Need Help!- Power Supply/Power Cord Sizing for Four 850 oz-in Servos, One is Slaved

[Plugger]

Power Supply/Power Cord Sizing for Four 850 oz-in Servos, One is Slaved

Could someone please give me feedback on power supply and power cord sizing? This is an aluminum machine made to cut aluminum plate. It is in a residential building connecting to 240VAC but will be moved to “light industrial” commercial space in the future. Here are some details:

I have four 850 oz-in Keiling servos.
Continuous current: 7.5 amps
Peak current: 33.5 amps

I also have
Spindle: 15 amps (3-1/4 hp router)

My gantry is 3 ft. wide weighing 215 lbs. with two servos driving it. A servo is on each side with one slaved to the other. Calculated servo inertia ratio was less than 10, with a 2:1 toothed belt reduction.

Servo cable is rated at 41 amps.

Power Supply Sizing Problem:
For a 3-servo machine, I believe a general rule of thumb that others have used is to take the peak current draw, multiply it by 3 servos and take 66% of that.

My 4-servo total current calculation for power supply sizing is:
(66% x 33.5 amps x 4 servos) + ( 15 amp spindle motor) = 104 amps

I took 12 amps off because one servo is slaved. So, 92 amps still seems like way too much for a machine with a 200 lb gantry. Life experience is telling me it’s a 65 amp machine. I don’t think it will be stiff enough to handle deeper cuts anyway. Am I off track?

In any case, I have one 1500W, 19.5 amp Toroidal power supply. The data sheet states that it can easily handle 200% overload for a few minutes. I don’t know how they come up with the 19.5 amps since it varies inversely with the voltage. So, I was planning to run it with just one power supply while checking the toroid for heat buildup. I think two of these should be enough. That will be 39 amps continuous with 78 amps for a few minutes. What do you think?

Power cord sizing should then be 78 amps + 15 amp spindle = 93 amps. In my residence, I can only get 240VAC ,50 amps. So, I’ll be limited on depth of cut until I move it to a “light industrial” commercial space. Does this sound like I’m in the ballpark and like it's going to work okay for now?

Thanks!

[Al_The_Man]

I think you are way over with your numbers, care should be taken not to run at the peak servo current, as this approaches the destruction point of the servo.
Your Toroidal P.S. will not be 19.5 amps even on 120v, even less on 240v.
It is extremely rare for all 3 motor to run all at once in continuous current mode, never mind peak current.
I think you need to revise those numbers some otherwise your cabling and fusing is going to be way over sized.
Al.

[mike_Kilroy]

I'm with Al; your numbers are way way way hi.

To your question will it work with the one power supply to start - yep.

I cannot say anything about your ps being rated different at different voltages; but assuming it is 19amp cont, sounds prob ok for this machine - or maybe one more.

here is how I would add it up:

case 1
-----------
4x 7.5a*50% = 15amps for all servos
10 amps for the spindle

so 25 amps total rms required - probably worst case in the real world.

keep in mind u prob will not be running spindle at 100% load most of the time - so say 10amps for 50-70% typical load.

u do not size servos for peak; just make sure ur ps can handle the peak short term, and not all at the same time.

no one runs all 4 motors at full rms load long term at the same time so dont size to that either.

no one machines at full speed! remember ur servo drive is a dc-dc converter -- ie., power in = power out. so if you run at full amps cutting - at what, 1/4 top speed typically, then since rpm = volts, motor volts =1/4 rated. so power in is 1/4 so your 7.5amps into motor comes from only 1.8amp in! THIS is the number you should consider for sizing. so 2a*4+10spindle=18amps typical case!

this is why i say one ps is prob fine - try it.

[Al_The_Man]

You mention everything is running off of 240v?
You rate the power devices by the primary current, on 240v the 1500va transformers will not consume half of 19.5 amps, I would allow 10a delay fuse on the primaries.
Al.

[Plugger]

Thanks Al and Mike!! My stress level just went down a few notches!!

Correction to previous postings: The spindle is *not* running off the power supply. (See attached Electrical Schematic in next post.) As a result, it seems the power supply should now be large enough. Also, I modified the suggested fuse values due to this and incorporated them into my schematic.

Here's my logic:
The power supply is 240VAC 1500W with 200% overload for a few minutes. Therefore, the amperage contribution of the power supply to each main is:
(1500W x 200% / 240VAC) / 2 mains = 6.25 amp

I would not normally use a 10amp delay fuse to run a 15amp spindle, but if that has been the experience of others, then I’ll give it a try. Did I interpret that right?

Further, this machine is going to be moved to a commercial space in the near future. So, I am trying to comply with NFPA 79. I believe that is the standard and the classification is “industrial machine” for use in light industrial space. If anyone with experience can check my schematic for errors, I’d appreciate it! I am trying to be error free and also use industry best practices. I don’t want an electrical inspector to shut me down or a future employee to get hurt.

I plan to use MTW (Machine Tool Wire) for the black and white conductors. Home Depot has it by the foot. If small quantities of other colors can't be found, I don’t think I am allowed to recode the termination points. I already have flexible, shielded cable for the spindle and servos. So, I really want these cables to enter the machine electrical enclosure, but I believe they will not meet color code standards either. How are others solving this problem?

Thanks!

[Plugger]

Right click on it/ save target as and then view it. Otherwise it's too small. Sorry!

[Al_The_Man]

You normally would have a disconnect or fused disconnect, the fusing here would be for the total panel, you would then set up subsidiary fusing from this for each of the other devices.
Attached is a typical schematic from NFPA79.
It shows 3ph set up, but the same applies for 1 ph.
You are allowed to run one colour wiring if the colour is identified at every termination point, you could use various colours of electrical tape if you wished.
The shielded cable colours will not be a problem when it is part of a jacketed cable.
If going this route for colour coding, it is best to ensure proper wire marking and documentation (control panel schematic) placed in the enclosure Document pocket or equivalent.
It also pays to follow Earth Grounding and particularly earth bonding conductors to all motor frames and metal parts of the machine Gantry etc.
The 10amp furs was for the PS not the spindle.
Al.

[Plugger]

Excellent post Al!! The information is much appreciated!!

To anyone who views this, my schematic in post #6 is NOT correct!! I cannot seem to edit or remove it. So, corrections are below. In any case, I highly recommend reading NFPA 79 to figure out how to safely build your system. It’s free to read at NFPA if you give them your e-mail. Otherwise, it’s $43 to buy. It’s great guide.

Corrections to post #5 and #6:

What I said…

“Therefore, the amperage contribution of the power supply to each main is:
(1500W x 200% / 240VAC) / 2 mains = 6.25 amp”

What I should have said or drawn in the schematic…
(1500W x 200% / 240VAC) = 12.5 amp
So a time delay fuse of 10amp in L1 and L2 should be enough. The fuses should be located just before the power supply. Additional fusing of a larger value should also be just after the disconnect in both L1 and L2.

Correction to schematic in post #6:
Neutral should not have a fuse. Neutral should not be disconnected by the main disconnect. Neutral should not be disconnected in the contactor. This is a safety issue!!