How to Reduce 3 Phase Voltage
How to Reduce 3 Phase Voltage
(OP)
I need to run some tests on my mechanical system which is powered by the star connected 208VAC 400Hz motor that I asked about in an earlier thread (thanks for help). As I don't have access to 400Hz power for these tests I would like to run the motor on 60Hz 3Phase. According to the FAQs, I should use about 30VAC on each leg.
Is there an easy way to reduce 220VAC 3Ph to 30VAC 3PH? As the real load will not be present, I don't need full speed or power from the motor for these tests. Or, is there a way to obtain 400Hz by using a modified alternator? Either way would be acceptable.
Is there an easy way to reduce 220VAC 3Ph to 30VAC 3PH? As the real load will not be present, I don't need full speed or power from the motor for these tests. Or, is there a way to obtain 400Hz by using a modified alternator? Either way would be acceptable.





RE: How to Reduce 3 Phase Voltage
That's simple. Many VFDs can be programmed for up to 400Hz output, that would be the easiest solution. A VFD takes raw power from a source, such as 208V 60Hz and converts it to DC. Then it inverts the DC back into whatever AC output you need up to and including the line voltage (it can't create voltage from nothing), so in your case 208V 400Hz. If the motor is completely unloaded, expect to need a VFD rated for at least 20% of the motor FLA, maybe a little more if you can afford it.
The alternative would be to find a 3 phase transformer with a suitable reduction ratio. 208-30V is roughly 7:1. The only standard transformer ratio that could work would be a 4160V to 600V, but I seriously doubt you will find one small enough to be affordable for a test. You could also try to find a 3 phase variac, a variable output transformer often used in test benches, but it will likely cost you more than a VFD. There are people that will make whatever transformer ratio you want, but that would IMHO be more troublesome than finding a VFD and actually testing it at 400Hz.
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RE: How to Reduce 3 Phase Voltage
jraef can you suggest a VFD that makes 400Hz? I can't think of one.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: How to Reduce 3 Phase Voltage
RE: How to Reduce 3 Phase Voltage
Schneider ATV71 / Toshiba AS1
ABB ACS550, ACS800
Siemens Sinamics
Even the lowly Teco FM-100 / Motortronics CSD drive will do that.
Need more?
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RE: How to Reduce 3 Phase Voltage
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: How to Reduce 3 Phase Voltage
RE: How to Reduce 3 Phase Voltage
If you can find two single phase transformers rated 240:48 volts, you may connect them for 35 volts, three phase, suitable for motor service. If the 48 volt winding is on the outside of the 240 volt winding , you may be able to peel off a couple of turns to get the voltage exactly right.
To determine the KVA rating, the 48 volt winding should be rated for about 125% of the motors anticipated current at 30 volts. If in doubt, go to the next larger standard transformer size.
Treat it with the same caution as you would full voltage, because one phase will be at full voltage.
If you want to try 24 volts, you can use two 120 volt to 24 volt single phase transformers. Connect the primaries to two legs of 120 volts and reverse one of the secondaries. You will get true open delta three phase at 24 volts. The 24 volt windings will each carry the actual motor current.
respectfully.
RE: How to Reduce 3 Phase Voltage
If anyone sees a problem with this unit, please don't hesitate to comment.
Regards
RE: How to Reduce 3 Phase Voltage
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RE: How to Reduce 3 Phase Voltage
Thanks to everyone who helped with my questions and made good suggestions. For $150, it would be hard to find a simplier or more cost effective solution.
RE: How to Reduce 3 Phase Voltage
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RE: How to Reduce 3 Phase Voltage
Just another point you should be careful of. Motors that are designed to run at high speeds/frequencies typically have a very low inertia fan connected. Unless you have a force ventilated fan, the efficiency of the fan will be very poor at slower speeds and you may run into cooling issues with the motor.
We used to supply VFDs to wood routing spindle that ran at 400Hz (typically 24,000rpm) and whilst they ran fine at rated speed, dropping it any lower than half would run into problems.
