Motor Control and Soft Starters

Hi there,

I would definately look at using a VSD/VFD to drive the two tandem motors.

You can take a look at the Danfos website. Download one of the VLT manuals - and you will get the typical circuit you are looking for.

Just make sure you use shunts and chokes. With a VSD/VFD you can be more certain that the motors are getting the same reference.

I would not use a softstarter for this application. Different soft starters work in different ways. Some makes thyristors only fire on two phases and some on all three. Some do not offer complete isolation in the case of breakdown etc etc.

Regards

Craig

Good info, THANKS/DANKE

First off, you cannot just put two motors in tandem like that unless they are EXACTLY the same motor. Any slight difference in slip will cause one motor to drag the other one as a load, overloading the first one.

Secondly, VFDs are unnecessary unless there is an advantage to changing speed. The only kind of crusher that can take advantage of that is a Vertical Shaft Impactor (VSI), which will change the product size and efficiency with speed. A Horizontal Impactor, Jaw, Gyratory (Cone) or Pulverizer (Hammer Mill) will actually work WORSE with a VFD unless you always run at full speed, because they work on HP and as you reduce speed, you reduce the HP. then if you are always running full speed, you will be wasting energy and increasing your operating costs. Besides, only the most expensive of VFDs with a closed-loop torque follower control capability can work to prevent the dragging issue on mis-matched motors, and I don't think anything from Danfoss is capable of that. By the time you finish paying for the 2 drives and sophisticated controls it takes to do this, it would have been cheaper to buy a brand new 150kW motor.

If the 2nd motor is the exact same make and model as the existing, and the slip value on the nameplate is the same, you can do it with one large soft starter feeding 2 separate OL relays going to the two motors. Then you just wire the OL relay aux. contacts in series to shut down the soft starter if either motor over loads. It's done that way all the time.

If you need more help, call this guy in Christchurch, he is one of the most knowledgeable soft starter resources on the planet.

http://www.motor-control.co.nz/

Cheers mate!

Hi all,

I said to use one VSD to control both motors. Of course I assumed that both motors were identical (assumption is the mother of all f#ckups).

I have used this configuration before - not for a crusher though. The Danfos VLT 5000 worked fine. Admittedly it is not cheap.

The VSD would work in closed loop tourque control.

As previously stated, there is a typical circuit in the VLT manual. They also give a few guidlines as to what setup is required.

Regards,

Craig