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# Variable frequency drive for high inertia loads

High inertia loads present some challenges in both mechanically and from a control standpoint. Suppose the load is a "cement block". We could use a better description as to how this "cement block" is being put into rotation, what the inertia value is, what the speed of rotation is, and mechanically how the proposed motor would be connected to this load. Will it be directly connected? If so, what type of coupling is being considered? If there is a gear reducer, what is the type and gear ratio? Will the reducer have significant backlash that could result in high maintenance if there are frequent changes in speed. This could also be a problem if the speed loop bandwidth is set up to be too high in an attempt to produce tight speed regulation. If it is belt connected you have introduced more spring into the system and could result in excess belt wear and resonances.

A NEMA Design D motor is often applied on high inertia loads because of it's high locked rotor and breakdown torque values and is usually associated with fixed-speed starting methods. The downside to using this type of motor is that you have to understand that this is also known as a "high slip" motor meaning that motor speed will be reduced significantly as it delivers high torque. It is for this reason that you should never apply a variable frequency drive (VFD) to a NEMA Design D motor. VFDs are designed to work primarily with NEMA Design A, B and sometimes C motors.

If you plan to apply a VFD to this application you have to consider how you will take the mechanical stored energy out of the load when you either want to slow down or stop the machine. Dynamic braking, using a chopper IGBT / resistor combination is cost effective on the front end but may be more costly when you consider the total cost of ownership. That is because the dynamic braking circuit takes the mechanical energy out of the load and dissipates it in the form of heat energy which is then just lost to the environment. If the load needs to start and stop or change speeds frequently a VFD with an Active Front End may be more costly initially but because the mechanical energy taken out of the load becomes electrical energy that is then sent back to the utility it may make the TCO less in the end. Another side benefit of the AFE is that the power system feeding the VFD will have a minimal amount of harmonic content impressed upon it. Depending on the size of the VFD and the overall power system this may or may not be a significant consideration. I would recommend staying away from DC injection braking because AC motors heat up really fast when DC is injected into the AC windings so it should only be used for short periods of time which wouldn't be applicable to a high inertia load.

A regenerative DC drive solution may also be considered. It gives you the same results in terms of braking as the AFE on a VFD solution. Something to consider would be the peak torque and current values that would be required. With a brushed DC motor there are limits as to the amount of current the brushes can effectively transmit to the commutator without excess arcing and / or heating up and lifting the commutator bars. The other downside to the DC drive is the rich harmonic content that is produced and the possibility of line notching on the AC voltage feeding the variable frequency drive. A VFD isolation transformer would definitely be recommended.

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