Motor Rated Switch Question

[Bob Coleman]

Hello All,

The switch on my shop vac burnt out this weekend, so I went to today to get a replacement. The new switch has the same electrical ratings, but a hp rating was not listed (there was just a "-" in that column). This was the only switch they had that was the correct size for the hole.

Is there any issue other than premature failure of the switch that I should be worried about with using the new switch?

Thanks in advance!

278395

[David L Morse]

This one http://www.digikey.com/product-detail/en/RA901-VB-B-9-V/432-1118-ND/483690 has a 3/4 HP rating.

[Jeff Erbele]

Hello All,

The switch on my shop vac burnt out this weekend, so I went to today to get a replacement. The new switch has the same electrical ratings, but a hp rating was not listed (there was just a "-" in that column). This was the only switch they had that was the correct size for the hole.

Is there any issue other than premature failure of the switch that I should be worried about with using the new switch?

Thanks in advance!

278395

It appears the switch on the left is your old one (fried near the prongs) and the one on the right is your new one that fits in the hole.

So it fits in the hole but it appears that the contacts are incompatible. Have you tried to plug it in; does your vac have a single socket or individual leads.

If everything else fits and works, don't sweat the HP rating as it converts to amps:
http://www.sawmillcreek.org/showthread.php?6316-Formula-for-converting-HP-to-AMPs-or-Visa-Versa
and your new switch is rated equal, 16amps @ 125volts and equal or greater amps @ 250volts.

Otherwise your greater concern might be the cause of the switch failure; cheap junk / old vac, coiled cord, exceeding duty cycle, plugged filter, bad bearings, brush or commutator issues if the motor is of that type, and so on.

If your vac has other problems those should be determined and addressed.

[Lee Reep]

As long as the replacement switch can handle the current that the old switch did (at the same rated voltage), you are fine. I would not worry about the HP rating -- most shop vacs have bogus HP ratings. Ever notice that they talk about having 4, 5 maybe 6 HP, yet the motors are tiny? And the big cyclone dust collectors that have big motors are typically 2 or 3HP.

Another way to calculate if you want is to know that 1 HP equals 746 Watts. So, for example, a 120V, 20A circuit can supply 2400W (W = V x A). That means the circuit could handle a little over 3 HP.

And that means a better way to know the power of your shop vac would be to see if you can find the current draw for it. If the motor drew, say 6A, then multiply that times 120, giving you about 720W (or about 1HP).

[Bob Coleman]

Thanks for the input!

The vac is "rated" at 5hp, yet the original switch is rated for 3/4 hp!

THe original switch has two standard 1/4 inch terminals and there are two wire leads running from the motor to the switch, so no issues with connection compatibility.

As far as the amps <--> hp conversion, I agree this is true for steady state, but my understanding is that motor rated switches are made to support the starting current of a given size motor, which may be quite a bit larger than the steady state current. My only thought is that this would damage the switch only, or could it damage motor? (actually I'm really only concerned that it start a larger fire since its not that expensive a vacuum)

I checked the motor, filter, etc. Everything seems fine. The spring return in the switch broke as well, so I'm chalking it up to switch internal failure.

Thanks again!

[John McClanahan]

It's possible the internal contacts were not the problem, but rather one of the spade plugs didn't plug tightly on the switch prong, creating a hot spot from the poor connection that melted the switch. Be sure the wires plug onto the new switch tight.

John

[Dennis Ford]

HP rating for switches is meaningful. A properly rated switch is capable of interrupting locked rotor current which is much higher than full load current (it does not have to be capable of carrying locked rotor current indefinitely. If the motor locks up and you flip the switch, it would be nice if the switch did NOT become a ball of flame. Biggest problem here is that the HP rating of the vac is bogus. Without information on the actual locked rotor current; it is mostly a guess.

[Jeff Erbele]

Biggest problem here is that the HP rating of the vac is bogus.

I agree with Dennis; the HP rating is bogus and over-stated.
1 HP = 746 watts x 5 HP = 3730 watts
If the power factor is 1 (an assumption), the current is almost 30 amps. My guess is the shop vac does not have a special outlet on a circuit rated for 30 amps; that it runs just fine on any standard household or basic shop 15 amp circuit. Most shop vacs do and especially the budget class.

Further if it were a 30 amp motor, it exceeds the factory switch amp rating by 3x. It is a pretty safe bet the vac-motor doesn't look anything like, or is built like a 5 horse motor on a table saw, air compressor, dust collector or any other machine in the shop, in regards to the stator, the windings, the armature or rotor, the shaft, the bearings and the collective total weight. I am discounting the frame and junction box because there is no correlation between the shop vac construction and the other machines.