How do I slow down an ac motor?

Question

How can I slow down an 120 volt ac motor it's a drift drive buffer. It is about 3500 Rpms I want it to be around 2500. Would a variable speed drive work? I'm in an electrical class but did not get that far.

Answer 1

Variable-speed drives are intended for situations in which the speed needs to be changed from time to time. If the speed reduction is to be a one-time change from one fixed speed to another you should first consider replacing an existing set of gears or drive-belt pulleys to change the final-drive speed. If there is no existing speed-change mechanism between the motor and the driven-machine, consider adding a mechanical speed reducer.



If the speed needs to be changed from time to time, you need a variable-speed drive. Since the existing motor is a 120 volt AC motor, it must be a single-phase motor. Single-phase motors are not usually suitable for use with variable-frequency drives (VFDs). There are exceptions, but I don’t think it is worth while to try to explain the exceptions.



A dimmer type device is not a good option. They are only suitable for specific types of motors and specific types of loads. It is very unlikely that you have a suitable motor.



If you need a variable-speed drive, you will probably need to buy a suitable motor and controller. Here is one place to start:

http://www.automationdirect.com/adc/Shopping/Catalog/Drives

Answer 2

An AC motor's speed is determined by the frequency of the commercial electrical supply, 60 hertz here in the US. If you had a 30 hertz source it would go half the speed. This is where a variable speed drive comes into play, commonly referred to as a VFD in industry (I happen to use them in my business). The VFD takes the commercial power input at 60 hertz and allows you to adjust the speed by changing the frequency (hertz) of the output which connects to the motor. VFDs have gotten much more reliable, cheaper, and smaller over the last 15 years or so. 2500 rpms will require around 42 hertz which should work for your motor. Some motors don't like going too low and we often limit the VFD output to 50% of max, in the US that would be 30 hertz on the output.



The other option would be a gear box. There are many companies that specialize in that type of thing. A VFD is easier for existing installations because you can mount it anywhere near the motor (replacing the motor starter relay), while the gear box has to be attached to the output shaft of the motor.

Answer 3

120v Ac Motor

Answer 4

a triac based dimmer device would do the job i suspect.. you will need to consider the inrush current at turn on when considering what rating to use.



take a look at > http://electroschematics.com/444/motor-speed-regulator-with-triac/



note it says DC motor, it means AC motor.. the circuit can be adapted for 120V ac and is a good starting point.