Overthinking it - perhaps a hammer would work after all

[fizzy]

Out of curiousity, I shall give a soft starter module a try. I'll probably start off with one of the cheaper ones, commonly used for chop saws and the like, apparently. Will report back in a couple of weeks.

My rage 2 saw gives a huge kick when it starts and did pop a breaker once. My compressor has a soft start device on it.

 

[richie00boy]

Out of curiousity, I shall give a soft starter module a try. I'll probably start off with one of the cheaper ones, commonly used for chop saws and the like, apparently. Will report back in a couple of weeks.

Check your compressor before you do, that first one is only rated to 2.2kW and the second to 25A, both would probably break on a typical 50 litre compressor which has a 2.5HP or 3HP motor.

I've been meaning to make my own soft start for a while, based on a meaty positive temperature coefficient (PTC) thermistor which is shorted by a relay after a second or so.

 

[daleyd]

Check your compressor before you do, that first one is only rated to 2.2kW and the second to 25A, both would probably break on a typical 50 litre compressor which has a 2.5HP or 3HP motor.

I've been meaning to make my own soft start for a while, based on a meaty positive temperature coefficient (PTC) thermistor which is shorted by a relay after a second or so.

I’m not sure I follow - 2.2 kW IS 3 HP, so anything rated for 2.2 kW would be Ok for 3 HP??

 

[skotl]

I’m not sure I follow - 2.2 kW IS 3 HP, so anything rated for 2.2 kW would be Ok for 3 HP??

Not that I know how they work but I guess that ~55A startup is still going to be relatively significant, even if reduced through a soft start?

 

[daleyd]

Not that I know how they work but I guess that ~55A startup is still going to be relatively significant, even if reduced through a soft start?

I don’t know the specs, I can’t see any links? But the point I was making was that a 3HP motor is the same as 2.2 kW. So if something is rated at 2.2 kW then it is naturally rated at 3HP.

I just didn’t understand the wording.

The 55A figure is significant, but you could only say if it was a problem if the specs for the soft start are known. And assuming it’s rated for a 2.2kW motor then it should be Ok?

 

[RaceDiagnostics]

Plug it into the socket on his cooker circuit.

 

[ajlelectronics]

So if something is rated at 2.2 kW then it is naturally rated at 3HP.

Startup current is many times higher than the normal run current. Any device "in the way" has to be able to withstand the inrush current.

 

[daleyd]

Startup current is many times higher than the normal run current. Any device "in the way" has to be able to withstand the inrush current.

Yes, that’s exactly what a soft starter does It’s literally their job to do this, and in fact limit the start up current.

I’ve fitted big ones on industrial fans, not on anything this small - but their entire job is to remove/smooth out the huge start up current when you start something on line.

 

[voipio]

A 3HP motor will draw around 10A from a 230V supply, when it is running at normal speed under full load, and much less than that under no load conditions.

The 55A figure, or thereabouts, is the brief maximum current that will be drawn by an induction motor when the rotor is stationary, and the supply voltage is applied. The motor essentially behaves like a transformer with the output terminals shorted. Once it starts moving, the current drawn falls dramatically, to a level determined by the phase slip angle. A soft start circuit limits the current to a level sufficient for the motor to start under load, and considerably less than the 55A, that's been quoted.

 

[daleyd]

A 3HP motor will draw around 10A from a 230V supply, when it is running at normal speed under full load, and much less than that under no load conditions.

The 55A figure, or thereabouts, is the brief maximum current that will be drawn by an induction motor when the rotor is stationary, and the supply voltage is applied. The motor essentially behaves like a transformer with the output terminals shorted. Once it starts moving, the current drawn falls dramatically, to a level determined by the phase slip angle. A soft start circuit limits the current to a level sufficient for the motor to start under load, and considerably less than the 55A, that's been quoted.

^^ this - the 55a is no longer relevant as the soft start prevents it getting this high. Typically it’s limited to a couple of times the running current (I only have experience with 3 phase soft starts not single phase ones talked about, but the principle of operation is the same)

 

[ajlelectronics]

the 55a is no longer relevant as the soft start prevents it getting this high

Agreed, but it will not bring the inrush current overhead down to zero though, will it?

 

[daleyd]

Agreed, but it will not bring the inrush current overhead down to zero though, will it?

