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6 Things to Consider When Adding a VFD to Your System

Adding a VFD to your system

Have you been thinking about adding a variable frequency drive (VFD) to an electric motor in your plant? VFD's are great controls that allows you to change the speed of the machine at the push of a button or you can program it into your automation platform. 

Before adding a Variable Frequency Drive to an electric motor in your plant, take a look at these 6 factors and see if its the right for for you:

1. Speed Range

All motors are designed to operate at a certain speed. If they are designed to operate on a VFD at certain speed ratios, those will be listed (dependent on torque type) on the nameplate of that motor. Each of the motors have been tested by the manufacturer and are then nameplated to be acceptable for use. If a motor does not have certain ratios on the nameplate, it doesn’t mean that it wont work. You may just want to consider if it is designed to operate that a different speed or not.

2. Critical Speed

Depending on the speed you want to operate your motor for your application, you have to consider various critical speed design points within the motor speed range. When the motor comes up to speed it has various resonant speed/frequencies based on its unique construction and mechanical design. This is important to consider when figuring out which speed range you want to operate in while using a VFD. 

3. Cooling

When you are changing the speed, cooling the motor is an important factor as you don't want it to get too hot. Most motors have internal and/or external cooling fans that are the main cooling points for the motor. If the fans are not operating at the designed speed of the motor, it may cause the motor to run hotter than it should. You want to avoid your motor running too hot as it cuts the insulation life in half by every 10°C rise in temperature.

4. Oil Film

On sleeve bearing machines, the shaft rides an oil wedge or film that exists between the shaft and the bearing inside diameter. This oil film is a critical part in providing a motor that runs smooth. The base speed of the motor decides which viscosity oil need to be used for proper lubrication. As the motor changes speed and if the speed changes enough, it could require you to change the oil viscosity. For example, if you go slower on the motor, the oil wedge could get smaller and require a higher viscosity oil.

5. Inverter Duty Wire

The winding must be insulated, suitable for inverter duty or VFD operation. To protect the motor from inverter type switching frequency voltage spikes, special insulation is required. The use of a proper wire strand and turn insulation protects the motor (such as inverter duty wire designed with a specially formulated film insulation for this type of application and drive).

6. Shaft Grounding

Shaft grounding brush/ring assemblies are critical to allow flow of shaft circulating currents that develop from a VFD. These circulating currents will cause early bearing failure from arching damage inside the races and balls of the bearing design (called electrical fluting). By grounding the shaft using the brush ring assemblies, such as an AEGIS Shaft Grounding Ring and properly insulate the opposite drive end housing, these damaging currents will be diverted and stopped from causing early bearing failure.


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Justin T. Hatfield, CRL, CMRP

HECO - All Systems Go



About the author:

Justin T. Hatfield, CRL, CMRP  is the President at HECO - All Systems Go. He is responsible for Electric Motor & Drive Sales, Electric Motor & Generator Repairs, Spare Solutions, and Predictive Services. Justin was instrumental in developing HECO MAPPS (Motor and Powertrain Performance Systems) which focuses on “why” you have a motor problem instead of simply “What” product or service should be recommended. Justin is a Certified Maintenance and Reliability Professional (CMRP) by the Society of Maintenance and Reliability Professionals (SMRP). HECO is an EASA Accredited Service Center for Electric Motors as well as a provider of predictive maintenance & reliability services and products throughout the United States.

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