Mike Dolt Enterprises
Mike Dolt Enterprises
User: newee, Location: Wisconsin
Asked on: Thursday, March 31, 2011 at 11:41 am EDT
Answered on: Tuesday, May 14, 2024 at 3:33 am EDT
Question: Everything I know, I learned in school and from the internet. I am at somewhat of a disadvantage in that my first job (been here less than a year) is at a company with no EEs, electricians, or anyone with electrical experience for that matter.Trying to size control transformers for some panels we want to build. This is really simple stuff, but I need someone to confirm that it really is as easy as it sounds.For example, let's say I have a 5HP motor, 460/3/60 supply. Any control product catalog will tell you what to use for a contactor, overload, and overcurrent protection. The catalog will also give the inrush VA of the contactor.The particular contactor I'm using has an inrush VA of 140 and a steady-state VA of 12. I don't have data on the lights or one DPDT relay that I'm using. Is it really as simple adding 140 to an overestimate inrush VA for the other devices, then adding the 12 to another overestimate? Based on that I can use a transformer mfr. table to get the appropriate transformer.Does a 100VA CCT sound big enough for a motor control circuit? 1 motor, a couple lights, a couple switches, and a relay.I am only slightly concerned because I have NO ONE from which I can get assistance on this simple matter. I'd hate to "engineer" a control panel and have an undersized CCT in there.
Yes, it is generally as simple as you have described for sizing control transformers. The following steps can help you confirm your calculations
1. Calculate the Total Inrush VA:
Add the inrush VA of the contactor (140 VA) to an overestimate of the inrush VA for the other devices. For this example, we'll assume 60 VA for the lights and relay, giving us a total of 140 VA + 60 VA = 200 VA.2. Calculate the Total Steady-State VA
Add the steady-state VA of the contactor (12 VA) to an overestimate of the steady-state VA for the other devices. Let's assume 15 VA for this example, giving us a total of 12 VA + 15 VA = 27 VA.
3. Select a Control Transformer
Using a transformer manufacturer's table, find a transformer that can provide at least the calculated Total Inrush VA (200 VA) and Total Steady-State VA (27 VA).
4. Example
A 100 VA control transformer can typically handle an inrush VA of around 200 VA and a steady-state VA of around 25 VA. Therefore, a 100 VA CCT sounds like a suitable choice for your application.
Additional Considerations
Load Factor
Most transformers are rated to operate at a certain load factor (e.g., 80%). Ensure that your calculations are based on a load factor that matches the expected operating conditions.
Voltage Drop
Consider the voltage drop in the wiring between the transformer and the control devices to ensure adequate voltage is available at the devices.*
Oversizing
It's generally better to slightly oversize the control transformer to provide some headroom for future expansion or unexpected demand.
Conclusion:
Based on your calculations, a 100 VA control transformer is likely a suitable size for your motor control circuit. Remember to consider the load factor and voltage drop, and slightly oversize if necessary. It's also important to note that this is a simplified approach, and consulting with an experienced electrician or engineer for more complex applications is always recommended. Flag for review