Gautham
Guest
G
We are building a Downspout machine for the first time.Actually we thought it would not be much difficult. But we now face a lot of problem.
our main drive(VFD) is rated for 11KW and the main motor is rated for 7.5KW.
Drive is configured for Sensorless vector. (autotune everything was done)
Now we wrote our PLC program and tested the machine without load on very low speed (100RPM). Our motor Max. RPM is 1750.
We have an Encoder which produces 1000pulse for 0.312m of sheet displacement.
Now we have different operations :
1. Straight tube cutting
2. Elbow ( left, right,up & down)
Below is the operation required for straight length tube.
The operator enters the length required and the no. of pieces that he wants.
Say if the operator enters length = 6m and no. of pieces =10
Our PLC has an Encoder card to read the pulse directly and an Analog output card which gives the desired speed command to the drive.
for the straight tube application for 6 m lenght;
from 0 to 5.5 m speed was kept at 100rpm and from 5.5 to 6m speed was reduced to 50rpm and at exactly 6m speed was reduced to 0 rpm.
Now without load everthing worked fine.
when we inserted the sheet 0.7mm sheet, the motor could not even pull the sheet at 100rpm and so we increased the speed to about 400rpm when the motor began to pull the sheet. Now we are actually using sensorless vector mode and i thought that the drive will manage to pull the load at 100rpm.
Below are the finding:
Without sheet the motor took 67% current and torque was 18 to 24% depending upon the speed.
With the sheet the motor took 130% current and torque was 150%.
Does that meant that our motor is underrated for the application.
Now the other thing is that as i said we tested our program without sheet and with sheet we had to increase the speed to minimum 400rpm.
But at 400 rpm i could not exactly stop the sheet at the desired distance.
Now our other option to stop the sheet at the required position is the use of DC injection, Mechanical brake and Dynamic braking.
Our drive manual says that DC injection is not possible for Sensorless vector motor. We donot have any mech brake attached and so our only option is Dynamic braking. Our drive has a brake switch only thing is that we have to try with a brake resistor.
We found one brake resistor which is 36 ohms, 500W. But our manual says a minimum load of 50 ohm or above to be used for a 11KW drive.What will happen if i use a 36 ohm resistor.
The other option that we are thinking about is to use the ramp function in our PLC to ramp down the speed.
Our PLC ramp function has many options and below is what i selected.
1.start value (this will be the current running speed)
2.End value ( we need stop condition so 0 rpm0
3. mode ( 0.01 sec deceleration)
4. Desired time ( if we enterd a time here the motor stops way before it reaches the required position). we actually need this to work as per the position instead of time. any idea on this.
Even if we succeed in the staright position we have lots of problem in the bending side.
As for the elbow.
First the tube of length 0.5m should come out and then the bending cylinder will operate as many times depending on the bending angle requred. Each bend will produce a 11Deg so to have 88deg bend we will have 8 bends and the motor should move the shee 0.025m for each bend. I could not produce that torque for the small lenght at slow speed and when i go to high speed i could not stop it.
So now we have come back to very bad situation that AC drive and motor is not the correct choice for Downspot machine.
Anybody have done the downspout machine with AC drive and motor please tell me the way to start and stop the sheet at exact position.
Many thanks
our main drive(VFD) is rated for 11KW and the main motor is rated for 7.5KW.
Drive is configured for Sensorless vector. (autotune everything was done)
Now we wrote our PLC program and tested the machine without load on very low speed (100RPM). Our motor Max. RPM is 1750.
We have an Encoder which produces 1000pulse for 0.312m of sheet displacement.
Now we have different operations :
1. Straight tube cutting
2. Elbow ( left, right,up & down)
Below is the operation required for straight length tube.
The operator enters the length required and the no. of pieces that he wants.
Say if the operator enters length = 6m and no. of pieces =10
Our PLC has an Encoder card to read the pulse directly and an Analog output card which gives the desired speed command to the drive.
for the straight tube application for 6 m lenght;
from 0 to 5.5 m speed was kept at 100rpm and from 5.5 to 6m speed was reduced to 50rpm and at exactly 6m speed was reduced to 0 rpm.
Now without load everthing worked fine.
when we inserted the sheet 0.7mm sheet, the motor could not even pull the sheet at 100rpm and so we increased the speed to about 400rpm when the motor began to pull the sheet. Now we are actually using sensorless vector mode and i thought that the drive will manage to pull the load at 100rpm.
Below are the finding:
Without sheet the motor took 67% current and torque was 18 to 24% depending upon the speed.
With the sheet the motor took 130% current and torque was 150%.
Does that meant that our motor is underrated for the application.
Now the other thing is that as i said we tested our program without sheet and with sheet we had to increase the speed to minimum 400rpm.
But at 400 rpm i could not exactly stop the sheet at the desired distance.
Now our other option to stop the sheet at the required position is the use of DC injection, Mechanical brake and Dynamic braking.
Our drive manual says that DC injection is not possible for Sensorless vector motor. We donot have any mech brake attached and so our only option is Dynamic braking. Our drive has a brake switch only thing is that we have to try with a brake resistor.
We found one brake resistor which is 36 ohms, 500W. But our manual says a minimum load of 50 ohm or above to be used for a 11KW drive.What will happen if i use a 36 ohm resistor.
The other option that we are thinking about is to use the ramp function in our PLC to ramp down the speed.
Our PLC ramp function has many options and below is what i selected.
1.start value (this will be the current running speed)
2.End value ( we need stop condition so 0 rpm0
3. mode ( 0.01 sec deceleration)
4. Desired time ( if we enterd a time here the motor stops way before it reaches the required position). we actually need this to work as per the position instead of time. any idea on this.
Even if we succeed in the staright position we have lots of problem in the bending side.
As for the elbow.
First the tube of length 0.5m should come out and then the bending cylinder will operate as many times depending on the bending angle requred. Each bend will produce a 11Deg so to have 88deg bend we will have 8 bends and the motor should move the shee 0.025m for each bend. I could not produce that torque for the small lenght at slow speed and when i go to high speed i could not stop it.
So now we have come back to very bad situation that AC drive and motor is not the correct choice for Downspot machine.
Anybody have done the downspout machine with AC drive and motor please tell me the way to start and stop the sheet at exact position.
Many thanks