Optimal VFD settings for single phase induction motor - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: January 19, 2025, 12:10:17 am
content: Hi folksI'm trying to find any reasonable information about the behavior of relatively big single phase induction motors 500W to 1500W @ 50Hz 230V when driven by VFD at lower speeds.I know it is somewhat of a compromise solution, but those are pumps and won't be replaced for a long time.They are pressurizing water systems and are run at short duty cycles most of the time, about 20-30 second pulses.My main aim is to achieve a soft start without a huge current spike as it is run from off-grid inverter and if possible regulate power in some reasonable band with a simple proportional correlation to pressure to extend the time they run but at lower power.But I cannot find any reasonable minimum frequency and voltage values and V-f ratios. Or any charts of how they behave at say 25Hz with about 60% voltage.Cooling is not a problem here as they are cooled by the water.

author: Martinn
author_rank: Frequent Contributor
timestamp: January 19, 2025, 11:28:18 am
content: Do you have a VFD already (which model)?AFAIK you need a special one phase VFD to run a single phase motor.You write "huge current spike" - do the motors have a starter capacitor (with centrifugal switch) or a permanently attached capacitor?Example:https://www.peter-electronic.com/wp/en/wp-content/uploads/sites/2/variable-speed-drive-versidrive-i-e3-3e3-0-37-22kw-commissioning-instructions-en.pdfI'd assume you can limit startup current by selecting a slow startup ramp.

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 20, 2025, 06:07:58 am
content: As already stated, single phase motors don’t lend themselves to variable speed control due to their use of capacitors to create a second, phantom phase to provide a rotational vector for the rotor.The only true single phase VFD on the market is build by Invertek.https://www.invertekdrives.com/variable-frequency-drives/optidrive-e3-single-phase

author: 2N3055
author_rank: Super Contributor
timestamp: January 20, 2025, 06:16:02 am
content: There are soft starters for that use.

author: Whales
author_rank: Super Contributor
timestamp: January 20, 2025, 06:34:02 am
content: > My main aim is to achieve a soft start without a huge current spike as it is run from off-grid inverter+1 to soft start.You can get DIN rail mounted delay timers with adjustable seconds delay. Wire them to control a DIN rail contactor.Start the motor with a big film capacitor in series, then use the contactor to switch over to direct mains connection. I recommend using "motor run caps" not "motor start caps" as thelatterformer do not overheat if used for more than a few secondsCapacitor sizing: there might be guides online (based on the kW or hp rating of your motor).

author: Siwastaja
author_rank: Super Contributor
timestamp: January 20, 2025, 07:12:33 am
content: Quote from: WattsThat on January 20, 2025, 06:07:58 amAnd that seems to be literally a single-phase inverter. Is anyone aware of commercial VFD designed to drive capacitor-run single-phase induction motors, by replacing the capacitor circuit with a true second phase?I did build one myself in the past for a specific application. After all, a capacitor-run motor is actually a two-phase machine, with capacitor providing the time shifting for the second phase (roughly 90 degrees). Instead of keeping using the capacitor, but using an inverter to generate the second phase, the advantages should be obvious: exact control of phase shift and voltage of the second phase. This of course requires rewiring the junction box of the motor. Maybe the advantages are not large enough to warrant a market for this kind of inverter?One disadvantage is that such inverter requires four half-bridges instead of the usual three, so off-the-shelf 3-phase VFD electronics can't be used with specialized firmware, as is the case with 1-phase Invertek.