increasing the V/F ratio 1

[fiaz1404]

Dear experts,

We have ID fan for cement plant operation, designed for 1070 rpm and required power is 1593 kw(data is taken from fan name plate).Calculated torque requirement for this fan is approximately 14280 NM . This fan is driven by 6.6kv, 60 Hz, 1600kw, 164amp and 1196 rpm motor, Calculated torque this motor can provide is 12775 NM and this motor is equipped with vfd power flex 7000. We have an issue that when operator want to increase the speed above than 950 rpm, motor reaches up to rated motor current 164 amp while power is 1294 kw. I think it is due to torque requirement of fan on certain process parameters is above than torque that motor can provide. Please see the attached screen shots of vfd for motor model and other parameters.
I have concluded that motor is under rated for this fan, I want to get a more power from same motor by increasing the V/F ratio so that fan speed can go above than 950rpm. Please advise me in this regards that this practice is ok or not and if it is ok up to which level I can increase the v/f characteristics.


Thanks,

Fiaz ahmed

 

[Skogsgurra]

The rated V/f is 110 V/Hz. What is the actual motor voltage and frequency when you reach the 950 RPM limit?

Also, is this a scalar or a vector drive? Vector drives can have their own ideas about V/f while scalar drives are more "obedient".

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Good morning Gunnar.
Scroll down in the attachment. It looks as if the 60 Hz drive is hitting the current limit at about 47 Hz.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Yes, the motor is under sized. You have to realize that a motor on a VFD is a constant torque device up to rated frequency, not a constant power device. The real world numbers you are giving work out almost perfectly.

1294kW / 950rpm * 1196rpm = 1629kW.

Motor is 1600kW, so 950rpm is the fastest speed your motor can turn the fan. You need a bigger motor to turn the fan faster, there is no way around it.

Well, if you have temperature monitoring in the motor (RTD sensors) and the ambient is reasonably cool you could always push the motor until the motor is running at its rated temperature (or say 90% or 95% of rated) and hope you get away with it without burning out the motor. This will require more current, so your VFD needs to be capable of sourcing this higher current. It's not really the proper way to do it, but if the motor is rated 1600kW in a 50*C ambient then it could produce more power when running in a 20*C ambient. Remember, the motor is really limited by the temperature rating of its insulation.

 

[Skogsgurra]

OK, seems to be taken well care of. I had to visit an aquaculture site to look at Bio Mass Counters so I couldn't follow up.
Bio Mass Counters. That is food for thought...

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

And yes,the PF7000 is a vector drive, for what that's worth.

In THEORY, you could change the input voltage to be higher and run the VFD at higher than rated motor speed, keeping the V/Hz ratio constant. We do that all the time on LV motors. The PF7000 drive will be rated for 7200V and probably 200A, so in theory you could squeeze another 9% speed out of it. But this is much much easier to do at LV. If you cannot tap your transformer to put out 7200V and/or there are other things connected to the same transformer that would be adversely affected, it would likely be cheaper to replace the motor than to replace the transformer and all of the other associated equipment just to get 9% more speed.

I like Lionel's idea better, but before you do anything, double check the output current capacity of the drive. It SHOULD be 200A, but may require changes in sensors to get more than it was programmed to do. It's a Current Source Inverter, so everything it does is based on accurate current sensing and control.

"Will work for (the memory of) salami"

 

[LionelHutz]

Jeff - I think you misread the question. No need for an increase in line voltage. The motor is not even reaching base speed or 60Hz. The motor is only running at 47Hz and 78% of rated voltage when it reaches rated current.

 

[iop995]

Maybe VFD stop to increase speed when motor reach rated current, so need to allow overcome this value by 20-50% for short time. To reach desired speed (1070rpm), VFD need to supply about 54.5Hz which coresponding to voltage about 5.94kV. At this voltage torque will be 8.5% higher that at 5.7kV, 14200Nm. Motor will working at rated power but slower speed and if motor have internal fan, need to check if cooling is enough at this speed (volume air is reduced with about 12%).

 

[fiaz1404]

yes lionel,s idea is ok as we have margin in winding temp of motor. motor insulation class is F and max winding temp during last year was 108°c. I think we can go up to class B temp rise, 130°c safely while vfd is 185 amp rated.
But i want to go for v/f change option because our plant management is considering risk in this option.
In this case at 47 hz there are 5170 volts, if I shall increase v/f ratio in such a way that at the same frequency i will get voltage up to 5300 volts then i can get more torque with compare to 5170 volts and hope it will decease the current while on same speed so we can go little above the 950 rpm. my concern here is to know about the v/f ratio that max can be used to avoid high magnetising current.

i can not write English well so sorry for any mistake while communicating on this forum.

fiaz ahmed

 

[ScottyUK]

Your English is very good, fiaz1404.

How old is the motor? Older motors are usually more generous in the design of the stator iron, which gives more margin for changing the V/f ratio. If it is a relatively new motor, and especially if it is a standard mass-produced type, then you will typically find that the design has been optimised so that there is very little excess iron and the core can easily start to saturate.

