Locked Rotor Calculation for Servo or Stepper motors

[RollaMO]

I am trying to size a UPS for my 208 VAC 3ph machine which has multiple 120vac, 3ph, and DC loads. First I am adding up the individual loads per phase and taking the largest phase and multiplying by 3 to get my total KVA and thus my FLA and UPS size.

How do I handle the locked rotor calculation? (Calculate Lock rotor KVA)

For the Servo motors should I use the nameplate FLA of the motor to calculate total KVA since the power dissipation of the controller will be small? What value would I use for locked rotor calculation? (Maximum controller output current, or 6x FLA)

For the Stepper motors should I use the controller nameplate rating for max peak current and inrush current for the KVA and then lock rotor KVA respectively?

Mike

 

[gar]

080811-1914 EST

RollaMO:

I think there are other questions you might ask.

Why do you want a large UPS? How critical is the machine? How critical is the part being machined? How expensive is the part? How often do you have a power problem? What is the nature of the problem? Do you want just enough time to shutdown the machine, or continue to work 24/7 lights out with no power? These require different solutions.

All our CNC machines are HAAS from 1993 thru 2000. The newer ones float thru short interruptions without any faults. I do not have quantative information on what short is, maybe 1 or 2 full cycles. When the northest blackout hit we lost several fuses. No major damage. We did lose an Ethernet switch but nothing failed that was connected to the switch.

The CNC controller is basiclly equivalent to a PC and could run on a small UPS. On our newer machines the spindle and servos all work from the same 320 or so DC bus. The spindle load is highest under short time loads, usually 200% for a short time, and much lower by current limiting at locked rotor. The servos also have peak loads greater than average capability and stalled or locked position.

You could add additional capacitance to the DC bus for a longer float thru time, or even use a large battery which has some advantage in some applications.

In the Detroit area supplied by Detroit Edison at large industrial plants, like 2,000,000 square feet, there is seldom a loss of power except from internal problems. Even in most local areas there is seldom loss of power. However, if a lightning storm is in the area it is wise to shutdown CNC machines.

In Kentucky power company system problems seem to occur more often. Dry Ridge in particular.

Provide more information on your known or expected problems and objectives.

On gaging equipment that I build I use a Sola input transformer and have never had a failure from power system problems, while in these same conditions PC based systems have all sorts of problems.

.

 

[RollaMO]

Locked Rotor Calculation for Servo or Stepper motors

In short we wind fiber on bobbins and when we have a power blip we have to discard the product and restring the whole line. I am looking at a 60KVA unit right now and only need to eliminate the 3 minute or less blips.

 

[realolman]

... For the Stepper motors should I use the controller nameplate rating for max peak current and inrush current for the KVA and then lock rotor KVA respectively?

Mike

I don't believe steppers have a locked rotor as such. The controller determines how much current flows through the windings, and usually has full current on at least one winding, sometimes more than one.

I think controller nameplate for the steppers.