What Are The Downsides Of Controlling A Brushed AC Motor Speed With A Dimmer Switch?

idbruce

Hello Everyone

I have several brushed AC power tools and I am wondering what kind of wear and tear I can expect by controlling the motor speed of these power tools with a dimmer switch.

Bruce

Duane C. Johnson

Hi Bruce;

idbruce wrote: »

Hello Everyone

I have several brushed AC power tools and I am wondering what kind of wear and tear I can expect by controlling the motor speed of these power tools with a dimmer switch.

Bruce

Generally no problems at all. That is how most "Trigger" type speed controllers work in these tools already.

The technical name for these motors is "Universal Motor".
These are very similar to "Permanent Magnet DC" motors except the field is an AC electromagnet instead of a permanent magnet.

These motors are reversible by swapping the leads on the field coils.

Duane J

idbruce

Thanks Duane

I particularly want to control the speed of a jig saw and a router. Considering that both of these tools are Milwaukee brand, the last thing I want to do is destroy them quickly, especially the router. I have found that cutting aluminum with this jig saw often ends up with the blade teeth getted clogged quickly. Both the jig saw and router have motors with uncontrolled high RPMs, so I am hoping that a reduction in speed and some cutting lubricant will help me cut more effectively and provide better results.

pjv

idbruce wrote: »

and some cutting lubricant will help me cut more effectively and provide better results.

Have you tried some methanol ?

You said you were stubborn...... but the same answers will come from the same questions !

Cheers,

Peter (pjv)

Duane C. Johnson

Hi Bruce;

idbruce wrote: »

Thanks Duane

I particularly want to control the speed of a jig saw and a router. Considering that both of these tools are Milwaukee brand, the last thing I want to do is destroy them quickly, especially the router. I have found that cutting aluminum with this jig saw often ends up with the blade teeth getted clogged quickly. Both the jig saw and router have motors with uncontrolled high RPMs, so I am hoping that a reduction in speed and some cutting lubricant will help me cut more effectively and provide better results.

Here is a list I compiled on "Back EMF" universal motor speed controls.

Thyristor Theory and Design Considerations
CONTROL OF UNIVERSAL MOTORS; Page 95, 96
http://www.onsemi.com/pub_link/Collateral/HBD855-D.PDF
See Figure 6.9

Mixer or Sewing Machine Motor Speed Control
http://circuit-diagram.hqew.net/Mixer-or-Sewing-Machine-Motor-Speed-Control_4884.html

Universal motor speed control
http://www.next.gr/automations/motor-control-circuits/universal-motor-speed-control-l12865.html

There is one in an antique GE SCR manual. mid '60s, but I haven't seen my copy for years. As I recall it was similar to the ones above.

These circuits work very nicely. I use one on a hand drill. Works even better than the internal trigger control. All the way from barely turning to 1/2 speed. (ya half speed as these are half wave circuits). When at no load the circuit "ticks" the motor every few seconds. Gripping the chuck causes more ticks. But the speed is quite constant.

I suppose some dimmers may work this way but the one I tested didn't really give the back EMF feedback like the above circuits.
Do they specifically make dimmers for universal motors, I suppose so, but I've not seen them.

Well worth making your own, you will like the action. Especially for the scroll saw which can be a bit sticky in aluminum and copper.

Duane J

idbruce

Thanks again Duane

That is a nice doc from ON-SEMI, saved that to my PDFs folder

One of those circuits is only rated fro 1.5A, and the other circuit does not mention a rating.

The router is 12.0A and the jig saw is 3.8A.

I believe the dimmer that I have available is rated at 1500W. I have tried it on the jig saw and it appears to work well. I just don't want to burn up the motor windings or melt the brush holders. I am not worried about the dimmer.

LoopyByteloose

Higher amps draw might indeed be a problem. The dimmer switches for lighting seem to provide lesser max watts for retail versions.

There is always a hazard that the startup of the motor has a peak current draw higher than the running power draw. So a lot of these switches start at full on and then you adjust downward.. just to get the motor to turn.

A Variac might work well and certainly are rated for more power. But motors may get very hot.

http://www.control.com/thread/1026237567

http://www.universal-lighting.co.uk/buying-guides/choosing_the_correct_dimmer_switch

LoopyByteloose

Grainger may have something more appropriate.. but not cheaper.

http://www.grainger.com/category/ac-speed-controls/motor-supplies/motors/ecatalog/N-9yj

Duane C. Johnson

Hi Bruce;

idbruce wrote: »

Thanks again Duane

That is a nice doc from ON-SEMI, saved that to my PDFs folder

One of those circuits is only rated fro 1.5A, and the other circuit does not mention a rating.

The router is 12.0A and the jig saw is 3.8A.

I believe the dimmer that I have available is rated at 1500W. I have tried it on the jig saw and it appears to work well. I just don't want to burn up the motor windings or melt the brush holders. I am not worried about the dimmer.

I believe the current rating of the back emf circuits is primarily defined by the ratings of the SCRs used.

Variacs and resistive lighting dimmer controls may work but they can't provide the feedback needed to regulate the motor speed in applications where the load is highly variable.

Duane J

LoopyByteloose

Well, thinking back on the cost of a 1 hp speed controller (about $250USD), it seems like that saner solution is to get a pair of belt drive pulleys that can shift the RPM down to what you desire. Some have multiple diameters and are used in pairs.

