Does amperage draw matter on bench grinders?

[larryq]

In looking at some older Craftsman bench grinders, I've seen some odd numbers. My 1/4 HP block grinder has an amp rating of 3.8, while my older-stype 1/3 HP block grinder has one of 3.6. And I've seen 1/2 HP block grinders rated at 5.8 amps, a fairly large step up.

How much does amperage matter when looking at bench grinder power?

 

[monkeyspanners]

Think it can depend on how efficient the motors are and also how they calculate the horse power.

 

[MoonRise]

More amps = more power

More power = 'better' (in general)

disclaimer : there can be variations in 'effective actual power', based on motor efficiencies and such.

Not much difference between 3.6 and 3.8 amps, noticable difference when the motor jumps up to the 5.8 amp one.

YMMV.

 

[signcrafter]

From what I understand HP numbers with electric motors are like HP numbers with small engines, manufacturers can use all sorts of formulas to come up with HP numbers that will make their stuff seem more powerful. Just like HP numbers on electric air compressors, can a 110 volt 30 gallon craftsman compressor really be 5 or 6 HP? Where a quality 220 volt 60 gallon compressor is only a 3 HP. To me the amps of a motor are more of a "real" or true measure of the motors power.

 

[littletoes]

1st, you have to have the available amperage to run it! But yes, usually more amperage means more hp. Read the name plate, if your motor is drawing more amperage than what is available, you have a problem....but this isn't what you were asking!

Heres one to think about; When in your shop, add all the amperage of all the equipment plugged into the same circuit, and if it is more than what the circuit is rated for, you then know what you can run together, and what you can't. Or what will blow a fuse.....it's basic math, and it does work!

 

[Davefr]

More amps = more power

More power = 'better' (in general)

...and if you plan to do any wire brushing or buffing you want all the power you can get.

 

[gsmornot]

You can say that again. Granted my grinder is nothing special but with the wire wheel on I can stop it. If I forget to install the spacer for the wire wheel it rubs the guard and may/may not turn on at all. I could use a better grinder but not until the one I have gives out.

 

[buening]

The craftsman block grinders varied in amperage over the years too. I'm not sure if it was an efficiency thing, or overstating horsepower rating. As seen below, 1/2hp ranged from 4.4a to 6.2a (seemed the older the block grinder the lower the amperage), yet the 1/3 shows 4.7a, the 3/4 shows 6.0, and the 1hp shows 6.8amps. I only randomly picked the non 1/2hp pictures for an example. Those amperages varied over the years too.

I'm not sure if the block grinders got less efficient over the years, or if they underrated their hp ratings. Typically the amperage ratings are spot on and the hp ratings are what "floats".

Examples:

Hard to read, but says 5.2amp

Now compare this to a 1/3hp one:

and a 3/4hp one:

and even a 1hp one:

As you can see, the amperage numbers don't exactly jive between them all in relation to the HP ratings.

 

[larryq]

The craftsman block grinders varied in amperage over the years too.

Thanks for all that, it's very interesting.

 

[Fuzz]

Using the nameplate rating on the grinder in the last photo:

1 HP = 746 watts

115V x 6.8A = 782watts

It looks to me that the horse power numbers are nominal ratings. The true rating is figured by the motor wattage. As with electricity in general, we use the nominal voltage of 120/240 volts for calculations, but meter readings will show the true voltage +/- of that number.

 

[Major Ramifications]

Yes, it matters a hell of a lot.
Especially if you use a wire wheel on that grinder.

 

[buening]

Using the nameplate rating on the grinder in the last photo:

1 HP = 746 watts

115V x 6.8A = 782watts

It looks to me that the horse power numbers are nominal ratings. The true rating is figured by the motor wattage. As with electricity in general, we use the nominal voltage of 120/240 volts for calculations, but meter readings will show the true voltage +/- of that number.

That would assume 100% efficiency. The formula for HP is V*I*Eff/746. Take the first picture for example. Backing out the efficiency using 0.5hp results in 52.3%. The last picture of the grinder with 4.4a results in efficiency of 73.7%. This all assumes the HP rating stated is correct.....

 

[PoorUB]

I have always assumed about 10 amps per HP on 120 volt, so 1/2 HP would draw around 5 amps. I would be skeptical of a motor that claims 1/2 HP and draws less than 4 amps. Looks like 1/3 HP to me.

 

[seber]

One hp = 746 watts. Fractional motors can range from 80% to 90% efficiency. So there is some variation but in general, amperage is power and far better indicator than some made up hp number. That is especially true since Sears came up with "developed horsepower" which has no meaning at all. That kind of garbage leads to six hp compressor that runs on 115 volts.