I went digging for this at Shopvac’s site, but came up empty. Other vacuums seem to run at anywhere from 10K to 30K rpm, but of course that depends on the fan design.
What’s the typical rpm for a ~2HP centrifugal fan wet-dry vac, running free with inlet and outlet both open to air?
Did Shopvac’s site say what kind of motor they use? The fan can run at any speed if gearing is used.
AC induction motors are by far the most common type used in domestic appliances. Their speed for a 60 Hz. source is 3600 rpm divided by the number of pairs of poles, minus about 50 rpm when the motor is driving its rated load.
A 2-pole motor will thus run at about 3550 rpm when fully loaded. A 4-pole motor at 1750, a 6 pole at 1150, etc.
I’m pretty sure ShopVacs use universal series-wound motors. These generally run at higher RPMs than AC induction motors, though I’ve no concrete numbers to throw at you, I’m afraid.
Vacuum cleaners almost always use universal motors with brushes, not induction motors (which are commonest on other appliances). It’s because they need high speed to create enough pressure. Running a single stage fan at induction motor speeds would take a fan wheel many feet in diameter to create the pressures vacuum cleaners need.
Universal motors from vacuum cleaners probably run 5000 or 10000 or so rpm, but they would run considerably faster without the fan (which is usually constructed of multiple stages of centrifugal impellers, with stationary stages between them). You can often ruin the motor by running it without the drag of the fan - it goes so fast that centrifugal force spoils the rotor winding insulation. You might have to run it at a higher voltage to do this.
Many small universal motors for 110 VAC are rated for 20,000 rpm or somewhat more.
The library’s copy of “Small Electric Motors” puts these in most vacuum cleaners. There’s no obvious way to to calculate the speed of these things, so I guess I’ll have to find a tachometer and measure it.
-Thanks guys.
It’s hard to convey how chagrinned I am at having forgotten all about the ubiquitous universal series motor. Particularly so since I worked for a short time in high school for a guy who repaired small motors. At that time machines were expensive and labor was cheap so repair was worth it and manufacturers hadn’t yet learned how to make them disposable. My job was to take the things apart, lubricate, install new brushes and reassemble the motor after he had dressed the commutator and undercut the mica.
I believe that such motors have no theoretical limit to their speed and without a load many of them will destroy themselves by centrifugal force on the rotors.
I’m not sure it takes a higher voltage to do it. A motor runs at a speed such that the applied voltage minus the back emf allows enough armature and field current to drive the load. With a series motor, at start the current is large so the magnetic fields are large and there is a large starting torque. As the rotor speeds up the back emf in it incrementally reduces the current and thus the magnetic field and therefore the back emf. So the rotor speeds up because the current has again increased incrementally. This increased speed increases the back emf which again incrementally reduces the current which reduces the back emf and so on and so on. It is a positive feedback system which is unstable in velocity. The thing that finally limits the speed is that the current finally falls to a point that it will just drive the load, and if there is no load except rotor friction and windage it might very well fly apart.
A ball park speed is probably the best you can do because as the motor bearing friction changes the speed will change since it is quite sensitive to the applied mechanical load.