Universal Motor Speed Controller

[made2hack]

Hi All,

I have a miter saw (230VAC / 50Hz / 2000W) that runs at 5000 RPM. I want to make it run at 2500 RPM. It is a Universal Motor.

I'm not sure what to search for on Google, what terms or topology to use. Too much confusion pops up and forum posts (elsewhere) that I'm having trouble navigating the results.

Below is an image of what I want to achieve. I've abstracted the motor in a black box in the event that there is a solution that I can implement externally (without modifying the internals of the electronics). However, you can if you think it is easier to recommend modifications to the internal electronics as well. I just assumed it might be easier externally.

But I don't know.

Thanks for you input,

P.S: Aside from the technical aspects, what effects will this have on the motor? Will halving the speed double the current? Or does the motor not necessarily maintain torque? Will torque just be halved if I halve the RPMs?

[Benta]

You need to post more info. What electronics are in the motor, if any? Does it have tacho feedback? Photos would be a great thing.

[drussell]

Are you trying to do something like this?

RIDGID abrasive cut off saw converted to dry cut. Maybe.

P.S: Aside from the technical aspects, what effects will this have on the motor? Will halving the speed double the current? Or does the motor not necessarily maintain torque? Will torque just be halved if I halve the RPMs?

There are several issues with trying to slow down a universal motor like that.  Yes, you will need some kind of tachometer feedback and controller to maintain reasonable torque, etc.

[6PTsocket]

Before you go all high tech, try a simple triac speed controller. It may be all that you need. People slow down routers with them all the time. If loss of torque gets to be a problem, a pulse width controller is better at lower speeds. If you just want to knock off a bunch of speed you can just stick a diode in series with the motor and get rid of half the sine wave. Try a simple solution first.

Sent from my SM-G900V using Tapatalk

[Kalin]

If  all you want is half speed ad are ok with losing power a single diode in series with either line is the way of go. I have seen it don in many different low power AC devices with a universal motor(ie: leaf blower, vacuum and a oscillating tool)

Sent from my SM-G965W using Tapatalk

[Zero999]

Yes, a diode will work. It won't necessarily reduce the speed by half though.

[t1d]

The diode approach seems like a reasonable and simple solution. I just wanted to point out that it would be extremely easy to make an external converter box, to keep you out of the motor. You just need a female socket output, for the saw to plug into, and a male wire plug input, to come from the wall outlet. Just connect the neutral and ground wires directly. Add the diode, in line, to the hot wire. You are looking at less than $20, for the box and everything else. Just use parts from the hardware store.

A/C wall-type power sockets come in different amperages; 15a and 20a are typical. Higher amperages may be available. Make sure that the socket and plug wire can handle what your motor will require.

The diode rating depends on what you are using it with, too. Make it everything hefty, so that you can use the adapter with a lot of different equipment. Be sure to label the device, clearly, so no one will think that it is just an extension cord.

IIRC, less expensive, decent motor speed controllers can be found at Harbor Freight.

[drussell]

If  all you want is half speed ad are ok with losing power a single diode in series with either line is the way of go. I have seen it don in many different low power AC devices with a universal motor(ie: leaf blower, vacuum and a oscillating tool)

Yeah, the diode idea is often used on blowers, even things like hairdriers to provide a "low" speed, however, I don't think this is going to work very well on a 2000W mitre saw that actually has to do some real torque when cutting rather that the air load of a blower fan.

Yes, a diode will work. It won't necessarily reduce the speed by half though.

I take it you guys haven't looked at AvE's video.    It totally didn't work.

No load speed remained the same, it lost torque, drew >30A from the AC line trying to run on half-cycles and heated the motor to the smoking point. 

[Zero999]

Yes, a diode will work. It won't necessarily reduce the speed by half though.

I take it you guys haven't looked at AvE's video.    It totally didn't work.

No load speed remained the same, it lost torque, drew >30A from the AC line trying to run on half-cycles and heated the motor to the smoking point.

No, I didn't watch it. If I watched every video posted here, I wouldn't do much else. If you post a video, please write a brief summary or what time the relevant part starts. Few people will have the time to spare to watch the entire thing.

Awhile ago, I used a diode to reduce the speed of a vacuum cleaner motor, used in an inflating device. It did work, but the speed wasn't reduced by half.

[drussell]

No, I didn't watch it. If I watched every video posted here, I wouldn't do much else. If you post a video, please write a brief summary or what time the relevant part starts. Few people will have the time to spare to watch the entire thing.

I thought it would be understood but I guess I needed to be more clear when I stated:

There are several issues with trying to slow down a universal motor like that.  Yes, you will need some kind of tachometer feedback and controller to maintain reasonable torque, etc.

I mean, based on things like <insert AvE's video>, if that is the kind of thing that the OP is trying to do, run a saw at half speed for whatever reason, a simple passive method like sticking a diode in there to chop the wave in half, etc. is not going to work.  The OP will most likely need some kind of a controller that can, for example, sense the actual current speed of the motor and adjust the power input accordingly.  While this could be done with a controller of some kind that produced variable DC from the motor's perspective, the power switch in the saw will likely not like interrupting DC so you would have to rig it up with a different kind of set-up rather than using the power switch and you're likely to not be able to just use some kind of simple method outside the saw itself.  I thought that's basically what the OP was asking.

