How to choose the correct VFD in 3 steps?

There is a lot of VFD available on the market. How to know, that you choose the correct VFD? I will try to describe how to do this in a smart way. To save your time and money.

1. Determine technical needs of a system

This sounds very easy, but this is the most important part of the process. You have to know what kind of physical process You will control, You need stable torque or You need to use a scalar mode? Or maybe vector mode will fulfill your needs? Find out the correct device with available settings for the physical process is the first important point. But not last. You have to find if You need additional control of the process. At this stage is also good to determine if You really need a VFD or soft-starter is enough.

how to find correct vfd

2. Correct VFD, with or without logic?

On the market are available many types of VFDs. But how to know if Your installation needs logic or not? This depends on how complex is your installation, or if this is just a modification of the existing installation. VFD can control a speed of a motor, but to automate the start and stop of the motor, You have 3 options.

The first is to build a complete control box with relays, timers, fuses, and contractors. If You have a free space on board for that, it is already great! Most of the time, You don’t. The advantage is that this is a relatively cheap solution, however, for a small vessel, it is not the best idea.

The second option, usually used for the bigger installation is a complex control over SCADA by PLC system. This solution is also the most costly, but definitely, it is the most flexible, and You can program a lot of interlocks and sequences. The only thing that is limiting You, in that case, is the amount of available I/O’s and Your PLC programming skills. The amount of I/O’s can be always extended by an additional module, so in that case, it is not a big issue.

The third solution is the VFD with logic built-in VFD. In other words, a small PLC is built in the VFD itself. In this case-control options are limited, with a limited amount of IO. However, we can control the process based on analog and digital signals.

Let say, You have founded a solution to control the process, now let’s focus on the parameters available in Your network and at Your e-motor.

3. Correct VFD – Electrical parametrs

The model of VFD has to be adjusted to the parameters. Both sides, input and output of the VFDs have to be adjusted to correct parameters. At first, we will focus on network parameters. so input to the VFD. Depends on the vessel, the power supply can be 230-400-440-690V. Important is also the frequency in our network, so we have to check if this is 50Hz or 60 Hz.

Now we can switch over to the output side of a VFD. However, in this place, we have to talk also about an important topic. Type of application which You need. Do You need a variable torque or constant torque application? What are Your maximum, and minimum speed requirements? What is the difference between these types of applications, You can find out this in our articles available here, here, and here. The most important is to find torque and speed requirements, then we can calculate the motor power. This gives us the correct motor size.

how to find correct motor size

Now is the last step. Check the plate from the motor will tell us in this case everything. VFD needs to adjust the output to the electromotor input.

correct vfd

Remember to check also non-electrical parameters of VFD and e-motor, like size, temperatures, etc.

Let’s summarize:

To choose the correct VFD You need to know:

  1. Define process which have to be controlled. What type of control is required?
  2. Application which You have, to choose the type of control of the process. How complicated is your system? Is this just a one pump or solution have to be more complex?
  3. Electrical parameters of Your network. Be sure that You focus on a solution adjusted to the needs of Your network. Sometimes onboad You have 690V instead of 440V. Be aware of it! ( Personally I experience delivery of a wrong equipment onboard!)
  4. Electrical parameters of electrical motor. Same as previous point, be sure that You have prepared proper mounting, and You have enough space for all installation.

With these steps, You reach the final moment. With all available data, You can choose the correct drive from the catalog.

Correct VFD

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  • Mateusz Białas

    Researcher of Autonomous and Remotely Controlled ships. Since 2014 in Maritime Industry. Experience gained at Ro-Ro, Ferry, Container, and Heavy Lifts, design of Navy Ships.

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