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Can we make a bypass on a variable speed drive ATV58? and if yes what is the required connections? we need to start and stop a high torque m/cs in ca...

Goals and Symptoms

Can we make a bypass on a variable speed drive ATV58? and if yes what is the required connections? we need to start and stop a high torque m/cs in cascade?

Causes and Fixes

The motor seems to need more than 1 seconde to be demagnetized.
As the time to open R2 is not adjustable, it's necessary to add an external temporisation to increase it.
As the application has a high inertia, the speed of the motor will not decrease to much between the time when the downstream contactor is opened and when the bypass contactor is closed. That should limit the motor overcurrent when the motor is connected directly to the line via the bypass contactor.
Tests are necessary to define the good value of the temporisation. It should not be much more than 1 seconde because sometimes with 1 seconde it works properly.
Remark: The bypass function is not included in the ATV58.

To bypass a drive the power sequence must be the following :
Starting :
- close the drive downstream contactor
- start the motor with the drive
- open the downstream contactor (drive isolation)
- wait the time necessary to demagnetize the motor
- close the bypass contactor.
- stop the drive

Stop :
- open the bypass contactor.
- wait the time necessary to demagnetize the motor
- close the drive downstream contactor
- start the drive (it will catch on fly the motor)
- stop the drive for decelerating.

The drive output must never be connected to the main (risk of big damage)

Before bypassing you must respect a time in freewheel to wait the motor demagnetization (Tm) otherwise there would be an inrush current peak and a violent motor breaking at bypass on the main.
Tm is equal to around 6 times the motor rotoric constant.
The rotoric constant is given by the formula :
- Tr = 1/(2*PI*Fs) * CosPhi / SinPhi
where Fs is the spliping frequency in Hz

There are other problems :
- Even without load, turning or not, when the motor is demagnetized, at switching on the main it will absorb the starting current (4 to 6In). The duration of this current is depending on the load
If the application has a strong resistant torque, during the demagnetizing delay, the speed decrease strongly and when you close the bypass there is long starting current necessary to reach the nominal speed again.

For upstream short circuit protection you must choose a breaker with a motor curve as for a direct starting on the main.
In some cases it could be necessary to add a tripping delay (depend on the inrush current value and shape).
i.e. add to a NS breaker an option STR53UE witch allows to set the tripping current (up to 10In) and a delay up to 300mS.

Remark : the interest of bypassing a drive, if you need variable speed uniquely for start and stop, is not so economically evident, in most of the case a softstarter (ATS) is a better choice.

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