VFD. multiple motor.long lead length

Also. What if I would put a correctly sized filter on each motor? That would be better than one large filter?

Addressing your real question here, one output filter behind the VFD is better than 7 smaller filters at the motor. Your VFD output is subject to 7x as much risk of load side circuit problems by adding those multiple motors, you want the load filter to protect the drive from all of the associated risks. Once it is there, it will have the added benefit of reducing the load side common mode noise issues going to the motors.

You want to over size the VFD by at least 10% of the combined motor FLAs because you will have a lot more capacitance in the cables and the charging current of all of that capacitance may exceed the capabilities of the drive transistors. The load filter will slow that rise time down, but there will still be the charging current itself. It's not much, hence the +10% rule.

The 240V single phase input issue is puzzling to me, especially when combined with the motors being 1HP 480V and the desire to have a bypass for the VFDs. Those factors don't jive.

• 480V 3 phase input to 480V VFD sized at 110% of combined FLA + bypass for the VFD = OK.
• 240V 3 phase input to 240-480V 3 phase transformer to 480V VFD sized at 110% of combined FLA + bypass for the VFD = OK.
• 240V 1 phase input to 240-480V transformer to 480V VFD sized for 200% of combined FLA, but no bypass of VFD = OK

Pick one of the above, but no combining them

I am sorry to confuse this more with the single phase. I am quoting to an OEM that has more than one project on the floor and their different installation sites have different power available. On the single phase service, a bypass would not be an option.

goto the vfd manufacturer and get his input and recommendations !!
if you goto a filter company and bypass the vfd manufacturer, it will be a finger pointing contest as to who is at fault.

in regards to the vfd size, I personally would go much higher than 10%.
I've always added a good safety margin just to be safe.

as I said, get with the vfd rep and tell him your application and see what he says.

stray voltage is a big concern. the stray voltage that pass through a cow's utters will cause them to dry up.

james

James Mcquade said:

in regards to the vfd size, I personally would go much higher than 10%.
I've always added a good safety margin just to be safe

Click to expand...

How much would you oversize it? What calculations would you use?

lesmar96 said:

we are feeling our way on the best way to protect ourselves.

Click to expand...

Hmm... might be wise to be more concerned about protecting the customer

lesmar96 said:

Since this is a 480V system and long leads

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You can filter all you want, but if you choose to go with long leads then you are not addressing the stray voltage concerns.
Install individual VFDs locally at each fan.
This obviously your decision, but I've just seen so much aftermath that I feel compelled to tell you this.
If you burn down the farmer's barn... at least there's insurance.
If you install a source of stray voltage... often the farmer is unaware of the cause and it's a long slow death of his livelihood. I'm not making this stuff up.

Based on my experience with dairy barns, and your information that you have seven 1hp 3ph motors, I would build the system this way considering all the issues with stray ground currents affecting dairy cows.

Select 230V three phase source power. Install a drive isolation transformer and connect the secondary as grounded wye. Locate the transformer as close to the drive as possible and place the drive in the middle of the various fans to minimize lead length. Size the drive at 15hp normal duty. 10hp is not big enough to start 7 motors simultaneously.
Immediately after the output of the VFD to the motors, install a sine filter. (Expect it to cost about as much as the drive).

Grounding technique is critical to success. Use shielded motor leads and bring all of the motor grounds and shield grounds, the transformer ground, the sine filter ground and the VFD ground back to a single grounding point. Do not mix or bond this grounding system with the rest of the grounding in the facility. (You may get some pushback from the local inspector on this but it is important).

If you want to provide full voltage bypassing in case of VFD failure, connect the bypass contactor from the secondary of the isolation transformer to the input side of the sine filter. Remember this contactor must be interlocked with the drive output contactor so you cannot feed line power backward into the VFD.

I know some of this sounds extreme but dairy cows are extreme. There have been fortunes lost over trying to put VFD's in dairy barns. Good luck and be careful.

DickDV

Do you think it is an issue for concern if the leads for the motors are different lengths? I think that when ever I had to run more than one motor from a single drive I was always concerned that motor leads should be about the same length for all motors. Does that make sense?

bruzynski said:

Do you think it is an issue for concern if the leads for the motors are different lengths? I think that when ever I had to run more than one motor from a single drive I was always concerned that motor leads should be about the same length for all motors. Does that make sense?

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No concern when the sine filter is in place. The motor leads have a sine wave on them downstream from the filter

With a sine wave filter like you are suggesting, it puts the pwm wave back into regular sinusoidal, correct? Then should have no problem with spikes. Lead length shouldn't be a problemm

bruzynski said:

Do you think it is an issue for concern if the leads for the motors are different lengths? I think that when ever I had to run more than one motor from a single drive I was always concerned that motor leads should be about the same length for all motors. Does that make sense?

Click to expand...

No, that's not necessary. Decades ago when people used Current Source Inverter (CSI) drives in LV applications, circuit impedance was important to the GTO thyristor firing circuit so in that case, motor lead length for multiple motors WAS important. But nobody has made a Low Voltage CSI drive now in a couple of decades, maybe more, yet things like that have a tendency to live on in folklore well past their truths. Modern PWM Voltage Source Inverters don't care.

