Calculating Motor Starting Time With Softstarter
#1
Posted 16 August 2007  07:55 AM
First posting for me!
I have an application where I require to know the starting time for a pump that we propose to use a softstarter on. I know that you do not use a softstarter to control the starting time of a motor but the application does not warrant the cost nor functionality of a VSD.
The information I have available is:
Motor torque details  locked rotor, breakdown and FLT
Load torque details
LRC and FLC
Motor and load inertia
I have tried several ways to calculate the acceleration torque but I have had varied results and no way to verify them.
Can anyone help?
I'd appreciate any comments.
Cheers
Eddie Munster
#2
Posted 16 August 2007  08:40 PM
Welcome to the forum.
You need to do several steps to calculate the acceleration time.
First, determine you start current profile that you will use with the soft starter.
Second, from that start current profile, plot the motor torque against speed. (reduced current torque)
Third, on the same graph, plot the load torque against speed.
The acceleration torque at any speed, is the difference between the motor torque and the load torque at that speed.
Next, break the speed range into small steps, say 10%. Assume the average acceleration torque for each step and apply that to the total inertia, and you can calculate the ime taken to accelerate through that step. Repeat this for all steps and add the results together. The greater the number of steps, the better the accuracy.
Hope this make sense.
Best regards,
Mark Empson  administrator
Skype Contact = markempson  phone +64 274 363 067
LMPForum  Power Factor  L M Photonics Ltd  Empson family  Advanced Motor Control Ltd  Pressure Transducers  Smart Relay  GSM Control  Mark Empson Website  Soft Starters
#3
Posted 17 August 2007  11:10 PM
Hello Eddie
Welcome to the forum.
You need to do several steps to calculate the acceleration time.
First, determine you start current profile that you will use with the soft starter.
Second, from that start current profile, plot the motor torque against speed. (reduced current torque)
Third, on the same graph, plot the load torque against speed.
The acceleration torque at any speed, is the difference between the motor torque and the load torque at that speed.
Next, break the speed range into small steps, say 10%. Assume the average acceleration torque for each step and apply that to the total inertia, and you can calculate the ime taken to accelerate through that step. Repeat this for all steps and add the results together. The greater the number of steps, the better the accuracy.
Hope this make sense.
Best regards,
#4
Posted 17 August 2007  11:42 PM
I suspected it would be something like this but it is not as easy as it sounds. How do I work out the reduced voltage torquespeed curve for the softstarter? I don't how to work out the current speed profile. I understand the relationships between voltage, current and torque during starting but I do not know how to relate all this to speed.
Does the current start at the point proportional to the initial voltage or from the current limit?
I assume it starts at, say, 0.3^2*LRC and then rises to the current limit before decreasing as the motor accelerates.
I have the motor torque curve at full voltage and the currentspeed curve.
A little more assistance....Pleeeeaaaase.
I appreciate your help.
Thanks and regards,
Eddie.
#5
Posted 18 August 2007  01:29 AM
At any speed, you can calculate the torque output from the full voltage torque and curent and your required start current.
You could easily set the start current to a constant value, say 400%, and then calculate the start torque for all speeds from zero to full speed.
As you reduce the voltage applied to the motor, the current reduces proportionally, and the torque reduces with the square of the current. i.e. half voltage will result in half current and a quarter torque.
Best regards,
Mark Empson  administrator
Skype Contact = markempson  phone +64 274 363 067
LMPForum  Power Factor  L M Photonics Ltd  Empson family  Advanced Motor Control Ltd  Pressure Transducers  Smart Relay  GSM Control  Mark Empson Website  Soft Starters
#6
Posted 23 August 2007  07:01 AM
With a closed loop controlled soft starter and a current ramp start profile, it is possible to accelerate a motor to rated speed within +/ 10% of the set ramp time, if ofcoure the ramp time is realistic for example 5 to 15 seconds.
Regards,
GGOSS
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