Fuses for machinery

[steamboat willie]

Hello!
I have modified my X2 mill and fitted it with a power feed for the x axis. To do this I have used a 24v geared motor which seems to work very well. I am indebted to others on this Forum who have done the same modification for their work and suggestions as to how they went about it. My question relates to the power supply. The motor I am using is 24volt, and I have a dedicated 24volt 5 Amp power supply to drive the motor. I am going to place an in-line fuse into the power line for the motor and am going to experiment starting with 1, then a 2 and then a 5 amp fuses. I am hoping to ensure that the fuse blows before anything else does. From my meagre knowledge it is the current that I am sensing, not the voltage and so I think I can use a 240 volt fuse of the correct current rating in the 24volt application. I have seen some comment about 24v fuses and thought that they would not do anything different than the 240 v equivalent.
Could I ask any learned members of this Forum to enlighten me as to whether I am able to use a 240v correct current rated fuse in a 24v application?
Thankyou for your time. Your comment will be most welcomed and appreciated.
Bill.
Bill.

 

[portlandron]

The fuse needs to have a voltage rating equal to or higher then the voltage of the circuit it is being used in.
If voltage rating of the fuse is lower then the voltage in the circuit there is a chance for an electric arc across the open fuse elements.

 

[steamboat willie]

Thankyou portlandron for your help. You have confirmed what I thought. The voltage rating of a fuse refers to the voltage up to which the fuse will resist arcing between the two poles of the fuse if it blows.
My thanks to you for your help and wisdom.
Bill.

 

[Wizard69]

Thankyou portlandron for your help. You have confirmed what I thought. The voltage rating of a fuse refers to the voltage up to which the fuse will resist arcing between the two poles of the fuse if it blows. My thanks to you for your help and wisdom. Bill.

For motor circuits you might also want to consider time delay fuses that can survive a bit of inrush current.

On another note you didn't mention what sort of power supply you have, some older motor drives didn't like seeing an open circuit on the motor leads. More than a few stepper drives had issues with self protection when the motor was disconnected. Modern drives pretty much have eliminated this issue but referencing manufactures fusing recommendations is a good idea. If you are running a plain old 24VDC power supply to the motor this likely isn't a problem.

 

[steamboat willie]

Thanks for your reply Wizard69. I am familiar with slow blow fuses and have a question to ask you - usually the damage is done quite quickly if there is a problem, and (theoertically) the fuse blows, or,as happened in my case the circuit goes PHUT (technical term). The circuit sacrificed itself selflessly for the protection of my 20 cent fuse. How kind. If the fuse was slow blow, wouldn't it all just happen at the same speed, then sensing a problem, the slow blow fuse would decide to do its thing and blow? If the peak current exceeds that specified for the circuit albeit ever so briefly, wouldn't damage still be done to the circuit in any case? I am aware that slow blow fuses are used for in motor startup circuits where the starting current draw is significantly higher than the current drawn when the motor is operating. If a slows blow fuse is used, what stops the peak current draw from causing damage? Is it because it is such a brief spike - fast enough to blow an ordinary fuse, but not slow enough to blow a slow blow fuse?
Many thanks for your thoughts and comments.
Bill.

 

[ddmckee54]

It's been my experience that slow blow fuses are used in circuits that are not controlled by circuit boards. Instead they are used in circuits controlled by a relay, simple switch, or that sort of thing. Usually when you are using an electronic control board of some type a fast blow fuse is used to keep the lands on the circuit board from turning into the fuse, just as you described. Different classes of fuses are designed to tolerate different levels of over-currents for different times. For instance one class of fuse may tolerate a 500% over-current for 4 seconds before opening while another class may only tolerate that current for 0.5 seconds before it opens.

Hope that helps, I've been designing industrial controls for 25 years and specifying a fuse still seems like black magic at times.
Don

 

[Tinkerer58]

I would have a close look at the specs for the power supply's output rating, generally when a power supply is rated at 5 amp or X amps it is based on continuous safe operating output. The output circuit is generally over rated in case of peak current demands eg. motor start up currents which are higher than constant running current and it should also state the period this peak current can be supplied by the power supply eg 20 mili seconds. Your current will also vary depending on the load on the actual motor in question. I would use an ammeter and put in series with the positive of the motor terminal and the power supply (always start with the meter set at the highest current range and work your way down till you get a reading). This way you will be able to monitor your startup current and running current of the motor under different loads. Your fuse should be at least rated at 25% above the maximum current draw from the figures you have collated from your above ammeter tests.

 

[steamboat willie]

ddmckee, and Tinkerer58,
Thankyou both for your invaluable advice and information. It will be a 5A fast blow fuse that I will use and I will connect the ammeter as advised to check the current draw.
Many thanks for your time and assistance gentlemen!
Bill.