I have a 3 speed single phase A/C motor inside an indoor air-con blower and it runs very noisy (not mechanical/installation issue but just the fan moving air is noisy itself) even on its LOW speed setting. Can anyone advise a way to slow the motor speed down to about 50% of its current speed. I'm not too worried about the MED and HIGH speed setting but if can just do for LOW speed setting would be great. (If slow for LOW, MED and HIGH together is also fine.)

I looked at VFD's but they all seem to be for 3-phase motors. Also someone suggested to join the HIGH and MED speed wires on the motor to the LOW speed and at the controller to put HIGH and MED to Ground.

Thanks in advance for any help. Picture attached.

enter image description here

  • \$\begingroup\$ What does the data for the motor tell you about possible modifications? \$\endgroup\$
    – Andy aka
    Jan 15, 2020 at 12:33
  • 1
    \$\begingroup\$ As a broad general statement, the speed of an AC motor is controlled by the construction of the motor and the AC line frequency. I'm afraid you're not going to get far without a VFD. That said, many VFDs will accept single-phase input, and you might find one that will also power a single-phase motor. But I suspect you're in the realm of serious control mods at that point, basically bypassing the entire existing speed control. It will definitely void your warranty. \$\endgroup\$ Jan 15, 2020 at 12:39
  • 7
    \$\begingroup\$ Gears. Or pulleys and a belt, probably quieter. \$\endgroup\$
    – user16324
    Jan 15, 2020 at 12:46
  • 1
    \$\begingroup\$ This is a motor with selectable taps for initial speed setting, rather than it already being a variable speed system under automatic control? If so, the speed should have been set to produce the appropriate delta T across the evaporator by balancing the fan performance against the duct resistance - assuming the blower was near correctly sized to begin with. If so, slowing the blower down further risks freezing up the evaporator. There are single phase VFDs available - effective they're really two phase, as they use the auxiliary winding as a second phase. \$\endgroup\$
    – Phil G
    Jan 15, 2020 at 14:57
  • \$\begingroup\$ Thank you for all your replies. Andy: There is no motor specific data. The air-con blower was pre-installed. Brian: Gears / pulleys are not so practical option as it's pre-fitted to size. It would be a big effort to have to re-mount the motor in the ceiling space. Phil: Thank you. Maybe single-phase VFD output is the way to go. \$\endgroup\$
    – RichB
    Jan 19, 2020 at 4:44

1 Answer 1


Reduce the voltage.

eg: connect it to 110V instead of to 220V

It's a fan motor it's designed to drive a speed-dependent load and run at reduced speed by slipping, reducing the supply voltage will increase the slip and reduce the speed without causing any damage.

  • \$\begingroup\$ May stall the motor and then cause unexpected heating because airflow is off for motor. \$\endgroup\$
    – KalleMP
    Oct 31, 2020 at 10:45
  • \$\begingroup\$ it's a blower, running the load slower will reduce the torque on a square law, it's very hard to stall a blower. \$\endgroup\$ Nov 1, 2020 at 2:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.