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.
Tek-Tips is the largest IT community on the Internet today!
Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!
-
Congratulations TugboatEng on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!
Primary fuses for control transformer 1
- Thread starter stuuk78
- Start date
-
1
- #2
jbartos
Electrical
- Jan 15, 2000
- 6,440
Comment: If 5A fuse blows when 1500VA 490V single phase transformer is at no load on the secondary side, then there is something wrong with the transformer. It should be properly tested. It may have inter-turn shorts or short over the transformer iron eliminating some turns. Normally, no load transformer current should be negligible, e.g. a small fraction of an amp in your case.
jbartos
Electrical
- Jan 15, 2000
- 6,440
Comment: NFPA 70-1996 NEC Article 450-3(b)(1) states that the transformer primary protection should not be more than 125% of the rated primary current of the transformer. There is not mentioned fast or time delay dual element fuse for the protection. In fact, the transformer primary can be protected even by a lower rated fuse than 125% of rated transformer primary current, if the transformer is not fully loaded.
- Thread starter
- #5
A faulty transformer will pop even a 20A fuse. Control transformers can have inrush currents up to 25 times.
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.
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.
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.
- Thread starter
- #9
PUMPDESIGNER
Mechanical
I know much less on electrical than most of you guys, but I do design logical controls for pumps and have to work with transformers frequently, under UL-508A and NEC.
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
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
Danfossegypt
Industrial
- Apr 3, 2004
- 26
hello stuuk78
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
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
- Thread starter
- #13
Yes i have checked the amps while there is no load on the secondary side and it is reading approx 1.3 amps steady, it is hard to see the inrush as it is so fast and my ammeter takes some time to display the results. It is as if the transformer is de-magentising once the mains power is switched off. The fuses did not blow on one ocasion but then the next time i turned it on they blow again. I have read on some transformer manufactures web pages and they reccomend a 15 amp fuse for primary protection while using secondary protection also.
I second the sentiments of PUMPDESIGNER. No 2 transformers are alike. You sometimes need to experiment with finding the lowest size that meets code but will not blow on inrush of THAT particular transformer. Most xfmr mfrgs have a chart showing what they know works. What is your xfmr?
"Venditori de oleum-vipera non vigere excordis populi"
"Venditori de oleum-vipera non vigere excordis populi"
Hello stuuk78
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
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
jbartos
Electrical
- Jan 15, 2000
- 6,440
Suggestion: Visit
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.
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.
- Status
Similar threads
- Locked
- Question
- Question
- Locked
- Question