# Thread: Power supply for servos

1. ## Power supply for servos

Okay, I have picked my drivers out...I want to supply 25 amps to each driver at 170 volts. I have 3 drivers so do i need to bridge rectifier that puts out 3 times those amounts in volts and amps or is it just the volts. Since most bridge rectifiers are standard amounts ..do you just round up the number required?
If i have 120v ac going into the transformer,how many amps does it need to be also,my common knowledge says 3 times the motor amount,but thats a large load. I have the DC volts= AC Voltage x 1.414...but is the total combined voltage or just 1 motor. Any insight would be welcome, and i must say this cnc build has become such a learning experience, i think i might have to YOU TUBE the build just for memory sake lol.

Thanks

2. Is this the motors on your other post?
They show constant torque at 6.5A?
If you used the numbers you have, you would need a 9KVa transformer!
AC secondary is .707xDC, so you could get away with a common 120V secondary transformer if you wish.
The bridge rectifier needs to be sized for current and something called PIV (peak inverse voltage), or what the bridge voltage is subjected to when the rectifiers are reverse biased.
Bridge rectifiers are cheap so you can oversize without the need to size tightly because of cost.
So you need the I, current value, lets say 50amps and a PIV of, in the case of a bridge, 200~250PIV.
I don't see you requiring anything larger than 1.5Kva for a supply.
You also need 250vdc rated electrolytic capacitor(s).
By my reckoning you would need ~8000µf-12,000µf, for a DC p/p ripple of =or<10vdc.
My suggestion anyway.
Al.

3. Thank-you Al....you are the man. I understand about 25% of what you said ,so after i do some research on what you stated so that i can understand the reason why you gave these numbers, i might ask a few more questions to get it down...i dont like just doing things, i like to know why they need to be done also. I will do some homework and get this figured.

Thanks

4. okay..you are saying the voltage of the secondary winding of the transformer is equal to .707 x the DC voltage required for 1 motor. So 1 motor requires 176 volts DC max. 176 x .707 =124 which makes a 120 volt secondary winding acceptable. Ok....there are 3 motors...does this matter or having the three motors share the load is ok?
Hope im not way off base.

Thanks

5. If using one P.S. for all drives, then the voltage is the same whether 1 or 3.
The current will be a product of the combined current for the total number of drives, it is going to be seldom, if at all, where all three motors require maximum current at one time.
This is generally taken into account when sizing the KVA rating of the supply.
(.707 is the reciprocal of 1.414 you mentioned earlier).
Al.

6. u r correct, u want 120vac secondary for about 160vdc bus. yes, all 3 motors work off this same dc bus. if 6.5a each rated continuous, AL was trying to tell u the xfmr would be around 6.5*3 motors*160vdc=3.1kva but not all 3 will work 100% at the same time so half that size shud be fine; thus his 1.5kva xfmr size. bridge rectifier rating shud be then 1500va/160v= 10 amp so buy the cheap \$ 3.oo 25amp one. u r going right direction, keep going.

hee hee - looks like i wrote this at same time AL gave his similar reply

7. Thanks guys...slowly beginning to make sense...if i have any more questions or aha moments i will let you know.

Thanks

8. i see where i went wrong now...i was using the 35 max amp as my amp used per each motor and i was thinking that all the motors where being driven hard non-stop lol. Well i learned alot today...but the day is young!!

Thanks

9. feel free to click on Al and/or my 'buy me a beer tab when you get-r-running!

10. I've been working on a similar power supply for an as-yet unbuilt system. So, one caveat: my knowledge is all theoretical, at this point.

From searching ebay and surplus shops, I've not encountered much in the way so-called 'isolation transformers.' That seems to refer to transformers that don't change voltage from primary to secondary, e.g. 120VAC primary, 120VAC secondary. I have seen a number of 'control transformers' and 'general purpose' transformers. Typically (in the US at least) these seem to be 480VAC or 240VAC primary (or both) and 120VAC or 240VAC secondary (or both). If you have a 240 volt circuit available, this would probably work best.

I like to use the web form calculator at Online Calculator .:. Linear Power Supply Designer

A few notes on usage:
1.) This calculation seems to be extremely conservative regarding the smoothing capacitor volt rating requirements (165vdc output suggests 300vdc rated caps. Elsewhere I've seen it suggested that a 20% margin is fairly safe to assume).
2.) A high voltage supply probably would not need regulation, so ignore that parameter. Rather, if you insert the max desired voltage ripple, it will give you a reasonable capacitor size value (i.e. 165vdc output x 10% ripple = 16.5vdc voltage in the 'regulator' voltage drop field).
3.) I've heard it suggested that rectifier ampacity ratings should be at least twice what you expect to use. I've bought 50 amp bridge rectifiers off ebay for a couple dollars a piece (albeit from china). There are some 150amp modules I've seen for less than \$20. If your volt ratings are enough (600v and 1000v units seem common) and you amp ratings are sufficient, it apparently comes down to having a big enough heat sink to keep it cool during operation.

I plugged in 37 Amps and it suggests an 8kva transformer is necessary. I've seen them on ebay at 7.5kva, 10kva sizes, mostly 480x240 primary, 240x120 secondary; I think these would work well. Since units this size weigh about 150 lbs, what kind of a deal you get might depend on whether you can find one locally.

Just curious, what drive are you planning to use? 25 amps per axis is some pretty serious power for any project. Is this for servo motors?

Let us know how it turns out when you get it figured.

-b2b

11. Originally Posted by born2bewilder
Just curious, what drive are you planning to use? 25 amps per axis is some pretty serious power for any project. Is this for servo motors?

Let us know how it turns out when you get it figured.

-b2b
Previous post suggest these are 6.5a @ continuous torque servo's, the 25a quoted was peak torque, usually max before demagnetization takes place, which should never be reached.
BTW, if the drive has isolated input control, and many do, it is possible and feasible to use 120ac direct supply without transformer.
This is done all the time with A-M-C drives and others that have built in P.S.
Obviously with this method you cannot connect P.S. DC common to earth ground, as you could with a isolation transformer.
But many do not anyway.
Also the link to the linear supply is a regulated linear, which is not necessary for stepper/servo supplies.
Al.

12. b2b that 25 amps was almost full load for those servos. Al the Man steered me straight and told me I only need to use the continuous rating for the servos which is 6.5 amps. They are large servos.I got an industrial morbidelli u46 router less most of the control for nothing so im turning it into a 4x8 flat vacuum table unit. I still want to retain all the spindles and drill blocks but it is becoming very fun to figure it all out. Its a heavy machine, the base was weighed when i took it out of the scrap yard and it came in at 4200 lbs. The bottom screw on the x axis is 2" around i believe. I could never do a build of this caliber if it was not for the free router in the first place.

thanks

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