Running multiple VFDs on RCD protected circuits

Jieve

Member
Join Date
Feb 2012
Location
USA
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274
Hello, I've been iteratively re-designing the controls for our conveyor system used for educational purposes and have another question, this time regarding the use of RCDs (residual current devices or GFIs) with VFDs. We want to build a motor control center with 5x VFDs (Siemens G120s) to drive 5x 0.37kW (0.5hp) 3-phase motors. This system will be plugged into a 3-phase 230/400V classroom wall outlet, which has an upstream 62A (30mA trip) RCD in the main wall cabinet (somewhat out of our immediate control).

The G120 manual says that current may flow through the earth conductors when using VFD's and can cause nuisance tripping. Therefore, they recommend a single 300mA trip RCD before each VFD for protection (no more than one VFD per RCD). In the main conveyor cabinet that we are building to distribute power to the MCC (and other cabinets), we would ideally like to put a 25A 30mA RCD in the main line right out of the wall plug and coordinate it with the main wall 62A RCD so it trips first (this would essentially mean two 30mA RCDs in series for convenience if tripped).

I have read a lot about people having issues tripping RCDs when running VFDs. I'm hoping someone more experienced with this can give some input as to whether or not they think it will be an issue trying to run all 5 of these low power motors off of the VFDs through a single 30mA RCD. If this is going to be an issue, what would you do as an alternative? Any input would be much appreciated.
 
Yes you are correct.
we always had problems with VFD's and 30ma RCD's so we start using 300ma RCD's and all the problems disappeared.
 
Thanks for the repomse, I just double checked the motor size, theyre all actually 0.18 kW (0.25hp). You think this would still be an issue even at this low power? What size motors are you running in those systems that tripped regularly?
 
Thanks for the repomse, I just double checked the motor size, theyre all actually 0.18 kW (0.25hp). You think this would still be an issue even at this low power? What size motors are you running in those systems that tripped regularly?
It has nothing to do with the size of the motor, it has to do with what is called Common Mode current flow that is created by all drives. Some people have had success using shielded isolation transformers ahead of the drives, which give the common mode current a place to go, leaving the individual line phases going INTO the transformer more balanced.
 
I have run 2kW ABB VFDs via a 30mA RCD, it is going to depend on how much earth leakage the VFD creates, I couldn't just spot that in the Siemens manual. You could look at using isolation transformers between the supply and the drives, that way the existing RCD will only be protecting the cabling to the transformer with little earth leakage. You would need to make sure that you add earth leakage protection after the transformer to protect the students. Adding transformers would give the added benefit of avoiding chucking a lot of electrical noise on to the classroom electric supply.
 
It has nothing to do with the size of the motor, it has to do with what is called Common Mode current flow that is created by all drives. Some people have had success using shielded isolation transformers ahead of the drives, which give the common mode current a place to go, leaving the individual line phases going INTO the transformer more balanced.

I have run 2kW ABB VFDs via a 30mA RCD, it is going to depend on how much earth leakage the VFD creates, I couldn't just spot that in the Siemens manual. You could look at using isolation transformers between the supply and the drives, that way the existing RCD will only be protecting the cabling to the transformer with little earth leakage. You would need to make sure that you add earth leakage protection after the transformer to protect the students. Adding transformers would give the added benefit of avoiding chucking a lot of electrical noise on to the classroom electric supply.
It should be noted however that using an isolation transformer is not a GUARANTEE of success, but it does help. Here in the US, we are allowed to use 100mA as "Equipment Ground fault" protection, but 30mA is NOT considered safe for personnel protection anyway, so nobody uses it. For us, the standard is 5mA (4mA minimum, 6mA maximum), which means using a GFCI ahead of a VFD is extremely problematic. Yet, some people HAVE had success using a shielded isolation transformer ahead of the VFD when Class A GFCI protection is required, meaning anything involving people coming in contact with water being in the vicinity of a VFD, i.e. swimming pool pumps, fountains etc.
 
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Thanks everyone for the responses.

Jraef, i assumed that the more current a motor draws, that likely the higher the common mode current would be. But i have searched for a decent explanation of what actually causes the ground current flow and have yet to find a good explanation. Would appreciate it if you could maybe give a clear explanation of how and why this hapoens to help my understanding and get a feel for the magnitude of these currents.

Also, could someone post a link of the type of isolation transformer i should look for for these drives? I'd like to use it as a starting point to study up on the application.

Here, 30mA RCDs are standard for personel protection and 300mA for fire safety, i believe.
 
Thanks everyone for the responses.

Jraef, i assumed that the more current a motor draws, that likely the higher the common mode current would be. But i have searched for a decent explanation of what actually causes the ground current flow and have yet to find a good explanation. Would appreciate it if you could maybe give a clear explanation of how and why this hapoens to help my understanding and get a feel for the magnitude of these currents.

