VFD Soft Starters - Methods of Synchronous Transfer
Abstract
Purpose of a VFD soft starter is to start and accelerate the motor up to the nominal speed. The final step is to synchronize the machine with the grid and transfer it to a grid operation. Two basic methods of synchronous transfer exist. They are closely described in this post.
Introduction
VFD soft start is the premium solution to start an electric motor before it is transferred to the grid and operated at fix speed. Final steps of the start-up are the synchronization and transfer from inverter supply into the grid.
There are two fundamental methods of synchronous transfer used in the industry. Both are analyzed in the next paragraphs with respect to the power hardware, quality of the transfer, safety and commissioning demand.
Purpose of a VFD soft starter
Across the line started electric motor is associated with high starting current (depending on the motor design up to 6 – 7 times the nominal current) and large motor torque pulsations. The starting current causes voltage sags in the grid that can negatively affect other consumers. Heavy duty start impacts the motor design as such.
Pulsating torque then stresses the shaft components, such as gear (if present) or couplings. While the mean value of torque is e.g. 40% of the rated torque, there are huge torque pulsations at the slip frequency (at standstill equal the grid frequency) with repetitive peak value of up to 500% of the rated torque. That means a significant stress of the shaft components present during every start-up event (the peak torque will be reached more than 100-times per start-up).
A soft start eliminates all those disadvantages: The starting current is below the nominal current of the motor and the torque ripple is drastically reduced. That is true for almost entire start-up. The performance in the final stage, synchronized transfer, depends on several aspects of which the synchronous transfer method might be the most significant.
Conditions for synchronization
Three conditions for a successful synchronization shall be fulfilled: matching magnitude, frequency and phase angle of the machine voltage supplied by the inverter and grid voltage. While the quantities will never be exactly
the same, there is certain tolerance as a parameter to be set in the synchronization device. The tighter the tolerance the smoother the transfer from inverter supply into the grid supply. A very narrow tolerance window might cause longer synchronization time. A common tolerance is around 2-5% of the nominal value. However, as explained in the next section, one synchronous transfer method may require some “workaround” to reach smoother transfer
Power switches involved in synchronous transfer
The power switches (typically circuit breakers) applied in a VFD soft starter are depicted in the figure below. VFD input breaker is closed during the entire start-up and synchronization process. The inverter output breaker connects and disconnects the inverter to and from the motor. It closes before the actual soft starting process is initiated.
The line breaker (motor breaker) is used when the motor is supplied directly from the grid of fix frequency. There are two methods of synchronous transfer (described in next chapter). One of them first disconnects the VFD from the motor before it is connected to the grid some milliseconds later. The other method keeps both breakers in closed position for a short time in parallel. The sequence of the breaker operation has major impact on the performance as well as system design or protection.
Methods of synchronization
Open before close
First method in our description is open before close, also called break before make. The designation refers to the sequence of the switching devices (breakers) in the VFD soft starting system and their interlocking. First, the switch between the inverter and motor opens. Some tens of milliseconds later the motor breaker closes connecting the motor directly to the grid. Open before close method is linked with a short interruption of current
to the motor. The “dead time” is typically ranging between 30 ms and 100 ms. During this time the motor is not supplied with power and does not produce any motor torque. One target of commissioning is to minimize the dead time by tuning of breaker timing.
The advantage of that solution is that the inverter is…
Drawbacks of the break before make concept include…
Close before open
Close before open, also known as make before break, is the other widely used method of synchronous transfer. The name again refers to the sequence of switching devices during the actual synchronous transfer. The motor breaker closes while the switch between the inverter and motor is still closed. There is a certain overlap time when both breakers are closed. It ensures uninterrupted power supply to the motor during the synchronous transfer. Thanks to that, the torque is smooth and has practically no bump. Thus, the method is also called
bumpless synchronization. This method is very “mechanical-friendly” as there is no (or negligible) transient torque. The inverter and grid supply can be synchronized very closely…
However, there are two consequences as well…
Comparison
Comparison seen from different perspectives (additional hardware, inrush current, transient torque, voltage design, control stability, inverter protection, commissioning effort, back synchronization etc) is available for our premium subscribers.
Summary
The article described two commonly used methods of synchronous transfer in VFD soft starters or drives with a synchronous bypass option: open before close (break before make) and close before open (make before break). While each of them has specific advantages and challenges, the close before open method is generally preferred.
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References
[1] Motor start with VFD, MB Drive Services, March 2020, available online, https://mb-drive-services.com/vfd-motor-start/
[2] VFD as soft starter, MB Drive Services, December 2020, available online, https://mb-drive-services.com/vfd-as-soft-starter/
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