480V 3-Phase Help - Unbalanced 3

[overrange]

This is my first post on this forum.

I'm a consultant working on a driver circuit for a new kind of actuator. We need to supply the load with a large slug of power initially, then taper off to a much lower value. Roughly 15KW for 0.5 sec, then back off to an average power of 80 - 200 W for extended periods. The load is mostly resistive. The high load will not be presented repeatedly (maybe once in 15 min) but the low power load will be continuous while it's active.

Not being a 3-phase guru, need some advice. Can I tap off of one phase of a 480 service to power this without causing a significant imbalance? What are the critical service parameters? The service would be at the lab bench at first but ultimately on a shop floor near robotic equipment. We will have to design this to work off of services typically available and may need several different algorithms.

Also any recommendations on good SSR's or other drivers for this device, similar to an industrial heater control. Thinking of doing zero crossing switching only to avoid EMI hassles. Many thanks!!!

Dave at USI

 

[jbartos]

Suggestions marked ///\\We need to supply the load with a large slug of power initially, then taper off to a much lower value. Roughly 15KW for 0.5 sec,
///About 18 Amps\\ then back off to an average power of 80 - 200 W for extended periods. The load is mostly resistive. The high load will not be presented repeatedly (maybe once in 15 min) but the low power load will be continuous while it's active.

Not being a 3-phase guru, need some advice. Can I tap off of one phase of a 480 service to power this without causing a significant imbalance?
///More info is needed. Namely, how is the 480V 3phase source rated in terms of amperes and VAs? Normally, 18A at 480V (assuming three phase load) should not pose a major problem in an industrial or power generation plant 480V power distribution system.\\ What are the critical service parameters?
///VAs and amps\\ The service would be at the lab bench at first but ultimately on a shop floor near robotic equipment. We will have to design this to work off of services typically available and may need several different algorithms.

Also any recommendations on good SSR's or other drivers for this device,
///Visit

http://www.thomasregister.com

and type Relays: Solid State under Product or Service, which will return 141 companies to approach to.\\
similar to an industrial heater control. Thinking of doing zero crossing switching only to avoid EMI hassles.
///Incidentally, various zero crossing controllers are already available. Surf the web for them.\\

 

[overrange]

Thanks jbartos!! Help me understand the current. If I pull from only one phase, and I have a 480V Wye, then it's going to be (480/1.73)* 54A = 15 kW if I use one leg and the neutral. If I connect across one phase, then wouldn't it be 480 * 31.25A = 15 kW. And if I used all 3 phases, then 1.73 * 480 * 18A = 15 kW.

So I'm thinking of using one phase and running 31 A +- and thus the concern.

What kind of VA rating would a service need in order to accommodate this load?

Thanks for the other tip as well.

Dave at USI

 

[Hertz]

I do not understand your concern about unbalancy. In any industrial facility there is always some degree of unbalancy. Also the 480 volt transformers are usually rated at 225 kVA and up, so a single 15 kW unbalanced load will not be a big problem . If you are planning to install many of these units on a factory floor, then you need to alternate the single phase load between the three phases and in the end come up with somewhat of a balanced load.

 

[RajT]

Your 480V 3 ph supply should be more then adequate to meet the requirements you state.
You simply need to connect your load as a single phase load and connect it across any one ph and N. The cable used can be between say 6-10mm2 ( or whatever size the load terminals permit)more to provide strength to a shop floor enviornment then current loading criteria.

The effect of high initial inrush value of 54A for 0.5 sec can be discounted as the time duration is small.
As for the VA rating of the transformer ( assume its a 3 ph unit)supplying the load, a 15kVA unit will be ample, but a smaller unit of say 7.5kVA should also be ok as the high current duration is small.
You do need to take into account the voltage drop across the supply cable if it is more then say 80-100m long as a smaller cable coupled with a smaller transformer may not do the job as the voltg drop at the load term may be too low for the actuator to function properly.