No, no one said it would - but it also won't be anywhere near 55a.

The only way you could get it to 0 would be a vfd…

 

[skotl]

Presumably the soft start module would need to *after* the pressure switch on the compressor? If it was simply at the mains input then the unit’s job was done 15 minutes ago, when the power was applied, but we’d see the inrush again once the receiver got below pressure?

 

[daleyd]

Presumably the soft start module would need to *after* the pressure switch on the compressor? If it was simply at the mains input then the unit’s job was done 15 minutes ago, when the power was applied, but we’d see the inrush again once the receiver got below pressure?

The soft start unit would go before the motor. The pressure switch is irrelevant, the soft start should simply be considered as part of the motor circuit that operates every time the motor starts. (The inrush current will always apply whenever the motor is started)

Sorry, I see what you mean, yes after the pressure switch, before the motor. Though in the case of a compressor it wouldn't make much difference where you put it, as the motor is literally the only thing in the circuit. If you put it just after the plug, it would probably still work OK...

 

[richie00boy]

I’m not sure I follow - 2.2 kW IS 3 HP, so anything rated for 2.2 kW would be Ok for 3 HP??

Hmm got my kW and HP a bit mixed up there... may possibly be OK then.

 

[daleyd]

Hmm got my kW and HP a bit mixed up there... may possibly be OK then.

I wasn’t sure, I thought I had missed something!

 

[voipio]

Out of curiousity, I shall give a soft starter module a try. I'll probably start off with one of the cheaper ones, commonly used for chop saws and the like, apparently. Will report back in a couple of weeks.

So, an update, as promised. It's been more than a couple of weeks, but I decided to give more than one module a try, and the last two only arrived a week or so ago.

The three modules tested were as follows: 20A two wire, 12A two wire, and 12A three wire, as shown in the pics below, all bought via eBay:

There were no datasheets for any of them. Using a test meter, none of the soft start modules appeared to be of the purely negative temp coefficient varistor types. At a guess, they all probably incorporate some sort of TRIAC power control circuit module, sealed in an encapsulant.

Test loads consisted of: 60W incandescent lamp, 325W halogen lamp, Vax vacuum cleaner (1000W), Dyson Animal vacuum cleaner (1300W), AEG induction motor 1/3HP driving a single stage vacuum pump.
Test power sources consisted of: a) domestic mains 235-240V, b) Kipor Sinemaster IG1000 generator, 230V nominal (rated at 900VA continuous, 1000VA maximum).

When powered from the mains supply, via the soft start modules, all of the above loads ramped up to speed or brightness over a period of a couple of seconds or so, and stayed there. When using the three wire soft start module, the induction motor got up to speed smoothly, and ran quietly for a few seconds before starting to buzz, and sound as if it was drawing more current. The measured current stayed more or less the same, at 1.9A, but the power consumed increased by about 50%, so perhaps the module's TRIAC triggering became less stable over time.

When powered from the Kipor Sinemaster IG1000, all of the soft start modules reduced the amount of initial throttle "grunt" heard from the generator, and all of the 1000W and under loads ran fine, although the induction motor again buzzed after a while, when using the three wire soft start module. Unsurprisingly, the generator hardly noticed the 60W lamp load, with or without soft start module.

Without a soft start module, the Dyson Animal vacuum cleaner completely stalled the generator. When the Dyson was powered via any of the soft start modules, the generator continued to run, albeit working hard, and the vacuum cleaner motor sort of got up to speed, but the overload LED came on and it dropped the generator's output. The generator's engine continued to run, though it needed to be stopped and restarted to reset the overload protection.

A few other loads were also tested, using mains power only. A 12W mains LED non-dimmable lamp, and a 7W mains LED dimmable lamp both misbehaved (flickered, and flashed on/off) when powered from the mains via any of the modules, whilst a 30W mains LED floodlamp behaved itself on the three wire module, but not on the two wired modules, unless the 60W incandescent lamp was wired in parallel with the 30W LED.

When driving the higher current loads, the modules did become somewhat warm to the touch, so probably a bit of heasinking, e.g. screwing it to a case, would be required.

In summary, in relation to the original post, and to the discussion that followed, a two wire soft start module, between genny and compressor, is probably worth trying.

