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UPS+VFD+Positive Displacement Pump


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I am intertested in knowing of anyone's experience with the following VFD application:


480VAC PWM UPS connected to a Constant Torque VFD starting and running at 60hZ a 25HP 460VAC motor driving a positive displacement hydraulic pump.


Speed control once started is not required.



What are the issues or comments related to:


1. The interface between the UPS with a filtered PWM output and the VFD.


2. Using the constant torque VFD as a soft start then running at base speed to limit the inrush and size of the UPS.


3. Max. Motor lead length with the VFD output filteres and un-filtered.


Great forum!




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Hello dithomas

Welcome to the forum.


There should be no dificulty running the VFD off the output of the UPS provided that the UPS has sinusoidal filtering on its output. - most do.

Your proposal is one of the best ways to get the motor up top speed with minimal current draw, however the loading at full speed is higher than with a DOL or soft start due to the harmonics of the input stage of the VSD plus the losses in the VSD.

The current drawn by the input of the VSD is not sinusoidal so although the cos thi may be "better than 0.95" the distortion power factor is not.

I would suggest that the UPS may need to be oversized by as much as 30% to cope with the current waveform. You would need to negotiate with the UPS supplier about that.


The maximum motor lead length is a function of the output circuitry of the VSD and the capacitance of the cable used. The VSD manufacturer can best advise you on this, but 100M is a typical length for a screened cable.


Best regards,

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I have done it for Air Conditioning equipment going into remote traffic control radar sites. They wanted to give the operators 1/2 hour to get to the site with a portable generator, so the UPS is used to keep the AC compressors running. The VFD was used strictly as a soft starter. We had to use a 25kVA UPS to run 10HP AC units. Later they changed the desin to have 2 smaller AC units (7.5HP each) and we could do those with soft starters instead of VFDs because they never had to start both at the same time, giving the UPS enough overload capacity to start the 2nd unit if the first one was already running.
"He's not dead, he's just pinin' for the fjords!"
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Thank you marke and jraef for the excellent replies.


The UPS manufacture and I have been discussing using either a 30 or 40 KVA UPS for the 25HP motor.


I am considering using a bypass contactor that would close once the motor is up to speed and then shut the VFD off in order to relieve the UPS of the VFD non sinusoidal current input while the motor is running at full speed.


Any comment?


I would rather not have to use screened motor leads. I have found a 2 - 8 KHz motor filter that would filter the VFD output.


Any Comment?


Are there any special considerations when performing the voltage drop calculation for VFD motor leads?




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Hello Dan


Unfortunately, you can not just bypass a motor connceted to a vfd in the same way that you can bypass a soft starter. The soft starter has SCRs in series with the supply to the motor and a bypass contactor just shorts these out. If you try the same with a VSD, there will be a big bang!!


With some VSDs, it is OK to have a contactor on the output that you can open and then close a separate contactor to the supply. You must ensure that the VSD is suitable for this type of operation. Most are not and will suffer damage from any switching on the output.

This type of switching is open transition and will result in a high current transient when the motor is "reconnected" to the supply. This could cause a problem with the UPS.


I would urge caution about this approach, it can be done, but must be done with care and in consultation with the VSD manufacturer and the UPS manufacturer.


Best regards,

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Just FYI, Robicon has an option for what they call "synchrouous transfer" of a VFD at full speed to a contactor. I do not know the details of how they accomplish this without damage to the transistors, but they are very successful with it. They are the only ones that I know of that offer it, although I am not sure of their ability to offer it in that small of a VFD. Their small LV VFD's are no longer made by Robicon, they come from the old Ansado group in Italy, who was bought out by Robicon a few years ago. If a soft starter will not work for this, you might try iving them a call to check it out.
"He's not dead, he's just pinin' for the fjords!"
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Hello all,


I am sure I have seen LV drives that provide a synchronising facility to simplify bypass. Certain product lines within ABB & Danfoss groups tend to ring some bells but I cannot confirm at this stage. Will certainly research and post the names and product codes once I have them.




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