Powering a DC Motor using a Bridge Rectifier

Thread Starter

minoush82

Joined Aug 2, 2020
7
Hello,

My treadmill has been sitting around ever since the motor-controller board broke. Since I could not find a replacement for the board, I thought why not build a power supply myself. I do not need fancy speed control; one single speed value is enough for me, probably the value that makes the motor rotates at 75% of its maximum speed.

The label on the motor indicates that it needs a voltage of 180Vdc to operate and draws 4.6A of current. My idea is to have something that turns my house's 220Vac to 180Vdc; and research in the web yielded that a bridge rectifier can do the job. I still haven't dug too much into it because something caught my attention. Quite a few people indicated that a bridge rectifier is likely to blow up when the motor starts because of surge/starting current that far exceeds the normal operating current in intensity. I believe 4.6A is the value of the operating current that the motor draws when already started.

My motor has a label on it saying LifeGear together with values for voltage, current, horse power, and rounds per minutes; and that's it! Not a single word more. LifeGear is the manufacturer of the treadmill. Unfortunately, I could not find the datasheet of the motor anywhere to confirm anything about surge current values.

Finally, my question: Is it possible to have a bridge rectifier to operate my motor at a fixed speed without blowing up?

Cheers,
Amine.
 

LowQCab

Joined Nov 6, 2012
1,078
It's not that simple.
Rectifying, and Smoothing, 220-Volts will get You ~308-Volts.
This 308-Volts DC then has to be "chopped-up", at a very high frequency
by some fairly strong Semi-Conductors (Transistors or SCRs/Triacs),
to reduce/control the average amount of Power that
the chopped-up DC Voltage will deliver to the Motor.
The preferred form of this control is called "Pulse-Width-Modulation" or PWM.

There is no problem getting Bridge-Rectifiers/Transistors that will handle the Power You need.

Since you are asking if this is possible, You can't do it yourself.

You can buy an Industrial Motor Controller that will do the job just fine,
but it will cost more than a new Treadmill.

On the next Treadmill You buy, install a Cooling-Fan on the Control-Board,
and it will probably last forever.
Aluminum-Transistor-Heat-Sinks are expensive parts,
so they scrimp on them as much as possible.
Heat is the biggest enemy of Semi-Conductors.
.
.
.
 

MaxHeadRoom

Joined Jul 18, 2013
23,585
If you want a real simple one, buy one of the Triac controllers off ebay, intended for power tools etc, and place a bridge rectifier after it.
I believe there is a Youtube video out there for it.
 

Irving

Joined Jan 30, 2016
2,056
It will give you double that and run the risk of electrocuting anyone near or on the treadmill if it doesn't set fire to it first...

NEVER connect mains electricity to something not intended for that purpose. You need a lot more electronics between the mains and the motor...

Something like this

 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
23,585
Thank you everybody.

This is the rectifier I have: KBPC2506. If I connect it to my 220Vac, will it give me the voltage I need for my motor (180Vdc)?
Don't use it direct, at a minimum, use one of the Triac controllers I mentioned or put one together from parts, the simple ones on ebay etc are cheap enough.
Place the rectifier on the output. Do a youtube search for details.
 

Irving

Joined Jan 30, 2016
2,056
Advising someone with no knowledge to connect a 1kW motor that'll draw up to double that on start up to a likely 250 or 400w triac controller with no isolation is at best going to end in magic smoke and worse in injury..
 

MaxHeadRoom

Joined Jul 18, 2013
23,585
Have you thought of trying to fix the existing board? I don't have any experience with the Lifegear version, but it might be worth checking the power devices, also if the belt sensor is not registering for some reason the computer shut the motor down.
Is this the board?

1627506545050.png
 

Papabravo

Joined Feb 24, 2006
16,772
So you are not left hanging. AC voltage is measured with an RMS (Root Mean Square) value which is useful in making AC power calculations. When you use a bridge rectifier on an AC waveform you get an output with a PEAK voltage about 1.414 times the RMS value. In the case of 220 VAC, that would be:

\( 220\; \text{VAC}\;\times\;\sqrt{2}\;=\;311.127\;\text{Volts-Peak} \)

That is not going in the right direction for you. If you had a way to convert that AC waveform to 180V DC them you might have something useful. Problem is, DC-DC converters generally have extreme difficulty with the variable load demands of a motor. This is not the stuff for weekend warriors and self-styled DIY'ers.
 

MaxHeadRoom

Joined Jul 18, 2013
23,585
So you are not left hanging. AC voltage is measured with an RMS (Root Mean Square) value which is useful in making AC power calculations. When you use a bridge rectifier on an AC waveform you get an output with a PEAK voltage about 1.414 times the RMS value. In the case of 220 VAC, that would be:
Keep in mind that most T.M. power supplies are rectified and smoothed direct off the mains grid.
They are controlled within certain limits by either SCR bridge supplies or PWM
From the OP, I gather the poster wants to re-institute simple manual control of the belt?
 

