Single inverter drive operating multiple motors

[NickA]

I am working on a design in which I have 10 identical 150hp inverter duty motors. I only need to operate 3 at any one time. I was thinking of only using 3 drives and some switchgear to choose which motors I want to run and save money on the drives. We will not need to operate them very often so switching back and forth will not be frequent.

Has anybody ever used a single drive to operate more than one motor by switching back and forth between them? I have a few ideas of how to do it but I'm not sure which would be the most cost efficient. If somebody has done this I would like to learn about their experience and maybe even see a schematic or single line diagram.

 

[wrsharper]

Assuming that you need speed control of the motors, and they have identical data plates, I do not see a problem as long as the contactors are electricaly interlocked and would even consider mechanicaly interlocked contactors to prevent improper operation. I don't think I would try using only mechanical switches.

 

[ozmosis]

A few basic rules:
1) Add the rated current of each motor and, as general advise, include a 10% margin on the total current value (I1+I2+I3 etc x 1.1) and the Itotal=the current rating of the VFD. HP should not be used ideally.
2) Ensure your total motor cable length is not beyond the limits of any EMC requirement or maximum length stipulated by the VFD manf. The total cable length is the distance from VFD to motor 1+VFD to motor 2+VFD to motor 3 etc.
3) Install some type of individual thermal overload for each motor. These should either interlock to shut the drive down or provide a warning that it has been taken out of the process. The drive will not be able to monitor the individual load of each motor.
4) it is advisable to fit some form of output reactor between the VFD and motors, especially if you have contactors that may come out of operation to a motor while the VFD is running.
5) Some method to ensure any contactor or isolator that is between the VFD and motor cannot be re-energised if the VFD is running. This will cause a few probelms with the VFD if done so. Especially at this power.
6) Obviously, if the motors are in parallel with one drive, they will run at a proportional speed as the frequency will be the same. However, slip will be different so even the same type of motors may run at different speeds. Not a big problem but it depends on your application.
7) Functions in the VFD such as Flying start will not work, if they are in the VFD being used.

I'm sure there are more but just be mindful of these.

 

[itsmoked]

sed2developer; Isn't he just using the VFDs ONE on ONE?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[HCBFlash]

Yeah, I have, but the swithgear I used to connect the different motors was in at least one instance just 2 disconnect switches fed from the drive, and the guy that ran things put his padlock on the unused motors disconnect switch. Since your motor parameters are identical there shouldn't be any trouble at all.

We all know that using different connected loads/applications would affect settings you might use in control functions of the drive.

Whether you use an MCC, a switchboard, or pipe from disconnect to disconnect, you're really only worrying about somebody closing the wrong switch in a given situation.

Please let us all know how this works out for you! and don't forget faq731-376

 

[DickDV]

Set up a Run Enable loop on the drive and equip each disconnect with a normally closed pre-action switch. That way, if any disconnect handle is moved, the drive switches off eliminating flash issues and possible stress on drive output IGBT's.

 

[wrsharper]

I should have added this in my first post. The reason I would steer away from mechanical switches is the confusion factor to someone who not as knowedgeable. At work we have two main switch gears that are set up to swap around. There is always confusion when different disconnects have to be switched for proper operation due to one switchgear without power and the other one with power (power company fault, although rare enough for people to forget). With contactors you could use relay logic or if budget permits a small or micro PLC and all the operator would have to do is push a button.

 

[ozmosis]

Keith
Yes, you're right. Guilty as found for reading the header and skimming throught the post.
Sorry, NickA. What I said is not incorrect if you are running a number of motors from one VFD but not quite what you are looking for.
The previous posts all provide the correct info on what you are looking at doing. I would add that in applications you are looking at doing it has been quite useful for applications I have done using a function in a drive called "Safety OFF". This is a recognised safety level (SIL) function that is embedded in a the drive functionality that allows the drive to remain powered but to safely shut down the output power section of the inverter section. An output contactor can then be opened and closed automatically in conjunction with the Safety logic timing to ensure that the contactor will not open whilst the drive is running and will close in a "safe" situation to prevent arcing and a high current surge.
We have used this function in stage/theatre applications for servo/high performance motors for lifting stage scenary. Time is critical and the powering up and down of drives is not acceptable, nor is the possibility of popping out drives due to motors drawing 5 times their current.

 

[NickA]

Thanks to all for the good advice. We do have a PLC in the system. I'll use it to control contactors and make it simple for the end users.

 

[controlsdude]

Did anyone suggest that the the VFD is rated 133% above the rated horsepower of all the motors for that drive?

take the total horsepower of all motors*1.33 = size of VFD

If you rate the VFD for just the total HP of drives you will have an undersized drive if all the motors are used at same time.

Also, each motor has to have an seperate overload for the motor after the VFD. Also the VFD has to have its seperate overall overload device.

 

[itsmoked]

Again jmounce; Let me repeat: The OP only plans to use one motor at a time. There aren't "all the motors for a drive"!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com