Need help with Control Transformer Fuse Sizing!

Thread Starter

3PedalMINI

Joined Jul 13, 2014
18
Hello! I am building a small automation controller for a single phase Baldor 5HP Motor. It will be controlled by a contact. When the contact closes I need it to turn on the motor and remain on until the contact opens. When the contact opens I need a timer to be initiated for 30-45 seconds. When the timer expires I need the motor to shut off. However I also need it so if the contact closes WITHIN the timer countdown period to clear the timer, keep the motor running and when the contact opens restart the timer.

Basically I have everything I need (including a 2 position holding switch(manual override and auto mode)) to make this all work. Where im stuck is the fusing for the control transformer. I dont even know where to begin on sizing for the primary side and secondary side. I found a formula but im not sure its the right one. Once I get the sizing right what style fuse do I need? I assume it needs to be a time delayed fuse to keep the inrush current from knocking them out.

These are the parts I have received:

Countdown Timer:
https://www.automationdirect.com/ad...er_Relays_1-z-16_DIN_(MS_Series)/MS4SM-CE-ADC

Control Transformer:
https://www.automationdirect.com/ad..._Transformers/240x120_VAC_to_24x12_VAC/PH75PG

Contactor:
https://www.automationdirect.com/ad...actors_-z-_Overloads/32_to_50_Amp/SC-E2-24VAC

Overload Relay:
https://www.automationdirect.com/ad...tactors_-z-_Overloads/32_to_50_Amp/TK-E2-2600

Thanks!
 

Reloadron

Joined Jan 15, 2015
6,987
Your control transformer: Hammond control transformer, 75 VA, 1-phase, 120/240 VAC primary, 12/24 VAC secondary, 50/60 Hz, panel mount.

So you have a 75 VA control transformer with a 24 volt secondary. VA ÷ VOLTS = Maximum amp load, 75 / 24 = 3.125 Amps. I would fuse the secondary at 3.0 Amps slightly below the VA rating. If you want to fuse the primary side then 75 / 120 (assuming 120 VAC mains supply) = 0.625 Amp so I would start with a 0.5 Amp fused primary. There is no need for a slow blow that I can see. Just make sure your fuse body is rated for the voltage.

Ron
 

Thread Starter

3PedalMINI

Joined Jul 13, 2014
18
Hi Thanks! Sorry, I should have mentioned the transformer will use 220 on the primary side. I think that changes the fuse rating?

Also, Would you mind terribly linking me to the correct fuse? my head is spinning with all the options. Pretty sure the fuses are standard but not sure about the bodies.

Really appreciate it!
 

Reloadron

Joined Jan 15, 2015
6,987
Yes, it does, so now you have 75 VA / 220 = 0.3409 Amp or about a 300 mA fuse. You may find a 330 mA fuse. The idea is to stay below the max current the primary will draw. Companies like Little Fuse offer a 3AG slow blow line which I think includes a 300 mA fuse. You can also find a pig tail version (axial leads) or cartridge version.

Ron
 

Thread Starter

3PedalMINI

Joined Jul 13, 2014
18
you guys rock! thank you! my transformer came with an integrated fuse holder. The 3AG looks too small for the fuse holder provided. Would you recommend I just use a chasis/inline mount? Its probably a bit better anyway?

On a second note: Im used to using (very limited experience) contactors that have blade terminals to jump off the line voltage over to the transformer. The specific contactor I got does not have these terminals (blade) What is the best way to jump power from the Line in over to the transformer? I would assume putting 10ga and 16 ga under the same lug is frowned apon :) Should I use a Din rail/jumper to feed both the contactor and transformer? I want this to not be "hacked" together lol

Thanks again guys!
 

MaxHeadRoom

Joined Jul 18, 2013
26,002
I would use a ceramic body fuse for 240V mains. There is enough energy in a good short circuit current on 240V mains to blow the glass to bits which makes it difficult to remove a blown fuse.
Never seen that happen in all the years I have used them, particularly the lower amperage ratings.
The 3AG has UL-CSA rating for 250vac.;)
Max.
 

Reloadron

Joined Jan 15, 2015
6,987
The following is taken from your transformer data sheet:
Standard secondary fuse kits utilizing 13/32” x 1 ½” CC fuse clips included with all transformers. Fuses are not included. (See Edison fuse section for MEN Midget fuses.) • Optional primary fuse kits available utilizing 13/32” x 1 ½” fuse clips (See Edison fuse section for HCTR fuses.)

I have used similar Hammond transformers which included fuse holder. I commonly refer to them as just midget fuses. The merit of using a front panel type fuse holder like Max posted is that it allows easy access to the control circuit fuse without opening the panel. As to a ceramic body? While I have never had a glass body fracture with low current applications like you have It certainly won't hurt to use one. If you go with a midget fuse they are also available from a variety of manufacturers with protection. Here is one such example. You have likely seen them before. The MEN types in the example start at 0.5 amps.

