How to Reduce 3 Phase Voltage 3

[gary10]

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.

 

[jraef]

gary10,
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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[itsmoked]

http://surplussales.com/Variacs/Variacs-4.html

Although a VFD would be more fun.

jraef can you suggest a VFD that makes 400Hz? I can't think of one.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[denison]

Omran has one, will go to 600 hz I think. I'll have to get the model # and stuff tomorrow if you're intrested.

 

[jraef]

Yaskawa G7 (that's what the Omron would actually be)
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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[itsmoked]

No sir! And thanks you two.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[aolalde]

The test at 60 Hz will drop the motor speed around 1/7 of the nominal at 400 HZ. May be you will need the 400HZ VFD recommended by Jraef for an effective mechanical test.

 

[waross]

If you go with transformers, I have had excellent results using two auto transformers in open delta for motor loads.
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.

 

[gary10]

Thanks to all for your comments and suggestions. As this device contains both a planetary and a bevel gearbox I would like to test at full rpm (10,000 motor). Therefore, I will go the VFD route. I have chosen a Automation Direct GS1-22PO unit which is reasonably priced and should do the job.

If anyone sees a problem with this unit, please don't hesitate to comment.

Regards

 

[jraef]

Just this. Try to get help on their toll free support line. Hint, allow PLENTY of time to wait on the phone. AutomationDirect was selling Hitachi drives exclusively for a long time and I was never that impressed from a programming and reliability standpoint, but this appears to be something else made in Taiwan, so I have had no exposure to it. If what you want is an inexpensive VFD that can deliver 400Hz, it looks like this one will do the job. That is an exceptionally low price for a 2HP 230V drive.

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[gary10]

Just a quick update on my situation. I purchased an Automation Direct GS1-22PO. I received the unit in two days as promised. The documentation must be good as I could understand it! So far the unit has worked perfectly and is easy to program from a laptop if needed. The ability to ramp up to full rpm (10,000) is also handy for my test initial purposes. After that, it is simple to go to the "quick start" mode. AD's technical suport quickly answered a question and I wasn't on the phone very long. As I have only a short amount of time using the drive, I can't say anything about the reliability.

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.

 

[jraef]

Sounds like you got a good deal so far.

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[ozmosis]

gary10
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.

 

[itsmoked]

Thanks for the feedback Gary!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Just FYI, I was looking at Delta Drive's website today and discovered that the AutomationDirect GPS-1 is in fact the Delta VFD-L Series drive. I had some serious QC issues with Delta a few years ago, let's hope they have improved.

 

[gary10]

With respect the the potential overheating problem I should mention that this motor has no fan and is completely sealed. The reason this isn't a problem is the the duty cycle is about 30 sec on and then a long period (hours) of inactivity followed by another 30 sec period of operation.

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.

 

[jraef]

Just so you know, in motors designed without cooling fans for short duty cycles such as that, the cooling is designed to take place out of the case itself for the stator, and out of the shaft for the rotor. Make sure that in your mechanical design you are not impeding that, i.e. a sheet metal box around the motor or the shaft coupled too close to the load. Hopefully it's not a flange mount to that gearbox!

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[gary10]

I now have a new problem. The vfd/motor combo works so well that we would like to use something similar in the final design. So far, I have been unable to find a vfd that will run with a 400 Hz input (208 VAC 3 Ph). 50-60 Hz is all I can come up with at this time.

Does anybody have a source for something like this? I have about a 1.3 KW power requirement.

 

[itsmoked]

Well if you can't find one.. (jraef may know of one) I'd unhook it from the motor and just try it. Measure the DC bus voltage. If it's happy and the display works you'd be in luck. Otherwise you may be able to rectify the 400Hz source yourself and then feed that to the VFD's DC bus.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

400Hz puts you into the military and aircraft industry. Be prepared for shelling out the big bucks!

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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