VFD bypass on Pump

[Golestan]

Hi,
We have a hot water circulating pump that is run by a VFD.

We are looking into failure senarios. If the VFD fails, can the power be connected directly to the pump (by- passing the VFD unit). Is it the same as running the pump at 100% roughly!

Thanks.

 

[MintJulep]

It would be the same as running the pump at line frequency.

There are commercially available bypass switches for exactly this purpose.

 

[ozmosis]

just be aware of the dynamics of starting your pump direct-on-line. If your system is used to starting with a VFD then this will generally be a gradual and less stressful start. Going into bypass, as mintjulep says, will start the motor at full voltage and frequency so the motor/pump will start immeadiatly. The effect is a high inrush of current and therefore an inrush of the medium you are pumping. (potential hammer occuring).
Depending on the size of motor/pump, it may be more practical to have a reduced voltage starter (Soft start for example) between the bypass contactor and motor.
Also, and I'm sure you don't need a sparky telling you this, as it is a HWCP, once in bypass there is no speed/flow control that the VFD provides so the consequences of full flow of HW will need to be taken into account.

 

[AbbyNormal]

Sometimes the overload protection is the VFD, out of curiosity what provides the protection in bypass?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[Golestan]

Thanks guys,
I am aware of the lack of control of speed and lack of any "soft-start" when we go into by-pass. I just wanted to know if this is even possible to do. We are looking at emergency failure conditions only. From the replies, then the speed of pump would by the line frequency of 60 HZ, is that confirmed?
regards,

 

[rbulsara]

Yes, it is possible to bypass the VFD and it will result in the motor running at rated speed (full capacity to the extent possible).

You can order VFD's with manual bypass switch or you can place an external bypass. Switching to bypass source generally is a manual operation.

For pumps, starting torque is generally is not an issue, but for blower (fan) etc., size of the breaker feeding the VFD needs to be sized to support a full voltage start.

 

[quark]

As suggested by sed2 and rbulsara, the problem is with overloading during VFD bypass condition. There will not be any problem with nonoverloading type pumps, otherwise, the motor trips due to the increased flowrates and thus power consumption.

Once you know the present flowrate by affinity laws, mark it on the original pump curve (full speed) and get the pressure drop required. You can throttle the valve to the required pressure drop and maintain same flowrate as that of running with a VFD and now you can safely bypass it.