One VSD for multiple motors

[ntr]

Hello all,

I'm designing a control panel for industrial machinery, and i want to use one VSD for multiple motors, just like in this article:
http://www.automation.com/library/articles-white-papers/motor-drives-control/vfds-can-control-multiple-motors


This means all motors are connected to the same VSD, but every single motor has it's own branche circuit protection. Now everything before the last overcurrent protective device is in the feeder circuit, and has to comply with feeder spacings. So in this case the VSD is in the feeder circuit. Most VSD's i know do not comply with feeder spacings, neither the filters or chokes.
If i use only thermhal protection for the motors, i guess all wiring after the VSD is to be sized to the overcurrent protection of the VSD. That's not a good option.

How can i do this to comply with the UL508A?

Thanks

 

[petersonra]

take a look at 31.4.

 

[iwire]

Why is there any reason for a breaker between the motor and VFD?

 

[petersonra]

Why is there any reason for a breaker between the motor and VFD?

depending on the motor sizes you might need it to meet the group motor rules.

 

[ntr]

Thanks for your response. I am aware of the motor grouping rules in 31.4.
I have about 90 motors of 1hp and 1.5hp, but they are not running all the same time, I want to use a contactor for every single motor to switch it on or off, and a thermal overload relay for motor protection. The VFD is 25hp and accepts fuses of max. 60A. I can't find any markings on the contactors or overload relays about group installation, so that leaves me with the first option as mentioned in 31.4.1 a), which dictates a branch protection of max 15A. That is why i need a breaker between the motor and the VFD.

 

[petersonra]

Thanks for your response. I am aware of the motor grouping rules in 31.4.
I have about 90 motors of 1hp and 1.5hp, but they are not running all the same time, I want to use a contactor for every single motor to switch it on or off, and a thermal overload relay for motor protection. The VFD is 25hp and accepts fuses of max. 60A. I can't find any markings on the contactors or overload relays about group installation, so that leaves me with the first option as mentioned in 31.4.1 a), which dictates a branch protection of max 15A. That is why i need a breaker between the motor and the VFD.

I might be inclined toward the use of a type F starter for this purpose. They are generally set up pretty well for group installations and have both overload and SC protection built in.

The markings are not always easy to discern. The manufacturer's instructions are generally a better place to look. I will warn you that as best I can tell S00 contactors can't be made into type F starters, but you have to look really hard to find this out.

However, type F starters are only listed for use at 480V on wye systems. So, if your system has delta power feeding it, you are out of luck using type F starters.

 

[kwired]

Is your drive in question designed to handle additional switching driven loads connected to it?

Most of your standard general purpose VFD's will not like that kind of activity, they may tolerate opening a circuit to a load to some degree but throwing a new motor into an already running output will look like a short circuit to the drive and it will go into fault and shut down. With most general purpose drives if you are going to run multiple motors you need to start and stop all of them together.

 

[ptonsparky]

Is your drive in question designed to handle additional switching driven loads connected to it?

Most of your standard general purpose VFD's will not like that kind of activity, they may tolerate opening a circuit to a load to some degree but throwing a new motor into an already running output will look like a short circuit to the drive and it will go into fault and shut down. With most general purpose drives if you are going to run multiple motors you need to start and stop all of them together.

My thoughts as well. FWIW.

 

[ntr]

Is your drive in question designed to handle additional switching driven loads connected to it?

Most of your standard general purpose VFD's will not like that kind of activity, they may tolerate opening a circuit to a load to some degree but throwing a new motor into an already running output will look like a short circuit to the drive and it will go into fault and shut down. With most general purpose drives if you are going to run multiple motors you need to start and stop all of them together.

Thanks for the tip, but this is not a problem. It is done this way before, the VFD is fine with this setup, but in European standards there is not such thing as feeder/branch circuit or motor grouping. You just have to make sure that your cables and components are protected, so an additional circuit breaker between motor and VFD is not a problem. I ran into this when we where ask to build the same panel according to UL standards in stead if IEC. Now it seems we have to change the design because of the feeder/branch circuit distinction in UL standards. I try to find a solution for this.

