Why use a VFD configured to provide 400VAC to a motor wired at low voltage ?

[VinceS]

Everything starts with the story:
Had a 15KW motor running a ring compressor on a VFD tripping, its faults were overcurrent related. I checked the normal things voltage and current in and out of the VFD. All voltage was as expected all three phase's were ballanced and ramped up as expected. Current was also ballanced.

Then I checked voltage and current at the Motor Junction Box, megged for ground faults, and checked for restance phase to phase between windings. I found ballanced voltage and current again. The motor megged good and restance was balanced phase to phase.

I then identified that the VFD was set up to provied 400VAC at 97HZ and FLA was set for 45A. The motor was a connected delta (low voltage 230VAC/45A). I noted that I could get the VFD up to 36Hz and I was at 45A. Like all good Electricians I tryed to identify a mechanical issue bearring or obstructed air flow in the ring compressor output. In doing this I removed the load of the compressor and the motor ran normal. Well having had VFD's have a week SCR or leaking Cap's I suspected the drive and replaced it.

The new drive was installed and the issue remained. Which lead me again to look at the motor. I noted it had been in a delta config I removed the shorting bars and rechecked restance and found a complete set of windings missing. Thus ballanced resistance now made sence. This is a first for me, I have never had all 3 windings open at once. ALSO a great lesson learned... The motor was acting like a 7.5KW motor and could no longer run the compressor.
So motor off to rewind shop.

NOW, the question.... I understand motors may be setup like this to provide better torque in the Volt/Hertz setup.
Is the intent of this setup to limit the Voltage to the low volt range(230VAC in my case) and never expect to arrive at the configured high voltage( 400VAC in my case)?
Since all motors have volts per turn limits in the windings, is this some trick used in the specific design by the EE to max out a known motor?

 

[topgone]

Is this a newly installed equipment? If this unit has been running for some time and tripped on overload, I would also check the driven compressor for possible damage. Your motor could have just been loaded more than it could take, IMO. Regarding the VFD presets, it's always done like that. You configure your drive by punching-in the parameter range that your drive is supposed to operate in, i.e. lower frequency limit and voltage as well as the highest frequency limit and voltage, etc.

 

[Besoeker]

NOW, the question.... I understand motors may be setup like this to provide better torque in the Volt/Hertz setup?

A 15kW motor can provide rated torque.

From
P=Tω
T=P/ω

In this case about 80Nm assuming a 60Hz four pole motor.

Connect it for the correct voltage and get the rated torque without exceeding rated current of the inverter or motor.

 

[Besoeker]

A 15kW motor can provide rated torque.

From
P=Tω
T=P/ω

In this case about 80Nm assuming a 60Hz four pole motor.

Connect it for the correct voltage and get the rated torque without exceeding rated current of the inverter or motor.

But given that you mention 230V and 400V maybe you are talking about a 50Hz application in which case the torque would be about 96Nm.
In star or delta.

 

[iwire]

I have removed a bunch of posts, lets stay on the OPs topic.

 

[topgone]

But given that you mention 230V and 400V maybe you are talking about a 50Hz application in which case the torque would be about 96Nm.
In star or delta.

@Bes,
Yeah, you are correct. I tried some calcs and 50 Hz it is indeed: 400/97 = 4.12; 230/50 = 4.6. Otherwise the v/Hz at 60Hz is 230/60 = 3.83, which will be exceeded when the setting is 400V at 97 Hz.

@VinceS,
Did you check the loading/unloading valve of the compressor? I have had an experience before when this component failed and kept tripping the drive. (Sorry if a little o/t)

 

[iwire]

The constant bickering stops now, it is done.

 

[Besoeker]

@Bes,
Yeah, you are correct. I tried some calcs and 50 Hz it is indeed: 400/97 = 4.12; 230/50 = 4.6. Otherwise the v/Hz at 60Hz is 230/60 = 3.83, which will be exceeded when the setting is 400V at 97 Hz.

The initial post indicates that the motor 400V star, 230V delta 50Hz for rated 15kW
The inverter is rated at 400V and the motor needs to be configured for that.

In delta, at 36Hz, the voltage should be about 165V. If it was more it would have moved the motor towards saturation and that may be what caused one winding to fail.

 

[Besoeker]

NOW, the question.... I understand motors may be setup like this to provide better torque in the Volt/Hertz setup.
Is the intent of this setup to limit the Voltage to the low volt range(230VAC in my case) and never expect to arrive at the configured high voltage( 400VAC in my case)?
Since all motors have volts per turn limits in the windings, is this some trick used in the specific design by the EE to max out a known motor?

And just maybe, just maybe, the EE is not altogether savvy about motors and VSDs

At rated voltage and frequency the available torque, power output, volts per turn are the same whether in star or delta. Provided the correct voltage is applied.