For the TL;DR (TL;DWatch in this case, I guess ), if you don't want to see what he was trying to do, the equipment used, testing methodology, scope traces, etc. I suppose you can just skip to 11:41 or 15:12 for an example of where it doesn't work:

https://www.youtube.com/watch?v=ZZjnbl1apeY&feature=youtu.be&t=701

Awhile ago, I used a diode to reduce the speed of a vacuum cleaner motor, used in an inflating device. It did work, but the speed wasn't reduced by half.

As I said before, for fans and things like that with a "fluid" load it will generally work since the load presented to the motor will decrease.  It doesn't really work like that with a saw.

Also, as for not being half, it is the same issue as trying to drop vacuum tube filaments with a diode... It doesn't do what you might at first glance expect, it will not be half power, etc. etc.  There are many resources, both print and video, which investigate and explain this.

[drussell]

The most important thing will be to wait for the OP to come back online and specify what it is that they're trying to achieve, then we can probably provide better suggestions as to what might work in that instance. 

[made2hack]

Hi all,

Thanks, I had forgotten about AVE's video. It is EXACTLY what I want to achieve, cut the speed of the saw by half (if not more). I don't necessarily mind to get into the electronics of the saw, I was just wondering if there was a straight forward way of doing it with a black box externally.

What exactly is it that makes the motor run at the specified RPM? Is it the internal winding? What I mean is, since the voltage drops by half, and the amperage goes up by 2x, presumably the winding of the motor is what causes a constant power to be sought? IE the winding will always try to draw 2kW, irrespective of the supply voltage?

So, short of rewiring the windings, what would be the next steps? I take it that the winding is sized for the specific voltage that the device is meant to run at, so for 230V, it would be say 18AWG but for 120V it might be 12AWG? Would I assume the same number of turns of winding? Say 100 turns in both cases?

[made2hack]

.

[Zero999]

Lower speed motors will have more turns on both the armature and the stator. Higher voltage motors also have more turns on both the armature and filed. In low speed applications a high speed motor is used with a gearbox to reduce the speed, which normally works out cheaper than using a lower speed motor.

A phase control circuit could be used to reduce the speed of the motor. If a fixed speed is desired, firstly build the phase controller with a potentiometer and adjust it to give the desired motor speed, measure the resistance of the potentiometer, at that setting and replace it with the nearest standard resistor value.

[made2hack]

Thanks a lot, I will look into phase control circuit.

[BrianHG]

The easiest thing you can try is getting a 2kw 240v to 120v transformer.  Yes, a real transformer, not the diode BS.  The diode will not give you a true speed reduction, instead it will surge high current, full voltage into the motor at 50% the phase cycle.  In this case, with no mechanical load on the motor, the motor will spin at around full speed.  A real transformer will deliver a true half voltage.  This does not guarantee half RPM, however, the speed will be reduced, but, also, the motor's strength will also be cut.

Check if you can adjust the brush orientation on the motor.  In some old DC motors, I use to be able to slightly rotate the brushes giving a lower RPM, yet, a higher torque with the same voltage which may be what you are truly looking for.  However, you need to observe the motor's idle draw current when doing this.  Lowering the RPM too low like this will cause a gain in current draw eventually to a point where you will burn out the motor.  (This means rotating the brushes axially (not turning the brush in it's socket) or rotating the outer electromagnets  Mechanically, most motors don't allow this, for the old small circular DC motors used to almost allow it by rotating the rear cap of the motor which held the brush orientation, there was usually a peg or screws holding the motor's back plate in the optimum brush orientation.)

[BrianHG]

Example, scratching out the protective plastic registry, rotating the illustrated motor's back plate, which holds the brush orientation, in the red direction slowed down the motor, but, increased it's torque.  Rotating the brushes in the green direction increased the motor's RPM, but, lowered it's torque.

The same would happen if you instead rotated the position of the internal magnets.

[Zero999]

Example, scratching out the protective plastic registry, rotating the illustrated motor's back plate, which holds the brush orientation, in the red direction slowed down the motor, but, increased it's torque.  Rotating the brushes in the green direction increased the motor's RPM, but, lowered it's torque.

The same would happen if you instead rotated the position of the internal magnets.

I've never tried that. Does it work in applications where the motor needs to be reversed?

[BrianHG]

Yes, the motor slows down, and down, eventually drawing too much current at a stop (no back emf lowering the current here...), then accelerates in the opposite direction.  At low RPM, the motor can gain so much torque, it is equivilant to having the motor on a geared down box, but efficiency eventually drops too much and heat is generated as the circuit begins to look like a short.  You still get some amazing play in speed before the short circuit condition begins to take over.

Rotating that rear face-plate 180 degrees will spin the motor in reverse at the same speed.  Or, you can just swap the wire polarity to do the exact same thing.

Careful on reversing large motors, the brushes may have an angle making them preferred to rotate in one direction.  In my treadmill motor, when reversing direction, to prevent the brushes from scratching the commutator, I had to turn them around (They were rectangle in shape, the motor had removable/serviceable brushes)

[BrianHG]

Remember what's happening here, rotating the brushes brings the charges magnetic poles on the core closer to the edge of the external magnet's north or south pole.  And what happens if you have 2 magnets closer together, the attraction is that much stronger.  The motor spins slower because the inductance of the change charge in the coils takes time to build current and a magnetic field instead of having the switch in magnetic field being around the optimum 45 degrees out of phase (electrically, this figure changes depending on the number of poles in the motor's design).

Careful, only around 10 degrees in rotation can double or half the RPM of the motor.