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

 

[mikekilroy]

Thanks Waross; the coffee helped. I see the problem now... I was looking it as changing the gear ratio so 47hz became 60hz for same fan speed was a fix; it still is not. As I said, the HP output to the fan is still the same. But I did not go the next step - to let the speed then go above motor base speed - assuming allowed by both the motor and drive, into the constant HP zone - to help the OP issue: get more speed.

Looking at it as today's operation is using 80% volts but 100% torque would suggest further mech ratio to reduce the 100% torque since we have some spare volts (speed) as you well point out... But perhaps increasing ratio to 90% speed (volts) & 90% torque (amps) may make more sense so motor does not have to overspeed or drive go above nominal 60hz either... then at 90/90% point he is back to same as today operation and has 10% speed and torque to increase some. To go all the way to 100% speed and thus any increase is at constant HP won't help either since now the torque avail begins to go down with any further speed increase over same as he has today... Perhaps the optimum ratio should be calculated if he wants to try a change: since torque goes up by square of speed, and speed goes up linearly, a little spreadsheet could calculate that something like 1/3 of the total 60/47 = 1.27 increased ratio makes best sense?

 

[waross]

You've got it.
I agree also with your comments on the ratio.
The optimum ratio will be one where the motor is running at rated speed and rated current.
Sort of like shifting to a lower gear when climbing a hill.

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

 

[fiaz1404]

waross,
thanks so much for explaining well to solve the issue mechanically, again thanks.

i am still stuck on v/f ratio increase because of current decrease in dol motors when supply voltage increas in plant, this decrease is upto limited increase in supply voltage. when voltage increase due to any reason while frequency remain same that mean increase in v/f ratio. some years back, i increased the supply voltage up to 3% by tapchanger for experimental purpose and observe the current trend of the important motors of plant and i found decrease of current in all motors except the lightly loaded motors. Due to this reason i am thinking maybe i can get some extra kw. Please help me to understand if there are some drawbacks in this option.
I am interested in this option as there is no major modifications required and 100kw power increas may fulfill process requirements.

fiaz ahmed

 

[LionelHutz]

I really didn't think this was so hard to understand. I guess some calculations are needed.

The motor when running at 950rpm is effectively a 1271kW motor, not a 1600kW motor. 1600kW x 950rpm/1196rpm

The motor is running at full load when operating at 950rpm, so it can be assumed the fan is requiring about 1271kW to operate at 950 rpm.

The desired rpm of the fan is 1070rpm. This is 1.13 times faster than 950rpm.

The fan power requirement increases by the cube of the speed change. So, 1271kW * 1.13^3 = 1834kW. This is the power the fan would require to reach 1070rpm. It's not very likely you'll get there using a V/Hz change or even by overloading the motor.

Now back calculate the speed increase possible by changing the drive ratio so the motor is 100% loaded at rated speed. The ratio = cube-root(1600kW/1271kW) = 1.08. So, you can increase the fan speed to 950rpm x 1.08 = 1026rpm by changing the drive ratio to get the motor running 1196rpm when the fan is running 1026rpm.

 

[iop995]

From fan name plate, nominal power required is 1593kW at rated speed of 1070rpm, so nominal torque is 14.224Nm. At actual speed (945rpm) power to shaft is 1294kW, and torque is 13.082Nm... not 9.798Nm as result from cubic power-spped relation. If fan data are correct, seem to be an aditional torque load in system.
Regarding HP=k*M*N relation, it's not valid for entire speed-torque curve and final operating torque is imposed by load not by motor. In IM motor, torque-speed it's a non-liniar relation; motor reach FLA at 945rpm because is undervoltage, developed torque is higher than nominal but still below to needed value to reach 1070rpm.

 

[LionelHutz]

iop995 - At 945rpm the motor is effectively 1264kW and 12773Nm rated. The torque of the motor doesn't go down and it's not a non-linear relationship. The motor rated torque at full rated speed is also 12773Nm.

The simple fact is that this motor is under sized for direct driving this fan. My take would be that someone saw the fan data 1593kW @ 1070rpm and simply picked a 1600kw @ 1196rpm motor thinking they were matching the motor to the fan. In reality, the motor chosen should have been 1800kW @ 1196rpm to properly match with the fan rating plate. A rather expensive mistake to make when the system can not reach the required operating point.

Now, to make matters worse, the fan appears to be running over the design rating. The fan should only be using 1097kW @ 945rpm if it was loaded to match the name plate. Since it is drawing 1294kW @ 950rpm, this means the fan requires a 2100kW @ 1196rpm motor to power the new expected load of 1870kW @ 1070rpm.

I really hope you don't need to reach 1070rpm because I doubt that your motor is capable of that.

 

[mikekilroy]

i am still stuck on v/f ratio increase because of current decrease in dol motors when supply voltage increas in plant, this decrease is upto limited increase in supply voltage. when voltage increase due to any reason while frequency remain same that mean increase in v/f ratio......

Please help me to understand if there are some drawbacks in this option.
I am interested in this option as there is no major modifications required and 100kw power increas may fulfill process requirements.


Fion, you do not have a v/hz drive. You have a vector drive. Your vector drive as wired and mechanically attached ALREADY HAS 20% EXTRA VOLTAGE - YOU ARE NOT VOLTAGE LIMITED.

You have NO **NO** control over the output voltage - this is a vector drive. It is controlling current, not the voltage.

You can only increase current limit to get more output from your present system.

Nothing else. Wishing will not make it work.

Either your present motor will be able to overload and do the job (not likely), or you can gain a SMALL amount more by tweaking gear ratio, or you need a larger motor. No way around it.

 

[fiaz1404]

thanks to all experts for their value able replies.

fiaz ahmed

 

[iop995]

Lionel - Torque is not a fixed or limited value and motor nominal torque is not maxim value for an IM. As you know IM have a maxim torque up to 2-3 times than nominal, so at 945rpm, motor develop 13.082Nm not nominal torque.
I hope that fiaz will let us to know why so high torque difference (fan data error or some system problem?).

 

[waross]

Torque should be limited externally by load selection or controls. If you allow a motor to develop 3 times nominal torque it may self destruck in less than one minute.
In this case the current is limited to rated current and this effectively limits the torque . At rated current the motor is generally developing rated torque.
I think that Lionel knows this.

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

 

[iop995]

I mentioned maxim torque vs nominal to show that motor have torque headroom to overpass even a load torque higher than nominal; this overpass process it's for (very) short-time (current and/or thermal limited); sure, working at higher torque than nominal will destroy motor in more / less time depending of overload and cooling conditions, so can't be a normal long-time operation mode but may be usefull to overpass an accidental overload.

 

[LionelHutz]

I expect the motor is under rated a bit and that's why it's torque when running at FLA is a little higher than rated torque.

The torque vs speed curve of a motor on a VFD with a current limit isn't the same as the full-voltage torque vs speed curve. In theory, the torque curve on a VFD set to current limit to FLA is rated torque at all speeds. Also, the motor breakdown torque might be 2.5 times the rated torque, but the motor won't be able to produce breakdown torque if the VFD can't supply the current required to produce breakdown torque.