VA ratings of transformers
- Thread starter realolman
- Start date
- Status
barbeer
Senior Member
- Location
- Pinellas County, FL
I would think that would violate any listing it may have, what does the manuf. say?
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
There are other forums?realolman said:
()The reason I asked is that Taco says you can run three of their hydronic zone valves on a 40VA transformer. Each valve draws .9 a @ 24vac.
That seems to me to be overloaded. Someone in the other forum who is usually knowegeable responded with the 200% reply.
There is a bit of weirdness involved, in that the zone valves work by heating wax in a sort of cylinder arrangement. When the cylinder extends, there is a switch that shuts off the wax heater. Then, as it contracts, the heater is turned back on. The heater is on/off as long as the thermostat calls for heat through the valve.
Now, I could maybe understand the on/off business allowing the use of a 40VA transformer better than the 200% business.
This is also the reason that I asked in a thread not too long ago about the ampacity of thermostat wire... If stuff might supply twice as much as it is rated, just what is the ampacity of it?
That seems to me to be overloaded. Someone in the other forum who is usually knowegeable responded with the 200% reply.
There is a bit of weirdness involved, in that the zone valves work by heating wax in a sort of cylinder arrangement. When the cylinder extends, there is a switch that shuts off the wax heater. Then, as it contracts, the heater is turned back on. The heater is on/off as long as the thermostat calls for heat through the valve.
Now, I could maybe understand the on/off business allowing the use of a 40VA transformer better than the 200% business.
This is also the reason that I asked in a thread not too long ago about the ampacity of thermostat wire... If stuff might supply twice as much as it is rated, just what is the ampacity of it?
broadgage
Senior Member
- Location
- London, England
This sounds like some sort of control valve for heating or cooling, in which case it may be acceptable.
I presume that the valves do not require full power continually, but only when changing position which probably only takes a few seconds. Once the valve has reached the correct position, it may require much less current.
In which case the controller may only drive one or two valves at the same time, not all three.
Alternativly if all valves are driven at once, the manufacturer may have determined that the overload is so brief that the transformer wont be overheated.
Also the system design may mean that two valves wont be open at the same time, for example heating OR cooling but not both together.
Provided that the equipment is UL listed, and is installed according to the manufacturers instructions, then I would not worry.
If however additional equipment has been/is about to be added beyond the instructions, then I would be rather more concerned.
I presume that the valves do not require full power continually, but only when changing position which probably only takes a few seconds. Once the valve has reached the correct position, it may require much less current.
In which case the controller may only drive one or two valves at the same time, not all three.
Alternativly if all valves are driven at once, the manufacturer may have determined that the overload is so brief that the transformer wont be overheated.
Also the system design may mean that two valves wont be open at the same time, for example heating OR cooling but not both together.
Provided that the equipment is UL listed, and is installed according to the manufacturers instructions, then I would not worry.
If however additional equipment has been/is about to be added beyond the instructions, then I would be rather more concerned.
Last edited:
Thanks for the reply... The zone valves are potentially all "on" at once. But they cycle even when they're "on" so maybe three on a 40VA transformer would be possible.
Here's a link describing their operation.
http://www.taco-hvac.com/uploads/FileLibrary/ZoneValveBroch.pdf
But, I am more confused about the statement saying that the transformer can be loaded to 200% of it's rating. Is that correct?
Here's a link describing their operation.
http://www.taco-hvac.com/uploads/FileLibrary/ZoneValveBroch.pdf
But, I am more confused about the statement saying that the transformer can be loaded to 200% of it's rating. Is that correct?
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080925-1500 EST
realolman:
Transformers have a moderately long thermal time constant. This means overloading for a short time will not exceed the maximum internal temperature.
Suppose the on-off cycle of the actuators is a 50% duty cycle, then your maximum average VA load is 3*0.9*24/2 = 32.4 VA. Well within the transformer rating. Also it is unlikely that the three units will have synchronous on-off cycles.
.
realolman:
Transformers have a moderately long thermal time constant. This means overloading for a short time will not exceed the maximum internal temperature.
Suppose the on-off cycle of the actuators is a 50% duty cycle, then your maximum average VA load is 3*0.9*24/2 = 32.4 VA. Well within the transformer rating. Also it is unlikely that the three units will have synchronous on-off cycles.
.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080926-1640 EST
Is it compliant? If you do not overheat the transformer there should be no problem. Other than for mechanical stress I would have no problem with 1000% pulse loads on a transformer. However, if I had a continuous pulse load such as that I would probably want to evaluate the transformer design.
The National Cash Register Co. was successful because Charles Kettering, later of Delco and GM fame, understood that you could greatly overload a motor for a short time without damage because of the long thermal time constant. Also ther basis of his automotive starter motor.
No a transformer could not be overloaded at 200% for any continuous application unless it was held at a very low external temperature and had good thermal conductivity to the maximum hot spot point.
.
Is it compliant? If you do not overheat the transformer there should be no problem. Other than for mechanical stress I would have no problem with 1000% pulse loads on a transformer. However, if I had a continuous pulse load such as that I would probably want to evaluate the transformer design.
The National Cash Register Co. was successful because Charles Kettering, later of Delco and GM fame, understood that you could greatly overload a motor for a short time without damage because of the long thermal time constant. Also ther basis of his automotive starter motor.
No a transformer could not be overloaded at 200% for any continuous application unless it was held at a very low external temperature and had good thermal conductivity to the maximum hot spot point.
.
- Status