Primary fuses for control transformer
Primary fuses for control transformer
(OP)
I have a 1500VA control transformer fitted with 5 amp primary fuses. The incoming voltage is 490V and the control voltage is 120V. The primary fuses will pop when the mains power is turned on, even when no motors nor controls are operational. If i fit 20 amp fuses, these will not pop, so i know i don't have a short circuit. According to the NEC i beleive i can go up to 250% i.e. 7 Amp fuse. Will this help or i am i missing something else.





RE: Primary fuses for control transformer
RE: Primary fuses for control transformer
What type of fuses are you using? When a transformer initally powers up there is an inrush current which could be blowing your fuse, if they are of the fast acting type. You should try some time delay or slow blow fuses.
RE: Primary fuses for control transformer
RE: Primary fuses for control transformer
RE: Primary fuses for control transformer
NEC is not against good engineering. As long as you have secondary OCPD sized to prevent the overload, the primary protection can be greater than 125% as long as you protect the conductors and vs short circuit. 20A fuse with #12 wires will be perfectly acceptable.
I do not have Code in front of me, I will check on Monday.
RE: Primary fuses for control transformer
RE: Primary fuses for control transformer
NEC Table 450.3(B) permits 7-amp hiside fuses where total secondary fusing is 15.6A or less.
As suggested by jbartos, it may be worth checking no-load current on the transfomer to verify that it is otherwise working OK. The Article 450 multipliers have evolved over years of application. The 250% allowance would not be there if code submitters hadn’t learned through experience.
RE: Primary fuses for control transformer
RE: Primary fuses for control transformer
RE: Primary fuses for control transformer
We always use the transformer manufacturer's formulas and tables. All transformers are not the same, some are coupled more closely primary to secondary, they have different inrush values depending on the type of transformer used (more or less isolation, de-coupling, etc.).
Either way, transformers are heavy inductive loads, require time delay fuses (we prefer circuit breakers for this reason of inductive load).
PUMPDESIGNER
RE: Primary fuses for control transformer
i hope the problem is that the transformer was the first use,so the core wasn't magentised so it draw a large current,try again the 5 amp fuses, i think it will work well
RE: Primary fuses for control transformer
RE: Primary fuses for control transformer
If possible, compare to no-load current of a similar transformer. 1.3 amperes versus 3.13 amperes calculated full load for a 1500VA 480V control-power transformer sounds like indication of a serious problem.
RE: Primary fuses for control transformer
RE: Primary fuses for control transformer
"Venditori de oleum-vipera non vigere excordis populi"
RE: Primary fuses for control transformer
RE: Primary fuses for control transformer
Is this a new installation, or is this a problem that has developed after a period of time?
I assume that this is new installation.
1. Transformers draw a very high inrush current at switch on. This inrush current is independent of the secondary circuit. The magnitude of the inrush current is dependent on a number of factors including the amount of flux in the iron. If the transformer is over fluxed, i.e. is wound for 460 volt and used on 490 volt, there will be an appreciable increase in the inrush current. The inrush current of transformers only lasts cycles, but can be in the order of 10 - 20 times the full load primary current of the transformer. If your transformer has been very conservatively rated, and is actually say a 2.5KVA capable transformer, then the inrush will be higher again.
2. Fuses fail when the energy through them is too high. You cna over load a fuse and it fails earlier the failure time tends to reduce with the square of the current, hence we talk about the i squared t of the fuse. (more common terminology with semiconductor fuses) A slow fuse will last longer with the overload, but will still fail. Sometimes, we just have to use a fuse that has an i2t greater than the switch on energy of the transformer in order to prevent failure.
3. If the transformer has a shorted turn or similar insulation fault, it will have a high primary current, will tend to be noisy and the open circuit secondary voltage will be low. It would also heat up reasonably quickly. Temporarily fit oversized fuses and run the transformer unloaded for a period of time. If the transformer primary current is reasonable, the transfomer does not make excessive noise or smells, and does not heat up excessively, then I would suggest that you just have a healthy inrush current.
4. In my part of the world, we do not operate under NEC, and I would fuse for the cable rather than the transformer. I would protect the output of the transformer, and use thermal protection in the transformer. (Usually a thermal fuse built into the primary circuit). I agree with Sanditech, about four times the rated primary current is about right in most cases.
Good luck,
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Primary fuses for control transformer
http://www.manufacturing.net/dn/article/CA150723
for:
TSE transformer soft starter is an electronic semiconductor with a microprocessor-controlled patented switch-on procedure. A short pre-magnetization of the transformer core results in a smooth power-on without any spikes. If operated for more than 2 sec, the internal electronic switch will be bridged by built-in bypass relays. Thus, the integrated heat sink in the transformer can be kept small. The usual type-B circuit breaker is used as a cut-out for the transformer and there is no need for a secondary fuse. Normally configured as a compact device for a nominal current of 16A, the TSE transformer can be combined for up to 63A using an external thyristor module.
Carlo Gavazzi AG, CH-6312 Steinhausen, Switzerland, FAX +41 41 740 45 40.