Sciencemadness Discussion Board

electrical advice needed: motor fuse

Magpie - 4-9-2016 at 17:42

I have a Craftsman ShopVac, 12 gallon, 120 VAC, 11 amps, for lab support and general cleanup of my garage. I've had it for 13 years with never a problem (despite it smelling like naphthalene for years) until 3 days ago when it just quit. Tearing it apart I found that a thermal fuse located on the motor had blown. Motor insulation is class B.

I want to replace this with the correct fuse. However, the parts list does not show it and the fuse and fuse holder have no identifying markings.

I have ordered a fuse rated at 15 amps and 121°C. Did I make a good choice? Your advice please.

highpower48 - 4-9-2016 at 19:17

No, not really. You should or replaced it with 10 amp fuse. By replacing it with a higher rated fuse you are taking the chance of destroying the motor and even the possibility of a fire.

Oscilllator - 4-9-2016 at 19:31

Quote: Originally posted by highpower48  
No, not really. You should or replaced it with 10 amp fuse. By replacing it with a higher rated fuse you are taking the chance of destroying the motor and even the possibility of a fire.

highpower48 I don't think you are quite right here. Magpie is talking about a thermal fuse, not a normal electrical one. Were it a normal fuse you would be right though.
Personally I think 121°C sounds a little high. If there is plastic around you would probably want one around 80-100°. I have a bunch I have scavenged from microwaves that are 10A 80C so if you have a spare one lying around you can tear it apart to get the fuse from that. They also have excellent contact switches.

Magpie - 4-9-2016 at 19:55

My reasoning on the higher amp rating was that if the normal current is 11a I would need something a little higher to keep it from blowing during normal operation.

For the temperature rating: I just realized that I saved the wire from the old fuse. Could I heat that in hot oil to see where it melts to get the rating?

wg48 - 5-9-2016 at 05:13

Quote: Originally posted by Magpie  
My reasoning on the higher amp rating was that if the normal current is 11a I would need something a little higher to keep it from blowing during normal operation.

For the temperature rating: I just realized that I saved the wire from the old fuse. Could I heat that in hot oil to see where it melts to get the rating?


Motors can have much higher starting currents than running currents so selection of the over current operated fuse must take in to account the time x current characteristics of start up ie a slow blow fuse. But as an other poster has stated thermal fuses are not current fuses.

Not all thermal fuses use a low temperature melting point metal alloy. Also it will be difficult to determine the melting point of a small peice of wire and in any case the operation my rely on mechanical failure prior to melting.

Personally I bypass the thermal fuse with a short when they fail on the assumption I will detect the smell of cooked insulation and notice the smoke ( I would strongly not recommend that for the average person who commonly ignore or don’t notice the warning signs until the item stops working or the flames get really high LOL)

Yes it’s a risk but I don't want to have to repeat the almost impossible task of disassemble and reassemble of modern consumer items. Do You?



Sulaiman - 5-9-2016 at 05:24

the 'blowing' of a thermal fuse could be due to a fault condition, or just 'worn out'
vacuum/fan motors get loads of dust/dirt/fluff/hairs in the bearings so overheating may be due to the bearings,
or worn out carbon brushes
or carbon from the brushes bridging commutator sections.

Magpie - 5-9-2016 at 05:55

The HEPA filter bag had a fair amount of dirt and the fluted cartridge looked like it may have been airflow constricted due a heavy dirt load. This may explain the fus e failure. I now have new items for replacement.

Actually taking this vacuum apart wasn't too challenging once I got the right screw driver (hex).

Edit: When I shorted the fuse terminals the motor took right off so there's no problem at the brushes or commutator. I don't suspect a problem with the bearings either as rotor spins freely and everything was clean around the motor.

But after 13yrs the bearings are bound to be a little dry.
[Edited on 5-9-2016 by Magpie]

[Edited on 5-9-2016 by Magpie]

[Edited on 5-9-2016 by Magpie]

Magpie - 5-9-2016 at 10:23

Quote: Originally posted by Sulaiman  
the 'blowing' of a thermal fuse could be due to a fault condition, or just 'worn out'
vacuum/fan motors get loads of dust/dirt/fluff/hairs in the bearings so overheating may be due to the bearings,
or worn out carbon brushes
or carbon from the brushes bridging commutator sections.


I ju st ran the motor about 2 minutes on the stand without the impeller. It only d rew 3a. However, the front bearing near the commutator was very hot. I'm now thinking this is the root problem.

hissingnoise - 5-9-2016 at 11:52

Worn brushes arcing can up the temp.!

Then again, when running in its housing, airflow can overcome some of these heat problems!

Magpie - 5-9-2016 at 12:32


The motor fan and front bearing are pressed in place - no maintenance or even inspection possible.

New motor ass'y: $83; new shop vac: $80. End of story.

Thanks for your replies.

Dr.Bob - 7-9-2016 at 05:14

Back many years ago, I worked in a lab where they ran a lot of large silica gel columns. Big enough that they were fixed and the silica was dumped in and then removed by vacuum. Occasionally, they would fail to dry the column well enough first, and the vacuum cleaner would shoot flames out of the exhaust due to the hexane fumes. That was very exciting. But the vacuum lasted for years despite much abuse. I would rebuilt the vacuum and use it until it fails again, then buy a new one. I have had several items last years on what I thought was their last leg.

Magpie - 7-9-2016 at 08:47

I like to repair things and believe in your philosophy Dr Bob. But that bearing was "hotter than a pistol" after running for 2 minutes and when I cut the power it made a strange noise as it coasted to a stop. The bearing is sealed and there's no way I can replace it. I have ordered a new shop vac, $80, free delivery to my house.