Convert DC drive system to AC drive system

Conversion of DC drive to AC drive is complex. Most important are the changes, impact or improvement in your power system (motor sizing, power factor, power quality, harmonics, efficiency, operation/maintenance, controls, etc.).

Many of the considerations for comparing AC and DC drive systems can be covered if you have a procedure for making an economic evaluation between the two technologies. Areas of comparison include: system purchase price, spare parts costs, motor cooling factors, location, cable costs, power factor corrections, harmonic generation, system efficiency, grounding, maintenance, and space requirements. So, if you have already selected retrofitting and made a choice, the cabling, space, structural, installation/layout and cooling requirements are the most important to solve (one by one). You can for example, reuse some existing equipment, space and conduits.

In general, before making the conversion (cost money) you need to know the reasons (unknown at present) for your conversion. Are your drive components getting absolute, do you have issues with torque requirements (unknown), operation/maintenance, reliability etc...? When you have acceptable reasons, then it is OK to move ahead. Otherwise, it is better to evaluate in more details since this may be your first conversion.
Environmental conditions for DC motor applications need to be reasonably clean and dry. For DC, it is acceptable to have:

  1. Motor speeds can reach up to 2500 RPM (acceptable in your case)
  2. Starting torques are greater than 150. You need to have some information about torque; otherwise you simply can not start your project. The most important is sizing your drive (mechanical/electrical sizing).
  3. Horse power requirements are low to medium (fractional HP to 1000 HP) . DC is acceptable in your case. 

In comparison, the AC drives are best suited for applications where:

  1. Environment surrounding the AC motor is dirty, corrosive, potentially explosive, or wet, (in more extreme cases special enclosures may be required) 
  2. Motors may only be available for limited maintenance 
  3. Speeds up to 6000 RPM, and precise closed loop speed regulation is needed 
  4. Applications require full load torque at zero or very low speed (AC drives without vector control do not have this capability) 
  5. Coordinated speed control if multiple drives is needed 
  6. Horse power requirements are medium to high (3 HP to over 10,000 HP) 

So before selecting AC drive, you need to know what is available (you can also have modern DC drive instead of AC drive).

In case you come up with acceptable reasons for your conversion (unknown at present), then the first task will be to check your mechanical components (gear box, shaft, inertia etc.) and coordinate your design efforts with all disciplines, establish cost as well as vendors. You can also have other alternatives with a new modern DC drive for example.

You need to evaluate in more details the mechanical / electrical sizing process (it is applicable to any drive system AC or DC) and the variable frequency drive manufacturers can provide you valuable information before moving ahead with your project. Sizing the motor and drive and collecting the data you need. Some field inspection and testing (torque) will then be require if you do not have the data you need.
When all components are correctly sized, then you can proceed with your conversion. The first task will be the mechanical considerations as explain before.

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