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Soft start for electrical motor


PaulF007

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Hi Guys

I remember seeing something on the forum about electrical motors that can be soft started ie. you don't have the peak power spike when you start a motor. 

Anyone here where you could buy something like this or is it possible to build a DIY version of it? It is a single phase motor and the inverter can handle it quite well but I noticed that sometimes it will have a startup spike that is a bit too high for my liking. 

P

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Soft starters work by temporarily reducing the voltage applied to the motor, causing it to have a slower start and hence less of a current spike. The mechanism by which it works is not unlike a dimmer switch, using triacs (or back-to-back thyristors) to chop away part of the waveform so you get a lower average voltage. So it is relatively simply to do, even DIY... BUT... Soft starting has limits. The torque required to drive your load is important, driving it with too little current to start moving the load is of course a bad thing (burn-out-kind-of-bad), so what you want to do is start at the minimum amount of current to just get the load going, and then ramp it up to full speed from there. This means that some loads don't benefit from soft starting at all, it needs too much current to get going anyway.

VFD/VSD is different. Here you have full control of the available torque and speed (via voltage and frequency control), which allows you to start much slower and with much less of a starting spike. But a VFD/VSD is essentially an inverter with the important difference that instead of making a flat 230VAC RMS at 50Hz, the output is adjustable.

While a sufficiently advanced DIYer can do both, for the most part a soft-start is much easier than a VFD, and even for the simpler option you need a basic understanding of electronics and electrical motors. And much as I sound all knowledgeable here (I hope I do!), it's something I'd much rather buy off the shelf than attempt to DIY :-)

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On 2/25/2017 at 6:09 PM, PaulF007 said:

3 phase motors

There is a neat trick sometimes used in single phase motors, I've seen at least one such setup (in high school) using a similar configuration. It had the old box on the wall with a little crank marked "start" on the one side and "run" on the other. What it does is wire some of those big coils inside the motor in series (causing a higher resistance, less current flow, lower current) and then as it picks up speed you switch it to the other setting which puts the coils in parallel, perfect for loads that have low starting torque.

Same with the 3-phase, it would connect them in star formation (230VAC over each winding) and then as it reaches working speed you switch to delta formation, so now you have 380+V across the windings (connected between phases). So it is a similar trick to start with a lower voltage, and works well if the starting torque of the load is less than two-thirds of the full output. Technically it isn't called a soft-starter, it's a star-delta starter. Or so says the internet. The old disclaimer still applies: I am not an electrician.

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8 hours ago, plonkster said:

While a sufficiently advanced DIYer can do both, for the most part a soft-start is much easier than a VFD, and even for the simpler option you need a basic understanding of electronics and electrical motors. And much as I sound all knowledgeable here (I hope I do!), it's something I'd much rather buy off the shelf than attempt to DIY :-)

As per usual your answers is detailed and logic. Thanks @plonkster! I agree with regards to the off the shelf option. My searches on the net have not showed to many suppliers around here that sells a "soft starter" any idea where I could start looking ? 

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8 hours ago, plonkster said:

There is a neat trick sometimes used in single phase motors, I've seen at least one such setup (in high school) using a similar configuration. It had the old box on the wall with a little crank marked "start" on the one side and "run" on the other. What it does is wire some of those big coils inside the motor in series (causing a higher resistance, less current flow, lower current) and then as it picks up speed you switch it to the other setting which puts the coils in parallel, perfect for loads that have low starting torque.

Same with the 3-phase, it would connect them in star formation (230VAC over each winding) and then as it reaches working speed you switch to delta formation, so now you have 380+V across the windings (connected between phases). So it is a similar trick to start with a lower voltage, and works well if the starting torque of the load is less than two-thirds of the full output. Technically it isn't called a soft-starter, it's a star-delta starter. Or so says the internet. The old disclaimer still applies: I am not an electrician.

