4 replies to this topic

[AB2005]

Dears;

My Friend has been working for designing a machine for paper cups. He needs a motion control system in which a motor would move and stop a head (say robot) again and again after 5-10 sec time interval. He asked to me which type of control would be better for that system. For me, stepper motor and its control system would be costly while a simple AC motor with VFD and feed back system can be a cheaper solution. The question rises in our mind that starting and stopping the AC motor through VFD again and again would be ok or motor would be heat up?

[marke]

Hello AB2005

There are a number of parameters that influence your selection.
You need to consider the torque requirements, speeds and accuracy.

If you use an AC motor and a VFD in open loop conditions, you will not have good accuracy or repeatability.
If you add a shaft encoder and a motion control interface on the VFD, you will get better accuracy and repeatability.

A stepper motor will be very accurate and repeatable provided that you do not exceed the maximum torque output of the stepper motor. The stepper motor does not need an encoder, so is simpler, but you will need to have a homing method for the stepper motor system.

Accelerating a motor and decelerating it using a VFD does not cause a lot of heating because the motor never operates under a high slip.

Best regards,
Mark.

[AB2005]

Thanks Mark;

We will install a shaft encoder and motion control system. Now I am cleared that starting and stopping the motor through VFD again and again is not a problem.


Best Regards

[AB2005]

Another question has risen that the motor will move an MS plate 2’ X 2’ and 12mm thick. I think we would have to use “Dynamic brake” system for stopping that plate at zero position (we don't need 100% accuracy). Recall that motor will operate after 5-7sec again and again. Question is that, can we operate the motor without any fear with that dynamic brake system?
Actually I don’t have any idea for that kind of job but have to answer to my friend.

[marke]

Hello AB2005

You may consider using an active front end VFD for this application as this will return the energy to the supply during braking rather than heating a large resistor. If the the operation is infrequent, then a resistor is OK, but if the operation is continuous, an active front end will reduce the losses and will save money.

Best regards,
Mark.