Question for Les or anyone else!

[Ed P]

I’m now getting more concerned that pirouetting Truss will renege on her commitment not to have power cuts this winter.

As I said elsewhere, I ruled out using a spare 1Kv UPS to run the gas boiler and central heating because of the motor start-up current draw. I then came across  ‘soft start’, which looks like it might solve the issue. Have you heard of ‘soft-start’, and if so, what sort of ramp-up time would be acceptable on a boiler?   I’d guess up to 10 seconds is probably ok, but 60 seconds feels like it might be too much and might burn out the boiler heat-exchanger.

[Les.]

Ed, I don’t know much about soft start for single phase, but I don’t see why not (up to a point)

As I think you all know, I designed a very simple single to 3 phase rotary converter without all the expense of a big transformer to run my lathe. I had an inverter that I was led to believe would run the lathe off single phase, but it was the wrong type. However, once I had my RPC working and the lathe running from it, I had the bright idea of placing the 400v to 400v inverter between RPC and lathe. This inverter will do everything except make tea, one of its features being soft start. It comes with a default time of 5 seconds, but I can set it from probably 0.5 second (maybe quicker) up to goodness knows what.

I remember working with 20 HP fans, which then needed to use star / delta start up system. As i assume you know, a suitable motor would be intended for 720v for full output in star connected mode, or 415v in delta connected mode. What happened was start up in star at 415v (so much under-run) and then switched over to 415v delta, where full power was available without the massive over-current situation had it been direct on line 415v in delta. I recall delay times of 20 to 30 seconds with big fans, whereas a 20HP pugmill only needed about 5 seconds or less.

Today these 400v inverters are everywhere in industry.

My earlier “up to a point” comment could be because of the use of capacitors for single phase. Some motors have a start capacitor, some a run capacitor, larger single phase motors often have both. For a typical capacitor start motor, the start windings are much lighter duty as they usually run for less than one second. Running it too long before the internal switch opened might not be a good idea.

I know nothing about modern central heating motors, so can not comment further, except to say I had a Grundfoss circulation pump (goodness, nearly 50 years ago) but that was a low powered capacitor run type, so no problem likely there.

Les.

[Ed P]

Thanks for the input Les.

I did a web search on UPS+Central Heating Boiler. Lots of probably Russian/Ukrainian sites (use Cyrillic alphabet) suggest it is ‘easy’.  However I worry about them a little as they suggest that a 0.7 power factor will take care of the startup, and that doesn’t gel with my knowledge of motors (admittedly all 3-phase). There are however .uk sites that also give general comfort and suggest a factor of x three is adequate for motor startup load. The link for a slightly smaller Grundfos gives me comfort that a 1Kv UPS with true sine wave output will be OK using just double the rated power for the pump even without a soft starter!

I think I’ll make sure that our spare 1Kv UPS is fully charged, and make the decision to use it or not when we get too uncomfortable.

[Ed P]

Again thanks Les – your comment about capacitors prompted me to delve into all the spec sheets I could find. The Grundfos IS fitted with a capacitor which I assume is a startup capacitor, it also states that it is  “Impedance-protected. Short-circuit-proof. No external motor protection required”.

About the only warning I could find states that a pure sine wave supply is required. As my Schneider UPS outputs a pure sine wave. I feel that I’m in good shape to use the UPS on the heating circuit, but I’ll keep the shower pumps on the mains system.

[Les.]

I have a strong feeling that the Grundfoss pump I had was a capacitor run motor. If a capacitor start, there is (normally) a switch inside which will audibly CLICK when the motor gets up to speed.

More likely today is to use a a PTC thermistor in series with the start winding which will quickly increase its resistance, thus maintaining a low or very low residual current in the start winding. In the latter case, you do NOT want repeated stop and start situations or the thermistor would remain high resistance and maybe not start up. Hmm.

Les.

[Ed P]

Les. wrote:

n the latter case, you do NOT want repeated stop and start situations —  maybe not start up

I’d make a wild guess that this does not apply as the central heating system is controlled by a thermostat, and is likely to have a situation where the heating goes on and off at frequent intervals (albeit minutes rather than seconds between on and off), but the pump ought to cater for thermostat ‘noise’ and frequent cycling).

This link implies that single phase pumps can be used with an UPS. They do not caveat it by pump brand, so I guess the capacitor is start or start.plus run. The Plumbers Forum talks about start capacitor.

Interesting – I now know more about the boiler circulating pumps than I ever did before – almost enough to be dangerous!

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