Hi everyone,
I'm currently working on a project where I need to power two three-phase motors using a single-phase 230V supply. Here’s the setup:

Motor Details:

1. Sanding Machine Motor:
   • Star Connection: 1.7 kW, 4 A, 0.89 cosφ, 2840 RPM
   • Delta Connection: 1.27 kW, 3.2 A, 0.83 cosφ, 1420 RPM
2. Dust Extractor Motor:
   • Power: 0.75 kW

Current Setup and Issues:
I've been experimenting with a single 80 µF capacitor to create the missing phase, but the power output seems quite low. Here are the specific issues I'm facing:

1. Low Power: The motor feels underpowered, and I suspect that the 80 µF capacitor might not be sufficient for both motors.
2. Switching Between Speeds:
   • If I start the sander on the lower speed and then switch to the higher speed, the motor runs but eventually trips the breaker (set at 6.3A Schneider GZ1M10).
   • If I try to start directly at the higher speed, the motor fails to start at all.

Questions:

1. How should I correctly size the running and starting capacitors for both motors? Are there specific formulas or guidelines I should follow for this application?
2. Would it be advisable to use a relay to control the starting capacitor? If so, what type of relay would you recommend?
3. What is the ideal value for a discharge resistor to safely discharge the capacitor after shutdown? How should it be connected in the circuit?
4. Are there other considerations I should keep in mind to prevent the motor from changing rotation direction when switching between star and delta configurations?

Any advice or insights you can provide would be greatly appreciated. Thanks in advance for your help!

 

How about a schematic of the setup?

 

Below is a formula for the capacitor size from this YouTube Video.
But the motor will not have it's full power and may run hot.

You don't need to discharge the capacitor as it will discharge through the motor winding it is connected to.

If you want full power you could use a VFD converter to generate the 3-phase power from the single-phase input.

 

https://en.wikipedia.org/wiki/Rotary_phase_converter

Since induction motors are sensitive to voltage imbalance, this is another factor in de-rating of motors driven by this type of phase converter. For example, a small 5% imbalance in phase voltage requires a much larger 24% reduction of motor rated power.

 

I built an RPC some years ago when they were popular before cheaper VFD's arrived.
Worked great, there were a few good design notes published at the time.

 

Like "C" stated: "If you want full power you could use a VFD converter to generate the 3-phase power from the single-phase input."
Running a 3 phase motor as a 2 phase-sort of motor is fine for an emergency, and the warning from "N" about reduced efficiency is valid. Check the "Automation Direct" website for deals with product support included.

 

I ran my RPC off of single phase, to produce 3ph for my mill and lathe in my shop for some years before converting to VFD !
There were some excellent build instruction's out there at the time.