# Capacitor Size for a DC motor. Do I even need one?

A couple months ago the AC motor on my wood lathe went out so, following a number of YouTube tutorials, I swapped it with a DC treadmill motor. On the surface everything seems fine and it works. The project got me interested in electronics again, so I've started taking online classes and trying to learn more. I've recently started learning about capacitors and inductors and it got me thinking about my lathe circuit. The circuit is very simple, it's basically a cheap SCR voltage controller ran into a bridge rectifier. The more I learn though, I'm thinking I might need to add a capacitor after the bridge rectifier to smooth the voltage to the motor. So my question is 2-fold. Do I need a capacitor and how would I calculate what size I need? The specs I have available for the motor are 2.25hp, 130V, 12.9A. The max I ever need to use it is maybe 80%, after that it's almost too fast to be useable. I've attached a rudimentary schematic of what I did below.

• As long as your AC/DC supply (it most certainly should have it already) and your voltage controller has enough capacitance for hold-up as is, your reasoning is sound. What’s the rated voltage on the motor and what’s your peak voltage after rectification? Commented May 1, 2021 at 17:18
• The motor is rated at 130VDC with zero load, I'll measure the rectified voltage when I get home.
– Dave
Commented May 1, 2021 at 17:28
• The motor itself should provide enough smoothing. Commented May 1, 2021 at 17:28

If it's running OK then I wouldn't mess with it.

Figure 1. Phase-angle AC control. Image source: LEDnique.com.

Your SCR control is most likely controlling voltage by changing the phase angle as shown in Figure 1. Adding any capacitance will tend to raise the DC after the bridge rectifier on longer than intended and would actually mess up the control depending on the amount of capacitance you add. In addition the SCR on turn-on would have to charge the capacitor on each half-cycle so it and the diodes would be stressed more.

Remember that the inertia of the motor is high enough that it doesn't respond in the < 10 ms half-wave (50 Hz) and it effectively smooths out the pulses of energy it receives giving a smooth rotation of the shaft.

The capacitor has nothing to do here, rather a large choke (inductor).if you want to smooth run. If you use a SCR, then also an EMI filter on the input side is needed.

Beside this, I would warn you about using cheap SCR rectifier, since it won't work for the DC motor. You need a controlled SCR rectifier made for controlling the DC motor and not a cheap lamp dimmer. Further the 4Q SCR control has everything, so you don't need an extra diode bridge and turnover switch as depicted.

EDIT:

A SCR rectifier has to be combined with a inductance, which you already have as motor inductance. The voltage can be smoothed with a capacitor only after the inductance - in your case impossible. The motor can exhibit torque ripple due to the current ripple and not due voltage ripple, so the capacitor is never needed. Right the opposite, if you want a very smooth current then you need to add yet another big choke.

• Thank you, yes I should have specified it further. The SCR controller is a motor speed controller rated up to 220V and 10k Watts. I mostly meant inexpensive (\$20) compared to a PWM controller rated for the same power requirements.
– Dave
Commented May 1, 2021 at 17:55
• @Dave Then you don't need nothing else (EMI filter if you want to be legal), specially not the capacitor, that would be a cardinal error. Commented May 1, 2021 at 18:20
• Thank you for the information and clarrification. The system works, I just wanted to be sure I wasn't shortening the lifespan of the motor by not using a pure DC signal. So much to learn.
– Dave
Commented May 1, 2021 at 19:08