I had forgotten about that method. As kids we had one of those motors running on the farm. I remember my dad promising all sorts of hell if the lever was left on the "Start" side and not pulled over to "Run". For a hammermill it was perfect but for a lathe it would be impractical. :) 

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10 hours ago, plonkster said:

Soft starters work by temporarily reducing the voltage applied to the motor, causing it to have a slower start and hence less of a current spike. The mechanism by which it works is not unlike a dimmer switch, using triacs (or back-to-back thyristors) to chop away part of the waveform so you get a lower average voltage.

Will it be possible to apply this to a geyser element?
Want to reduce the startup current on my element (1kw startup at around 3kw)

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1 hour ago, viper_za said:

Will it be possible to apply this to a geyser element?
Want to reduce the startup current on my element (1kw startup at around 3kw)

That's a little odd. Resistive elements don't usually have such a big inrush current. A quick google (just now) suggest that there is definitely some inrush current, because the element has a lower resistance when cold, but I didn't think it was this big. I should measure that some day.

It's not one of these PTC things?

A soft-starter on a heating element should work just fine. There is also no start value related to the starting torque of the load (as with the motor), you can literally scale it up from zero. Also, since heating elements generally have a unity power factor, there is no phase angle current shenanigans, your current is proportional to the voltage and power is  proportional to the square of the average voltage.

I have no idea how the inverter will handle it though. Such wave chopping especially at high power levels always concern me. I have not been able to find a good answer on this topic yet. Reading on the AEVA forum how the PIPs go into some sort of bouncy oscillation when the vehicle battery chargers start to chop the power, and having seen an old Nelson Adams inverter do that same thing in the 90s, I am not filled with much confidence.

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26 minutes ago, plonkster said:

That's a little odd. Resistive elements don't usually have such a big inrush current. A quick google (just now) suggest that there is definitely some inrush current, because the element has a lower resistance when cold, but I didn't think it was this big. I should measure that some day.

It's not one of these PTC things?

Yes it's a PTC element.
Geyserwise spec suggest a 15A max inrush current for the element.
If you have some time would you be able to recommend a soft-starter to get and test it out.
Would like to reduce that "spike" my bank needs to absorb each time this happens.

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  • 4 months later...

I have a very easy way to control geyser elements

its called a diode

you place it in series with any element and it will reduce its power by half( half wave rectification)

if you really want to you could have it controlled with a contactor or manually with a switch   so it bypasses diode and then you have half or full  power

trade secret

PS don't try this on a motor

cheers

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20 hours ago, maxomill said:

you place it in series with any element and it will reduce its power by half( half wave rectification)

It also messes up your power factor something horrible... :-)

And because it allows through the entire half of the wave the peak the lowest you can do is 50%, at around 70% power factor, so this will only reduce the load on the inverter by 30% or so.

Well, let me explain. It will reduce the power use by 50% (real power, as measured at the battery), but in terms of the inverter's VA capacity, it will use 70% of the original apparent power.

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Interesting , so far the best "real life" explanation of the power factor and va vs w. So on a 1000 w element you will get 500 w  with the diode but you will get 700 va. 
Now something else I have been looking at is if one could use something like a PWM to reduce the elements draw.  So ideally you would either need to reduce the input voltage to the element to reduce the output watts. This way , if it is doable , you can write a little program that can increase the power output to the geaser element as you solar surplus increases.

But sofar I haven't found the right term to look for. Keep in mind you don't want to control the temp by switching on or off.  You want a variable watt control so ideally a range between 500 and 2000 w in my system :) .

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2 hours ago, PaulF007 said:

something like a PWM to reduce the elements draw

I've thought about that too. If it is a resistive element, it is not going to care whether you feed it AC or DC. So if you use a couple of suitably sized diodes (20 amp, 400V type things) it is easy to make DC, and then once again adding the right kind of high voltage MOSFET you could PWM it. But the moment I got this far, I realised that an SCR (aka Triac) is so much easier to use, cheaper to buy, etc... but then you're back to a dimmer. But if you were to use a PWM setup with a MOSFET, that's still going to create a pretty bad non-linear current waveform (ie bad power factor) so the next thing you want to be doing is add a capacitor and an inductor to help with that, and then.... bingo you have a buck converter :-) And it is not going to be cheap.

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