yobbo II
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Motor starting
Hello,
Thanks for the replys on this question in Quick questions...
It takes a generator that is approx. 4 times the rated power of an ordinary squirrel cage 3 phase motor to start the motor. A 20Kw motor needs an
80Kw generator (approx.)
When starting a motor from a generator that is too small to supply the start up current could you connect the motor to the generator (switch the motor
on) when the generator is at (say) 30Hz, or even zero Hz. The generator will be putting out less voltage and the motor will not have to go up to full
speed (50Hz supply assume) so I hope a 4x current is not needed. It's a bit like using a variable frequency drive. The engine of the generator is
then reved up to output 50Hz at the proper output voltage. Lets assume the engine power is not a limiting factor. The current will build up much more
smoothly?
Another method I conjured up was to use a smaller motor to get the large motor spinning and then connect the big motor to the generator. I believe
this will not work as you would need the generator and spun motor (the large motor) in sync. If they were exactly out of sync. the motor would demand
a very high current (more that 4x the running current) at startup.
Another method conjured up was to have a (squirrel cage) motor running with a flywheel (or just the flywheel effect of the motor itself). This motor
and generator will be running at line frequency (full speed).
When the large motor is now switched on the generator will supply power but the running motor(s) will also supply power to get the larger motor up to
speed. (squirrel cage motors can be used as generators when there is circulating power in the system)
It's not a very practical solution but would it work?
I suppose another was of asking this question is this. If you had a workshop and there is a number of motors in the shop. You have one larger motor
(too big for the generator at startup). Would it be practical to start up this large motor LAST so that the other motors can supply some power along
with the genrator to get the large motor up and running?
Thanks for your time.
Yob
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Rainwater
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Quote: Originally posted by yobbo II  | When starting a motor from a generator that is too small to supply the start up current could you connect the motor to the generator (switch the motor
on) when the generator is at (say) 30Hz, or even zero Hz.
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When a generator is speeding up, its power output is much less than when at full rpms. This would cause excessive heat in the windings, possibly
resulting in a stall of the motor.
Assuming the generator could properly operate in this setup, the motor winding current is limited by inductance,
XL = 2πƒL
The lower the frequency, the lower the impedance , the greater the current
| Quote: | | Another method I conjured up was to use a smaller motor to get the large motor spinning and then connect the big motor to the generator.
|
This will not work with a direct couple between the motors. A gear box, with an increase in torque to drive the high powered motor up to some speed
which will reduce its starting current. After the motor is at startup speed, the "starter motor" decouples, it does not have to, but leaving the two
connected will cause problems
The current reduction is roughly directly proportional to speed. For example, with a normal rmp of 3600, FLA 20, LRA 80.
80-20 = 60 extra amps for startup
The faster you spin the motor, they less extra amps you need to reach fullspeed
Rpm = 25% 20 + 60*(1-0.25) = 65 amps
Rpm = 60% 20 + 60*(1-0.60) = 44 amps
Rpm = 95% 20 + 60*(1-0.95) = 23 amps
Motor winding parameters, star delta configurations, all effect this math, but this is good enough for rough calculations.
This is commonly used with appratus containing large flywheels. Rock crushers at the local quarrys use this type of setup.
| Quote: | | If you had a workshop and there is a number of motors in the shop. You have one larger motor (too big for the generator at startup). Would it be
practical to start up this large motor LAST so that the other motors can supply some power along with the genrator to get the large motor up and
running? |
If the motor was compatible, a soft starter might lower the startup current enough to prevent overloading the power supply. But special precautions
must be incurred to prevent damage to the starter and motor. The motor must be started with no load. Centrifugal pumps and hydraulic drive couplings
are about the only acceptable application.
For the price of all the extra equipment, it will likely be cheaper to get a new generator. A soft starter, limits the startup voltage to the motor,
thus reducing startup current at the expense of increased startup time, lower starting torque, and a massive heating of the motor windings.
Properly implemented these can extent the life of a moter greatly.
These are good units,
https://www.se.com/ww/en/product/ATS22D47Q/soft-starterats22...
