Most all manufacturers of garage vacuums and shop vacuums (shop vacs) are marketing their vacuums using horsepower - typically 4 to 5 Peak Horsepower (Peak Horsepower or PHP).
The problem is that horsepower is just horse hockey when it comes to measuring the performance or suction of a vacuum cleaner and has no relationship to the performance or cleaning ability of the product.
Horsepower is not based on the nominal operating current of the vacuum cleaner, but rather on the in-rush current to the motor when first turned on. That PEAK in-rush current is what they base the HP number on and lasts no more than 0.008 to 0.012 seconds. It is often inflated by bring the temperature of the motor down to as low as -20F. If you convert a motor with a 4 horsepower rating it would have to draw about 2983 Watts, or 24.9 Amps, at 120Volt under ideal conditions. This would GREATLY exceed the voltage supply circuits and blow the circuit breaker in a home.
For the technical person: Horsepower is not based upon the normal operating current of the motor. It is calculated using the maximum in-rush current. In-rush current is the current a motor sees when first started up, before the effects of winding inductance come into play. A motor with a nominal operating current of 12A can easily have an in-rush current of 30A to greater than 50A, depending upon the windings. It typically lasts for only 8 to 12 milliseconds (0.008 to 0.012 seconds).
Some manufacturers attempt to maximize the in-rush current by conditioning the motor at very low temperatures (as low as -20F). This lowers the motor resistance, thus increases the in-rush current. Power is equal to volts times amps. Even ignoring the efficiency and power factor of the motor, a motor with a nominal 12A current @ 120V will only produce 1, 440 Watts. There are 745.7 Watts per horsepower. Thus, under the best of conditions, the motor will only produce 1.9 hp. A motor touting 4 hp would have to produce 2983 Watts. With that power, the motor would be drawing 24.9 Amps. Horsepower is determined for the motor (without any fan blades), not the vacuum cleaner system, and does not take into account any normal system losses (air leaks, restrictions, piping and the like).
This became such an issue in the late 1980s and early 1990s that Underwriters Laboratories and the Canadian Standards Association put wording into the vacuum cleaner standard prohibiting horsepower from appearing on the products electrical rating label. Horsepower has no bearing on the performance of the product.
If you really want to know the best way to measure performance of a vacuum cleaner, you need to find out what the Airflow (CFM) of the machine is as well as the Vacuum Pressure (IL - Inches of Water Lift). The real problem is getting that info from the manufactures.
Even with that info be careful. Just because you get that rating on the MOTOR does NOT mean that the machine is capable of those performance specs. The way the air travels through the machine will affect the final numbers. In my next blog I will delve more into this issue.
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