powering a dc motor from ac?

[moose842]

I need help powering a DC motor from AC. Trying to power an old vacuum motor running 120V DC at 8A. I know I need a bridge rectifier and then some sort of smoothing capacitor, HOWEVER doing the calculations I found, I'd apparently need somewhere in the neighborhood of 66,000 microfarad capacitance to smooth it out. Formula: C=I/(2*f*vpp), where I=load current, f=AC frequency, and vpp=desired voltage ripple. I used 1V for voltage ripple, but honestly i have no idea how much ripple is acceptable to still run a DC motor like this without too much issue or overheating.

Unfortunately, I did not keep or even look at the circuit powering said motor, so I can't reverse engineer this thing, but I SERIOUSLY doubt a simple household vacuum had 66,000 uF worth of capacitors in it... (I'm no engineer, but it just don't seem right..)

Can anyone answer 1: how to smooth out my rectified DC for my needs and/or 2: how much voltage ripple is acceptable for running my motor.

 

[Martaine2005]

Typical back of the envelope calculation is 1000μF for each Amp.
I have lots of blender motors and Hoover motors, none have smoothing caps just X2 caps.


martin

 

[Bluejets]

Can be done but not a good approach however you look at it.
Get a motor designed for the supply and loading on ac.

 

[Harald Kapp]

1: how to smooth out my rectified DC for my needs

You don't have to.
On the contrary, this would be detrimental. Rectifying 120 V AC gives you ~ 170 V peak. Using a large smoothing capacitor means you'd operate the motor from 170 V (DC +- ripple) which is way to high for the motor. Without capacitor the effective voltage (RMS) after rectifying will still be 120 V. Perfect for the motor.

2: how much voltage ripple is acceptable for running my motor.

100 %. DC motors are controlled by pwm signals in many applications.

 

[Ylli]

As others have said, no smoothing capacitor is necessary. In fact, if this is a "universal" motor (brushes, wound rotor and stator), then it most likely run on the raw AC just as well as DC.

 

[Minder]

Plus a typical DC motor only needs a rectified supply, no cap necessary.
From the other site, it IS a Universal motor.