Lucky to Have a Face

Bought the old girl a new Hoover upright last week. Went at the old one today (hoover that is) to see what I could salvage. Removed the motor and housing. The impeller was broken so I took that off (turns off CW - left hand thread) and turfed it. I could not find any markings regarding RPM on the motor. So I dug out my digital tach and powered up the motor. I guess my eyes must have just about bugged out of my head: 31,998 RPM accompanied by a small jet engine whine.

Online I found that the factory motor spec was just slightly under 20,000 RPM. So where did the extra RPM come from? I determined through my online research that one must never run the motor with the impeller and cover removed. 'Tis hard to believe but he air resistance on the impeller keeps the RPM at the lower value: 20,000 RPM. There are documented cases of salvaged vacuum cleaner motors disintegrating when run without the impeller in place. A new impeller is $10, so I will weigh the expense against the usefulness of the motor.

I guess I'm fortunate that I still have a face.

Bill

I stripped the motor from an old Bosch grass strimmer and measured the no load speed at a fraction over 16,000 rpm.

Hi Bill,
This is the way a series wound motor behaves. The torque produced by the motor is roughly proportional to the square of the current being drawn by the motor. The motor speed increases until the voltage across armature is equal to the supply voltage minus the voltage across the field windings. If there is a load on the motor then an equilibrium is reached where the current though the field coils is enough for the armature to generate enough voltage to maintain this state. (The voltage developed across the armature is roughly proportional to the field current times the speed.)
If there is no load on the motor then the current is very low. If the current in the field winding is low then the armature needs to rotate faster to generate enough voltage to try to reach the equilibrium state. The motor speed will increase until the torque due to friction is enough for the equilibrium state. This may not be reached before the motor disintegrates.

Les.

Our washing machine has a 1600 rpm spin speed, that motor must be going pdq, considering that the poly vee drive is from a very small motor pulley to a large pulley on the drum

The legendary 'Duplex' wrote up a home-built grinder based around a series wound motor that relied on a friction pad to keep . Utterly insane if you ask me.

Neil

I tried using the series wound motor out of a Hoover Twin Tub washing machine, with an electronic speed reducer to drive a flexible drive shaft over my work bench, but to get it slow enough, it had virtually no torque under load, I did burn out a 1/8" drill bit. It needs a more complex controller than my simple one.

Ian S C

Les Jones description is spot on, the fan and the back pressure in the bag provides sufficient resistance to keep the rpm down, take it off, and they will run to destruction! another fun thing we used to do at tech college was to rig a flourescent tube with a switch that removed the choke from the circuit by shorting it out, the tube gets brighter and brighter until..............................................Dont try this at home!!!

I have a hotpoint wahing machine motor labelled as 15000 rpm, and that will be with the load of a full drum - certainly a candidate for 30k+ unloaded.
I think a vacuum motor might have cooling problems but a washing machine motor is also designed to be run slow as well as fast and to do so for periods of up to an hour with a medium duty cycle. I just wish there was documentation for the speed controller connections.

I really haven't considered using the Hoover vacuum motor to drive any tools. I am kicking around the idea of using it as a source of blown air (rather than having to hook up my compressor) or a maybe a dust extractor for my woodworking and carving activities . I suppose it could also be used to quickly inflate air mattresses; maybe even the odd Zeppelin. I have heard that some people make leaf blowers from them while others have used them to build battery operated hovercraft.

Bill

All of the series wound washing machine motors I have seen are driven by a closed loop speed controller so they do not have the problem of speed runaway. The moters have a magnet and pickup coil on the end of the shaft that generates a few pulses per revolution to provide speed feefback to the control board. This is needed to provide the wide range od speed between the wash cycle and the spin cycle.

Les.

There was an article in MEW some years ago regarding altering the motor speed board from a front loader washing so that the motor and board could be used for other purposes in the home workshop. I have couple some where, I put the bits in plastic box and mounted the speed potentiometer on the front, the motor is very noisy and is designed to run in a horizontal position and has little protection from bits getting inside. So the idea never got used, however the motor bearing are good for other uses in the workshop. Better to get a 3 phase motor and inverter if you need motor speed control. Back

To Ian, these motors have a small Taco on the end of the shaft for feed back so that as the motor slows down more current is applied to the motor, maintaining the selected speed. So not quite the gutless straight series motor as was fitted to the twin tub.Ted

Vasily Zakharov wrote an article on using wasjhing machine speed controllers in MEW 86 with a wiring diagram for a Hoover controller.

There was a further letter on the subject in Scribe a line in issue 89.

Obviously all due precautions to be taken following this approach, especially as these are open frame motors as John points out..

Neil

John, it depends on how old the motor is as to whether it has electronic control, the early ones didn't.

Ian S C

The same circuit board and modification will work on most ex washing motors. I have looked at other makes of washers and they are much the same. I can't exactly remember, but I put an extra resistor on the board limiting top speed, regardless of the speed potentiometer, a bit of safety feature. The motors are well balanced, and can get some revs on, but noisy at top speed. More importantly the aluminium heat sink on which the Triac is mounted is at MAINS VOLTAGE, user beware. That is why I put the board in a 12" X 8" X 4" plastic box which I already had.Ted

Please note Haynes used to do a service manual on washing machines its supprising how many bits are interchangeable

Roy

Out of interest what would happen if you did let it run out to full rpm with no load?

I suppose it would be something like this: **LINK**

Warning if you are concerned about the well being of others or the preservation of electric motors this video is not for you!

Edited By Thomas Gude on 01/03/2015 20:55:11

I once ran one of my heli-nitro engines up without the blades attached - snapped conrod, bent crank, smashed head....didnt do it again!

I have a 91/4" Ozito angle grinder it had a soft start on it which went faulty so I decided to take out and just run it without ,it must have had some sort of rpm limiter in it as the speed increased (I could not check as some one liked my rev counter more than me ) I could use a cut off wheel in it but not grinding wheel as it was way out of balance at the new speed as luck would have it a mate gave me a Ryobi grinder with built in speed controller for sanding and polishing functions it had a broken pinion in the gear box giving a speed control between 1500 and 2800 ( I think) I made a steel box for it and leed in ,mounted normal switched outlet on it and can plug my grinder in to it and can control the speed now, I have also used this on an electric chain saw motor driving my drill press works well! I got as scrap as it had no belt pullies ,it had a crude system of bicycle and industrial sprockets and chains , I wish I had taken a photo