# Powering a DC motor from AC?

#### moose842

Joined Jan 13, 2022
2
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.

Joined Jul 18, 2013
24,975
You do not need to create 'pure' DC to run a DC motor from AC.
e.g TM's, they have used a SCR bridge in the past and operate fine, also if you look up KB DC motor controllers, they also work in the same way.
IOW, directly from a controlled bridge rectifier output.
It is not a Universal motor is it? Most of these use this style, which are AC/DC operation.

#### moose842

Joined Jan 13, 2022
2
Ahh, good catch! It was indeed a universal motor. I went back to my setup and and found that the plug I wired to it had a 2 amp fuse in it (didn't catch that), which explains why it didn't power up and thus why I thought it was a straight DC motor. Swapped the cords and it spun right up straight from the wall.

#### LowQCab

Joined Nov 6, 2012
1,913
Running a "Universal", Series, Brushed-Motor will work fine on DC,
but it will shorten the life of the Brushes substantially to run it on DC.

A "real" DC-Motor will have much heavier Brushes and Commutator for this reason.
.
.
.

Joined Jul 18, 2013
24,975
Ahh, good catch! It was indeed a universal motor.
You have to take care to always run it with a load, a series field motor runs at very high rpm, uncontrollably with no load.

#### Carl_MIT

Joined Jan 26, 2022
5
Originally I was thinking how to solve this problem, but I laughed out loud when I saw that you found out that it is a general purpose motor

Joined Jul 18, 2013
24,975
Originally I was thinking how to solve this problem, but I laughed out loud when I saw that you found out that it is a general purpose motor
?? What is a 'General Purpose' motor?

#### LowQCab

Joined Nov 6, 2012
1,913
Usually they're called a "Universal" or "Universal-Series-Wound" or "Universal-AC-DC-Motor".
.
.
.