Lionel's idea is a good one and you can almost certainly raise the current limit a little if you have stator winding RTDs to protect the machine.

 

[waross]

The motor is maxed out on current. The voltage is below rated voltage so a higher voltage will not be used anyway.
Yes, you can get more power by increasing the voltage and frequency, but the power comes from the greater speed at the same max current.
My understanding is that when a motor is driven over-voltage to achieve greater power, the drive ratio is lowered.
Try a larger sheave on the fan and the motor will be able to run faster and develop it's full HP and a greater speed.
Direct drive? Trim the impeller blades to lower the torque required at a given speed and then you will be able to use the voltage headroom to spin the fan faster.
You are running at 81% power and 78% voltage. You have to drop the torque requirement so that the motor can spin faster in order to use the surplus power.
More a mechanical problem than an electrical problem.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

No such thing Bill. All mechanical problems are electrical in origin.

 

[waross]

Grin!!!
I can tell, you have that tee shirt too.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

You'll have to test. It should be fairly simple. either set the motor rated voltage higher or the motor rated frequency lower. I would not be surprized if the current increases rather than decreases though.

 

[fiaz1404]

before posting my question on this fourm i have idea to increase voltage or decrease the frequency in motor parameters but now I will try combination of three things
1. increase of voltage or decrease freq of motor parameters in drive.
2. increase the rated current up to 105 %.
3. increase the ramp up time to reduce the dynamic torque requirement when changing the speed from one setpoint to another setpoint. what is expert,s opinion about this.

is there any way to monitor the motor when it will saturate ?

fiaz ahmed

 

[LionelHutz]

Well, #3 is pointless.

Monitor the current. You'll know if changing the V/Hz ratio is working almost as soon as you start it.

 

[mikekilroy]

of course changing v or hz is an option as in #1, but not a good one: this is a vector drive. that means these values are used to tune the vfd to the particular motor specs. changing these from nameplate data is the wrong way IMO to go since you will simply be lying to the vfd about what your motor is. remember that ur vector drive does NOT control the voltage at all - it simply goes where it must to make the currents right.

#2 is the right way to get same result: all u want is MORE current for more torque so simply turn current limit up while watching the thermistors and motor temp. also, changing mechanical ratio on belts won't help; HP is a figure of speech only: your motor/vfd is limited at 47hz due to the torque/amp rating of the motor; more gear ratio simply moves your limit point from 47 to 60 hz - no difference in final fan speed.

#3 is not the issue from your math since the motor is at rated amps already - slower accel wont change that. slowing it down is simply an exersize.

 

[LionelHutz]

changing mechanical ratio on belts won't help; HP is a figure of speech only: your motor/vfd is limited at 47hz due to the torque/amp rating of the motor; more gear ratio simply moves your limit point from 47 to 60 hz - no difference in final fan speed.

I don't agree. HP actually is important. More HP is required to move more air. So, increasing the drive ratio to increase the motor RPM gives more HP and allows the fan to be turned faster to move more air. Remember, you get a torque multiplication when you change the drive ratio.

 

[mikekilroy]

It is so easy to stand on one's head to help see the results of linear, square, & cube results. So easy for me to be confused! I will happily stand corrected if I am wrong!

That said, my thinking comes from my 10' dia windmill "fan" I built. I figured torque went up by square of the speed, thus HP went up by the cube of the speed.

Also 20 years of my life as engineer selling, applying, helping design, torque motors, where we could output 1,000#ft of torque - at zero speed, yet mechanical result is 0 HP! Hence my belief HP means nothing. Sorry.

Am I wrong saying if you increase gear ratio so that FLA is reached at 60hz to rather 47hz, mechanical HP will remain the same? Please show how I err....

60/47hz= 1.277 speed increase. Means same load torque on output goes down by same 1.277.... So HP=N*T/5252 or [(N*1.277)*(T//1/277)]/5252= what? same?

Please correct me! Thanks.

 

[waross]

Have a cup of coffee and slow down Mike.
Round off 47 Hz to 50 Hz and 1500 RPM.
Round off 60 Hz to 60 Hz and 1800 RPM.

HP = Torque times speed times a constant.
Take any torque you want and plug it into 1500 RPM and then into 1800 RPM. You will find that 1800 RPM always gives a higher HP for a given torque than 1500 RPM.

Torque and current are related. The motor is hitting the current limit at about 80% speed, 80% voltage and 80% HP.
A lower ratio will give more torque on the fan for a given torque on the motor. This will allow the motor to use some of the excess voltage and HP and deliver more air before hitting the current limit.
A mechanical problem.
The only electrical solution is to try to justify an over current on the motor. Calling it a change in the V/Hz ratio does not change the fact that you intend to increase the current above the maximum nameplate value.
If this is a direct drive fan, there are methods to reduce the torque demand and let the fan run faster.

Bill
--------------------
"Why not the best?"
Jimmy Carter