Here's a drawing I did that might it explain better:

The differentiating factor is the current that the supply has to provide.
In this case, the inverter.

 

[stew]

Why are we trying to run a 230 v connected motoer on 400 v? seems like a recipe for disaster to the motor. this will oversaturate the windings and failure will occur rapidly . overloads will trip due to high sat current not overload.

 

[topgone]

Why are we trying to run a 230 v connected motoer on 400 v? seems like a recipe for disaster to the motor. this will oversaturate the windings and failure will occur rapidly . overloads will trip due to high sat current not overload.

Did the OP categorically say the VFD tripped while running at 400V / 97Hz? My take is that the 400V / 97Hz data was a parameter setting read out from the VFD settings menu.

Besides, operating the motor at 400V / 97 Hz (V/F = 4.12) does will not saturate the iron core being a bit less than running the motor run at 230V / 50 Hz ( V/F = 4.6)

 

[cadpoint]

The bottom line is that one needs to address the electrical requirements!

As I read it you never got to the spec's of the machine, and that's not what is required!

 

[kwired]

Besides, operating the motor at 400V / 97 Hz (V/F = 4.12) does will not saturate the iron core being a bit less than running the motor run at 230V / 50 Hz ( V/F = 4.6)

That is true but isn't the motor also going to run at a much higher speed at 97 Hz vs 50Hz? Will likely not have the same torque and probably not be enough torque to drive a constant torque load unless the motor is oversized.

 

[topgone]

That is true but isn't the motor also going to run at a much higher speed at 97 Hz vs 50Hz? Will likely not have the same torque and probably not be enough torque to drive a constant torque load unless the motor is oversized.

You are correct. Unless the driven equipment is very, very much smaller in capacity compared to the 15kW motor or the duty is not speed dependent, there will a drastic increase in power output. BTW, we don't size VFDs based on driven equipment ratings but on the motor limits. If the VFD was actually set to operate the pump motor at 400V/ 97 Hz, with the current limit set at 45 (literally from the OP), then the tripping proves that the control logic is functioning properly.

Please note that if you operate motors above base speed, you must also check if the motor manufacturer approves operation for the specified speed range. Overloading is usually taken-cared of by the electronic overload relay and or with the current limiting logic built-in with the VFD (if properly configured). Else, some VFD brands offer features for closed-loop feedback such that the VFD responds to signals from the served process and prevents eventual tripping of the unit.

 

[Joethemechanic]

What kind of torque increase does a ring compressor require with an increase in speed?

While it isn't a centrifugal device, speed increase would have to require some torque increase just to overcome friction head on the discharge. Even the flow within the compressor is way WAY more turbulent than laminar. That's gotta eat up a bunch of torque with an increase in speed.

 

[kwired]

I might have had it wrong. Torque requirement may not change but HP requirement will change if speed changes. If you reduce frequency to get rated speed then you will have reduced HP from the motor but it will also be at reduced voltage and you will still overload it beause the load will be demanding somewhere near its rating - unless you have an oversized motor you have a problem.

 

[Joethemechanic]

Hmmmmmm, I'm a little lost.

Has it been determined if the cause of this overload is the motor itself, or is it the load?


If torque from a motor remains constant, and said motor is 15 HP at 1,500 RPM @50Hz,,,,


And we double the frequency to 100Hz with the motor running at 3,000 RPM, if torque remains constant, the motor will be producing 30HP.

But most mechanical loads will require a torque increase with an increase in velocity (RPM)

 

[kwired]

Hmmmmmm, I'm a little lost.

Has it been determined if the cause of this overload is the motor itself, or is it the load?


If torque from a motor remains constant, and said motor is 15 HP at 1,500 RPM @50Hz,,,,


And we double the frequency to 100Hz with the motor running at 3,000 RPM, if torque remains constant, the motor will be producing 30HP.

But most mechanical loads will require a torque increase with an increase in velocity (RPM)

We need some of the motor experts to straighten us out.

I'm sure you can increase the voltage and frequency and the motor will be able to handle that, but then you need to consider the load. Higher frequency will mean faster RPM. Faster RPM will increase HP demand from the load with no other load reducing efforts being made. Lowering frequency to get back to rated speed will put the load back to rated HP, but the available motor output HP is now reduced if you don't want it to overheat. Beause it is operating at lower frequeny, speed and voltage, current is going to go up - especially if load is still demanding original HP rating - which it should be.

 

[Joethemechanic]

T = HP x 5252
rpm​

T = torque (in lb-ft)
HP = horsepower
5252 = constant
rpm = revolutions per minute
​

I love copy and paste, so much easier than typing from one of my books lol
​

 

[Joethemechanic]

But this ring blower, while not having a speed/torque curve as steeply increasing as a centrifugal blower still can not have a straight line.

Was this thing operating correctly and then failed, or is this a new installation that never worked right? I think someone asked, but I don't think I saw an answer to that