Changing the Speed of an AC Motor

Assuming its a single phase motor, it depends on the type of motor. If its a universal type, just varying the voltage will change the speed, but it will also reduce the power. A synchronous motor, would have to be rewound to add a tap to the wiring.

Know what you mean! I did 10 years in NY, but I'm a Louisiana man.

The most common and reliable way for A.C. motor speed control is a variable frequency drive. They're readily available from all major electrical component manufacturers but a bit pricey! However, they've dominated the industry in speed control. The older way (and still around) is using d.c. motors with SCR drives. That's pricey too!

If it is a single-phase motor, you may be able to find an economical VFD, perhaps from an HVAC supplier. I'm not sure, but I think they are used for controlling small condenser fans in your HP range. Note that this a variable-torque (less torque at low speed) application, which might not work if you need full torque at the lower speed.

hi

you are telling that its a 1/3 hp motor that means its a fractional hp AC motor. here u didnt mentioned the type like induction motor, universal motor or synchronous motor.

for fractional hp motor its better to use voltage control only. this will be cheap and reliable.

this can be done by using solid state devices like thyristor or triac..but here the power is going to reduced by reducing the voltage...

v/f method is also there but its bit reliable for high power motor.

If it is a 4 pole motor it is possible to bring out winding connections and by using switches/ relays/ contractors you can use it at two speeds. If it is 6 pole winding then you get 3 speeds. If it is already 2 pole winding then you have to rewind the entire thing. In 1986 I have converted many small 3 phase 1440 (50Hz) motors to 2920 RPM by converting 4 pole to 2 pole winding.(for M/S Gillette Inc.)

Get a multi speed motor, less expensive than VFD and available most anywhere.

If your spec states that it is a 2-speed motor, then all you need to do is apply voltage to the low speed tap or the high speed tap when desired (easiest way to do this is with a selector switch). If your motor does not have leads designated for low and high speed in the make up enclosure, then the motor is not intended for multi-speed control.

Most single phase motors designated as "2-speed" automatically switch from low speed to high speed during startup when a centrifugal switch is triggered as the motor picks up speed. The low speed winding gives the motor higher torque to help with start-up then the switch to high-speed reduces torque making the motor run more efficiently. I would not recommend deliberately removing the centrifugal switch from a single phase motor in order to control speed on command because the windings are not rated for continuous operation at high torque. I also would not recommend attempting to vary speed to such a motor with a VFD because the centrifugal switch will cause undesirable operation as the speed of the motor increases and decreases.

Multi-speed AC motors have internal taps specifically for speed control. These motors typically have 2-4 sets of windings that act as magnets in the stator. Different positioning of the magnets yields different speeds. The switching between speeds occurs either as the result of a logic circuit or more simply with a manual multi-position switch (like a box fan).

The speed of standard AC motors that do not have internal centrifugal switches can be controlled using a VFD to change the frequency of the AC going to the motor. This gives true variable speed, but cost is an important factor (a VFD for a 1/2 HP motor costs around $300).

brar47; buy a 1/3 hp 3 phase motor & a vfd @ 1 hp 120 volt single phase or 230 single volt single or 3 phase & program the speed you want. perry

Use a TRIAC/DIAC motor speed control. Should be able to Google and find a circuit for variable speed control.