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)

Question

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

Answer 1

Restricting air flow by adding an obstacle means, paradoxically, less work for the fan. The more air a fan is moving, the more it has to work. If you restrict the flow, it’ll get less air to move and it’ll draw less power/current. The extreme scenario is removing all the air, i.e. placing it in a vacuum, and then it’ll barely draw any power.