I'm building a supply for a 90 VDC 1/2hp motor.

So far I am using a garage sale variac (until I can find a regular 120/90VAC step-down transformer) and a bridge rectifier.

It runs the motor just fine, but I'd rather be kind to the motor and take out as much ripple as reasonable. The motor may pull as much as 5 amps (max) but the load probably just won't be too much. I was hoping someone could just say "get this and it will work fine." The motor will just be turning an observatory dome, so there might be a bit of a pull to start it but then probably not a lot of load.

Mostly it's going to be a full on and off situation. A control board and relays will turn on the power to the motor. Then if it needs to go the other way the other relay will turn on the reverse power. Mostly it's job (after homing) will be to move the dome incrementally to keep the opening in front of the telescope. So on/off 90vdc.


You don't need to do anything.

Normally 90VDC motors are run by unfiltered power from a phase control type speed controller, operated from 120VAC (or 180VDC from 240VDC). A properly designed driver will also give you significantly better torque at low speeds via IR compensation.

Here is a typical speed controller used for this purpose.

If you do choose to add capacitance you'll increase the voltage by approaching 40% as the ripple is reduced, and add a surge to the variac wiper which it may not like (or the fast blow fuse may blow to protect it). Peak to peak ripple is:

VRp-p = \$\frac{Tc I}{C}\$ where Tc is 1/120 or 1/100 for 60Hz or 50Hz repectively, I is the motor current and C is the capacitance in Farads.

For example, if I = 1A and C = 330uF then at 60Hz the ripple would be 25Vp-p. You also need to ensure the ripple current rating of the capacitor is sufficient. But I don't suggest doing this.

  • \$\begingroup\$ Capacitors (ESPECIALLY for DC motors) reduce the power factor of the motor, and will increase efficiency of the driver, no? \$\endgroup\$ – tuskiomi Oct 28 '19 at 19:29
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    \$\begingroup\$ @tuskiomi Should increase line harmonics and harm efficiency by increasing peak current. \$\endgroup\$ – Spehro Pefhany Oct 28 '19 at 19:32
  • \$\begingroup\$ Ah so caps should be before the rectifier, then? \$\endgroup\$ – tuskiomi Oct 28 '19 at 19:33
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    \$\begingroup\$ @tuskiomi There's a case for AC-rated capacitor banks connected to large AC motors to improve power factor but usually we don't get charged for power factor in residential situations and it otherwise mostly benefits the power company (negligibly for such a small motor). \$\endgroup\$ – Spehro Pefhany Oct 28 '19 at 19:35
  • \$\begingroup\$ That's a really good point. \$\endgroup\$ – tuskiomi Oct 28 '19 at 19:36

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