Mechanical difference between 1725 and 3650 AC induction motors?

[hylourgos]

What's the essential mechanical difference between these two standards? Are there minor mods you can make to switch them?

Curious, H

[djs]

Induction motors run slightly slower than the synchronous speed they are designed at. For example, the windings can be designed to make the stator's magnetic field rotate around the rotor once per 60 Hz cycle. In one minute at 60 Hz (60 cycles per second) the field rotates around the motor 3600 times. The rotor, which consists of bars running sort of parallel to the shaft, has current induced in these bars (that is why they call it an induction motor) which reacts with the spinning magnetic field to produce a force, and thus a torque.

At "no load" (a freely running motor) this induction motor will spin at nearly 3600 rpm. When load is applied the motor will slow down slightly. At the rated load the motor will slow to its rated speed, something like 3500 - 3550 RPM.

An induction motor rated at 3550 RPM is designed with two poles in the stator (the windings on the outside of the rotor) such that the magnetic field rotates around the rotor once per 60 Hz cycle.

An induction motor rated at 1750 RPM is designed with four poles in the stator such that the magnetic field rotates around the rotor once every

2 cycles of the 60 Hz.

There is no way to convert 3550 RPM induction motor to run at 1725 RPM.

There is also no way to run it at 3650 RPM unless you use an electronic speed controller that can increase the frequency of the power driving the motor.

DJS

[Robatoy]

[snipped for brevity]

Excellent explanation.

r

[Lew Hodgett]

RE: Subject

2 pole motors = 3600 RPM 4 Pole motors = 1800 RPM 6 Pole motors = 1200 RPM 8 Pole motors = 900 RPM

The above are synchronous RPMs.

Induction motors require some "slip" to develop torque thus ave values are as follows:

3450, 1725, 1150, and about 825 RPM.

Lew

[hylourgos]

Hey DJS, thanks, that was lucid (you too Lew). I guess the only way to approach this is from a drive perspective.

Again, Thanks, H

[hylourgos]

I ran across this dual-speed motor, but there doesn't seem to be any room for drives in order effect the speed change. How do they do it on this model?

H

[Robert Bonomi]

you can design a motor with a switchable number of poles. it will run at different speeds, depending which sets of coils, etc. are hooked up to the power at any given time.

Multi-speed motors for GFA furnaces are one of the more common examples.

[hylourgos]

Thanks, H