RE: How to Reduce 3 Phase Voltage
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: How to Reduce 3 Phase Voltage
RE: How to Reduce 3 Phase Voltage
During the tests I have been running, I notice that the motor housing begins to warm to the touch after about 1 minute but this is with no appreciable load. I will be interested to see the temperature rise with the working load applied. I have attached a range of temp tabs to the housing for reference and I plan to use those during the loaded tests.
Again, thanks to everyone for all the good advice and comments. Without your inputs, I wouldn't have been able to do any of the preliminary tests. It was fortunate that I have been able to run the motor at full speed as several minor problems arose (and were easily rectified) that wouldn't have shown up until the real runs. One was a gearbox oil leak out a vent which wouldn't have looked good to the customer.
RE: How to Reduce 3 Phase Voltage
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RE: How to Reduce 3 Phase Voltage
Does anybody have a source for something like this? I have about a 1.3 KW power requirement.
RE: How to Reduce 3 Phase Voltage
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: How to Reduce 3 Phase Voltage
You might first consider trying an off-the-shelf VFD anyway though. If it is a pure Diode front end it may work just fine. 400Hz is actually used partly because it rectifies to DC much better than lower frequencies, less ripple. You just need to be careful because some VFD mfrs use SCR front ends in order to avoid a pre-charge resistor and the SCR gating circuit would be line frequency dependant.
I worked with Yaskawa years ago on trying to get them to modify a VFD for an on-board application at Boeing, but I changed jobs in mid stream so I don't know if anything ever came of it. The only real tricky part though was actually just the control power within the VFD. At that time Yaskawa was using AC transformed off of the line input for control power feeding a separate SMPS. Now a lot of drive mfrs just tap off of the filtered side of the DC bus with a DC/DC converter for control power.
So a VFD with a passive diode front end converter and a DC bus control power source would probably work. If the VFD mfr still has a problem with that, get or build your own 400Hz rectifier and just feed DC into the drive. A lot of them provide terminals on the DC bus for just such an application.
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RE: How to Reduce 3 Phase Voltage
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: How to Reduce 3 Phase Voltage
Standard rectifier diodes should be fine at 400Hz. If this was a very large drive I would be more concerned with the rectifier's behaviour because very large semiconductors are s-l-o-w, but not at the kind of power rating you would supply at 208V. Avoid using a drive with an active (power factor corrected) front end. The controller IC used in PF-corrected drives might be capable of operating at 400Hz but probably not without component changes.
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Sometimes I only open my mouth to swap feet...
RE: How to Reduce 3 Phase Voltage
RE: How to Reduce 3 Phase Voltage
As I learned it, the Ripple Current rating in a capacitor is based on the power generated in it (heat loss), calculated
Pgen = Irms2 x ESR where:
Pgen is the Power generated
Irms is the magnitude of the current ripple, and
ESR is the Equivalent Series Resistance of the cap.
ESR goes down as frequency increases and ripple rms current goes down as frequency increases into a bridge rectifier, so essentially if a capacitor is designed for a lower frequency it will work fine at a higher one. The problem as I see it would be using a cap designed for a 400Hz application in a 50/60Hz supply. There are lots of other factors, but this image sums it up nicely.
Image taken from this paper: "Deriving Life Multipliers for Electrolytic Capacitors: A Brief Explanation" which can be downloaded in .pdf from the following website (direct link to paper is disabled). http://www.cornell-dubilier.com/
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RE: How to Reduce 3 Phase Voltage
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: How to Reduce 3 Phase Voltage
Good link - thanks for the reminder. Looks like gut feeling was right!
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Sometimes I only open my mouth to swap feet...
RE: How to Reduce 3 Phase Voltage
The data that was posted from Cornell-Dubilier indicated to me that I probably wasn't going to see any problems.
Thanks again to all who replied.
Gary Knutson
GLK Design