Soft starting a single phase induction motor/RPC - Page 1
Forum section: electronics / projects

author: mc
author_rank: Regular Contributor
timestamp: September 06, 2013, 12:42:30 pm
content: I'm looking at rebuilding my rotary phase converter (takes 240V single phase in, and outputs 415V phase), which will involve fitting a larger idler motor, however I'd like to minimise the current draw during startup.At the moment it's all controlled via a contactor and a relay. You turn it on, and the step-up transformer gets powered up (240 to 415V). You then hit the start button, which then closes in the main contactor, sending 415V to the idler motor. As the idler motor comes upto speed, a 240V N.C relay, which connects the start capacitors, connected between the star point and the generated/false leg, gets powered up disconnecting the strat capacitors.Essentially the same as the bottom diagram on this page -http://www.waterfront-woods.com/Articles/phaseconverter.htmbut with a step-up transformer added to the input.As it stands, the transformer is 5KVA, and the current idler motor is a 3KW. I'd like to upgrade the motor to about 4KW to give me a bit more power, however I'd like to reduce the light dimming start-up surge.Soft starters are available for single phase motors, but nothing much above 2KW, and I'm not sure they'd work by simply placing them between the contactor and idler motor.Has anybody got any suggestions to consider?

author: CalG
author_rank: Contributor
timestamp: September 06, 2013, 04:55:39 pm
content: As you are transforming the single phase input to the idler, it would be possible to soft start on the 240V and then switch in the transformed higher voltage when nearly up to speed.You would need an additional potential relay, an additional contactor, and rethink your starting capacitors (perhaps).It would be a reduced start voltage starting scheme, without the dropping resistors. by taking advantage of the two voltages you presently have.Browse for "delta -wye starting configuration, or "motor soft start circuit", or "reduced voltage motor starting".There will be some detailed diagrams posted. Pick and choose ;-)cheers

author: mc
author_rank: Regular Contributor
timestamp: September 06, 2013, 06:30:16 pm
content: The issue with that is switching smoothly from the 240 to the 415. There would need to be definitive gap between switching over, otherwise there's a risk of the 240 and 415 being connected, but with a gap there's the possibility of the motor and being out of sync causing a big surge when connected.However that has got me thinking about a different simpler solution.I've got various other transformers lying around, and I'm thinking if I use one of them connected in series with the motor during startup, it would limit the starting current, then once things are upto speed, have a contactor bypass the transformer.

author: peter.mitchell
author_rank: Super Contributor
timestamp: September 07, 2013, 08:27:03 am
content: a massive switched NTC thermistor?

author: mc
author_rank: Regular Contributor
timestamp: September 07, 2013, 01:39:08 pm
content: Been thinking about this, and a 220ohm 200W resistor is only £20, and will save playing around with old transformers. The resistor is capable of 10x rating forI've so far came up with the following schematic.When S1 is switched on, 240VAC is applied to K3 (it'll be a Crydon A1230 - a zero cross turn on 240Vac controlled SSR), which then powers up the TX and a 24V DIN rail mount power supply. The sole purpose of the SSR is to control the inital switch on surge, which is more a 'nice to have' to stop the light dimming surge when switch on happens to coincide with a voltage peak.Once the start Switch (S2) is pressed, the main contactor (K2) shuts and is latched, powering the Idler Motor (M1) via a 220ohm 200W resistor.As M1 comes upto speed, the generated voltage between the start point and the false leg (L3) rises, eventually powering up the start capacitor disconnect relay K4. When K4 switches, the start capacitor is removed from the circuit, and 24V is applied to the resistor bypass contactor (K1) causing it to close and latch.K1 has to latch once upto speed, as when a large load is applied, the voltage on the false/generated leg can drop low enough that the start capacitor kicks in briefly (this is one reason I want to swap to a larger motor), and If K1 didn't latch, it would mean that it would reconnect the start resistor causing a major volt drop.To power down, the Stop Switch (S3) is pressed, which disconnects the latching circuit to both K4 and K1.I know the diagram isn't ideal, but I've just grabbed the first reasonably appropriate components in DipTrace.