DickDV,
The only thing I think is important to change (possibly) in your excellent treatise is that the power cable shields and motor grounding conductors should always be routed direct to the VFD ground terminal, not a ground lug or bar. I have seen quite a few issues with Common Mode noise being introduced in that short little circuit conductor from the VFD ground to a panel ground or an MCC ground bus. Remember, Common Mode noise always has to return to its source, and in the case of a VFD, that's the VFD ground plane. Any added connections increase the likelihood of it finding some other unintended path of lower resistance, like a cow's udder...

But to Dicks point, the issues of dairy installations losing production due to stray currents has been shown to be the result of people not paying attention to ALL recommended grounding practices as promoted by the VFD manufacturers. People are used to what the NEC requires and fail to understand that the NEC is only concerned with not starting a fire, there is nothing in the NEC that is intended to address Common Mode noise, and CM noise abounds in VFD installations.

DickDV said:

Select 230V three phase source power. Install a drive isolation transformer and connect the secondary as grounded wye. Locate the transformer as close to the drive as possible and place the drive in the middle of the various fans to minimize lead length. Size the drive at 15hp normal duty. 10hp is not big enough to start 7 motors simultaneously.
Immediately after the output of the VFD to the motors, install a sine filter. (Expect it to cost about as much as the drive).

Grounding technique is critical to success. Use shielded motor leads and bring all of the motor grounds and shield grounds, the transformer ground, the sine filter ground and the VFD ground back to a single grounding point. Do not mix or bond this grounding system with the rest of the grounding in the facility. (You may get some pushback from the local inspector on this but it is important).

If you want to provide full voltage bypassing in case of VFD failure, connect the bypass contactor from the secondary of the isolation transformer to the input side of the sine filter. Remember this contactor must be interlocked with the drive output contactor so you cannot feed line power backward into the VFD.

I know some of this sounds extreme but dairy cows are extreme. There have been fortunes lost over trying to put VFD's in dairy barns. Good luck and be careful.

DickDV

Click to expand...

I greatly appreciate your advice on this stray voltage thing. I know it is a big thing and I have heard big discussions about it. In this case, mounting the drives at the motors just isn't much of an option.

Can you explain a bit of the science behind the 230V supply? Spikes just not as high?

jraef said:

DickDV,
The only thing I think is important to change (possibly) in your excellent treatise is that the power cable shields and motor grounding conductors should always be routed direct to the VFD ground terminal, not a ground lug or bar. I have seen quite a few issues with Common Mode noise being introduced in that short little circuit conductor from the VFD ground to a panel ground or an MCC ground bus. Remember, Common Mode noise always has to return to its source, and in the case of a VFD, that's the VFD ground plane. Any added connections increase the likelihood of it finding some other unintended path of lower resistance, like a cow's udder...

Click to expand...

I have come to this conclusion from other research, that high quality grounding is nearly the most critical feature. I have heard of grounded EMT etc. but things like that arent practical in a dairy barn. No one runs metal conduit because it will rust. Using shielded is a plus, but I wonder how many contractors even use that.

What about this things of long lead length then there is no way to get around stray voltage?

Lesmar96
If I were doing this job I would use NEMA4 VFD’s located close to each fan.
Network them for rum command and speed command. “Drive software is free”. Or you could simplify the control by a simple master slave arrangement. Set the speed in the master unit then have the rest of the VFD’s follow its speed get the rum command from the master running output.
Get a NEMA4 enclosure to house a breaker for each fan that way you have several fans on a branch circuit. Use a minimum of 2 branch circuits or as many as needed to handle all the fans. That way a single failure will not take the whole system down.
I know you are tempted to use EMT to run the power do not do it. EMT over time will separate and you will not have the protection or shielding you want. Use Ridged or IMC for all runs. It’s harder to run but you will have a much better job in the end. The conduit will provide the necessary shielding you want.
In a farm environment you would not have to be concerned about rodents destroying the wiring.
I understand you are designing this for an OEM. A word of advice be very careful, most machine builders don’t get involved I the installation wiring due to liabilities and local code requirements. They like to leave that to the local contractors so if something goes wrong they have somebody to take the heat. With any farm installation double check all the grounding everything has to be grounded no floating systems. They even bond all the metal parts and plumbing just I case. And last thing you want to do before turning it over to end customer get all the work inspected using a local UL inspector. Do the job right the first time and save yourself a lot of trouble. With all these types of installations no one solution will work for every job you have to take each job as it comes.

I know an electrician for years that every job he did the inspection was built into the cost even just changing a rec in a house. He got burnt once somebody else’s work failed but he couldn’t prove he wasn’t involved ended up paying out big bucks.

I remember a few years back that SEW offered a VFD in the motor terminal box, and I am sure other manufactures used to offer the same, maybe this is a way to go as the VFD is part of the motor so you do not have long motor cables just long supply cables.

Alan