 

[skotl]

So, an update, as promised. It's been more than a couple of weeks, but I decided to give more than one module a try, and the last two only arrived a week or so ago.

The three modules tested were as follows: 20A two wire, 12A two wire, and 12A three wire, as shown in the pics below, all bought via eBay:

View attachment 431988View attachment 431989View attachment 431990
There were no datasheets for any of them. Using a test meter, none of the soft start modules appeared to be of the purely negative temp coefficient varistor types. At a guess, they all probably incorporate some sort of TRIAC power control circuit module, sealed in an encapsulant.

Test loads consisted of: 60W incandescent lamp, 325W halogen lamp, Vax vacuum cleaner (1000W), Dyson Animal vacuum cleaner (1300W), AEG induction motor 1/3HP driving a single stage vacuum pump.
Test power sources consisted of: a) domestic mains 235-240V, b) Kipor Sinemaster IG1000 generator, 230V nominal (rated at 900VA continuous, 1000VA maximum).

When powered from the mains supply, via the soft start modules, all of the above loads ramped up to speed or brightness over a period of a couple of seconds or so, and stayed there. When using the three wire soft start module, the induction motor got up to speed smoothly, and ran quietly for a few seconds before starting to buzz, and sound as if it was drawing more current. The measured current stayed more or less the same, at 1.9A, but the power consumed increased by about 50%, so perhaps the module's TRIAC triggering became less stable over time.

When powered from the Kipor Sinemaster IG1000, all of the soft start modules reduced the amount of initial throttle "grunt" heard from the generator, and all of the 1000W and under loads ran fine, although the induction motor again buzzed after a while, when using the three wire soft start module. Unsurprisingly, the generator hardly noticed the 60W lamp load, with or without soft start module.

Without a soft start module, the Dyson Animal vacuum cleaner completely stalled the generator. When the Dyson was powered via any of the soft start modules, the generator continued to run, albeit working hard, and the vacuum cleaner motor sort of got up to speed, but the overload LED came on and it dropped the generator's output. The generator's engine continued to run, though it needed to be stopped and restarted to reset the overload protection.

A few other loads were also tested, using mains power only. A 12W mains LED non-dimmable lamp, and a 7W mains LED dimmable lamp both misbehaved (flickered, and flashed on/off) when powered from the mains via any of the modules, whilst a 30W mains LED floodlamp behaved itself on the three wire module, but not on the two wired modules, unless the 60W incandescent lamp was wired in parallel with the 30W LED.

When driving the higher current loads, the modules did become somewhat warm to the touch, so probably a bit of heasinking, e.g. screwing it to a case, would be required.

In summary, in relation to the original post, and to the discussion that followed, a two wire soft start module, between genny and compressor, is probably worth trying.

Thanks for this. Not sure if I'm reading this correctly but did you get a chance to try the soft-starts on the compressor itself? i.e. would a compressor be able to start on (say) a B16 circuit without throwing the breaker?

 

[voipio]

I didn't try a compressor with the soft start modules. An induction motor driven vacuum pump was the nearest I could get, and the soft start modules definitely reduced the surge. The induction motor started with a definite "clunk" when the soft start modules weren't in circuit, and that "clunk" wasn't there when a soft start module was in place; just the click of the centrifugal switch could be heard. I would say that, with a soft start module fitted, a compressor is going to be less likely to trip a B16 circuit breaker. The only complication with a compressor might be how quickly the dump valve closes and pressure starts to build and load the motor. The modules I tested were relatively inexpensive.

I have a clamp meter with a max/min function, but wasn't convinced that it was capturing the initial surge currents properly, so didn't carry on using it, beyond an initial test. The bargraph "analogue" display on the clamp meter gave readings that were quite a bit higher than the digital display "max" values.

 

[bourbon]

We tested this in the shop. A guy wanted to run a compressor on a Genny. So, our lads experimented. They used a 5.5 Kva Genny. The compressor was a 3HP 150 litre. Never missed a beat. BUT, and the reason we THINK it was ok, was that it was a belt driven compressor. Our theory is that the belt takes a little bit of the effort that is needed on the start up voltage. A direct drive, did bog the genny down, but didn't stall it on start up. But it was a 5.5kva. That's our real world experience.