Motor modification for VFD

Although I like to think myself to be reasonably knowledgable I need a little advice from the experts. Having been gifted a 3/4HP 1425rpm 3 phase motor I have decided to upgrade my Super 7 to a VFD drive. The motor in question is a Brooks Gryphon rated a 0.75HP and, according to the data plate, draws 1.27A/phase under full load. Wired as built in a star configuration the motor was successfully tested for electrical safety and winding continuity before being tried under no load on a nominal 415V three phase supply. This all seemed satisfactory.

The motor has now been modified to a delta configuration taking care that the phase relationship between winding is retained. A DC resistance test results in all three terminals showing equal resistance between each pair.

Although the motor runs when connected to a 0.55kVA Yaskawa VS mini J7 inverter it trips out with an overload alarm after a few minutes running. It informs me the motor is pulling about 5A which seems exceptionally high.

Any thoughts gentlemen?

What voltage is the invertor set to supply ?

I've tried various voltages between 200v and 120v.

Yes I appreciate that but I wouldn't have expected it to rise by the amount it has done, hence the query. I should add the motor isn't under any load.

Edited By Brian Oldford on 20/03/2017 19:31:08

There was a similar query a couple of weeks back, which eventually was put to bed on the basis that the motor wasn't suitable for VFD drive, and the high frequency component of the VFD drive output was generating circulating currents in the motor. Sounds rather as if this might be another one.

1.27A x 415V x 3 = 1,581 Volt Amps

5A x 200V = 1,000 Volt Amps.

Bear in mind the inverter is measuring total current, not current per phase.

Neil

I cannot claim extensive experience with motors and VFDs but I have wired up and configured about ten different setups mostly with very old motors and secondhand VFDs. In every case the motors and drives have stayed cool, run smoothly, and been trouble free.

I'm not sure what you mean when you say you have 'maintained the same phase relationship' as that is actually determined by the mains supply. Has the motor got six separate wires/terminals? and if so are the start and end of each winding wired correctly?

If the motor has tested OK on 415V then it should work as well on delta.

Ian P

Thanks for that. It still makes for what seems a very inefficient 3/4HP motor.

If you rewire the motor for 220V it will draw twice as much current as 440V so 5-6A is not excessive. Reset the inverter current limit and try again. Run without load and check the motor temperature, it should be warm, running under maximum load may make the motor too hot to hold comfortably but the paint shouldn't melt.

Where does the 200 Volt figure come from, it seems a low value?

Ian P

Thanks for the input. By the general look of it this is a fairly old motor. The VFD is a new one.

By phase relationship I mean the star/neutral point had to be accessed by dismantling the motor and some of the twine lacing undone to find and unsolder the star/neutral point. To create the delta configuration each "inner" end was joined to an adjacent "outer" end to complete the triangle of windings to be phased correctly IYKWIM.

Edited By Brian Oldford on 20/03/2017 20:42:43

Does the motor plate have a current rating at 240V?

055kw should be OK for a modern 3/4HP motor designed for VFD operation, but may be a bit tight

The Inverter has a max continuous current output of 3A at 240V for the 0.55kw unit so not surprised it is tripping if it is drawing 5A

Nothing for 240V on the plate. It was "hard-wired" for 415V.

Hmmm. Now you've set me thinking. Having an older (maybe less efficient) motor maybe I should have gone for a little more slack in the electrical ratings department. Perhaps 0.75kVA. may have been a better choice.

Brian

I see that the inverter allows you to set its 3 phase output voltage. Its not a feature I have come across before as I would just have expected the inverter generated three phases to be created from the VFD 'DC bus' so is a fixed value somewhere near 240V.

I hard to know where the current and voltage measurements mentioned in the various replies are derived from,, (whether total drawn from the mains, or individual phase output currents etc)

Ian P

Was the motor on load? If unloaded the motor will not draw anything like its rated current - if it did where is all the power going? If it's tripping and drawing 5A but not driving anything there is something very wrong with it.

You can't multiply volts by amps to get power with an inductive load. You have to multiply VA by a power factor to get power. This is because the voltage and current are not in phase with each other.