Also, could someone post a link of the type of isolation transformer i should look for for these drives? I'd like to use it as a starting point to study up on the application.

Here, 30mA RCDs are standard for personel protection and 300mA for fire safety, i believe.
Common mode current in VFDs is the result of the high speed switching of the transistors used to create the PWM output that makes it all possible. Without going into the gory details of how PWM works, the issue is that the DC bus is switched in pulses by transistors to make it happen. In the olden days of yore, BJTs (bipolar junction transistors) were used on the output, which had a turn-on time that could be measured in milliseconds. But over time, advancements were made in transistor technology and we went to IGBTs, (Insulated Gate Bipolar Transistors) that switch up to 40 times faster than BJTs did. This improved efficiency and decreased losses, which led to the smaller-cheaper-faster drives we all enjoy today. But it came at a price: the ultra fast rise time of those pulses causes the conductors, AND the motor itself, to start to act like a set of capacitors, charging and discharging into everything connected to them. Invariably some of the capacitive coupled energy makes its way into the ground plane, which is what becomes Common Mode Current.

Slight variations in the capacitive coupling effects of different components in the system, such as the placement of conductors in a raceway or next to other conductors, leads to unequal current flows referenced to ground, which is exactly what an RCD (Residual Current Device) is designed to detect and prevent. The amount of capacitive coupling has very little to do with the magnitude of the current flow through the conductors, it's more about the CHANGE in voltage potentials between them. So the Common Mode Current is almost the same on a 1/2HP motor as it is on a 200HP motor under the exact same circumstances (although they are rarely EXACTLY the same circumstances). Carrier frequency, the basic pulse rate of the transformer firing, affects it as well and typically the larger the drive, the lower the maximum carrier frequency, which actually DECREASES the capacitive coupling effect a little. So in reality, it is often WORSE on smaller motors than it is on higher power motors. Adding to the issue is that the latest generation of IGBTs, called "Gen IV" (generation 4), are now even FASTER than they were, and since those are only made in small sizes so far, it makes the issue even more prominent at the lower size range.

What you want to look for is called a "Drive Isolation Transformer". I could post some links relative to North America, but I have no idea what you have for resources where you are. Often times companies that make transformers also have Drive Isolation Transformers, so you could try your favorite supplier.
 
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There is another source of earth leakage, at least here in Europe, a lot of drives come with in-built line filters. The filters clean up some of the electrical noise that would otherwise be transmitted on to the mains. The filters are made of capacitors and inductors, and they essentially dump some of the noise down to earth, which means there is a current down to earth.

www.mtecorp.com/pages_lang/wp-content/uploads/SR-033.pdf

There isn't anything particularly special about isolation transformers. Normally with a transformer you put one voltage in and get a different one out, you need one that has the same voltage in and out, and is capable of carrying the current required by your motors. It might be easier to go for single phase supply drives, then you only need single phase transformers, but the current will be higher. Your best way is to contact transformer manufacturers and tell them what you want to do. Typical:

http://www.block.eu/en_UK/products/transformers/

It will add significant weight and heat to your panel.
 
Guys, thanks for the detailed answers.

Jraef, thanks for that detailed explanation, I understand PWM but didn't realize that it was mainly capacitive coupling that creates the earth leakage currents. I would assume that shielded motor cables would maybe help this to some degree, or is that not the case due to the fact that shielded motor cable shields should be grounded at both ends?

In order to test if we could get away with using a single 30mA trip rated RCD, I hooked up 5x 1hp async motors through 5x Siemens G120 VFDs. I ran the mains through a single type A RCD, and ran all of the motors at different speeds. At no point did I have any issue.

HOWEVER: there are 3 common types of RCDs, type A is for use with AC and pulsed DC currents. As I think I may have mentioned before, 300mA type B is recommended by Siemens to prevent nuisance tripping. Type B includes detection of constant DC currents as well. It seems to me that logically type A would be just fine for use with VFDs, as we're talking about a pulsed signal. Is this a fair assumption? And if I can get away with a 30mA type A RCD as a first component in our main supply cabinet, is there really any need to be adding extra 300mA type B RCDs before each VFD? Our main issue here is student safety, which is why I want to keep the 30mA RCDs in the main system if possible.

Bryan G: thanks for that response and information. In our case, there is no extra built-in main line filter in the VFD power supply model we are using.

Regarding the isolation transformer: this may be a somewhat different topic, but out of curiosity, if we were to end up using a single 3-phase isolation transformer to feed all 5x VFDs, would this be an ok solution? And I'm assuming that the output should be wired in wye for safety purposes, and just use the phases to drive the VFDs. How should the transformer input be wired, wye or delta?
 

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