$1500
Youll need the fan mount to go with that.
https://www.se.com/ww/en/product/VW3G22400/fan-option-size-a...
$170
Then a start stop station for the 12v controls.
Not sure what the PTC cost will be, they dont name what type in the datasheet. But comming from square d, it isnt gonna be cheap.
Youll need a special breaker for that rig to.
If you ever have trouble with it, youll need their software package, cant buy it, only get a yearly subscription. You can skimp that cost by sending
them a programming sheet of how you want it configured. But if you want to change anything, they got you by the ;!)) sack.
I recommend a nema 1 cabnet with dust filters. Nema 3 if its a wet location, but then you'll have to add a thermopipe to get the heat out of the box.
At 60c she will turn off. And without active cooling, it will get hot quick.
"You can't do that" - challenge accepted
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yobbo II
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Thanks for reply.
I am going to try some resistors and see how I get on.
https://industrial-electronics.com/Motor_Control_5-26.html
Yob
[Edited on 23-5-2023 by yobbo II]
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Rainwater
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Using a fixed resistor, or better stages resistors, allows you to only have set steps you can vary the impedance for a specific load.
As the load conditions change the voltage drop will vary, to much impedance added and your motor will experience oscillations in torque and speed.
To little added and the starting current will jump to high
Resulting in an overload or longer start up time and higher coil temperature.
If your going the diy route 100% then look into triac's ( link here )
This is the technology the professional soft starter is built on, and its basicly the same price compaired to using a heating element (a.k.a resistor)
By using 3 units, (1 for each leg), slapping in 2 phase shifters ( A-0, B-120, C-240 degrees apart) you can operate all 3 triac's in tandom
Large current devices are available but cost around 50-75usd a piece. Around what you would pay for quality elements. The controls will cost about
$2-$50 + an electronics education.
The design is slightly more complex, but much more versatile.
Using a triac allows you to direct control the RMS voltages applied to a load.
With a proper switching source to limit the transition state of the devices, high efficiency can be achieved, a bypass relay can be added so the
started is only loaded during startup, making the unit as ideal as possible.
Thermal monitoring of the softstarter and motor are a must, voltage and current monitoring are optional. Might total around $400 after its complete
and void every warranty you motor has. It can also kill if you touch it or short something out and it explodes.
"You can't do that" - challenge accepted
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Dr.Bob
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Power inverters are getting cheaper, so it might be doable to have a battery setup that would run an inverter to generate the surge current, then
switch over to a smaller generator. I wish more effort would be put into electric circuit panels that would be able to combine multiple sources
and multiple loads such as to allow a house or business to have the grid, a generator, solar or wind sources, battery backup, etc so that it would
know how to distribute the load based on preset priorities and load characteristics to be able to run everything as best as possible, store the excess
power when it is readily available, and then power up loads as possible, even helping to smooth out the grid loads. That would dramatically help
make the grid work.
Also, some loads are easily modified to work when power is available. Old time windmills pumped water up to a tower, where it was available 24/7,
solar could be used much like that. A/C is mostly needed when the sun is shining, so that could be powered most when solar is working. But having
a balanced system would be great. 99% of solar systems are not able to work when the grid is down, which seems silly. But there just is not a
practical system for using them yet.
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Rainwater
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Nice find, I seriously dought its legit
£180 for the 20kw version
£40 for the heatsink and fan
£220 to usd is 270.
No thermal protection.
No overload protection.
No circuit fault detection.
No bypass relay.
So the thermal protection when using a softstarter is required here, to meet NEC.
So is overload protection as these will stop you building from catching on fire.
Fault detection is required for UL listing and is very nice to have. If this setup will not be used in a professional environment then it can be
omitted.
Bypass relays just help improve efficiency because a triac cant be turned fully on, 100% there is always a diode drop, and likely a small delay in the
RC timer circuit that controls the diodes.
But all this can be fixed very easily, use something like that and add on all the bells and whistles.
Edit: i sware there was another reply, maybe the fumes is getting to me
[Edited on 26-5-2023 by Rainwater]
"You can't do that" - challenge accepted
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Texium
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Thread Moved
5-12-2023 at 13:57 |
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