Supplying a three phase VFD via DC bus connection and external rectifier?

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
Just looking for a sanity check here. I'm evaluating the feasibility of picking up a nice 10 horse air compressor for my home shop and would like the ability to vary it's operating speed (within published design specs). It's already got a three phase motor, so I need only furnish a VFD.

In order to run a 10 horsepower VFD off of single phase, I'll either have to de-rate the whole installation or see about installing an external single-phase rectifier to feed the DC bus terminals. That's why I'm slowing down and asking a question here.

Are the DC bus terminals on drives generally intended for external rectification? Or are they there for e.g. external filter caps, etc? They are usually sized for 100% of the input and output terminals' wire size, which leads me to believe they are there for a power application of some sort. I know some drives utilize features like active rectification and single-phase lockout which might not play nice if there is no voltage on the AC line terminals.

Any thoughts?


Joined Nov 6, 2012
If You don't have the Spec-Sheet and a Schematic-Diagram,
you'll just be guessing with a very expensive piece of equipment.

Keep in mind that you will need a VERY substantial Single-Phase-Service to the VFD.

Being able to control the Motor-Speed on a Compressor is a huge bonus,
as this enables much smoother Pressure-Control, instead of the usual "On-Off" BS.

Also keep in mind that the Compressor will become more and more inefficient as
it's running rpm goes down, but much quieter.
The Volume of Air can be relatively linear with speed variations,
but the maximum pressure that it can generate will go down very quickly,
so don't go thinking that you're going to get
~175psi while turning the Compressor at ~100-rpm.

Also, the Compressor Internal Bearings and Piston-Rings rely on "Splash-Lubrication",
which will not work properly at very low RPMs.
The reputable Compressor Manufacturers will specify a minimum continuous RPM.

If you are looking for quieter operation, there are 2 approaches ........
Build a HUGE Intake-Air-Muffler,
95% of the noise from a Compressor is caused by the Air-Intake.
Most factory stock Car-Mufflers will work well. Bigger is Better.
Make sure there are NO LEAKS between the Air-Intake and the Muffler.
Put the Muffler outside if possible.
Use over-sized Pipe,
1.5" Schedule-40 PVC Pipe works fine,
but may melt if used within ~12" of the Cylinder-Head of the Compressor.
Use a 36" section of High-Temp, High-Pressure, Cloth-Reinforced-Rubber-Hose
between the Compressor-Head and the PVC Pipe.

Additional Sound deadening ........
Create a 1/4" Steel platform for mounting the Motor and Compressor(s),
then mount this Steel-Plate to the Pressure-Tank with large Rubber-Isolation-Mounts
located as far apart as possible to minimize the movement relative to the Storage-Tank.
After this is done, the ridged high-pressure-outlet-pipe must be replaced,
using a much-much longer, and preferably larger diameter, Copper-Pipe,
and giving special attention to allowing some degree of movement between
the Compressor(s) and the Storage-Tank to prevent stress-cracking of the Pipe.
This will help to reduce the noise of the pulsating
high-velocity Air flowing from the Compressor into the Storage-Tank.
Running this Copper-Pipe around the entire circumference of the Tank should
provide enough freedom of movement to prevent any problems with Stress-Cracks.
Provide a flexible support-clamp every ~2 to 3-feet of Pipe.
Caution, the High-Pressure-Discharge-Pipe will give You a severe burn.

An alternative is to add-on a Fan-Cooled-Dehumidifying-Radiator between the
Compressor and the Storage-Tank,
this is almost mandatory in the Southern-States due to heavy Humidity year-round.

In addition to the Car Muffler ..........
Get 2 Compressors that each have the Capacity You need ..........
Preferably get the type that are a "Twin-Inline-Cylinder" design, not a Single-Cylinder.
Then cobble-up a Mounting-Plate for both that will allow You to
"lock" the 2 Compressor's Flywheels together,
BUT 90-DEGREES APART in their rotational positions,
this will basically create a 4-Cylinder Compressor,
with a much smaller pressure-pulse every 90-degrees, (instead of a big one every 180-degrees),
this also cancels-out almost all the imbalances/vibrations in the Compressors and
produces a much smoother Torque-Load for the Motor and Drive-Belt(s).
Almost all Compressors can be run in either direction of rotation, with equal performance.

If You are wanting to retain the "Single-Compressor-Rated-Motor"
but drive 2-combined Compressors, You can,
simply try smaller and smaller Motor-Pulleys until the
Current required by the Motor remains in an acceptable range.

If You need proper "Shop-Air-Pressure", ( ~175-psi ),
you will probably be running a "2-Stage", "V-configuration" Compressor.
Doubling them up can also be done, as with the lower-pressure "In-Line" style above,
but they must be 180-degrees out of phase with each other,
instead of the 90-degrees which is required with the "In-Line" Twin-Cylinder-Style.

Bonus ...... You will also find that Doubling-Up on Compressors does not
require "twice" the Motor Horsepower,
because start-up-loads are much lower and smoother, as compared with a single Compressor.

( I would drill bolt-holes in both Flywheels and Bolt them together, (alignment is critical),
pressed-in "Roll-Pins" are recommended for accurate alignment.
This Drilling and Pinning should be done by a proper Machine-Shop for adequate accuracy.
Then, mount the individual Compressors on 8-Rubber-Isolation-Mounts to
prevent any binding caused by any possible mis-alignment between them,
this will further reduce the generated noise as well ).

After all this is accomplished,
the Compressors can run at ~half their normal speed, or even less,
and, if needed,
and if,
You have a Motor with adequate reserve Power-Capacity,
You can allow them to run at "normal-speed", or higher,
and get twice the CFMs for heavy Air-Consumers, with only a moderate increase in noise,
and only when it's actually required.


Joined Jul 29, 2016
The DC bus on a vfd is for chaining multiple vfds from one input power. The Allen Bradley servo drives have a similar setup with a dc link connection that can be used to run a string of drives from one ac input supply. Youll need the dc bus voltage to be at or near to the peak voltage of the designed input ac. So for 480V its up near 670ish, I forget. 170VDC for 120VAC. If you're getting a loss of input phase fault on the VFD the dc bus voltage is too low.
So to answer your original question, they are for external rectification from another vfd. I would DEFINATELY verify with the manufacturers manual, to be sure.