Variable frequency drive

Voltage spikes do not usually present a problem in 220V AC drive applications. The reason this is true is that all NEMA standard motors use an insulation that is designed to operate at 600V continuously and withstand intermittent voltage spikes of 1000V (a new motor can often withstand spikes to 1200V). Even if a spike were three times normal peak voltage (325V), it would still be within the 1000V limit. The net result is that standard motors are well suited for operation in 220V AC drive applications as long as the AC drive and motor manufacturer's cable length guidelines are followed. Always consult the manufacturer if the cable between the AC drive and motor will exceed 200 feet. If higher than normal ambient temperatures exist, you may want to upgrade to a motor that uses Class F insulation but, "special" inverter duty insulation is not required.

Unfortunately this is not always the case for 380V applications. Due to the high peak voltage (650V), even relatively small spikes can exceed the 1000V limit of standard motors. If the cable length between the AC drive and a standard motor exceeds 25 feet, a load reactor or dv/dt filter is always recommended. Always follow the manufacturers cable length guidelines closely when using standard motors in 380V AC drive applications.
A servo drive system, as opposed to a variable frequency drive (VFD) or AC motor drive (AMD), has the ability to position with extreme precision and repeatability. While a VFD or AMD can be configured for position control, it cannot attain the exactness of a servo drive. VFD's and AMD's are also available in much higher horsepower ranges than servo's. Selection of external feedback sensors such as encoders, resolvers, linear position sensors, etc., required to achieve closed-loop control and each having its own inherent precision, will enable the system accuracy accordingly. Your application requirements will always dictate which system configuration will give you the best results!

Many of today's VFDs are fully capable of doing positioning when using encoder feedback; as with many other things, the lines have become blurred. What separates servo motor from induction motors is their low inertia and their ability to accelerate and decelerate much faster, the trick is to decide when you need full servo performance.
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