Dual motors / VF drives on a common shaft

[jraef]

I am about to embark upon a new adventure in VFD applications, at least new for me. We are going to put 2x 400HP VFDs and motors (identical, new installation) onto a machine where they are both close-coupled to a common shaft, spinning a platten. The reason for 2 motors is because of physical size (800HP motor is too big for the machine). Several other vendors have insisted on Closed Loop Vector for this, stating that it will not work any other way. Unfortuneately Closed Loop is not an option because the environment is too harsh for any known encoders to survive (fine dust, corrosion, weather exposure and high vibration). We are going to try it in V/Hz mode. The stated goal is to accomplish load sharing between the motors, but +-10% would be fine as far as the end user is concerned. Does anyone have experience in this and be willing to share expected pitfalls / solutions?

"Venditori de oleum-vipera non vigere excordis populi"

 

[aolalde]

Hi Jraef.
As far as you keep the Volts/HZ constant identical and the driver output reasonably close, the motors will share the load.
I have experienced problems with two 3500 HP, 3740 rpm motors in tandem moving a boiler water feed pump. One motor overloaded more than 20%, the problem arose because the spare motor was not identical to the original. The main difference was the speed torque curve. Since both motors must have identical speed, the delivered torque will be a function of the speed torque characteristics. If the rotor slots, resistance or shape and the stator flux are not identical, the curves depart from each other.
When a motor with the same design parameters was replaced, the motors shared the load as expected.
I think that in your application with VFD, the initial set-up of the drivers could be adjusted to define any desired share of the load. I wonder if the temperature change will make the original setup unsteady and further adjustments will be required?!.

 

[niallnz]

Hi Jraef

I've commissioned a drive system on a crusher that has 2x250kW motors coupled to a common shaft. As it was a new installation the motors were identical, but the mechanical torque sharing was managed by the two drives, they were set up as master/follower with a fibre optic communication link between them. No encoder feed back is necessary. There were no more issues with this application than you would get with any other installation of drives that size.

 

[DickDV]

Actually, jraef, as long as you don't need precise speed or torque control to operate the platen, operating both motors on a single VFD is the preferred way to do this.

With both motors nameplated the same, you can expect that they will load share naturally with the 10% or so that you mention.

With motors of this size however, I would recommend having thermostatic switches buried in the stator windings and returned to the drive "External Fault Loop". This is better motor thermal protection than could be expected from the drive software or even from Overload Blocks placed in the individual motor leads.

I you need more precision than the above offers (speed regulation will be slightly less than motor nameplate slip), you can power each motor from its own VFD. Be sure each VFD is capable of sensorless vector operation. Set one motor up as a speed regulator and the other as a torque follower to the first.

Sensorless vector requires no encoders or extra hardware on the motors. Be sure to choose a "high end" sensorless vector brand. I am prejudiced but would prefer to use an ABB ACS800 model with DTC in this application. You could acheive speed regulation down to about one-tenth of motor nameplate slip using that type of drive.

 

[jraef]

niallnz,
By any chance were they PDL drives that you used? I mention that because you mentioned a fiber-optic link.

"Venditori de oleum-vipera non vigere excordis populi"

 

[buzzp]

The installations I have seen that do this use tandem motors but only one drive (PAM drive). Of course seperate protection is required for each motor. The application was a submersible pumping application (oil well about 10k ft deep).

 

[niallnz]

Hi jraef

They were ABB ACS800's, ABB has used fiber-optic coms for a lot longer than PDL. As I work for ABB and as this is an open forum I didn't want to show my bias.

The reality is that any two sensorless vector drives will do, provided one is set up for speed control (master) and the other for Torque control (slave/follower), and that the master can provide an analogue torque reference to the follower drive. The nice thing about the comms link between two drives is that all the start/stop and fault interlocking is done for you.