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Killer Shop Vacuums

How can a $150 vacuum have a 6.5 horsepower motor?

Last update: August 2, 2010

If you're looking for a new shop vac, one of the comparisons you might be checking out is the horsepower rating. And, if you know anything about machinery, you might be totally mystified at how shop vac manufacturers get such amazing horsepower from such small, inexpensive motors.

Take a look at your tablesaw. It has a massive, heavy motor that's much larger than what's in those shop vacs, yet it apparently produces one-half or less of the power. And if you've had to replace one, you know that even a cheap 3hp motor costs several hundred dollars.

Also, think about the electrical requirements. Every wonder why 3hp and up machines always run on 240 volt power? It's because the power draw of a motor is directly related to the mechanical energy it produces. The power drawn is a function of volts multiplied by amps or "watts". Those watts can be provided by using lower voltage at higher amperages or higher voltage at lower amperage. It would be possible to create a 120 volt motor producing 3 horsepower, but the amperage would be really high and require you to use extremely heavy wires for the outlet as well as the power cord. So they just use 240 volts and drop the amps so you can use normal wires.

So, back to our shop vac. It says it has a 6.5 hp motor, yet it runs on everyday 120 volt power with a fairly thin power cord. A 6.5 hp motor would require at least 5000 watts of power. At 120 volts, that's over 40 amps! Even if the circuit breaker didn't pop, 40 amps of power through that little cord would cause it to heat up and burst into flames!

How do the vacuum manufacturers do it? What special magic are they using? Or are they just lying?

Well, they can't really lie because there are government agencies that are supposed to keep companies from flat-out telling untruths. What they are doing, however, is using the word "horsepower" in a way that no normal person would by adding the word "peak".

When normal people talk about horsepower, we're thinking of the concept of how much work the motor is producing. However, vacuum manufacturers ignore how much work the motor is producing and instead measure how much power the motor is consuming, which you could rationalize as indirectly measuring the output by measuring the input. Which would make sense, in a way, except for their rather unique definition of "peak".

To normal people, "peak horsepower" would be the amount of power when the motor is working really, really hard. However, vacuum makers (because they're measuring power usage) choose to use the peak amount of power being used by the motor. And with electric motors, that peak occurs when the motor first turns on. This startup surge or "inrush" is tremendously higher than normal runtime power consumption. By concentrating on this "peak" the manufacturers can claim a much higher horsepower rating than should realistically be stated.

Now another question may pop into your. mind: If this shop vac is pulling 40 amps of power, why doesn't the circuit breaker pop? Well, it's because the inrush current peak only lasts about 0.01 seconds. That's right, your monster 6.5 horsepower vacuum only produces that much power (theoretically) for 1/100th of a second.

Let's take a closer look at our example "6.5 HP" vac above. (I'm not trying to single out the Shop Vac brand, all vacuum manufacturers do the same thing. After all, in this case, being truthful would just hurt them) The specs say that the unit is pulling 12 amps at 120 volts, which gives us 1440 watts. Even if the motor were perfectly efficient, it would produce less than two horsepower. So, in reality, this vacuum is probably producing around 1/4 of the power it claims.

The bottom line is that when shopping for a vacuum, you should totally ignore the horsepower rating.

 

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