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06-10-2012 11:12 AM

Variable frequency drive bypass modules work

Has anybody used one of these variable frequency drive bypass modules that the drive manufactures offer? It appears that they are used to keep the motor running (at rated speed) while the VFD is being serviced/replaced.
06-11-2012 10:34 AM
Top #2
06-11-2012 10:34 AM
Bypasses are fairly common in HVAC systems in North America which is why you would see manufacturers such as Eaton offering these Bypass packages with their variable frequency drive. The benefit of a bypass is that you can virtually eliminate downtime, but sometimes maintenance costs and good old fashioned forgetfulness can easily outweigh these benefits.
As you know, VFDs save money when running motors only at the required speeds so if a drive is taken offline for repairs, there is a historical tendency to leave the system bypassed instead of immediately replacing the VFD in question. Most modern drives are designed to be fairly quickly replaced, so you may see people moving away from bypasses in exchange for a couple backup drives in stock. Additionally, the initial purchase price for VFDs+Bypasses is higher than VFDs+Replacement due to several output power offerings in a common package.
06-12-2012 09:49 AM
Top #3
06-12-2012 09:49 AM
If you are looking at an online bypass system, then there are essentially two choices, 1: open transition switching and 2: closed transition switching.

Open transition switching requires the drive to be frequency synchronised to the supply voltage, a contactor then disconnects the motor from the variable frequency drive and a second connects the motor to the supply. There is a short period of time when the motor is disconnected from the supply and during this time, the motor can slow down and will generate voltage due to the slow decay of the rotating rotor field.
When the motor is connected to the supply voltage, there is an inrush current that is dependent on the phase difference between the voltage of the supply and the voltage generated by the spinning motor. There may also be a "start" current to accelerate the motor back to the line speed if it has decellerated during the open transition period.
The switching transient will also result in a torque transient which can be quite damaging. The VFD must be capable of sustaining live disconnect on it's output, or the output must be disabled before the switching occurs.

Closed transition switchings requires that the VFD output is perfectly synchronised in voltage and phase angle to the supply voltage. When synchonisation is perfect, the supply voltage is shunt connected to the motor and then the VFD is disconnected from the motor. An element of current limiting either in the output of the variable frequency drive, or the supply voltage will allow more tolerance in the synchronisation. One solution is to use a three phase reactor in series with the supply that can be bridged once the VFD is disconnected. - essentially like a primary reactance starter.
If the impedances of the VFD output voltage and the supply voltage are both very low while they are shunt connected, severe damage can result to the variable frequency drive.
Current source inverters lend themselves better to shunt switching than voltage source inverters.
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