Static phase converter too big?

Your question sort of opens a can of worms. The answer is “well it depends on . . . .”. Plenty of views and opinions. All the views presented above are good. Some considerations to include are how comfortable you are with electrical systems, or what resources can you access to assist you? All the options require that something be wired up. How many 3 phase motors are you running? If the mill only has the one main motor (i.e. does not have a 3 phase feed motor), then the VFD becomes simpler than if you needed to power it as well (the VFD is not a whole machine power source, it is only for one motor). Do you have, are you planning to have other 3 phase equipment? Certainly you can put a VFD onto each 3 phase motor - but if you have (intend to have) a bunch of 3 phase machines, then converting them to VFDs, or to single phase motors is a chore - that with one phase converter is “one and done”, just keep plugging more machines in. I probably have 20 3 phase motors across the fleet, a bunch of VFDs makes no sense. I have converted two machines to VFDs for shop wiring and layout convenience - and I got the VFDs for free (and I am comfortable with doing the wiring on about any machine power options).

The right answer depends on you. Let us know how you make out.
 
Last edited:
Ulma Doctor said:
the major problem can be the starting capacitance.
generally speaking it only requires between 50 and 150Uf of capacitance per HP to start a 3 phase motor on single phase supply
that being said, at worst case your 2 hp motor would need a 300 Uf capacitor
if the static converter is rated for 8 HP, it is likely to have capacitance upwards of 600 to 800 Uf
at best case you'd be putting 2X the required capacitance to the windings of your 2HP motor, which is not a good thing...

Compounding the problem is the potential relay found in most static converters.
the potential relay will be called into action of dumping the start capacitors charge through the potential relay to the motor winding.

that's all well and fine if the workload doesn't take much power.
but as soon as the line voltage dips below somewhere around 165V, the start capacitor's charge is dumped by way of the potential relay into the motors windings, ad infinitum, until either the load is reduced, the cap blows up, the potential relay fails, motor windings fail, or the motor voltage is otherwise cut off.

a quick get around to the potential relay issue is the installation of a normally open momentary switch to the generated leg of the static converter.
the momentary switch would block unwanted winding activation, past the initial starting operation
the motor would last longer, provided that long periods of heavy loads were not demanded or necessary.

my advise would be to have a look at the starting capacitor inside the static converter and reduce the starting capacitance to somewhere around 300Uf, and install the normally open switch before i'd go any further

start capacitors are about $20
a momentary switch could be around $35

good luck on which ever road you travel
Click to expand...

After researching VFD’s for a while and seeing the name brand pricing, I figured I would dig deeper into the static converter and down sizing option.

Rather crude and simple d ices to be honest. It looks like this model has two 400uF starting caps in parallel.

If I disconnect one of them, I should drop this close to your recommendation. If it was rated at 4-8 HP, this would drop me nicely into the 2-4HP range?

Do I still need the momentary switch? I see a bleed resistor in there
 

Attachments

  • image.jpg
    image.jpg
    319.7 KB · Views: 9
  • image.jpg
    image.jpg
    267.1 KB · Views: 9
  • image.jpg
    image.jpg
    270.2 KB · Views: 8
butter said:
After researching VFD’s for a while and seeing the name brand pricing, I figured I would dig deeper into the static converter and down sizing option.

Rather crude and simple d ices to be honest. It looks like this model has two 400uF starting caps in parallel.

If I disconnect one of them, I should drop this close to your recommendation. If it was rated at 4-8 HP, this would drop me nicely into the 2-4HP range?

Do I still need the momentary switch? I see a bleed resistor in there
Click to expand...
400Uf is in the range-you are good to go there.
You certainly can operate exactly as the static is now that you reduced the capacitance.
The momentary switch is not 100% necessary, but it will increase the service life of the motor and the static converter
 
The bleed resistor is there to slowly discharge the capacitor when power is no longer applied.
it is a safety hazard to handle live capacitors.
The bleed down resistor makes it a little safer.
Always discharge capacitors before any service of electrical circuits
 
Thank you both.

The momentary push button goes on the third leg just before the motor? Hmmm… this would mean i would need to push it at every motor restart.
 
butter said:
Thank you both.

The momentary push button goes on the third leg just before the motor? Hmmm… this would mean i would need to push it at every motor restart.
Click to expand...
Yes, the button will need to be manually depressed when the machine is operated
You can liken it to being the start button to operate the mill
 
Both capacitors show evidence of leakage, really question the use of cardboard and fiberboard as an insulator. Might be better off just replacing it then risk a capacitor letting go or fire risk. At the cost of a 2 Hp generic VFD, as well as getting full power out of your 2 Hp motor, I would go that route. Do not go with a DPS digital phase converter commonly sold on Amazon, they are overpriced static converters.
 
Hi All, just closing the loop on this in case anyone else reads this in the future.

I swapped out the two leaking capacitors for one new 330uF cap. Added a momentary pushbutton and it works well.

Thank you all for the help and the opinions.

button.jpg
 
You must log in or register to reply here.
Back
Top