I guess these are either 4 step V belt pulleys or variable pitch V belt pulleys.

http://www.grainger.com/category/v-belt-pulleys/sheaves-and-pulleys/power-transmission/ecatalog/N-hxd#nav=%2Fcategory%2Fv-belt-pulleys%2Fsheaves-and-pulleys%2Fpower-transmission%2Fecatalog%2FN-hxdZ1yzy3olZ1yzy3ok%3F_%3D1398237175958

$WMc%

Remember that a AC brush motor is just a DC motor with a Rectifier.
'
There is a lot of code for PWM.
'
Also 120volts is the RMS value...So at least double that for your switching semiconductor ratings.
'

$WMc%

There is this http://www.harborfreight.com/catalogsearch/result?q=router+speed+controller
'
This is a router speed controller list $19.95

Duane C. Johnson

Hi Walt;

$WMc% wrote: »

Remember that a AC brush motor is just a DC motor with a Rectifier.

That is clearly not true. See:
Universal Motor

Sure, rectified AC Permanent Magnet DC motors do exist but are not what is used in these applications.

Duane J

$WMc%

Clear as MUD
'
1st Most AC/DC motors don't have permanent Magnets....(Too expensive)...(The larger ones 1hp and up)
'
2nd It's way beyond this post to explain the phase angle method used to reduce or eliminate the need for diodes.
'
3rd Just pulse some DC of the rite voltage and very the motor speed for the old sander motor.
'
Is it a full moon tonight?

Alex.Stanfield

Duane C. Johnson wrote: »

Hi Bruce;
Here is a list I compiled on "Back EMF" universal motor speed controls.

There is one in an antique GE SCR manual. mid '60s, but I haven't seen my copy for years. As I recall it was similar to the ones above.

These circuits work very nicely. I use one on a hand drill. Works even better than the internal trigger control. All the way from barely turning to 1/2 speed. (ya half speed as these are half wave circuits). When at no load the circuit "ticks" the motor every few seconds. Gripping the chuck causes more ticks. But the speed is quite constant.

I suppose some dimmers may work this way but the one I tested didn't really give the back EMF feedback like the above circuits.
Do they specifically make dimmers for universal motors, I suppose so, but I've not seen them.

Well worth making your own, you will like the action. Especially for the scroll saw which can be a bit sticky in aluminum and copper.

Duane J

Thanks Duane, you saved me a lot of work since I was planning to build an intelligent dimmer for speed control of cutting machines (drills and chopsaws). Now that I found the TDA1185 everything looks simpler. Have you used any other chip with the same function? I found the tda chip on e-bay and aliexpress but not on traditional ic supplies. Does somebody know who carries them?

Thanks Alex

Duane C. Johnson

Hi Alex;

Alex.Stanfield wrote: »

Thanks Duane, you saved me a lot of work since I was planning to build an intelligent dimmer for speed control of cutting machines (drills and chopsaws). Now that I found the TDA1185 everything looks simpler. Have you used any other chip with the same function? I found the tda chip on e-bay and aliexpress but not on traditional ic supplies. Does somebody know who carries them?

Thanks Alex

No, I haven't used a chip to do this.
I have used the SCR circuits that use the back EMF from the universal motors as feed back.
The TDA1185, apparently, uses the motor current to increase the TRIAC conduction angle.
Similar chips are in the trigger control on AC drills.
Should work fine.
BTW, the TDA1185 and TDA1185A are now obsolete, but some are still available.
There must be replacements for it.

Duane J

Alex.Stanfield

Duane C. Johnson wrote: »

Hi Alex;
No, I haven't used a chip to do this.
I have used the SCR circuits that use the back EMF from the universal motors as feed back.
The TDA1185, apparently, uses the motor current to increase the TRIAC conduction angle.
Similar chips are in the trigger control on AC drills.
Should work fine.
BTW, the TDA1185 and TDA1185A are now obsolete, but some are still available.
There must be replacements for it.

Duane J

Thanks again Duane, I found the U2010B from Temic which seems to do the job and it's currently available. I'll have to study the specs in more detail.

Edit: one datasheet says Temic but apparently it's from Atmel

Alex

ErNa

$WMc% wrote: »

Clear as MUD
'
1st Most AC/DC motors don't have permanent Magnets....(Too expensive)...(The larger ones 1hp and up)
'
2nd It's way beyond this post to explain the phase angle method used to reduce or eliminate the need for diodes.
'
3rd Just pulse some DC of the rite voltage and very the motor speed for the old sander motor.
'
Is it a full moon tonight?

It's not the case for an UNIVERSAL Motor. This motor normally has connected rotor winding (via the brushes) and stator winding in series. So, whenever the current changes direction in the stator (and so polarity) also the direction in the rotor is changes (and so polarity). Therefor the magnetic fields are always equally oriented and create torque to a direction independent of current direction. The only difference to driving such an motor with DC is, that torque is pulsating with 120Hz and the motors are noisy. But there is no secret sauce, no need to explain something related to diodes. So it might be a good idea to drive the machines from a adjustable dc-source

Mark_T

$WMc% wrote: »

Remember that a AC brush motor is just a DC motor with a Rectifier.
'

NO! Read above, universal motors do not have rectifiers!!!

The field windings mean the magnetic field changes with the armature current sense so the
motor direction is constant. The field current rising with the armature current means that
the speed isn't strongly affected across the AC cycle, only the torque has apprecable ripple
at mains frequency x 2.
.
They behave fairly resistively under load so aren't too bad a load for a triac to control.