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

On a given fan speed, the pressure drop influences the amp, ECM or not. The ELI5 way to explain is that the fan always want to run at the speed it is designed to, and any back pressure will give resistance to the fan and the fan needs to use more power. By shortening the duct there are now less duct to push air through hence less pressure drop and the power and amp of the fan reduces.