# Thread: Yet another power supply question 38vdc

1. ## Yet another power supply question 38vdc

I need some help constructing a power supply for 38vdc Ametek dc motors.
Please correct me if I am not correct on my assumptions.
Motor DATA
900 RPM AMETEK, #965922-101. Permanent magnet DC motor rated 38 VDC nominal. Reversible, ball bearing. Max. rated rpm is 1300 max. Armature current before permanent demagnetization is 12 amps. Torque constant is 50 oz-in per amp. Terminal resistance 20 ohms +-20%. Voltage constant is 37 VDC per 1000 rpm. When tested on 38 VDC, we recorded the following data: No-load spedd 1100 rpm @ 0.260 amp. With a 100 oz-in load the shaft speed was 900 rpm @ 2.3 amps. When tested as a generator at 1000 rpm, the output was 5.8 amps @ 12 VDC.

1/So the desired DC voltage is 38vdc.
2/To calculate the desired transformer voltage 38vdc/1.4 = 27.14 VAC
3(i)/The desired transformer current and Volt amp (VA)rating needed is using four motors would be 872VA. 4 x 12 amp x 0.67 x 27.14VAC
3(ii)The desired transformer current and Volt amp (VA)rating needed is using three motors would be 655VA. 3 x 12 amp x 0.67 x 27.14VAC
3(iii)The desired transformer current and Volt amp (VA)rating needed is using two motors would be 436VA. 2 x 12 amp x 0.67 x 27.14VAC

AM I CORRECT SO FAR??

Because I am a Canadian Smuck (I hate paying brokerage fees), I would like to purchase a toroidal transformer from Plitron.
Looking at Plitron standard toroidal transformers, they have 400VA, 500VA, 625VA,750VA, 1000VA, which one would work best?

Any help would be appreciated

P

2. Do the calculations for 25 amps and you'll be okay. The 750VA one will work fine, the 1000VA will give you alot of room for growth. Transformer voltage what you've calculated is correct, using a bridge add another 1.4 volts for 2 diode drops...does Plitron offer custom voltages? If not then pick something between 28 and 30 VAC.

3. Plitron offer's custom voltages but 4-5 week lead time

custom 51v 15a for ~\$100 <--- hard to beat
1 week

Dan

4. Thanks
>Do the calculations for 25 amps and you'll be okay. The 750VA one will >work fine, the 1000VA will give you alot of room for growth. Transformer >voltage what you've calculated is correct, using a bridge add another 1.4 >volts for 2 diode drops...does Plitron offer custom voltages? If not then >pick something between 28 and 30 VAC

So I am looking for a 750VA toroidal, the ratings from both Plitron and SumR are 30 VAC with 12.5Amps.
Question
Will this be enough amperage to operate two, three or four motors??
It is going on a cnc router table.

Now, the filter capacitor if C=(80,000*I)/V
then C = (80,000*12.5amps)/V,
Question
but which voltage do I use here?, the 38vdc of the motor requires, the voltage of the toroidal in AC,or dc voltage of the toroidal VAC * 1.4????

P

5. >Now, the filter capacitor if C=(80,000*I)/V
>then C = (80,000*12.5amps)/V,
>Question
>but which voltage do I use here?, the 38vdc of the motor requires, the >voltage of the toroidal in AC,or dc voltage of the toroidal VAC * 1.4????

O.K. I think the voltage to be used is the output voltage of the supply.
So C = (80,000*12.5amps)/30, C = 33,333 microfarads minimum filter capacitor size.

But if the toroidal has dual secondaries the output voltage is still 30volts, but the amperage is now 25amps
So C = (80,000*25amps)/30, C = 66,666 microfarads minimum filter capacitor size.

AM I ON THE RIGHT TRACK???

P

6. C=80000*I/VDC

Regarding the motor what are they rated 12amp max?
1300rpm = how many amps?

1000 rpm @37vdc
1300 rpm @48vdc

How fast are you planning to run these at 1000 or 1300

IF this is the case, assuming 3 motors, how often will
probably never and prob only during rapids = little or no load

assuming 12amps you'll prob need 5-8amps/motor
I'd say 15-18amp transformer

Maybey some of the pros here could help
shed some light

Dan

7. > [QUOTE=DACMACHINE]C=80000*I/VDC

>Regarding the motor what are they rated 12amp max?
>1300rpm = how many amps?

>1000 rpm @37vdc
>1300 rpm @48vdc

>How fast are you planning to run these at 1000 or 1300

Not sure, I have not purchased them yet!!

>IF this is the case, assuming 3 motors, how often will
>probably never and prob only during rapids = little or no load

Probably never, I was planning on building a cnc router for machining wood(mainly), thoughput and rapid travel speed is NOT the main criteria.

>assuming 12amps you'll prob need 5-8amps/motor
>I'd say 15-18amp transformer

P

8. You're on the right track 28 to 30 VAC @12.5 Amps with dual secondaries you'll have the required voltage and amps.