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nelsonhoover (Electrical) (OP)
14 Aug 07 10:39
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.
Helpful Member!  Skogsgurra (Electrical)
14 Aug 07 10:44
I visited a very rural place in southern France once. They had a single phase motor that was incredibly hard on the very weak mains.

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
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

Helpful Member!  fsmyth (Electrical)
14 Aug 07 10:51
Well, at over 150A run current, the start current will well
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.
Helpful Member!  jraef (Electrical)
14 Aug 07 13:21
You have a 60HP 1 phase motor? Wow.

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.
nelsonhoover (Electrical) (OP)
14 Aug 07 16:09
No, it's a 3ø motor being used as a rotary phase converter to generate 3ø. It is powered off 1ø using a stack of start capacitors to get it going. The final output is further tweaked with some run capacitors.
jraef (Electrical)
14 Aug 07 19:22
Well, from an electrical standpoint, ALL of the power still needs to come from the 1 phase source, so unless you plan on seriously derating that motor it is going to be drawing the 3 phase current, times 1.732. So assuming you have a 240V 1 phase supply, the 1 phase current will be 154 x 1.732 = 266A! So even with a soft starter, your current will get to 200% minimum even if the motor is unloaded (maybe more), so now you are looking at a starting current of no less than 532A. That's a lot to ask of a single phase source.

itsmoked (Electrical)
14 Aug 07 21:52
  Maybe its time to seriously think about a propane fueled generator?  Get a WhisperGen or something similar.  You can then jettison all the phase converting and loss of HP the supplied motor will experience.  You will also avoid all the potential problems you cause the neighbors when you press the start button on your 1ph Frankenstein.

You'll be the only one with power when the "lights go out".

Keith Cress
Flamin Systems, Inc.-

Helpful Member!  waross (Electrical)
14 Aug 07 23:57
I  like the first suggestion. Spin the rotary converter up to speed with a small motor. You may consider a small three phase motor and a VFD capable of running on single phase.
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.
Skogsgurra (Electrical)
15 Aug 07 2:02

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
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

itsmoked (Electrical)
15 Aug 07 2:47
How do you deal with the bad phase match on the open transition?  You wait some time before the second closure?

Keith Cress
Flamin Systems, Inc.-

Skogsgurra (Electrical)
15 Aug 07 4:32
Yes. We took it a bit above sync speed. Then coasted down to right speed and phase angle before closing. There was a device that took care of that part.

Gunnar Englund
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

nelsonhoover (Electrical) (OP)
15 Aug 07 11:10
Thanks, I think the idea of using a VFD to get it up to speed is probably the best idea.

Skogsgurra, you say you had a device to get the proper speed and phase angle before switching directly to the line?
Skogsgurra (Electrical)
15 Aug 07 12:15
Yes. It was an old ABB device. But that is only needed if your motor is a sychronuous one. An induction motor does not need to be exactly the right speed. And the phase angle does not exist until connected (no flux in rotor).

Gunnar Englund
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

nelsonhoover (Electrical) (OP)
15 Aug 07 16:15
Yes, this would be an induction motor. Thanks for the help.
waross (Electrical)
15 Aug 07 22:16
The capacitors must be out of the circuit when accelerating on the VFD.
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.
jraef (Electrical)
16 Aug 07 1:16
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.
itsmoked (Electrical)
16 Aug 07 3:37
Succinctly clarified Jeff.

I now know enough to feel like I could pull something like this off.

Keith Cress
Flamin Systems, Inc.-

jraef (Electrical)
16 Aug 07 20:05
Sorry, never addressed the configuration issue.

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.
Helpful Member!  Marke (Electrical)
16 Aug 07 21:08
There can be an issue if you have a current limit set below the magnetizing current of the motor.
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

waross (Electrical)
16 Aug 07 22:46
I'm always pleased to benefit from your knowledge Marke and always happy to see you visit the forum. Please come more often.
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.
Marke (Electrical)
16 Aug 07 23:18
Hello Waross

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

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