Papabravo

Joined Feb 24, 2006
16,772
Keep in mind that most T.M. power supplies are rectified and smoothed direct off the mains grid.
They are controlled within certain limits by either SCR bridge supplies or PWM
From the OP, I gather the poster wants to re-institute simple manual control of the belt?
I wasn't drawing any conclusions about how to get to the final goal, I was just trying to show the function of the bridge rectifier, and to offer the suggestion that DC-DC converters I am familiar with leave a great deal to be desired when it comes to driving motors, because of startup current demands and frequent load transients. I wasn't saying you couldn't make one, but I'm suggesting that it takes a fair level of engineering skill to do it.

The TS is, of course, always free to do anything he sets his mind to.
 

LowQCab

Joined Nov 6, 2012
1,078
Any Controller for a Treadmill MUST have RPM-Feed-Back, and
preferably a Switch-Mode Current-Regulator.
This is way over the capabilities of the Thread-Starter.
.
.
.
 

Thread Starter

minoush82

Joined Aug 2, 2020
7
@MaxHeadRoom: My power supply is different than the one you show. I have tried to test components as much as possible, but I gave up because it seems that some components must be desoldered to measure them correctly. This was a lot of work for me to do, so I gave up the idea of fixing the power supply.

The solution you suggested of using a Triac controller + bridge rectifier seems adequate. Would you mind letting me know if my understanding of the solution is correct?

1. The controller's purpose is to transform the main's AC voltage to a new AC voltage value.
2. The rectifier's purpose is get me the DC voltage value by multiplying square root of 2 by the AC voltage coming out of the Triac controller.
3. Some suggest connecting a large capacitor (possibly one from the board) across the output of the rectifier for better results.

So if I change my 220v AC to something like 127v AC (using the controller), I can get 127*1.41=180v DC using the rectifier? Is this how the components are meant to work?

@Everyone Else: I appreciate the caring because high voltages are lethal/destructive, but I am not going to try plug anything without fully understanding things. Having said that, I do not appreciate judging people as not having knowledge and withdrawing information because they might hurt themselves.
 

Irving

Joined Jan 30, 2016
2,056
A standard 'mains dimmer' is 250 or 400W and will still output too much voltage at too little current for the top 50% of its span - it doesn't reduce the voltage, just chops off part of the cycle. Your motor is rated at 1000W input and could need twice that to start - a standard dimmer will overheat and fail, the output voltage won't be clean DC, the motor won't run smoothly and you run the risk of burning it out - its not just about voltage, its about power transfer and torque generation. In a nutshell, this 'simple' solution isn't going to be reliable or safe for you or your motor. I'm not withholding information, just explaining why I wouldn't recommend you do it. I wouldn't do it myself that way, IMHO it's a wrong and unsafe solution.
 
Last edited:

Irving

Joined Jan 30, 2016
2,056
My power supply is different than the one you show. I have tried to test components as much as possible, but I gave up because it seems that some components must be desoldered to measure them correctly. This was a lot of work for me to do, so I gave up the idea of fixing the power supply.
Fault finding on a power supply of this nature is rarely about testing components. First step is a close inspection of the board preferably under a microscope or high-powered magnifier to look for burnt, damaged or discoloured components. Step 2 is, with an understanding of how such supplies work, and a healthy respect for the 300 - 400v DC generated internally, is to work methodically through the common stages of the supply; input filter, input rectifier, switching controller, switching element, output rectifier, feedback/output control - using datasheets for the controller chip and reference designs to give clues about the layout as most SMPS/motor controllers follow a common theme - measuring voltages and observing waveforms (with appropriate hi-voltage probes) to determine what has failed.
 

MaxHeadRoom

Joined Jul 18, 2013
23,585
The solution you suggested of using a Triac controller + bridge rectifier seems adequate. Would you mind letting me know if my understanding of the solution is correct?

1. The controller's purpose is to transform the main's AC voltage to a new AC voltage value.
The Triac controller as shown in quite a few Youtube videos (via Google) is used mainly for other off-T.M. purposes, although you can control it this way with a bridge on the output, it will be rough to control for exact speed in T.M. usage.
Motors will tolerate quite a bit higher voltage than their rated value as long as the rpm is not exceeded, Most N.A. T.M. have 90vdc motors but the power supply is 120vac rectified for the SCR bridge versions, and in the PWM version, capacitively smoothed to 160VDC!!
What is the Manuf. part number on the board?
 
Top