reversing ac motor rotation

[jason random]

Hi all, I am trying to reverse the direction my circular saw rotates in. It uses a soft start ( no capacitor ). Is it just a matter of reversing the two stator wires or is something else needed? Thanks for any meaningful guidance.

[amyk]

You'll have to determine what type of motor it is first. Does it have brushes?

If it's a brushed series (universal) motor, you can reverse the two brush leads to reverse the direction.

If it's a single-phase induction motor, those usually can't be reversed unless they're designed for it with symmetrical windings (e.g. PSC).

[jason random]

thanks for the great reply. It's a brush type. Now I know they are called universal. So just remove the wire one side and install it the other side and vice versa?

[Gyro]

Yes, with a brushed universal motor, the direction is set by the relative polarity of the magnetic fields of the field winding and armature. If the wires are long enough, reversing the connections to the brushes will reverse the spin.

It's hard to tell from the photo whether the angle of the brush gear is rotated slightly to increase power in it's normal rotation direction, but if the brushes are at 90' to the stator pole pieces then it should run equally well in either direction (the brushes will be noisy for a while until they bed in to the new direction).

[Kleinstein]

One can not see from the mechanical position of the brushes if the motor will work OK in the reverse direction. The extra angle to get a preferred direction of rotation can be at the rotor.

Chances are relatively high that the motor has a prefered direction of rotation and runninng it the other direction would need a reduce voltage and reduced power.

[Benta]

^ this.
Practically all universal motors for power tools are timed for a certain rotation direction.
If you reverse it, you'll lose a lot of power and see a lot of brush fire.

[langwadt]

^ this.
Practically all universal motors for power tools are timed for a certain rotation direction.
If you reverse it, you'll lose a lot of power and see a lot of brush fire.

yep, and OP said concrete saw so the nut holding the blade will probably loosen when running the wrong way

[Gyro]

Ah yes, nut loosening could be a serious problem!

[jason random]

thanks everyone for your valued input. Some food for thought there but I think I'll just swap over the brush wires and keep the fire extinguisher handy.  . It came with a spare pair if needed. It's to be used for a small job so with luck performs ok and lasts the distance. I'm not really concerned with the loss of power for this particular application, it just needs to rotate CW.
The brush wires are too short for a simple swap over but can be easily extended to make the distance.
I'm aware of the nut issue but have plenty of loctite handy and are capable with a welder so one way or another it's staying on.

[Kleinstein]

Excessive sparking at the brushs can damage the motor. If run reverse with a preferred direction winding, one should reduce the power / voltage for that. As a relatively simple though not ideal bodge maybe a series diode.

[Gyro]

... As a relatively simple though not ideal bodge maybe a series diode.

The OP doesn't say what country he's in, but passing large DC currents like that will temporarily blind (saturate the current transformer of) most AC spec RCDs / RCBOs / GFCIs(?) disabling their ground fault protection while the tool is in use. This may be an issue for a portable tool, particularly if the concrete cutting blade is water cooled. Something to be aware of.