Question

Can I use a motor starter protector with Variable speed drives and inverters?

Answer

They are only recommended for multiple motor applications from the output of a single drive. The drive handles the aggregate current monitoring, not the individual motor branch current.

NEC article 430 53c “group motor rating” deals with the concept of conventional across the line motor starters and the requirements of short-circuit protection from feeder to motor branch circuit. These guidelines do not mention variable speed drives. The recommendation is always to follow the manufacturer specification. In this case Hitachi recommends J class fuses.

You would never use a MSP, because it is not rated as a SSPD (short circuit protective device) in North America, in place of these J class fuses. There is also no need of an overload on the output side of the drive unless there are multiple motors being used.

Always check with your local inspectors and municipalities before an installation to verify their local requirements which may differ from the National Code.

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FAQ Subcategory: None

This item was last updated on 2002-01-22
Document ID: 899

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