Transformer types for driving a half horsepower 120VAC motor from within an enclosure

[rh2]

We have a new requirement to add a liquid pump (120V, 0.5HP, 8A, M code ~5.7x LRA) to an equipment skid. The skid has a weatherproof controls enclosure that is already used to house a VFD for another process, a PLC, some relays, terminal blocks, and branch circuit protection for various instrumentation. The majority of the power in this enclosure is of course consumed by the VFD (modulation of the 480V mains) and we will have enough mains power from our generator to accomodate the new pump. Inside the enclosure there exists a single phase, type T 0.5kVA instrumentation transformer (Pri=480, Sec=120) that is used to power the smaller things like the PLC and instrumentation. It would be very convenient for us to use the existing footprint that this transformer occupies (about 6"x6") and put, in its place, what I believe should be a 2kVA unit to power everything along with the pump.

My question is simply regarding the type of transformer this is (T) and then any implications within NEC of using a type T for what is now powering a characteristically different load (motor start).

1.) Is this considered "illegal" or an improper type of transformer to employ for this use case? Should I be looking at other types? If a traditional isolation transformer is still OK, should I avoid certain types? (T vs. TF vs. ??)
2.) My understanding is that the 115C rise rating of the currently installed transformer should require me to find the 2kVA transformer in a similar 115C rise rating so as to not introduce a change in the heat dissipation profile of the inside of the enclosure. Should I be wiser in my assumptions?

Thank you

 

[rh2]

I posted this in calculations because I also wanted to ensure my approach to sizing the transformer was appropriate. The PF on the nameplate is specified as 0.63 @ 100% and then LRA would be 45.6A. I have used various tables on websites that estimate a transformer VA rating (and de-rated in accordance to PF) and this is where I initially determined 1.5kVA, to which I conservatively just added the 0.5kVA and concluded with 2.0kVA. This seems to follow the anecdotal/general guidelines I've read on these forums about 3 x HP in kVA for any concurrently started loads that could occur on top of any existing load.

 

[jim dungar]

The NEC does not use the terms T and TF. Those sound like something from a manufacturer like Schneider Electric.

Terms like 'control power' and 'general purpose' transformer are from manufacturers and listing companies. They do not prevent you from applying them per NEC 450. Unless shown differently on their label, all transformers are isolation style.

The 115°C temperature has nothing to do with the amount of heat a transformer produces. Your 2kVA will have more losses than a 500VA does, you will likely need to provide additional cooling.

Most manufacturers have control transformer sizing charts that provide sealed (continuous) and inrush values.

 

[Jraef]

The added heat in a sealed enclosure with a VFD and PLC inside will .likely give you more headaches than you are wanting. What I and many others typically do in those situations is to get a weatherproof encapsulated transformer and hang it on the outside of the enclosure.

As to size, my rule-of-thumb is that to handle the motor starting current, the transformer must be sized at roughly 3.5x the motor kVA. So your motor is 1/2HP, but .63PF, so 592kVA, x 3.5 = 2.072kVA as a minimum transformer, so 2kVA is probably OK, I would go 3, but then again, voltage drop on the secondary side is not likely a problem for this, since you already have a 500VA CPT. If you eliminate the internal 500VA CPT, then absolutely use a 3kVA external transformer.

 

[retirede]

I would consider mounting an enclosed transformer adjacent to the control panel.