Ron
 

Thread Starter

3PedalMINI

Joined Jul 13, 2014
18
Thanks, I am probably just going to go with the panel mount fuse holders 3AG. Even though it will only ever be me that accesses this it would be nice todo it from the front. Also the 3AG fuses seem to be a little easier to get a hold of locally if need be! Thanks again all!

My last and final question: Im used to using (very limited experience) contactors that have blade terminals to jump off the line voltage over to the transformer. The specific contactor I got does not have these terminals (blade) What is the best way to jump power from the Line in over to the transformer? I would assume putting 10ga and 16 ga under the same lug is frowned apon :) Should I use a Din rail/jumper to feed both the contactor and transformer? I want this to not be "hacked" together lol
 

Reloadron

Joined Jan 15, 2015
6,987
I would assume putting 10ga and 16 ga under the same lug is frowned apon
You can "piggy back" on the mains side of your contactor. I am assuming based on your contactor being 3 pole that you are switching 220 volt phase to phase to your motor. Then yes, you can piggy back your control transformer power off the contactor lugs. Personally I generally would use a terminal strip or block where my mains entered my control panel and get my power there but you can also do it as mentioned.

Ron
 

MaxHeadRoom

Joined Jul 18, 2013
26,002
Is this going to be installed in an industrial or commercial setting? IOW is it operated by someone other than yourself, if so there are some things that are mandatory when building a automatic system, such as a recognized E-stop station, this must be easily accessible by any operator.
And possible other safety features.
Max.
 

ebp

Joined Feb 8, 2018
2,332
For a transformer of that size I wouldn't fuse the secondary at all. Sometimes with very small transformers the primary "doesn't know what the secondary is doing" because of winding resistance and other factors. With a 75 VA transformer, you won't be able to overload the secondary without the current being accurately reflected in the primary. Secondary fusing is something that is a good idea if you have multiple secondary windings that are being used separately, but if you intend to use the secondary windings in either series or parallel there is no concern with overloading a particular winding.

You definitely need a slow-blow ("time delay") fuse for the primary. If there is residual magnetism in the core when power is applied and the applied power pushes the magnetism in the same direction you can get a very large current spike because the iron core "saturates.". The Wikipedia article on transformers explains an illustrates this quite well. Your contactor coil also takes a lot of "inrush" power, so you will have a transient overload on your transformer. This is perfectly OK because the time will be very short but it might cause nuisance blowing of a non-delay fuse that was selected to be reasonably close to the normal full-load input current for the xfmr.
 

MaxHeadRoom

Joined Jul 18, 2013
26,002
Your contactor coil also takes a lot of "inrush" power, so you will have a transient overload on your transformer.
One reason I never use AC inductive devices where at all possible, a simple bridge rectifier on the output and 24vdc coil can be used.
Same for solenoids.
Max.
 

Thread Starter

3PedalMINI

Joined Jul 13, 2014
18
You can "piggy back" on the mains side of your contactor. I am assuming based on your contactor being 3 pole that you are switching 220 volt phase to phase to your motor. Then yes, you can piggy back your control transformer power off the contactor lugs. Personally I generally would use a terminal strip or block where my mains entered my control panel and get my power there but you can also do it as mentioned.

Ron
Cool, Yeah I think thats what im going todo! Theres a DINrail in there anyway so might as well do this! Thanks for all of your help!

Is this going to be installed in an industrial or commercial setting? IOW is it operated by someone other than yourself, if so there are some things that are mandatory when building a automatic system, such as a recognized E-stop station, this must be easily accessible by any operator.
And possible other safety features.
Max.
Nope, its just me! Its for my garage. I will be the only person using this. Although it wont have a "Estop" it will have a a 3 position held switch for OFF-Manual-Auto. Its for an electric pressure washer im building!

For a transformer of that size I wouldn't fuse the secondary at all. Sometimes with very small transformers the primary "doesn't know what the secondary is doing" because of winding resistance and other factors. With a 75 VA transformer, you won't be able to overload the secondary without the current being accurately reflected in the primary. Secondary fusing is something that is a good idea if you have multiple secondary windings that are being used separately, but if you intend to use the secondary windings in either series or parallel there is no concern with overloading a particular winding.

You definitely need a slow-blow ("time delay") fuse for the primary. If there is residual magnetism in the core when power is applied and the applied power pushes the magnetism in the same direction you can get a very large current spike because the iron core "saturates.". The Wikipedia article on transformers explains an illustrates this quite well. Your contactor coil also takes a lot of "inrush" power, so you will have a transient overload on your transformer. This is perfectly OK because the time will be very short but it might cause nuisance blowing of a non-delay fuse that was selected to be reasonably close to the normal full-load input current for the xfmr.
Thanks! I was struggling with the reasons to fuse the Secondary side. I might just do it because its like $3 bucks more and 4 extra crimps. But for low voltage like 24v I cant see a reason for it, especially just powering a timer and a contact! Ill make sure to get a slow burn fuse for the primary. Although all its running is a pressure washer pump!
 
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