Replacing Star Delta with Inverter (3 phase)

[flyers]

Hi,

We have a project to replace one Star Delta connection for 90kw motor (3 phase, 415V) with an Inverter. Before dismantle the S/D, the current before the contactor is around 80Amp, after replacing with an Inverter, we measure the same incoming cable (before Inverter), the current is only 18Amp. We measure the cable after the Inverter, the current is 68Amp, is that possible that the incoming current (before Inverter) is so low?

The speed is the same (full).

Thank you.

 

[jraef]

What are you using to measure current? Cheap meters do not do well with the harmonic rich current signature of a VFD, my general rule is that if you paid less than $3,000 US for the meter (do your own conversion), then the readings will be meaningless.

 

[T Gibbs]

Your VFD (inverter) will give you the output amps on the display.

 

[Jeff23spl]

yes a 1-200$ clamp is useless with a VFD unless it is an analogue one slow enought to read average.
VFD display will give it but keep in mind when the motor is unloaded or at low speed, the incomming power is it still at full voltage (W=VxI) when the output could be at a lower voltage by VFD ramp. more amps with less voltage would equal more voltage with less amps. Power factor is also something to look into, unloaded motor will have a very bad PF (inductive current read goes in and out without being all used) (VA) but the capacitor inside the VFD should take care of a good part of this and releasing only the real energy demand from the power source(watts)

I'M just not sure by how much the VFD will correct power factor and it may change from brand to brand...someone could confirm ?

 

[James Mcquade]

i would suggest to that you investigate what the star delta motor does. by that i mean what the star configuration does and then the delta does to the motor.

it has been a long time since i worked with star delta, but it would be worth your time to make sure the drive can put out the power requirements for the motor based on the star configuration and the delta.

it beats having to put it back in and explain what happened.
i will admit that there are many others woh can answer the question better than i can, but it would be a good learning experience.

regards,
james

 

[T Gibbs]

When changing from a star/delta to a VFD, I've always just hooked the motor up delta as if it was a standard 9 lead motor. The star part is just for starting with less inrush than across the line. The VFD accomplishes that task with the ramp feature by controlling the frequency.

 

[flyers]

Hi guys,

Thanks for the replies. We are measuring the current using 2x clamp meter & Siemens power meter as well, both are shows the low reading at the Inverter input power cable. I admit there are not expensive clamp meters.

We put 1x clamp meter at incoming (reading: 18Amp) and 1x at outgoing (reading 68Amp), the Inverter outgoing display also showing 68Amp.

Can we say that incoming shall be 68Amp or slightly lesser maybe ~60Amp, and impossible to be as low as 18Amp. Assuming the Inverter display (68Amp outgoing) is correct.

Correct me if i am wrong.


The customer does not want to use the initial S-D connection (the previous S-D is to reduce the inrush current during start up) together with the Inverter. They want to replace the entire S-D with Inverter, to minimize their spare parts and reduce the large space consumed. All the new Inverter will be located in an air conditioner room instead of putting the panel besides the machine like the initial S-D design (due to dusty environment).

 

[DickDV]

Be sure the motor is wired in delta connection. Read output amps from drive keypad. You cannot spend enough money to buy an ammeter that will read reliably on the VFD-motor leads.

At full speed, the input current should be pretty close to output current. Something is wrong with the input current readings, I would say.

Does the motor have enough load on it to match the 68amp current?

 

[maxketcham]

a good fluke or amprobe true rms amp clamp will read your output currents to the motor without any problems. reading output voltage now is a different story. the motor will act as an inductor to the pwm and filter it to a readable rms value. it sounds as though you are reading the current of the motor connected in delta. you have to remember an inverter will convert the incoming 3 phase to DC before it changes the dc back to ac. so the input currents and output currents can be very different depending on the load of the motor. as the motor loads the incoming currents will increase. the thing you want to check right now is what is the frequency and what is the output voltage that the drive is running the motor at. It sounds like you are running the motor at less than 60 hz and 415 volts. remember that the wattage in will always equal the wattage out (with a little conversion and control power loss)so make sure your drive is actually running the motor at rated speed and voltage. the base voltage should be whatever you have in yur country 50/60 hz and whatever the motor name plate says.