I asked the manufacturer (Schneider) of the overload relay's for the grouping markings, but they could only point me to the marking on a manual motor controller, which we don't use. Considering the amount of motors, we want to choose the least and most cost efficient parts per motor.
I'm not sure about the advantage of a type F combination over a group installation.

 

[petersonra]

I asked the manufacturer (Schneider) of the overload relay's for the grouping markings, but they could only point me to the marking on a manual motor controller, which we don't use. Considering the amount of motors, we want to choose the least and most cost efficient parts per motor.
I'm not sure about the advantage of a type F combination over a group installation.

Usually, a type F starter consists of a manual motor controller and a contactor listed to be used together.

The big advantage to me for type F starters is that they usually have bus bars and terminals available that readily meet the feeder spacing and tap rule protection requirements for group installations.

One of the problems I have with your setup is that you are being rather vague about what you actually have. If the VFD is small enough that it can be fed with a 15 A CB, that is all the motors require for short circuit protection and you only need overload relays for each motor to provide the individual overload protection. Beyond that, it is application dependent and you have not provided enough information with which to provide you with useful advice.

Off the top of my head, I am not real sure how you would go about calculating the short circuit current rating of such a setup, and that is a UL508a requirement. I don't recall any overload relays that have short circuit current ratings above the default ratings without having a contactor, and fuse or CB in front of it.

 

[Besoeker]

Why is there any reason for a breaker between the motor and VFD?

Safety perhaps.
Or if you wanted to disconnect that motor to carry out maintenance on it.

 

[ntr]

Sorry for being vague, i don't really know what information you need. As mentioned, the VFD is 25hp and accepts fuses of max. 60A. Is has to feed about 90 motors of 1 and 1.5hp, not running all together at the same time. We use one main control panel and a bunch off small panels with the motor contactors and overload relays in it. This reduces the amount of wiring in the field as we can feed every small panel with only one supply coming from the VFD. The wiring to this small panels is sized for the max current of simultaneously running motors. For European installations we use one circuit breaker for a few of these boxes to provide short circuit protection for the wiring/cables. For UL, this will put the VFD in the feeder circuit. Maybe we have to switch to a few smaller VFD's of max 15A so no additional protection is needed between motor and VFD?

The overload relay's (Schneider LR3D07/LR3D08) are able to withstand 65kA SCCR when protected with a 15A circuit breaker and in combination with the contactor LC1-D09 we use.
Thanks for thinking along.

 

[petersonra]

Sorry for being vague, i don't really know what information you need. As mentioned, the VFD is 25hp and accepts fuses of max. 60A. Is has to feed about 90 motors of 1 and 1.5hp, not running all together at the same time. We use one main control panel and a bunch off small panels with the motor contactors and overload relays in it. This reduces the amount of wiring in the field as we can feed every small panel with only one supply coming from the VFD. The wiring to this small panels is sized for the max current of simultaneously running motors. For European installations we use one circuit breaker for a few of these boxes to provide short circuit protection for the wiring/cables. For UL, this will put the VFD in the feeder circuit. Maybe we have to switch to a few smaller VFD's of max 15A so no additional protection is needed between motor and VFD?

The overload relay's (Schneider LR3D07/LR3D08) are able to withstand 65kA SCCR when protected with a 15A circuit breaker and in combination with the contactor LC1-D09 we use.
Thanks for thinking along.

I am a little concerned about your statement of 90+ motors of 1 and 1.5 HP on a 25HP drive. Is that a single 25Hp drive for all 90+ motors?

I don't have a problem with feeding several boxes off one CB as you are suggesting as long as it meets the group motor rules, the tap rules, and the motor short circuit and overload protection rules.

