By-pass Capability

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hurk27

Senior Member
Ability to send full voltage and current to a motor to bump it past a jam or to keep a machine running that isn't really dependent upon a slower speed, in a case where a drive fails, there are other reasons, but these two come to mind, when I was looking for a drive, I read about this.
 

rcwilson

Senior Member
Location
Redmond, WA
Bypass capability uses contactors that can bypass the VFD and run the motor as a normal motor when the VFD fails. It was a popular design for large motor drives in a critical service and when VFD's had high failure rates.

Some VFD's have programming to automatically transfer the load between the bypass and the VFD or to transfer automatically and go to bypass on VFD failure.

It sounds like a nice feature but it adds complexity and costs. Three contactors are needed: bypass, VFD input and VFD output. The bypass circuti also needs motor overload protection.

With the reliability of modern drives, there is not much payout for adding the bypass. This is especially true if operation at 60 Hz means the fan or pump has to have a damper or a control valve to control the process. Deleting the flow control devices and eliminating their energy losses is one of the economic benefits of using variable speed. Adding a bypass eliminates those cost savings and increase the initial costs.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Overly-simplistic analogy: Imagine wiring a switch in parallel with a dimmer, just in case the dimmer goes bad, and you really, really need the light right now.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Overly-simplistic analogy: Imagine wiring a switch in parallel with a dimmer, just in case the dimmer goes bad, and you really, really need the light right now.
Decent enough analogy.

But bypass is actually very common in HVAC fan drives. The reason is, if the VFD has to be serviced or replaced they can run the motor full speed and go back to damper control, keeping the building open and occupied. If you don't, having even one fan shut down for a week waiting for a replacement can necessitate a partial building closure if the air balance gets so out of whack that you cannot open the doors against a vacuum, and building owners don't like to get calls like that from tenants or worse yet, have to give back some rent because the building was unable to be occupied. Most industrial users would be smart enough to have a spare on hand. That's virtually unheard of in the HVAC world where "cheap rules".
 
Ability to send full voltage and current to a motor to bump it past a jam or to keep a machine running that isn't really dependent upon a slower speed, in a case where a drive fails, there are other reasons, but these two come to mind, when I was looking for a drive, I read about this.

Current bypass mode applications are municipal or other water pumping applications where multiple staged pumps provide service for widely varying flows. Several parallel pumps are switched in-out from and onto full speed - across the line start - and an ASD can vary small portion of the total flow. The starting and stopping of ANY pump is done via the ASD - for limiting the water hammer of FVNR start and also eliminating the shock wear on the motor/pump/bearings - and when it reached full speed the ASD is switched out and another pump starts to vary the flow as/if the demand rises.
 
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