 

[maxketcham]

the fact that you are reading 68 amps on a 90 KW motor is the clue that you are running the motor unloaded or at least partially loaded as the nameplate currents for a motor this size at 514 Vac should be approximately 156 amps at full load. Are you sure you have the motor connected in a delta configuration? you do not want to run it in star (or wye) with this probably being an older wye-delta motor you also do not want to run it at anything other than base speed. it will not have sufficient cooling at a lower speed to keep it from burning up. If you want to run it at any speed besides base speed make sure you contact the manufacturer and find out if it is capable of doing what you want. they may suggest you do not run it at anything besides base speed or they may offer an upgraded cooling kit to allow the lower speeds. My concern right now is that either you have connected the motor in wye or you are trying to run it unloaded at less than base speed. you need to check that the drive is ramping all the way up to line frequency and voltage at the output of the drive.

 

[DickDV]

There is no reason to believe that this motor cannot be slowed down without overheating. I would assume at least 3/1 with this size IEC motor. And, while it is possible that the motor base speed is 415V @ 60Hz it is more likely to be @50Hz. Check the motor nameplate carefully.

Finally, you can trust your "true RMS reading" meter if you wish but prepare to be mislead at least part of the time no matter how much you spent for it. And your meter better be rated for 1800V because that is the voltage that is possible on a 480V VFD system. That's why MG1 Part 31 Inverter Duty Motors are now tested at 1800V. Why not use the VFD display? It's more accurate and way safer!

 

[Jeff23spl]

even Fluke say that a good ampprobe is not accurate and designed a special tool based on a scope design for that and being costly. But certainly yes, a good amp probe can give an average with a 10-15% margin error to compare 3 lines or to know if it run or not but for accuracy, it fail.

A good point about 68amps on a 415v 90kw motor mean it isn't loaded at full speed and in that condition, comparing input with output is difficult because for sure the output voltage is lower and output PF also.

If the amps read would have been reliable and motor is wired delta correctly
18amps x 415v x 1.73 = 12 923 VA
68 amps x 105v x 1.73 = 12 350 VA

I'm not sure if i'm right but adding theoric power factor to that would give:
95% PF for VFD input 12900va = 12 255watts
50% pf for motor being very unloaded
11900watts (heat loss of 355w maybe) / 0.5PF = 23 800 VA for the motor
23800 / (68*1.73) = 202volts at the motor maybe

so this is why it could be way easier to test it at full speed and load because output voltage and PF will be similar at input and output.

 

[jraef]

Even if the speed is the same, the issue of power factor looms large in VFDs and most people are unaware of this, so it catches them off guard.

When running DOL (Across-the-Line), if you put on a clamp on ammeter, you read current, ALL of the current going to the motor, whether active or reactive. A simple ammeter cannot discriminate. On an unloaded motor the power factor is very low, so the current reading you see has a low active current component, plus a high reactive current component, however you will see only the sum.

Once you add a VFD, the rectifier of the VFD corrects the displacement power factor, so all you see by measuring the LINE side is the active current component (plus a little reactive from the distortion), not the reactive due to the displacement power factor of the inductive load. But on the LOAD side of the VFD, you see both again, so it appears higher.

 

[Gil47]

A 90 Kw 415 volt 50 Hz motor would have a full load current in the region of 170 Amps'

So your test of 80 amps whilst still on the Star Delta starter indicates this motor was not loaded and my guess is the belts were remove during the test
The resulting power factor of that test would have been in the region of 0.2 Pf

Putting that unloaded motor at that power factor on an Inverter could probably result in the readings you are seeing.

Check as DickDV says and make sure the motor is connected in Delta to your inverter

You may need to do some tests with the motor loaded by reconnecting belts

 

[Jeff23spl]

in the case of a 25%pf, my exemple will became

11900watts (heat loss of 355w maybe) / 0.25PF = 47 600 VA for the motor
47600 / (68*1.73) = 404volts at the motor (So full speed with the drive belt or mecanical load removed