AC Motor Selection and Cabling

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.

One way to get around the use of a reactor or filter in many 380V applications is to use a motor specifically designed for AC drive applications. Special inverter duty motors utilize insulation that is designed to accommodate spikes up to 1600V and has a temperature rating of Class F or H. In some cases, these motors can be up to 300 feet downstream of the AC drive and still not require reactors or filters. Again, always follow the manufacturers cable length guide lines.

I will end this section with one final cautionary note. Avoid 380V operation when using standard motors with a frame size of 180 or smaller (180 = 5 to 7.5 HP, 56 = 2 to 3 HP). The reason I say this is that these motors tend to be manufactured with price as the major objective. The insulation system is not as robust as that of larger frame motors and they have an inherently high impedance. Since these motors are usually wound for 230/380V operation and therefore utilize 600V insulation they can be used, safely in 220V AC drive applications.
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