Newbie- Servo and power supply requirements?

[LazyMan]

I am new to the servo arena. I am looking at Nema 34 850 oz/in Peak Torque 72VDC Servo from homeshopcnc and the current ratings are 8.5amps continuous and 38amps peak. I have not seen any power supplies that even come close to these figures and am wondering what the deal is. Are the large capacitors on the unregulated power supplies able to provide the peak currents?
What are people using for power supplies to drive three motors?

[Bubba]

Here is some information that Mariss posted some time back. Think of it this way, just because you have a 400 HP motor in you car, you don't use it all the time. So using his method, figure what you will actually need. Then for sake of argument, lets assume a 3 axis machine and total up the amperage that you caculate and multiply by 2/3, this will be a good size for the amperage rating of your power supply. Further assume Gecko drives (max 80V) and use a voltage rating of 72 volts for your power supply. You don't want to go much above this in any case for the listed servo motor anyhow!

"If you are designing a machine and you get to motors, the first thing you should do is calculate the power you need. Never buy a motor (stepper or servo) first and then figure out if it will fit what you need. That is the sign of an amateur or hack.

Motors are motors. They couple power to your mechanism and power is what makes things happen. The choice of a motor comes after you know what's needed.

Power is velocity times force or torque times RPM. It doesn't matter if the motors are steppers, servos or a gerbil in a spinning squirrel cage at the start.

To separate what motor need (neglect the gerbil), is the power your mechanism needs.

Rule #1: If you need 100 Watts or less, use a step motor. If you need 200 Watts or more, you must use a servo. In between, either will do.

So, how do you figure the power you need?

Method 1: You have a plasma table, wood router or some other low work-load mechanism. You have a clear idea of how many IPM you want but you’re not sure of what force you want at that speed.

Pick the weight of the heaviest item you are pushing around. If it weighs 40lbs, use 40lbs. Multiply it by the IPM you want. Say that's 1,000 IPM. Divide the result by the magic number "531". The answer is 75.3 Watts so use a step motor.

Eq: Watts = IPM * Lbs / 531

Method 2: You have a Bridgeport CNC conversion you are doing. The machine has a 5 TPI screw and you need a work feed rate of 120 IPM. 120 IPM on a 5TPI screw 5 * 120 or 600 RPM.

How about force? Not a clue? Use your machinist's experience on a manual machine. The hand crank is about 6" inches in diameter. How much force would you place on the hand crank before you figure you're not doing something right? I hear about 10 Lbs.

!0 Lbs is 160 oz, 160 oz on the end of a 3" moment-arm (6" diameter, remember?) is 480 in-oz (3 times 160) of torque on the lead screw.

The equation for rotary power is: Watts = in-oz * RPM / 1351

For this example, Watts = 480 in-oz * 600 RPM / 1351 or 213 Watts.

213 Watts is servo territory. You have to use a servo motor to get that, about a NEMA-34 one.

OK. Long post, late night. If anyone cares, let me know. Proper application of servo motors is an entirely different topic, it's involved but not particularly difficult. Servos are not steppers and they are not interchangeable. Let me know if I should continue.

Mariss"

Hope this helps.

PS: Yes, big caps help with the required power surge. An unregulated power supply works very well for this purpose and again, Mariss recommends "C=80000 x I/E where C is in microFarads, I is in Amps, and E is in Volts"

[Kelinginc]

Originally Posted by LazyMan I am new to the servo arena. I am looking at Nema 34 850 oz/in Peak Torque 72VDC Servo from homeshopcnc and the current ratings are 8.5amps continuous and 38amps peak. I have not seen any power supplies that even come close to these figures and am wondering what the deal is. Are the large capacitors on the unregulated power supplies able to provide the peak currents? What are people using for power supplies to drive three motors?

What is your application?

You can check here for 72VDC , 20A power supply
http://www.kelinginc.net/SwitchingPowerSupply.html
The servo kits cost more
http://www.kelinginc.net/CNC34ServoMotorPackage.html

[LazyMan]

Thanks Bubba,

I appreciate the help. The application is cnc related but not for a machine tool. The servo is going to turn a crank arm.

Originally I figured at 72v and 8.5amps continuous current the absolute max continuous power would theoretically be 72vx8.5a= 612 watts, but with the formula you using lets say I run the servo at 4200 RPM, with 170oz-in or continuous torque I would get 170oz-in * 4200rpm/1351 = 528 watt of rotary power?

Just realized the specs given for the servo says oz/in not oz-in, are they the same thing?

Another question. If a servo has a max rated power of 4200rpm, do you try to gear the servo so that its running as close to max rpm as possible? For example, for rapids on a CNC router, would I want the servo to be running as close to 4200rpm as possible

Now I wish I actually paid attention in physics class...

[Bubba]

Yep, the is supposed to be oz-in. sorry for the typo!

As for servo design, I once again defer to Mariss and info gained from him:
"1) Start by finding out what your motor's no-load speed is at its rated voltage. Use a strobe tach if you don't have the data.
2) Design for a speed equal to 80% of your motor's no-load RPM when your mechanism is moving at its maximum speed.
3) Calculate the RPM of the driven element (leadscrew, etc, etc) when the mechanism is moving at its max speed.
4) Divide that RPM into 80% motor no-load speed. That will be the optimal reduction ratio from motor to load. Use a toothed-belt for anti-backlash reduction.
5) Determine what is your software's maximum step pulse rate. It'll be published somewhere. Convert your motor no-load RPM into revs per second (divide RPM by 60)
and divide that by 4. Divide that into your software's max step frequency. The result will be the maximum encoder line count.
Pick the next lowest standard mfg encoder line count. In other words, if your result is 566.66 lines, pick a 500-line encoder."

So as you can see, you will use 80% as your maximum speed. Yes, I also wish I had paid more attention in physics class, but do wish I could remember what little bit I was taught those many years ago:})