2KW power control


Hi to all at the forum, I am completely googled out !!
I have been searching with no luck, for a circuit diagram for a simple soft start circuit. I have just purchased a new 2KW compound mitre saw, but when i switch on, the kick off it is vey hard, unlike my 2.2KW router, which has a soft start and speed control, ( i had a look inside the router, but the circuit is encapsulated in black resin ). does anyone have a circuit diagram for such a project ? Any other method of preventing this kick ? searching only seems to find industrial sized units, any help would be greatly appreciated.

Regards, wildbill.


Senior Member
This may not help you but it's a thought... A thermistor in series with the unit starts out high resistance when cold and then as current goes through it it can get to very low resistance - from self-heating I^2 x R losses - [these used to be marketed as 'Surgistors' I think though those may have been resistors with a bimetal contact unit in parallel] - The problem is sizing both physical and electrical for the thermistor unit - obvioulsy a small one will just burn up so you need something with a lot of bulk and need to estimate the cold resistance... For your power level I don't know what it use - I have used this method up to about 200 watts... BTW when you said 'router' I was thinking Internet - but I know about power tools as well! :)
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Senior Member
Simple and quick: an electric heater with 1/2 the wattage rating of the motor and a relay with a 1/2 to 1 second delay.
The heater is wired in series with the motor and the delay relay's contacts bypass the heater after the delay to apply full voltage to the motor. Just remember to let the motor come up to speed before trying to cut wood...

This design is not as small as the router's soft start, but the router typically has a universal (AC/DC) motor and the soft start is just another way of using the speed control electronics (SCR/Triac) - simple circuits that do NOT work on large AC-only motors.

Or you can search the forum for the thread on variable frequency drive (VFD).



Senior Member
Maybe you could just put a Variac in line with it, and just slowly dial up the voltage. I'm pretty sure you can get variac's that high without too much difficulty (although, i'm not really sure about price!). I have a 1 KVA variac that can go from 0-120V AC.


Senior Member
We use soft start circuits in most of the high power broadcast transmitters at work.

Most of the circuits are on the 240v or 415v mains and simply use a a BIG (say 200 watt) resistor from 50 ohms to 250 ohm in parallel with a breaker (relay) on a time delay.

When the circuit first starts up the current flows through the resisitor, limiting the inrush current. Then after a time delay the breaker closes and shorts out the resisitor allowing full current to flow.

Same as what papaof2 said I guess.


Senior Member

Just learned something about "variacs". We ordered several 120V/2KW ones (China knock-offs with the Variac name on them) from MPJA, to control some heaters in a Physics research lab. Almost every time you turned one on it would trip the 20A breakers...loaded, no-load, wiper at 0V out, or any where in the range. After much head scratching and several attempts at soft-start circuits, I talked to an engineer at the real VARIAC company. He said that's likely to happen, even with theirs. Because there few turns of very large wire and have a very low resistance, so the initial current surge is very high. They will only work if we put them on branchs with higher current breakers.

With that new information, I don't think using a 2KW variac is going to help limit the current surge of the 2KW miter saw.

I learn something new every day...but I'm not always happy about it. ;)



We use the method described by Premelec on a 3-ph 10KW motor controller.
Three black discs about 1" diameter. They start off at just a few ohms and very rapidly drop to almost short when passing current. Not comfortable to hold but not scalding hot so probably about 40-50C running temperature. In our case they were fitted to prevent breaker trips rather soft start.


Wildbill, it may be a good idea to find out what type of motor your compound saw has. This may affect your choice of method for soft start. My Elektra 'David' Beckum has an inuction motor.

Michael 2727

Senior Member
I'd leave the kick there, it will remind you that
a 12", 250mm tungsten tipped blade spinning
at 4,500 RPM is a serious tool and not a toy.