Schneider VFD.. load sharing

SiemensAutoAnsari said:

Yes Mr. Gronvall,
I realized the mistake. I will make parameters changes and will come back with the results.

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rupej said:

Can you get both motors amp measurements to the controlling PLC? If so, you could make a PID control of the drive to adjust the frequency from say 47-50Hz to make the load on both motors equal. That would allow you to re-enable slip compensation.

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SiemensAutoAnsari said:

Hi,

Motors currents are almost equal now within acceptable range. However, Control mode from Vector to V/Hz has caused increase in starting current.

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Gronvall said:

Hello,

The problem is clearly because you have activated slip compensation. Then the VFD will output higher frequency than line frequency to compensate for the motor slip.
If you deactivate this function then they should run at the same frequency and the load will be shared more equally.

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GaryS said:

I actually seen this before many years ago before vfd’s were popular
It was on a stone crusher they used 2 identical high slip motors
What they did was start the first motor then after it got close to speed, start the second motor.
Both motors were connected on line starters

Most load shearing configurations are done with 2 matching motors and vfd’s.
It can be done with motors of a different size referred to as a helper motor

You may be able to do what you want it depends on capabilities of the vfd I don’t see where a plc would be needed. The vfd must be able to catch a spinning load and be able to be setup as a torque follower configuration.
You will need a good load meter on the main motor ( Across the line starter motor)
Setup the vfd to be torque controlled with the output frequency set to about 65hz (Assuming the line freq is 60hz ) it must be above the expected run frequency
Then the load output signal from the main motor is used as the torque reference for the vfd
You will need to set both the load signal from the main motor and the torque reference to the vfd are scald in % of full load
Example if the main motor is pulling 50% load the vfd motor will run at 50% of its load
I have seen this work with motors of a different size 100hp main and 50hp helper
The way you will have to start the 2 motors is first start the main motor then after it is running at speed before you load the system you start the vfd. The vfd must firs catch the already spinning motor the vfd motor will try to run at the set point but the torque will be limited to the same torque as the main motor is producing . you will see as the vfd pick up it’s share of the load the the torque produced by the main motor will be reduced . after both motors are running then you can add the load to the system. As the load increases the torque required and generated from both motors will increase to meet the demand of the system

It would be an interesting project to work on
Let us know how you make out
Good luck

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SiemensAutoAnsari said:

Thanks Mr Gary for your detailed response. Actually the main purpose of our VFD installation was to reduce starting current. If we would start first motor DOL/SD method, it would serve no purpose.
VFD motor is our first motor to start and DOL motor will follow.

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Saffa said:

One of the motor experts like Jraef might be better to chime in here, but I would have thought that starting the DOL motor second would cause excessive mechanical stress, as there is no synchronization between the DOL and VSD motor. If the VSD motor is started second and is using catch on the fly, then the drive should detect the motor rotation and sychronize with the DOL motor, which will be rotating the VSD driven motor since they're connected to the same belt. I think this is what GaryS is getting at too?

I'm a pump and fans guy, but I am rather curious about this.

I'm not sure what applying voltage to the already rotating DOL motor will mean in terms of "starting" current. Is the start current lower because the initial torque to get the motor up to speed is reduced?

I remember that accidentally applying voltage to a high inertia centrifuge load that was still in spin down, made a rather nasty noise and its made me cautious ever since

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GaryS said:

The way you want to start the 2 motor is a disaster waiting to happen
It’s not a matter of if it will happens it’s only a matter of when.

While a spinning load may only pull 25 % of the starting current it also may pull 250 % you have no way of controlling it. If the rotating motor is phased matched you will have low current but if the phase in reversed the motor stop momentarily they would not be too bad if both motor were DOL starting but when that happen to a vfd that already running at full speed the miss matched phase will cause a very large surge to output transistors / SCR’s .
When a vfd starts a motor the output voltage is very low about 50v or so and that keeps the current low as well but once the vfd get up to full speed the transistors would be full on and the output voltage would be at line voltage it would be about the same as putting a copper bar across the output of the vfd (very bad idea). Modern vfd’s have a function called catch a spinning load that is designed to synchronize the output transistors to the rotation field of the motor already in motion. It does this by the injecting a small current into the motor and measuring the counter emf of the motor and phase rotation of the feedback from the motor when it gets things synced up it will them allow the output transistors to fully turn on. It can do this even if the motor is in reverse rotation, It will first stop the motor and start it in the correct rotation. But it can only do this if the vfd is starting while motor is already in rotation.
This all also depends if the vfd is capable of catching the spinning load.

A DOL starter just has no way to control the current delivered to the motor it is whatever the line can supply in most cases that inrush can be as large as 10,000 amps or more. And if the line phase is not synced with the actual motor rotation a large inrush current will be generated. The vfd will do it’s best to keep the motors running at set speed but it will fail and could go up in smoke in the process.
If you are trying to limit the inrush current when the load is started then your best solution would be a vfd on each motor or 1 vfd that capable of running both motors. I know that expensive, but then so is replacing multiple vfd’s until you get it worked out.

A solution that may work would be to use 2 why / delta starters or reduced voltage starters. They have been around and prove to work since the first large motors were used. The normal startup those would keep the inrush current to about 50% certainly not more that 100%.
Another option may be (and I don’t recommend it) would be to stager the motor starting using the 2 why / delta starters start just 1 motor then when starter switched from why to delta start the second why / delta it should limit the current inrush to something manageable.

You may also want to look at using 2 soft starts and set them up to limit the starting current to 100% of motor capacity

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