Before proceeding I should state that this is potentially a very dangerous activity.
High voltage DC is capable of more harm than AC although both are quite capable of causing death and severe unjury.
Unless you really know what you are doing with mains power I strongly recommend not doing this.



I have viewed several vids on YouTube that show running a DC motor at various speed from mains power via what is effectively a large dimmer switch and a bridge rectifier.Because I have a horsepower meter I though it would be interesting to measure the actual motor horsepower from this setup.


DC motors are readily available from old running/walking machines and while they can be run using the walkers power supply and controller these are not always functioning and they have the limitation that the speed is reset to zero every time they are switched off.
The motor I used came from an old running machine I found by the side of the road and is rated at 1.3HP @180V DC pulling 6A to generate 4500rpm.
Setup2.jpg

MotorSpecs.jpg


The dimmer switch controller (VarAC in Photos) I am using is a bit beefier than the usual switch used to control lights which are limited to a couple of hundred watts.
The one I am using was purchased from ebay for $12 (https://www.ebay.com.au/itm/AC-220V-4000W-SCR-Controller-Electric-Voltage-Regulator-Speed-With-Fan-SR/282658437804?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649) . It's quoted power rating is 4000W, although the fine print says that the 4000W is for intermittent/short bursts and for continuous use the max recommended power is 2000W. The unit has a small 12V cooling fan but no earth connections which means mounting it inside an insulated box.
The Bridge rectifier (BR) was a KBPC3504 400V 35A Metal Bridge Rectifier purchased from Altronics for ~$5.
Esetup.jpg


Here is the setup showing the connections.

The dimmer switch has the usual two sets of input and output leads and varies the output from zero to mains input levels.
The bridge rectifier takes the variable output from the dimmer and converts it into a crude form of DC. A meter on the DC output shows a DC that varies significantly so it is a sort of variable DC so unlike AC it is always going in one direction.
A higher quality, high current DC power supply will use devices like capacitors to smooth out the variable DC and make it steadier.
Despite the fact that it’s variable DC the motor seems to run fine with this form of DC.

I've shown my HP measuring rig (It's called a De Prony Brake) before but I include it here for completeness.
1: is the motor with a 77 mm radius flywheel.
On the LHS of the flywheel where there is a red spot this is the laser Tacho (4) reflection - the tacho is used to measure the flywheel RPM
2 and 3 are luggage scales
Setup.jpg
5 is a multimeter set on V that measures the output V of the dimmer switch (7)
6 shows the input voltage and current being drawn.

My hand on the RHS of the picture on a bike brake pulls luggage scale 3 and a steel band that wraps around the flywheel upwards which applies a load onto the motor.
The difference between the force measured by 2 and 3 , times the radius of the flywheel is a measure of the torque.
When the RPMs are factored in this enables the motor output HP to be calculated without using currents and voltages and then subtracting efficiencies.

Measuring sequence
1) adjust the V of the dimmer to output say 60V and then apply some force on the brake until I see a rise in the current being drawn.
2) Take a photo using a camera remote connected to the camera which thus simultaneously records all 6 digital readings.
3) increase the load with the brake until the current being drawn increased by about 1A and hit the camera remote again.
4) The previous step was repeated until the current reached about 6A or the motor started to stall or shudder.
5) Increase to the next Voltage eg 80V and repeated the above 1) to 4)

Then I downloaded all the photos and while viewing each photo I transferred the data to a spread sheet and plotted up several graphs.

Here is the main one that shows the Motor Output HP versus RPM, and the Motor Output HP versus Voltage drop.

The Solid line curves refer to the motor output HP curves for different starting voltages (60 ... 200V) as the load is applied and refers to the left hand side axis.
The dotted lines are the actual voltages measure coming out of the dimmer switch as the load is applied - refer to the right hand axis of the graph.
As the load is applied for each nominal voltage the RPMs and dimmer switch output voltages drop.

The large red dot is the manufacturers spec for the motor - which is easily met at the 180V dimmer switch output.
A major disadvantage of this type of simple power supply is that the RPMs drop quite dramatically (by as much as 50%) when a medium heavy load is applied so hoping to get reasonably constant RPM using this type of power supply on a DC motor is not going to happen.

HPVRPM.jpg

The next thing to note is that the horsepower output using this cobbled together type of power supply is only able to meet manufacturers spec for a relatively small Range of V's and speeds. In practice there is only 1HP available between about 140 and 180V which is between about 2000 and 5000 RPM which is about all one can get from a V/F VFD although the RPM will be much more stable using a VFD. A proper well regulated DC power supply would be able to drive a DC motor much better than a dimmer and Bridge type power supply.

The 200V curve shows what can happen if these motors are pushed too hard - what happens is the RPMs and Voltages don't drop as the load is increased instead the motor goes into a runaway
effect which can be disastrous. You can see the beginning of this effect on the 200V black curve with both the V and HP increasing - the amount of current drawn also rises exponentially and can damage motors dimmers and blow breakers. During a preliminary test the current drawn reached 21A in less than a second (I only know this because the camera caught the current value and it showed up on a photo) and then the breaker tripped on the 10A expander board the dimmer was connected to. If this had not happened the high current would most likely have damaged the dimmer and motor.

Most proper DC motor setups (such as on a walking or running machine) will have built in protection mechanisms that prevent this happening but the YouTube dudes don't mention this.
If you are going to try this with a dimmer and a rectifier I recommend adding a fast acting fuse to prevent this runaway effect.

Summary.
Assuming you have taken the necessary precautions of adding a fuse, the dimmer and bridge method is super cheap way to get a basic DC motor up and running as long as you are not concerned about being that specific with RPMs.