How can a high-power AC-DC Power Supply maintain a fairly constant voltage (for a DC motor)?

Thread Starter

Electric-Gecko

Joined Dec 10, 2016
26
I am making a power supply and control circuit for this high-power brushless brushed DC blower fan. It is said to run at 9.4A at 12V DC, so it presumably has a resistance of 1.276Ω, and consumes 113 watts of power at full load.

I am using a transformer to step down the 60hz, 120V AC mains power to an appropriate voltage. But the problem I see is that single-phase AC does not supply continuous power; it goes past 0v every time it changes direction (120 times per second). A bridge rectifier would turn this into a pulsating current/voltage. But how can I maintain a fairly constant voltage at such high power draw? A constant (rather than pulsating) voltage would probably result in less wear to the motor, less noise, and more predictable behaviour.

Here are the solutions I can think of to supply a large constant direct current from an AC source:
  • Polyphase (eg 3-phase) power source and rectifier.
    • (This option is not available to me.)
  • Single-phase transformer and rectifier with huge capacitors.
    • According to my simulation on LTSpice, it will take 14mF of capacitance to stop the voltage from going below 2/3 of the peak. The capacitors should have a high ripple current rating.
  • Rectified single-phase with huge inductor.
    • This inductor would be quite huge. Actually, this method does not work at-all according to my simulation.
  • A combination of huge inductor and capacitor.
    • I have heard that inductors combined with capacitors often have oscillation currents. Would this be a problem even if I had a diode between them?
  • Using a transformer with a higher output voltage and stepping down the rectified voltage using PWM (when the voltage is more than 12 volts). A more modest-sized capacitor can be used for when the voltage dips below 12V.
    • I would need a PWM control that works with a pulsating input voltage. The fan I'm using is not said to be rated for PWM, but I'm guessing it would be fine if I use some good capacitors to absorb the voltage ripple and an inductor.
I'm sure there are power supplies out there that somehow manage to turn a single-phase input into a rather stable high-power DC output. But how is this normally achieved? Which options should I consider?
 
Last edited:

Thread Starter

Electric-Gecko

Joined Dec 10, 2016
26
I already have the blower. I chose a DC blower because I read that DC brushless motors are quieter and have better efficiency than single-phase AC motors. Now I just want to make a good power supply for it. I can't possibly be the first person in the world to face this problem.
 

Thread Starter

Electric-Gecko

Joined Dec 10, 2016
26
With a switching power supply like this one.
Hmmm. That one isn't so expensive, so maybe I should just buy one of those.
But how does that power supply achieve a stable voltage with a high power draw? Does it have a large capacitor? Does it use PWM to drop a higher voltage down to 12V (like the 5th method I mentioned)?
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
19,163
You don't need a SMPS to run a DC motor, a simple transformer and bridge will do it.
If you already have the transformer then you are already there.
There are many DC motor controllers that are ran this way, if variable speed is needed, a Triac control on the AC side, this is how many TreadMill drives work in a similar fashion, only SCR's.
Alternatively there are many PWM controllers on Ebay for a few $$'s. if you need rpm control.
Max.
 

OBW0549

Joined Mar 2, 2015
3,013
How does that power supply achieve a stable voltage with a high power draw? Does it have a large capacitor?
It has a relatively small capacitor, on the mains side; since the current drawn from the mains is only a little more than a tenth of the output current, it doesn't have to be very big.

Does it use PWM to drop a higher voltage down to 12V (like the 5th method I mentioned)?
Yes, through an isolation transformer which, since it operates at a very high frequency, doesn't have to be very big.
 

shortbus

Joined Sep 30, 2009
7,078
You don't need a SMPS to run a DC motor, a simple transformer and bridge will do it.
If you already have the transformer then you are already there.
There are many DC motor controllers that are ran this way, if variable speed is needed, a Triac control on the AC side, this is how many TreadMill drives work in a similar fashion, only SCR's.
Alternatively there are many PWM controllers on Ebay for a few $$'s. if you need rpm control.
Max.
And many motors especially steppers are not supposed to be powered by switching supplies, since they don't have the reserve capacity needed, like a linear supply does.
 

Thread Starter

Electric-Gecko

Joined Dec 10, 2016
26
You don't need a SMPS to run a DC motor, a simple transformer and bridge will do it.
If you already have the transformer then you are already there.
My hackspace already has 2 free 125VA Hammond transformers. They have a 25V output with centre tap. I suppose I can use PWM to drop the 25V down to 12V.

The project is an air cleaner for my hackspace. The project was shelved many months ago as I temporarily moved to a different city for a few months for work. I actually have a pretty complete design from back then for a power supply & control circuit. But now that I can return to the project, I was thinking of scrapping that and going for a different design, as that design uses a 12-12.5V transformer for power and uses the "huge capacitor" method to absorb the voltage ripple.
I remember you [MaxHeadRoom] replying to a topic I posted back then for this same project.

if variable speed is needed, a Triac control on the AC side, this is how many TreadMill drives work in a similar fashion, only SCR's.
Max.
Yes; I do want variable speed. I didn't mention it before as I wanted this topic to focus on the power supply.

Are you suggesting PWM on the primary side of the transformer? Interesting. I never considered doing that. But I was going to use the same transformer for the control circuit. I would have added an additional secondary winding to the transformer for the control circuit, which should be achievable without dissasembling the transformer since I can use very thin enamel wire.
 

MaxHeadRoom

Joined Jul 18, 2013
19,163
No depending on the set up, I was recommending PWM on the secondary, you do not need exact motor plate voltage when controlling via PWM as you can control the maximum rpm.
Max.
 

Thread Starter

Electric-Gecko

Joined Dec 10, 2016
26
Now I am trying to figure out how I can use a buck regulator for motor voltage control. There are buck regulator IC's available on Digikey, but they don't have a current rating high enough. Fortunately, there are also buck regulator controllers which use an external transistor. This is good, but the problem I have is that I can't figure out how to adjust the output voltage on them. The "Typical Applications Circuit" on the datasheets show how to use them with their fixed integrated reference voltage. But I see some buck regulator modules such as this which use a potentiometer to adjust the output voltage. But I want to supply an external voltage for the buck converter to match.
 
Top