1
\$\begingroup\$

Good afternoon, I hope that you are all well not withstanding COVID-19.

I have a 230/24v 250VA Transformer which is being used to power 4 x motors drawing a maximum of 20.3 Watts each.

I would like to know what amperage of fuse to use for primary and secondary and whether or not they need to be slow blow?

The data sheet for the transformer can be found here: https://media.automation24.com/datasheet/en/86345_Data%20Sheet_en.pdf

Many thanks in advance and stay well.

\$\endgroup\$
2
  • \$\begingroup\$ What specifically are the fuses intended to protect? \$\endgroup\$
    – Andy aka
    Commented Apr 16, 2020 at 12:32
  • \$\begingroup\$ I'm no expert, but I would have though that it would be to protect the transformer's primary windings (in the event of an internal short) and the secondary windings in the event of an internal short, or a short in one of the motors? \$\endgroup\$
    – user5822039
    Commented Apr 16, 2020 at 12:39

2 Answers 2

Reset to default
1
\$\begingroup\$

Your output is only drawing about 3.4A maximum assuming your accounting for start-up current, so if your goal is to protect the motors, a 4 or 5A fuse, if your protecting the transformer, a 10A fuse on the secondary, slow blow is more needed when both the motor maximum and the transformer maximum are close, e.g. all 4 motors spinning up at the same time, the transformer under say a 50% overload will not burn out faster than the fuse, so a slow fuse is selected for the short spinup period.

Input fuse about 2A conventional.

\$\endgroup\$
3
  • \$\begingroup\$ Many thanks for youR answer. Just to confirm - you don't advise using a slow blow for this application? \$\endgroup\$
    – user5822039
    Commented Apr 16, 2020 at 12:46
  • \$\begingroup\$ Not unless you forgot about the motors startup current, if your total power consumption per motor includes that, then any time above that power amount is unlikely, as such no need for slow blow. Its for cases where say the motors will draw 15A for 3-5 seconds while starting, but you want to protect the transformer against shorts above 10A, here a slow blow would be needed. \$\endgroup\$
    – Reroute
    Commented Apr 16, 2020 at 12:52
  • \$\begingroup\$ Thanks again Reroute. \$\endgroup\$
    – user5822039
    Commented Apr 16, 2020 at 17:59
1
\$\begingroup\$ 
What specifically are the fuses intended to protect? – Andy aka

I'm no expert, but I would have though that it would be to protect the transformer's primary windings (in the event of an internal short)

No, you should be trying to protect the infra-structure wiring that feeds the transformer in case the transformer fails short circuit and melts the infra-structure wiring causing a fire and lots of damage, cost and risk. You can use a slow blow fuse.

So, you need to understand what the building/site wiring is rated at and choose a fuse of suitable value. You can't protect the transformer against itself failing - if it fails it's trash and no amount of fusing is going to make it anything less than trash.

And, by the same token, the transformer and wiring to the motors are the "new" infra-structure that need to be protected from the motors going short circuit and drawing too much current. If you deem the transformer to be the weak link in that new infra-structure then you could use a thermal fuse that blows when the transformer gets too warm or just a regular slow blow fuse.

In all situations of using a fuse, the fuse is to protect infra-structure.
\$\endgroup\$
9
  • \$\begingroup\$ I follow a variation on that, fuse for the lower, the maximum expected use case + some margin, or the infrastructure, If I have wire that can handle 100A, but things have gone bad at 20A, I would be selecting the 20A fuse. \$\endgroup\$
    – Reroute
    Commented Apr 16, 2020 at 12:54
  • \$\begingroup\$ Fusing for use_case+margin OR infrastructure is NOT protecting the infrastructure in the even that the use_case+margin exceeds the rating for the infrastructure. Be careful here what you recommend. \$\endgroup\$
    – Andy aka
    Commented Apr 16, 2020 at 12:58
  • \$\begingroup\$ "Fuse for the lower" as in, which ever is the smaller value. \$\endgroup\$
    – Reroute
    Commented Apr 16, 2020 at 13:01
  • \$\begingroup\$ This is not really correct. The building wiring is protected by the building circuit's fuse/breaker. Device fuses are not intended for that. Primary and secondary fuses also serve two different purposes. Primary fuses protect (upstream) against a short occurring in the transformer. Secondary fuses protect (the transformer coils) against being overloaded. In a mains-AC connected device, the primary fuse is intended to protect the socket and power cord, nothing else. Therefore, you only need to care about what those can handle (typically 15A in the US) when limiting the max value. \$\endgroup\$
    – Foogod
    Commented Nov 25, 2023 at 17:54
  • \$\begingroup\$ @Foogod which part of my answer are you having problems with specifically? \$\endgroup\$
    – Andy aka
    Commented Nov 25, 2023 at 19:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.