Bypass contactors setup for vfd thoughts?

What is everyone's thought on vfd bypass contactor setup? My application is blower pulling air through a pipe and set of critical flow venturies. The vfd will be for energy savings, etc. I'm thinking that for the hardware cost of 3 contactors, etc that having dol bypass to keep running in the event of vfd failure is a good idea. This is for a 100HP motor.

Bypass contactor schemes wouldn't be so popular were they not a cost effective backup plan for mission critical motors.

I haven't run into this, just bypass contactors around soft starts but if you decide to go ahead you'll want a contactor that auto disconnects the VFD outputs from the motor when the bypasses kick in AND disconnect switches on the input and outputs of the VFD so you can swap it out without shutting your process down. Sounds obvious but you'd be surprised on how often that's missed.

If the purpose is for a manually operated bypass, you can just use a double throw safety switch on the output of the drive, then have a full separate enclosed combo starter as the bypass. That way you can physically remove the drive to service it while the motor runs in bypass. The three contractor setup is only necessary if you want it to happen automatically, but that can have other repercussions regarding a sudden surge in pressure. Rethink how that is all going to be dealt with first.

I've done dozens of bower/VFD applications. With modern VFD technology the failure rate is low, and the probability of having three simultaneous failures is very, very low. I have rarely used bypass contactors, and this hasn't been an issue.

Inrush current on a 100 hp 480/3/60 motor is going to be significant. If you really think you need a bypass it should be a reduced voltage starter. I've seen throwing an across the line starter on motors this size trip the main breaker and take out an entire MCC. On larger motors, I've seen across the line bypass take out an entire plant by blowing the fuses at the main transformer.

In cases where I've provided a bypass for VFDs, I have opted to put one on each VFD. This is a standard option for most drive suppliers. By the time you get done with all the safety interlocks and such the cost savings for using one starter to serve all three blowers won't offset the added complexity for installation and operators, in my opinion anyway.

Tom Jenkins +1

If the blower feeds a venturi it will probably be a high pressure centrifugal fan with a large diameter turbine and a lot of inertia. This type of machine with a power of 100HP should never be started directly ... it would stop the whole plant

lfe said:

Tom Jenkins +1

If the blower feeds a venturi it will probably be a high pressure centrifugal fan with a large diameter turbine and a lot of inertia. This type of machine with a power of 100HP should never be started directly ... it would stop the whole plant

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I agree that a soft start drive would be a superior backup, but it wouldn't be quite as cost effective. As for whether starting a 100 HP motor across the line would shut down the entire plant, I suppose that would depend upon the plant in question. I've certainly witnessed multiple motors of even higher HP being started across the line in some plants.

The motor is actually 75HP. I'm planning g on putting a 100HP vfd (for protection if we need to upsize). This unit is coming from another one of our facilities where it was ran on a starter.
It sounds as though most do not worry about bypass anymore since vfds last so much anymore. For the cost of the contactors/etc I may still do it.

g.mccormick said:

The motor is actually 75HP. I'm planning g on putting a 100HP vfd (for protection if we need to upsize). This unit is coming from another one of our facilities where it was ran on a starter.
It sounds as though most do not worry about bypass anymore since vfds last so much anymore. For the cost of the contactors/etc I may still do it.

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The problem lies in getting an exact replacement within a reasonable time frame. That's much easier with a contactor than with a drive. Many drives are likely to become obsolete models prior to the end of their life cycles.

g.mccormick said:

The motor is actually 75HP. I'm planning g on putting a 100HP vfd (for protection if we need to upsize). This unit is coming from another one of our facilities where it was ran on a starter.
It sounds as though most do not worry about bypass anymore since vfds last so much anymore. For the cost of the contactors/etc I may still do it.

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I've been using the Siemens 3RW3 Soft Starters with excellent success on larger hydraulic pumps. Feature built in bypass contacts.

One of the very significant reasons for specifying a softstarter on a 100hp motor today is that the standard-efficient motors of yesterday had around a 6 times FLA inrush current for across-the-line starting which was generally manageable. In 1992, high efficient motors became the law of the land which raised the inrush to 8 to 9 times FLA---high but still manageable generally. The current rules from 2010 mandate premium-efficient motors with inrushes in the neighborhood of 14 times FLA. That is most certainly not manageable generally. Let's see. Putting some numbers to it, a typical 100hp 460V motor will have an FLA around 124amps. Fourteen times that is 1736amps inrush.

Are you sure you don't need a softstarter?

I may be confusing some people, but this is 1 motor. The 3 contactors would be for a three contactor bypass arrangement.
I would not probably use a soft starter as it is highly unlikely that I will ever need to start the motor dol. The bypass is only a backup safety.

How will you start the motor then if the drive fails without any sort of starter?

g.mccormick, if you do go with a bypass check with your local utility. They often mandate using reduced voltage starters above a certain hp motor - 40 hp is a typical across the line limitation.

And, one more clarification. If you do lose the VFD and use the bypass, you have also lost the motor overload protection that the VFD provides. Strictly speaking, a "contactor" doesn't include overloads, and you really should have them. Adding overloads turns a "contactor" into a "starter".

Tom Jenkins said:

And, one more clarification. If you do lose the VFD and use the bypass, you have also lost the motor overload protection that the VFD provides. Strictly speaking, a "contactor" doesn't include overloads, and you really should have them. Adding overloads turns a "contactor" into a "starter".

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Yes I'm aware. I guess I could have stated contactor and starter bypass for vfd.

2 contractors will isolate the vfd. The third contactor/starter would be backup for failed vfd.


Specific implementation aside, would you bother with installing a backup starting method in a vfd application? I'm getting the sense that no, most would not bother.