Single phase VFD-nevermind

I thought you wanted to keep the old motors? Motors with capacitors don't lend themselves to VFD.

No.But if you feed them two power sources with a 90 degree phase shift, you dont need a cap , and you CAN control the speed.

Please see the link:

The link says nothing about cap run motors. What happens if the run cap is left in circuit? THanks, Eric

Capacitor run motors use the capacitor to shift the phase of the current applied to one of the windings.

this phase shift is both load and frequency dependant so you get low starting torque (due to rediuced phase shift) and if you reduce the drive frequency you'll also get reduced pahse shift and thus reduced torque.

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I know how cap run only motors work, how they have low starting torque. What if the frequency is increased? Does that increase starting torque? I can see the drive starting the motor at 60 Hz or above and then once up to some certain speed shifting to the programmed torque. The description of the drive says that it supplies 1.5 times running torque for starting torque. Did you follow the link? I don't understand these drives well, the single phase ones. Do you? Thanks, Eric

Torque on the bottom end for AC motors of such type requires a constant DC current to be in the mix called magnetic current. This current needs to be around 35% of the full current of the motor for good slow RPM operations and torque at the bottom end.

You would do better if you can get your hands on a 3 phase motor, a small one and then get an inverter that operates from single phase that has the VECTOR mode in it. With VECTOR mode you can program minimum current that gives you a stable low RPM and some torque. You also want to make sure the motor is rated for an inverter, one that is designed to operate at any RPM with woudings that are fit for the PWM mode..

Of course, you could simply use a miniature PIV gear, the V-Belt style for simplicity.

Jamie

Increasing frequency increases the speed not the torque. Overall power increases since it is a factor of speed and torque.

The 90 degree phase shift is frequency dependent the capacitance and inductance of the winding will only shift 90 degrees at one frequency.

All the VFD's I've used have three phase where each output has a voltage that's 120 degrees phase shifted from the others. If there is a 90 degree VFD I haven't encountered it (but haven't looked either) It would require rewiring the motor and removing the cap but that's not impossible....

check out:

Microchip component that can be programmed for 2 phase operation if you wanted to build such a device - the application note also discusses speed torque etc..

The site I posted a link to is a typical Chinese to English translated site. But I was able to find two circuits on the site for cap start as well as cap run motors. I was not able to find any other single phase VFD makes that offered these circuits. Anyway, I'm gonna buy one of the things because they say it will work on the motor in question. I'll try it in my shop first on both type motors. Since it turns out the customer's motor is a cap start motor the centrifugal start switch will need to be bypassed if the motor is to be operated below 60 Hz. But I think instead I will set it up to go from 60 to 90 Hz. That should be enough speed differential to bridge between the existing speed selections. Eric