Motor soft starting
 

Hi,
I have a water pump that uses a single phase induction motor with a capacitor permanently in circuit of about a quarter hp at 230 volts. (Is that called a split phase motor?)
I would like it to start at a reduced torque to avoid pressure shocks to the pipework.
I've been kicking ideas around such as a resistor, diode or even a transformer with a bucking winding that is shorted out after a short period when the motor is at speed.
I've looked on eBay, but most soft start units there seem to be for star/delta starting of polyphase motors.
Ideally it would ramp up to full revs (which, of course, it won't with my simple ideas), but I'd happy if it started in two steps.
Another thought is, maybe, a gadget that reduces resistance as it heats up (I can't think what they're called), then is shorted by the aforementioned timer.
Any thoughts appreciated.
Cheers, Pete.

Re: Motor soft starting
 

It's a 'permanent split capacitor' motor. It can be started at reduced torque using reduced voltage or by adding series resistance or reactance as you suggest (although not with a diode, as the resulting DC current in the winding will screw things up royally). You can also buy purpose made single-phase soft starters that ramp up the conduction angle when powered on, then cut themselves out of circuit. E.g. RS 2.2kW single-phase soft start

Note that only the torque can be readily controlled in this way; induction motor full running speed cannot practically be reduced below around 80-90% by voltage control. Therefore, to be sure the soft-start would work correctly, it is important to have an idea of the torque/speed curve of the load. What type of pump is it? Centrifugal / positive displacement?

But, taking a step back, if the pump startup is causing a hydraulic shock, how about fitting an accumulator or pulsation damper that will absorb the shock? Obviously this will depend on the application (which we don't know yet), e.g. 5 litre accumulator

Re: Motor soft starting
 

I have been using a "collapsible hose" in the garden (the type that shrinks and expands with pressure, quite amazing), a length of that would act as a shock damper. Available at all good stores.

I don't do a lot of gardening, I was just watering plants under instruction!

Re: Motor soft starting
 

Hi

Agree, accumulator after the pump should solve the problem.
You can easily make a small one with an upside down T fitting, the upright has a length of pipe on it with a blank end.

Richard

Re: Motor soft starting
 

Accumulators have flexible bags inside which are filled with air which is compressed by the water. Such devices are used in central heating systems to keep the radiators pressurised and need to be pumped up if there is a leak.

Re: Motor soft starting
 

They can also be a simple closed vertical pipe (closed at the top!), the problem there is eventually all the gasses get dissolved in the water. This kind are used a lot in the USA and when "hammering" becomes a problem the solution is to drain the system and let the air back in. Saw that in an episode of "This Old House".