Motor Speed Reduction with VFD

As you probably know, induction motors don't run at synchronous speed, but slightly slower, a factor called slip. The amount of torque produced by an induction motor depends on the amount of slip. An induction motor will produce no torque at synchronous speed, but as it slows down, slip increases and more torque is produced. It slows to the point where the amount of torque produced is equal to the amount of torque required by the load.

https://en.wikipedia.org/wiki/Induction_motor?sa=X&ved=0ahUKEwi0lI_M_OLJAhWFJI4KHS_nDUUQ9QEIHzAA

So, two things come to mind. Either your VFD voltage is dropping, reducing the amount of torque the motor can supply, or the mechanical load on the motor is increasing, causing it to slow down.

The slip portion i also assumed while programming the drive and have calculated it using the formula slip = frequency - rpm x poles/120. And as you mentioned voltage drop on the VFD there is no voltage drop across the VFD and the mechanical load remains the same throughout as the problem i mentioned above , is happening throughout the operation under no load condition.

I would look at the acceleration rate and start delay parameters.

Because of the size and weight of the crane, the program most likely has time delays built in to avoid "plugging" of the controls and to allow smooth ramping during any change of crane movement speed so that structural stress is minimized.

A small movement change at the fulcrum point on a tower crane is tremendously magnified at the end of the bridge because of the leverage factor.

I would not change any of the settings without consulting the crane manufacturer and obtaining their approval and guidance as a change might cause the crane to be damaged or at worst case it might collapse.

the acceleration sate and the start delay parameters i would only change if i am facing problem at the time of start. this problem is actually coming when the motor has actually reached the given speed reference. As i mentioned in the first post, it is happenning when the motor is already running at the given speed reference.

Vector control would have been able to maintain more accurate speed control. But as you may have already surmised, using one VFD to control 2 motors results in not being able to utilize Vector Control. A better way to do accomplish slew control with two induction motors is to use a master-slave setup with two VFDs in Vector Control, with the master as a speed follower from your control system, and the slave as a torque follower to the master. But your crane mfr chose to do this less expensively with one drive. We can assume they knew what they were doing, so you may just need to learn to live with the limitations that entails.

Some drives, when operating in Scalar (V/Hz) mode, offer what is called Slip Compensation, and/or Torque Boost for temporary low speed operations like this. If not already employed, you might try those features to attempt to improve performance. But I agree with not changing anything without consulting the crane mfr, you don't want any unintended negative consequences.

Slip compensation i did apply already. Torque is already adjusted by the VFD as it is the one specific for the crane application.

The crane manufacturer can answer all these questions, and more, over the telephone.

the actual VFD that came with the crane has the same problem. And we are changing from that brand VFD to the one we deal with. Moreover chinese crane so dont expect much support

What would be the change in the performance if i change the motor connection from delta to the star connection?

The "star and "delta" motor connections are specific to applied voltage levels. (see motor nameplate.)

If the applied voltage level requires the motor connection to be delta then arbitrarily changing the motor connection to star will decrease available motor horsepower resulting in poor motor performance, excessive motor winding temperature, and motor failure.

The delays and erratic operation have to be caused by VFD programming, crane control logic, and/or VFD operation.

You should be able to use a two channel oscilloscope and recording voltage test meter to isolate and determine what is causing the issue(s).

Monitor the VFD control input and feedback signals from the crane control while simultaneously monitoring the VFD output to determine which is the culprit.

Have you verified the drive configuration matches the crane operation control protocol?

Example 1:

Analog crane signals (4-20mA)=> Analog VFD control (4-20mA) (or 0-20mA) (or?)

Example 2:

Digital crane signal (-10VDC/+10VDC) => Digital VFD control (-10VDC/+10VDC) (or 0-10VDC) (Or?)

I have commissioned and worked on more than one crane that was not set up properly at the factory before shipment to the site.

If the VFD, crane control parameters, and configuration match then I would look closely for any delayed or ramped control signals that are a result of program parameters or configuration.

I keep coming back to the fact that crane control is required to be smooth and linear with no abrupt change of speed and/or direction to avoid inducing unstable load conditions and subsequent damage to the crane and/or the load.

There is a crane control function called "inching" wherein the crane movement is limited to a specified speed of operation and for a specified length of time.

There is a crane control function called "anti-plugging" wherein the operator is prevented from making any abrupt change in drum direction and/or bridge or carriage direction.

Verification of all crane movements is in order. (Full step-by-step commissioning procedure should be executed.)

Your screen name is remarkably apt. Welcome to CR4.

We might find the anomaly writing the algebraic equation of VFD and load motors

Using Law of conservation of energy and its rates

Say

1. power _(VFD) = Power_(motor1)+Power_(motor2)
2. VI_(vfd) = VI_1(motor1)+VI_2(motor2) or
3. I²R_t = I₁²R₁+I₂²R₂ ; R_t = R₁+R₂; R₁= R₂=R_t/2 likewise, I = I₁+I₂ where I₁=I₂= I/2, so
4. I² = (I/2)²+(I/2)² = 2I²/4= I²/2, then
5. I²≠I²/2

Now, where are we?

Despite the VFD having "torque control" in the programming, if it is running in Scalar mode, there is no torque control; you cannot execute a torque feedback loop without Vector Control. Most drives that are capable of Vector Control have the parameters for accomplishing torque control in the programming list, but enabling them while in Scalar control is pointless, they are ignored. Since you have already tried the slip compensation and that didn't help, you may be in a situation wherein there is nothing you can do with this setup as it is, because of having only the one drive running two motors. Scalar control of two motors from one drive just cannot maintain the kind of accuracy at low speeds you are looking for. That's why Vector Control exists. But Vector Control requires the drive to create and maintain a mathematic model of the motor and compare performance against that to be able to adjust the output to correct for errors. This can only be done with one motor, the second motor and circuit to it introduce too many additional variables and errors into the model.

As what Jraef said is like above 2 RLC networks paralleled . What happens is, you review your basic electrical circuits theory.

It might have the same C, but L's and R's may vary even if motor used is identical.

... WILL vary...

hi guys , thanks for all the suggestions and comments. was able to sort out the issue today. the problem was coming when both the motors are sunning together . figured it out by running the motors individually. moreover the motor brake is not controlled via vfd so I had no option to use any parameter from the vfd to put the delay. so had to use an output contactor with 2 seconds of delay time . as the power comes from th vfd one motor runs straightaway and the second motor runs after the delay of 2 seconds thus running at the same speed as that o the reference given to the vfd . all your suggestions have been really helpful. thanks again guys. cheers ☺