I am a little concerned with your statement that the wiring is sized for the max current of simultaneous running motors. While there are some provisions that allow you to size wiring in that way, you still have some minimum conductor sizes the NEC allows for.

 

[ntr]

Bob, wiring sizes will be coordinated with NEC and circuit breakers, don't worry .

It is still not clear for me if a circuit breaker between motor and VFD is possible. If not, i have to use multiple smaller VFD's. The 60A before the VFD is too much. That is my main question at the moment.

 

[petersonra]

Bob, wiring sizes will be coordinated with NEC and circuit breakers, don't worry .

It is still not clear for me if a circuit breaker between motor and VFD is possible. If not, i have to use multiple smaller VFD's. The 60A before the VFD is too much. That is my main question at the moment.

I can't find any prohibition in the NEC or UL508a on locating a CB downstream of a VFD. There is a specific provision in the NEC that allows for multiple motors on a single VFD.

430.124
(C) Multiple Motor Applications. For multiple motor application,
individual motor overload protection shall be provided
in accordance with Article 430, Part III.

Part III starts at 430.31.

I will point out one other thing. The NEC requires that you follow the manufacturer's instructions.

I copied this out of an AB manual but there are a lot of drive manuals that say something very similar.

Output Disconnect
The drive is intended to be commanded by control input signals that will
start and stop the motor. A device that routinely disconnects then
reapplies output power to the motor for the purpose of starting and
stopping the motor should not be used. If it is necessary to disconnect
power to the motor with the drive outputting power, an auxiliary contact
should be used to simultaneously disable drive control run commands.

 

[ntr]

Thanks for your investigation. My concern is that the VFD is pushed in to the feeder circuit by adding a circuit breaker downstream, or do i miss something here? I foresee problems with spacings. VFD's i haven seen are not prepared for feeder spacings. Maybe we can attached some barriers on the terminals, but i don't know if that is allowed or practicable.

Thanks for pointing to the requirement to follow the manufacturer instructions, i will check the manual for this point.

 

[petersonra]

Thanks for your investigation. My concern is that the VFD is pushed in to the feeder circuit by adding a circuit breaker downstream, or do i miss something here? I foresee problems with spacings. VFD's i haven seen are not prepared for feeder spacings. Maybe we can attached some barriers on the terminals, but i don't know if that is allowed or practicable.

Thanks for pointing to the requirement to follow the manufacturer instructions, i will check the manual for this point.

It is an interesting question and one I have never considered before. If I think of it and have the time to do so, maybe I will call UL and ask one of their guys about it.

 

[ntr]

My concern is that the VFD is pushed in to the feeder circuit by adding a circuit breaker downstream, or do i miss something here? I foresee problems with spacings. VFD's i haven seen are not prepared for feeder spacings. Maybe we can attached some barriers on the terminals, but i don't know if that is allowed or practicable.

I took a close look at the statement in the UL508A. Is says that "spacings at and within a component or device shall be investigated based on the requirements for that component or device.". The VFD is investigated to UL508C. Looking in the UL508C I see spacings of only 3/8 inch trough air and 1/2 inch over surface at 301 – 600 V.

The spacings for feeder circuits as outlined in table 10.2 of UL508A are only required for "Spacings between uninsulated live parts of adjacent components, between uninsulated live parts of components and grounded or accessible dead-metal parts, between uninsulated live parts of components and the enclosure, and at field wiring terminals".

That means that I don't have to worry about feeder spacings on a listed component itself.

I also looked in the UL489 for circuits breakers. The spacings for circuit breakers are resp. 25.4 and 50.8 mm at 301 – 600 V. That is the same as in table 10.2 of UL508A. That explains the larger barriers on circuit breakers.

Any comments on this explanation? Am I thinking right?

 

[petersonra]

That means that I don't have to worry about feeder spacings on a listed component itself.

I suspect that to be the case. I don't see any requirement in UL508a that says you should or even can modify a listed component for any reason.

But, the way it is worded is not 100% clear to me.