For 415V 1.27 seems very high per phase or low if for all three phases. I found a plate for a 0.5hp BG on line and it was 3 phases, 1A total, which would suggest 1.5A for a 0.75hp motor.

Anyway, 0.75 hp = 560 watts and your inverter is 0.550 kVA.

A bit of googling shows that this is done 'to provide a more meaningful figure for inverter output'. I wonder if it is done because it makes the inverter sound more powerful than it really is - I doubt your 0.55 kVA inverter can put out anywhere near the 0.56 kW your motor needs.

Another source says kVA rating is then demand on the supply, not the drive, which should be specified in kW. They state that a 700 kVA inverter is only good for motors up to 500W so it looks like you need a 800 kVA inverter for your motor.

Sorry for the bad news. I can see this kVA <> kW catching out a lot of people.

Neil

What's with this kVA?

I have fitted literally 100's of inverters when we used to build inkmixing machine from IMO, Fuji, Omron, Yaskawa, Telemechanique etc and never come across this before.

So interested in why I went and looked at a few still in stock which included a couple of J7 mini's and all was made clear.

The Input line lists the input in voltage, Hertz and input amps.

The output line list the output in voltage, phase, hertz, amperage and kVA

Seeing as no motor I have seen list the spec in kVA only amps why would anyone want to read the spec in kVA when the direct comparison in AMPS is on the plate ???

One reason why I had never seen it is we matched amps for amps.

Hi Brian. I have the 1.1KW version of the Inverter you are trying out. I use it with a 1KW 3phase motor from an old multi-platter disc drive - those things that stored 100Mbytes on 10 platters...

This is all fitted to my Maximat V10P lathe, and when I select the AMPS display on the Digital Operator ( front panel thingy) it shows around 1.6amp when the lathe is spinning the chuck at 3000RPM, and around 3 amps @ 1500 RPM taking a nice deep cut in BMS with carbide tip tooling. Neither the invertor nor the motor are beyond warm to the touch, even after a morning's work.

Some questions -

Did you buy the invertor new? If not, have you verified the default settings for the V/f curve? ( page 37 to 39 in the user manual, if you have the same small manual I have) . If the Torque boost curve is set to give a higher voltage than normal at 50/60Hz, then amps will of course be greater. You said you tried voltages between 120V and 200V? Can you explain how you set up the constants to achieve this?( I presume you did this using the V/f curve setup)

How did you determine the current was '5 amps' ? By selecting the AMPS display via the pushbuttons on the digital operator?

Did the Invertor display a fault code ( oL1, oL2, or oL3). If so, was the displayed code blinking?

What are the operator constants 60,61 and 65 set to? ( See page 51 for reference-Overtorque detection)

Did you measure the amps drawn when connected to 415V in STAR configuration ( A 'Tong-Tester' - an Electrician will have one..) Measure each phase..Can you supply the motor from Utility 3phase DELTA and measure the amps draw from plain 50Hz AC? Again, measure each phase. This will right away indicate if the motor has an internal wiring problem, or the DELTA has not been phased correctly, and will aid in putting to bed issues such as " and the high frequency component of the VFD drive output was generating circulating currents in the motor"

I have NEVER had a problem feeding any old 3phase motors from many varieties of the Yaskawa drives, from 0.55KW up to 10KW, the lower ranges 200V AC single phase inputs, the higher ones 3phase input. I have implemented maybe around 40 such drives on all sorts of machines , some of them more than 40 years old! I have yet to find a 3phase (working..) motor that misbehaves on these drives. Never suffered from 'high frequency circulating currents' either.

The switching frequency of the drives are in the 5KHz to 15KHz range ( typically) and the inductance of the motor windings is SO high relative to this frequency that Di/Dt simply creates an RMS 'average' of the current required to flow. There is NO magnetic field in the rotor or stator plates varying at the switching frequency AT ALL...The only AC magnetic field is the RMS value resulting from the PWM voltage.

Let me have some answers Brian, and I will see if I can help..

Joe

+1 Older motors may have trouble with a VFD. The other thread was this one and the symptoms are similar.

Dave