Soft start for rotary phase converter
Soft start for rotary phase converter
(OP)
Is there any way to soft start a 60 HP rotary phase converter on 1ø? This is a more rural area and 3ø isn't available. The electric company here didn't really like the idea of starting a 60 HP motor without a soft start. There wouldn't be any load on the motor at startup, of course.





RE: Soft start for rotary phase converter
They solved this by wrapping a rope around the shaft. Two or three guys pulled the rope to bring the unloaded machine up in speed. They then made sure that the rope was clear from the shaft before they hit the ON switch. Worked quite well.
But that was in rural France. In the sixties...
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Soft start for rotary phase converter
exceed most 200A service capabilities, even with a slow ramp
on a VFD. Probably the most practical would be a 5-10 hp
pony motor to get it up to speed before applying line voltage.
RE: Soft start for rotary phase converter
To directly answer your question, no. It isn't a good idea to try to start a 1 phase motor with a soft starter. There are small 1 phase soft starters on the market, but they are restricted for use with PSC type or shaded pole motors. Cap start or split phase motors with a starting switch get in trouble with soft starting, plus the caps wreak havoc on the SCRs in the soft starter.
But you should probably contact this outfit if your motor is that big.
http:/
RE: Soft start for rotary phase converter
RE: Soft start for rotary phase converter
RE: Soft start for rotary phase converter
You'll be the only one with power when the "lights go out".
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Soft start for rotary phase converter
Question Jraef and/or Skogs; Can a very small VFD be used to ramp up a fairly large motor using current limit? When the motor is up to speed unloaded, the VFD would be disconnected and the motor would be connected to the single phase line. It would save the price of a pony motor. This would be the twenty-first century equivalent of the "Rope on the shaft" trick.
respectfully
RE: Soft start for rotary phase converter
I did just that about six months ago. We used an 800 kW inverter to speed up a 6 MW motor and then transfer to the 11 kV bus.
It worked there. So it should certainly work in this less demanding application. Scaled down, of course.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Soft start for rotary phase converter
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Soft start for rotary phase converter
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Soft start for rotary phase converter
Skogsgurra, you say you had a device to get the proper speed and phase angle before switching directly to the line?
RE: Soft start for rotary phase converter
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Soft start for rotary phase converter
RE: Soft start for rotary phase converter
Thanks Gunnar. Now I seem to remember your comments about the large motor on the inverter. I hope that I didn't plagerize you. If so, I swear that it wasn't intentional.
Jraef: With your hands on knowledge of currently available VFDs, can you make any comments on the suitability of "off the shelf" VFDs to slowly accelerate an unloaded motor 5, 10, or 15 times their rating? Will it be an issue to configure an off the shelf VFD for this service? Recommendations?
Thanks Jraef.
Respectfully
RE: Soft start for rotary phase converter
I've never done anything greater than 5x the rating (a 10HP VFD on a 50HP motor) and it took about 3 minutes to accelerate it that way. All it had on it was a clutch assembly that eventually would couple in to a centrifuge. That centrifuge motor was not standard however, they tend to have more rotor than normal because of the expected heat they expect starting X-Line, so it may have been able to accelerate faster or with a smaller VFD if it had been std.
I have heard of someone using a 25HP VFD to start a 200HP pump motor, but I didn't witness it and the guy was kind of a braggart so I didn't put much stock in it. If you think about it, at some point you are going to be at full speed and that means full voltage from the VFD. So the VFD output will need to be able to deliver the full UNLOADED current of the motor, i.e. the magnetization current, with iron losses, windage, friction etc. We know that some unloaded induction motors can draw as little as 20% FLA, so that shows you technically will be able to get it to full speed with a 5:1 ratio in those cases, but 10:1 or 15:1? At 15:1 that means the unloaded current can be no more than 6.67% FLA. I have never seen a motor draw that little even uncoupled.
RE: Soft start for rotary phase converter
I now know enough to feel like I could pull something like this off.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Soft start for rotary phase converter
What I did was to set up the drive for 100% current limit, even though technically the VFD was capable of more. Most drives are capable of 150% for 1 minute, but they don't necessarily have a way to engage the current limit down below that 150% after a specific time, so if you exceed that 1 minute rating it trips off (some of the newer more sophisticated drives can now do this however). In the drive I used, I turned on the stall prevention feature as well, which overrides the acceleration ramp time in order to allow the motor to continue to accelerate while under current limit. Without that, the accel ramp might continue to push the output frequency gain while the current limit keeps the voltage down, meaning that you start to saturate the motor. Stall prevention will detect that disparity and keep a constant V/Hz ratio, inhibiting the frequency gain instead.
RE: Soft start for rotary phase converter
The VFD may accelerate the motor to a point and stop accelerating due to current limiting on the mag current. If this happens, you will need to drop the V/Hz ratio, i.e set the rated motor voltage lower than line voltage. This will reduce the torque capacity, so there is a limit.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Soft start for rotary phase converter
I guess the consensus is that the VFD must be able to deliver the magnetizing current demanded by the motor.
Off the wall, What happens if we set the volts per HZ ratio so that the Voltage is 50% or less at rated frequency? Will this let us fudge on the magnetizing current?
As I understand motor starting, we must supply the no-load losses (mainly windage on an unloaded motor), and the magnetizing current.
Would this technique let us get the size of the VFD below the 20% limit or is it time for me to go off-line and get a coffee infusion?
Thanks to Skogs, Jraef and Mark.
RESPECTFULLY
RE: Soft start for rotary phase converter
The classic motor starting theory does not apply in this situation as you never actually "start" the motor, you run it from zero Hz. By that I mean that it never operates under high slip conditions so we can ignore all that theory.
Reducing the V/Hz will reduce the voltage at any given frequency. This will reduce the magnetizing current and the flux in the iron. The net result is that the maximum torque output from the motor shaft will be reduced. This could be a problem in getting the motor rotating in the first place.
You would need to use a VFD in an open loop V/Hz mode. If you ran it in sensorless vector mode, it would probably get totally confused as the measured characteristics would be well outside of the acceptable values for that size of drive.
Provided that less than 20% torque was sufficient to accelerate the motor and load, you should be able to use a drive rated at less than 20% of the motor rating.
Best regards,
Mark Empson
http://www.lmphotonics.com