather old. Some of the older motors were high voltage motors and the newer motors, although they are lower voltage, along with newer drivers will give much more performance. What matters more is the torque rating of the old motors versus the new motors.

You might also look into some brushless motors (would have to gear them down, but that increases the useful torque) as they beat steppers hands down.

The old power equation doesn't tell the whole story with steppers as they are generally driven with chopper type drives which use voltage to magnetize the coils, but actually put out very low current.

Evan
12-13-2012, 02:55 AM
Higher voltage reduces I^2*R losses by a factor of 2 for a doubling in voltage for the same power. That reduces the heating of the coils by that same amount. Steppers are usually operated at far above the rated voltage listed on the label for that reason and one other. The average power is maintained within acceptable limits by chopping the waveform using pulse width modulation. Using higher voltage also overcomes the inductive reactance of the coils allowing for faster stepping with same total power draw.