Some companies provide additional circuits inside the drive to allow the input of closed-loop feedback signals from encoders or resolvers. These circuits will decode the signals from the sensors and scale them to the level required. This feature makes the drive operate more like a package system. This also makes it easier for design engineers to put a complete system together where all of the components match, Allen-Bradley calls its drive with closed-loop capability a vector drive. Its scalar drive can be operated as either an open-loop or closed-loop drive. Other manufactures provide similar packages. It is important to remember that the vector and scalar drives are more expensive and more complex than a simple variable-frequency drive. A fractional horsepower variable-frequency drives cost less than $600 today.
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