Why do Non-ECM fan motors draw less amps if connected to a duct tube after decreasing girth of the duct work at the end? (Assume fan motor is literally sitting in middle of a room and has a duct tube running couple feet – ie not a fan motor how it would be in usual hvac system)

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Hey everyone!

Why do Non-ECM fan motors draw less amps if connected to a duct tube after shortening the girth of the duct work at the end? (Assume fan motor is literally sitting in middle of a room and has a duct tube running couple feet – ie NOT a fan motor how it would be in usual hvac system)

NOTE: Or would they only draw less amps actually if in a system where the occlusion is “behind them” so to speak ie where its less air they are taking in)?

Thanks!!!

In: Engineering