Schneider VFD.. load sharing

SiemensAutoAnsari

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For a stone crusher, which is driven by 2 identical motors (each 160kw) has VFD (SCHNEIDER ATV930C22N4C) for 1st motor and 2nd motor starts DOL after 1st motor has achieved full speed.
As both motors share common belt, load should be equally shared and both motor should have equal current at full load.
But in my case, VFD motor takes almost double the share w.r.t 2nd motor which is still on Contactor.
VFD motor 250-300 amps (on display)
Contactor motor 100-150 amps (Clamp meter)
Can I do something in VFD to make both motor share load equally???
Mechanical belt adjustments we already tried...
 
Well the load sharing feature in the drives is meant for 2 vfds where one will be a master and the other a slave.
When the vfd motor is up and running what hz is at running at? 50hz?
 
Yes Mr. chud, both the motors are set at 50hz. Actually, previously in one of our other machines for the same application, we had 1st motor with S/D Starter and second on DOL. And we would adjust belt tightness to keep ampere rating of the motors equal. However, considering starting current, we decided ro replace S/D starter with VFD.
On full load RPM is maintained at 1500 by VFD. Torque keeps adjusting itself. But second motor is totally independent and provides no information other than current on Ammeter.
 
Normally with crushers we use soft starters exactly for the reason you describe above. The vfd is going to try and keep motor on the set speed. So your current and torque will fluctuate. With a soft start it just ramps up and kicks in a bypass contactor so it essentially becomes a direct online
 
Hi,
Yes we had soft starter for one of our Jaw Crushers. However, soft starter would still take around 800-900A starting and would load generator at max. So we decided to replace everything with VFDs.
Moreover, VFDs don't have bypass function, otherwise we would have switched to contactor like soft starters.
 
Since mechanical adjusting options are exhausted, it seems to me that the only option is to control and adjust RPM on VFD drive, but the speed (frequency) needs to be maintained depending on the load. So I don't see a degree of freedom that you can use to balance motor currents.
 
Hi Pandiani,

Will running VFD motor at lower speed (say 80% of rated speed) help?
If VFD won't able to handle load, burden would shift to 2nd motor??
But is it good to run motor continuously at lower speed??
 
Last edited:
SiemensAutoAnsari said:
Hi Pandiani,

1. Will running VFD motor at lower speed (say 80% of rated speed) help?
2. If VFD won't able to handle load, burden would shift to 2nd motor??
3. But is it good to run motor continuously at lower speed??
Click to expand...

1. I don't know, you should try it and see.
2. Also needs to be tested.
3. I have seen many applications where VFD runs in a range 50%-100% without any problems (for example coal feeder control), but that depends on the motor. You should have information about the motor connected to VFD.

I have seen application where two VFD control under pressure in a boiler. The Setpoint is the desired under pressure and there is a additional "balancing" logic. Average current is calculated Iavg= (I1+I2)/2. There there two additional deviations (like cascaded or nested control) Iavg-I1 and Iavg-I2 are formed and added with limiters to a main control deviation (pressure).
 
Pandiani said:
1. I don't know, you should try it and see.
2. Also needs to be tested.
3. I have seen many applications where VFD runs in a range 50%-100% without any problems (for example coal feeder control), but that depends on the motor. You should have information about the motor connected to VFD.

I have seen application where two VFD control under pressure in a boiler. The Setpoint is the desired under pressure and there is a additional "balancing" logic. Average current is calculated Iavg= (I1+I2)/2. There there two additional deviations (like cascaded or nested control) Iavg-I1 and Iavg-I2 are formed and added with limiters to a main control deviation (pressure).
Click to expand...

Thanks... I would try it..
 
Is the vfd is v/hz mode or is it in "scaler" mode? The DOL motor will run at its slip frequency. If the vfd is doing speed control on the motor and not v/hz, then it could be running the motor at 50hz shaft speed instead of providing 50hz output to the motor. If the VFD motor is running at 50hz, it could be pulling along the DOL motor (which would like to actually run at slower than 50hz shaft speed when running due to slip). For this application, I would think you would want to run the VFD in V/HZ mode. So that it outputs 50hz and then the motors slip should have it running the same as the DOL motor (well within reason). This is assuming the motors are the same.
 
Pandiani said:
3. I have seen many applications where VFD runs in a range 50%-100% without any problems (for example coal feeder control), but that depends on the motor. You should have information about the motor connected to VFD.
Click to expand...

This may require an inverter-duty motor, it's obviously possible to do this with a regular motor but I believe inverter-duty motors are designed to account for the increased heat when running at lower speeds, among other factors. It's very possible that the existing motor is an inverter-duty model, but like you said, the catalog number/documentation should be checked.
 
g.mccormick said:
Is the vfd is v/hz mode or is it in "scaler" mode? The DOL motor will run at its slip frequency. If the vfd is doing speed control on the motor and not v/hz, then it could be running the motor at 50hz shaft speed instead of providing 50hz output to the motor. If the VFD motor is running at 50hz, it could be pulling along the DOL motor (which would like to actually run at slower than 50hz shaft speed when running due to slip). For this application, I would think you would want to run the VFD in V/HZ mode. So that it outputs 50hz and then the motors slip should have it running the same as the DOL motor (well within reason). This is assuming the motors are the same.
Click to expand...
Yes g.mccormick,
my vfd control mode is 'Voltage Flux Vector Control with automatic slip compensation' at present. Would try U/F mode. Fortunately, both motors are identical. And my vfd is ATV930C22N4C.
 
SiemensAutoAnsari said:
Yes g.mccormick,
my vfd control mode is 'Voltage Flux Vector Control with automatic slip compensation' at present. Would try U/F mode. Fortunately, both motors are identical. And my vfd is ATV930C22N4C.
Click to expand...

Hello,

The problem is clearly because you have activated slip compensation. Then the VFD will output higher frequency than line frequency to compensate for the motor slip.
If you deactivate this function then they should run at the same frequency and the load will be shared more equally.
 
Gronvall said:
Hello,

The problem is clearly because you have activated slip compensation. Then the VFD will output higher frequency than line frequency to compensate for the motor slip.
If you deactivate this function then they should run at the same frequency and the load will be shared more equally.
Click to expand...

Yes Mr. Gronvall,
I realized the mistake. I will make parameters changes and will come back with the results.
 
g.mccormick said:
Is the vfd is v/hz mode or is it in "scaler" mode? The DOL motor will run at its slip frequency. If the vfd is doing speed control on the motor and not v/hz, then it could be running the motor at 50hz shaft speed instead of providing 50hz output to the motor. If the VFD motor is running at 50hz, it could be pulling along the DOL motor (which would like to actually run at slower than 50hz shaft speed when running due to slip). For this application, I would think you would want to run the VFD in V/HZ mode. So that it outputs 50hz and then the motors slip should have it running the same as the DOL motor (well within reason). This is assuming the motors are the same.
Click to expand...
Hi,
I changed control mode from Sensorless Vector to V/Hz. And I could achieved quote significant improvement in sharing load. Now the difference between 2 motors is more or less 40A which much better than the previous one. Having said that, now my starting current goes upto 500A which was earlier only 150A or so.
I didn't make any profile as such for V/Hz mode. In Schneider VFD, it has 5 points V/Hz.
Could you please help me make a profile for Constant Torque Stone crusher application???
Hope this profile would improve my starting current....
 

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