VFD Single Phase in and out

FoTenX · Jul 6, 2020

Hi there
I preface this query with: I am an almost complete electrical novice. I need to control the speed, startup and slow down of a 2100W induction AC motor through a VFD. My workshop has ordinary Australian 240v single phase supply. I can find plenty of VFDs that convert single phase to 3 phase but I can't find a single to single phase unit.Can this even be done? What other options are available?

pc1966 · Jul 6, 2020

Usually VFD drive three-phase motors as it is hard to get a single phase motor to run well over a wide range.

In reality most "single phase" motors normally uses a capacitor to generate a 2nd shifted phase to get going, and that shift is frequency dependent so not great outside of a moderate range around the designed speed.

Depending on the equipment and your needed speed range it might be worth seeing what it would cost to get a 3-phase replacement motor for it. Typically motors have fairly standard frames (i.e. shaft and fixing means) so it is frequently an easy and reasonably priced job.

Lucien Nunes · Jul 6, 2020

As PC1966 says the performance of single-phase motors under VFD control is definitely inferior, making single-phase output VFDs something of a necessary evil. They do exist, but if you plan to use one, beware the need to derate the motor quite heavily and the limited speed range over which it will still provide adequate torque.

Strima · Jul 6, 2020

What's the application for?

If it's a lathe, for example, then three phase VFD is better as it offers good torque at low speeds where a single phase motor won't. Poor torque equals a lot of chatter and poor finishing.

FoTenX · Jul 7, 2020

Thank you for the thoughtful replies. The applications are a 2x72" grinder and a powered hacksaw (home built). I mainly wanted to cushion the start a little (especially for the hacksaw) but I can see now that a 3 phase motor would have perhaps been a better choice. We live and learn!

marconi · Jul 7, 2020

Re#5; You mention you mainly wanted to cushion the start of the machines. You could use one of these:

RS PRO Single Phase Soft Starter - 12 A Current Rating, 2.2 kW Power Rating, 230 V ac Spply Voltage | RS Components - https://uk.rs-online.com/web/p/soft-starts/2096340/

wired between the motor's terminals and the 240V supply/starter. It has a rating of 2.2kW and you stated your motor is 2.1kW. One per motor or you could share it between motors by suitable switching or plug-socket if you only use one machine at a time.

FoTenX · Jul 7, 2020

marconi said:
Re#5; You mention you mainly wanted to cushion the start of the machines. You could use one of these:

RS PRO Single Phase Soft Starter - 12 A Current Rating, 2.2 kW Power Rating, 230 V ac Spply Voltage | RS Components - https://uk.rs-online.com/web/p/soft-starts/2096340/

wired between the motor's terminals and the 240V supply/starter. It has a rating of 2.2kW and you stated your motor is 2.1kW. One per motor or you could share it between motors by suitable switching or plug-socket if you only use one machine at a time.

Thanks Marconi! That is the answer!