Bypass contactors setup for vfd thoughts?

Not sure why this hasn’t been suggested....why not add a second VFD??

We just recently priced a starter for a 125HP motor and a Powerflex 750 was cheaper.

Your talking one starter and two contractors.

You could then set the second vfd to flying start enable. It would reconnect to the already spinning motor so you don’t miss a beat. But it would need to be disabled if starting from stop.

g.mccormick said:

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.

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In in-plant applications I would only provide bypass at a customer's insistence, always recommended against it, and never felt the customer got their money's worth from them.

The exception was remote duplex (two pump) sewage lift stations. These were often at the end of the power line and more prone to lightning strikes than most installations. With only two pumps the problems of failure (flooded basements) were significant. These were generally low hp as well, making the cost of redundancy reasonable.

Seth's suggestion has merit, and if I were still providing systems I would definitely examine that.

seth350 said:

Not sure why this hasn’t been suggested....why not add a second VFD??

We just recently priced a starter for a 125HP motor and a Powerflex 750 was cheaper.

Your talking one starter and two contractors.

You could then set the second vfd to flying start enable. It would reconnect to the already spinning motor so you don’t miss a beat. But it would need to be disabled if starting from stop.

Click to expand...

Someone was ripping you off for that starter then... The cost differential should be significant at that size. Remember, when comparing a combo starter against a VFD (assuming that is why the price was higher), you must compare it to a VFD price that includes the enclosure and breaker.

If you put a second VFD in the same box for the same motor, you must STILL include the isolation contactors.

As I said earlier, if you want the lowest cost backup and will only use it manually if the drive fails, use a double throw safety switch to feed the motor and a simple starter as the bypass.

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've seen a 4,000kW (5,333hp) motor started DOL. It didn't stop the plant.

Of course, that's not to say that you don't need to consider that - only that the assessment needs to be around the DOL starting current of this particular motor relative to the rest of the load on the MCC, and the rating of the MCC, etc - it's not as simple as "that's too big a motor to start DOL". If you've got enough headroom, you can do whatever you want.

hi Guys
I have 8 mixer and each one 50 hp motor. We are using power flex 700 and I replace and send them every 2 years. My condition we are apliying inrush current continues.
Mixer on and off we have slow star and using digital inputs and after time is done fast. I don’t see any electronic fail. Your case is blower when u energize vfd keep continue run. Vfd doesn’t fail easily endless u have makanical fail or motor related issues. If it is my project I would invest my money phase reactor.

ASF said:

I've seen a 4,000kW (5,333hp) motor started DOL. It didn't stop the plant.

Of course, that's not to say that you don't need to consider that - only that the assessment needs to be around the DOL starting current of this particular motor relative to the rest of the load on the MCC, and the rating of the MCC, etc - it's not as simple as "that's too big a motor to start DOL". If you've got enough headroom, you can do whatever you want.

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I said in the case of a centrifugal fan.
What forces to use a soft start more than the size of the motor is the inertia that must drag, a large turbine has a big inertia. An uncoupled electric motor does not have it. An electric motor for a hydraulic pump does not have it.
Have you seen a train go from 0 to 100 mph in 2 seconds?

On the other hand, I have seen and installed many big blowers that need a minimum of 20 seconds to reach the nominal speed and they should not be started more than 4 times per hour since that long start heats up the motor and the starter as well.

An example:
https://www.youtube.com/watch?v=-JkKl0UgPTE

jraef said:

Someone was ripping you off for that starter then... The cost differential should be significant at that size. Remember, when comparing a combo starter against a VFD (assuming that is why the price was higher), you must compare it to a VFD price that includes the enclosure and breaker.

If you put a second VFD in the same box for the same motor, you must STILL include the isolation contactors.

As I said earlier, if you want the lowest cost backup and will only use it manually if the drive fails, use a double throw safety switch to feed the motor and a simple starter as the bypass.

Click to expand...

That’s true, you would probably be in it for more money adding another VFD. Ones a half dozen, six of the other.

I agree, I probably would not bother with the cost and complexity of a bypass contactor setup... My normal response i that 3 contactors replace 3 wire nuts

Especially when you know there's a spare 5 minutes across town...

We have bypass starters on some of our critical applications VFDs(WR plant). These are in areas were overflowing a tank or station is not an option. These overflows will end up in a canal or river. DEP just doesn't like that to happen.

They are installed on 75 HP and lower motors. For our 200hp aerators, the VFD backup is a soft start located in a cabinet next to these 2 VFDs.

It takes about 15 minutes to wire the soft start to either motor. We also have a 200 HP portable soft start on a moveable cart that can run any of our motors in case of a VFD failure. We only have 3 motors this set up can not cover. And that's 3 250HP motors on our Deep Injection well. With 3 motors, we are not too worried about a single VFD failure.