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jclemmojoe

Member
Location
VA
Occupation
Retired
Reducing speed of a 240 volt single phase motor from 3450 rpm to 1725 rpm by changing input voltage to 120 volts?
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
jclemmojoe said:
Reducing speed of a 240 volt single phase motor from 3450 rpm to 1725 rpm by changing input voltage to 120 volts?
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Welcome to the forum.

No, definitely not. You'll just burn it up.

You need either a speed controller or a different motor.

Unless you can alter the drive ratio in some way.

What's the application?
 
C

Carultch

Senior Member
Location
Massachusetts
jclemmojoe said:
Reducing speed of a 240 volt single phase motor from 3450 rpm to 1725 rpm by changing input voltage to 120 volts?
Click to expand...
Generally speaking, an AC motor is meant to rotate at a speed that is directly determined by the frequency of the AC waveform. If it doesn't have enough voltage to maintain a near-synchronous speed, the rotor will stall and the windings become a short circuit. So you'd also have to run it on 30 Hz power to run the same motor at half speed.

A DC motor can run at a variable speed by varying voltage, but a standard AC motor cannot. An AC motor needs a VFD drive to provide a custom frequency, for operating it at a custom speed.

3540 RPM comes from 3600 RPM, which is directly determined by 60 Hz. It doesn't precisely match 3600 RPM, due to its slip ratio. 1725 RPM has a similar relationship to 1800 RPM, which is also determined by 60 Hz. The geometry of the magnets and coils is what make the difference between ~3600 RPM motors and ~1800 RPM motors, allowing the latter to rotate at a speed that's about half the grid frequency.
 
G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
In a related story... A friend of mine once controlled the speed of a tape machine by powering it from the speaker output of a large solid state bass guitar amp with an oscillator on the input. He turned up the amplifier volume until the voltage was 120V and set the oscillator at 60Hz to get it rolling; after that he controlled the speed of the tape machine with the oscillator frequency.
 
D

Drod04Mustang

Member
Location
Avon Park Florida
Occupation
Industrial Electrician-Residential Electrician
Change gear box sprocket would be easiest or add a VFD(a little more expensive but can really dial it down or speed to to full potentiall of motor) We have these at certain intervals throught the packing jar facility where we need more fined tuned control.. TECO WestingHouse L510 or Durapusle10 are pretty cheap and straightforward to program. I just had a 3450 electrical motor 1/2 hp 230v 3 phase on conveyor belt that was going to fast. 3450 like the label specified. A welder changed out the spocket(gear box spocket from a 26 point sprocket to a smaller -18 point -20 point spocket and toned it down to about 1950-2000rpm roughly. I did not have a RPM tachmometer to measure the exact speed but it did the job.
 
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