I am using a 24v transformer with an output of 40 VA to open and close 3 individual 2-pole 20amp contactors. Does anyone know how many VA each contactor requires to operate? Right now everything is functioning fine but I want to make sure I am not stressing out the transformer. Thank You!
Transformer powering Contactor.
- Thread starter aptman3
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ptonsparky
Tom
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- EC - retired
You can take a current reading with an amp clamp as you would any load. One contactor at time is a normal furnace or a/c load for a 40va. 40/24 will give you rough current capacity. Check voltage drop as the load is applied and after seal in.
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You should be able to get the "pull in va" and the "holding va" from the contact manufacturers spec sheet.
T
T.M.Haja Sahib
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If you please explain further how the above data may be used,it would be very nice on your part.
ptonsparky
Tom
- Occupation
- EC - retired
http://forums.mikeholt.com/showthread.php?t=141653&highlight=sola
Try the reference above, if I did it right. The OP transformer sounds like what is used on the typical older home heating system and will most likely differ somewhat in its capabilities.
Jump to Kwireds post 10.
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As Don has pointed out, there is a difference between pull-in and hold-in VA. The pull in is generally the much higher of the two but lasts usually not more than a few tens of ms. If your transformer is managing to pull the contactors in, I would think it unlikely that is being stressed by the running VA.
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- PE (Retired) - Power Systems
Also be careful of the type of transformer you are using. Cheap 'doorbell' transformers have poor voltage regulation and thus do not have high 'pull in' capability. The common 'furnace' transformer is usually fine when you are turning things on all at one time. A machine tool (also called a control power) transformer has the largest capacity for 'pull-in' and 'seal-in', tand can easily handle loads turing on/off while other loads remain on.
T
T.M.Haja Sahib
Guest
Thanks.I just wanted to know the formula for calculation of transformer VA,given coils pull in VA.
Actually these ratings are required for not damaging the transformer but to prevent contactors chattering due to low voltage on start up.
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