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08-03-2012 05:47 AM

AC drive VS soft starter for high torque application

Hi everyone,
I want to accelerate a 22kW motor from 0 speed to full speed in 12 seconds. I have Danfoss MCD 3000 series Soft Starter having no function to vary motor frequency.
Is there any way to use soft starter having no method to vary frequency, for high staring torque applications?

I only manage to set acceleration time to 3 seconds.
When i limit the current to 35A at 380V, the rotor can not be able to rotate. The rotor starts to rotate after 60A and catches full speed at 120A. In this case if i increase acceleration time to 15 seconds than it might damage the motor winding.

Is there any solution to run this application using same soft starter?
According to my knowledge i need to install AC drive for this application, as AC drive works like running a car on gears.
08-03-2012 05:52 AM
Top #2
08-03-2012 05:52 AM
Fall all my knowledge, I think you need VFD drive with vector control for your application to be able to control the torque at the starting. With the starter, you can start progressive but without a specific speed or torque control.
08-05-2012 10:04 AM
Top #3
08-05-2012 10:04 AM
If the requirement is truly to control the ramping time to full speed a RVSS (Reduced Voltage Soft Starter) is not the way to go. You will need a VFD drive which has capacity to control velocity over time. It looks like 60 Amps is about the region that the motor takes to begin rotation. Having that knowledge tells you that if you continue on with a RVSS you want to have the starting current within that area to minimize the possibility of the motor sitting in a stall condition under current.

Once at that point of initial rotation you can set up the ramp time to 15 seconds, however it must be realized that you cannot control the velocity during this ramp period. Once at full speed the soft starter should have a bypass mechanism to shunt it to line power. The function of the soft starter is to minimize inrush current to the motor and shock loading to the mechanical parts of the system by ramping the current / voltage to the motor. The only way to get close to a velocity ramp is to have a very constant system load and controlled source which is typically not possible.
10-10-2012 10:47 PM
Top #4
10-10-2012 10:47 PM
On the assumption that the motors' FLA is around 45Amps and that it is a high torque load, sitting at anything close to 60 Amps will never allow acceleration to full speed. My guess is that the 120 Amps that Sana saw at the end of the ramp is what is going to be needed from an RVSS. I think the 60 Amp level seen at breakaway, is just that, it's beginning rotation. I believe that setting a current limit on an RVSS at anything close to 60 Amps is simply going to result in a slow speed rotation until either the motor cooks itself or the thermal I2T overload trip activates.
I thing you need a AC drive.
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