# DC motors

Discussion in 'Home Power and Microgeneration' started by mark Ransley, Oct 10, 2003.

1. ### Nick PineGuest

Leviton's 6639 AC motor speed control seems cheap enough.
Grainger's sells it for \$16.38. I think it's essentially a lamp dimmer.

Nick

2. ### daestromGuest

Reduce power factor from 1.0 to 0.5, current to supply the same 'real' power
must double.

Losses in windings, wiring, transmission are I^2R, so double the current,
four times the losses.

daestrom

3. ### daestromGuest

Uh, losses are I^2R, so double the current means 4X losses.
daestrom

4. ### daestromGuest

These types of AC motor speed control only work with 'universal' motors that
have brushes (like hand-drills or 'skil' saws). They won't work for
capacitor-start motors.

Lowering the voltage to a universal motor (by phase-shifting a triac) works
by reducing the voltage applied to both the field and armature windings.
But induction motors will overheat if you try to get rated hp from them with
much-reduced voltage. To change their speed requires a change in frequency
(or a very specially build 'high-slip' rotor design).

And universal motors aren't available above a small fraction of a hp

daestrom

5. ### Nick PineGuest

The Grainger catalog says they are for "split capacitor or shaded-pole motors."

Nick

6. ### Steve ThomasGuest

You are absolutely right.
I can be a bit foggy before the morning coffee.

7. ### Mike GloverGuest

*doh* (slaps head) You're right, or course. Thanks.

8. ### Steve SpenceGuest

a ac motor may be 80-90 % efficient. if a dc motor was 50 more efficient,
that's only 90-95% total. dc motors tend to be lower voltage requiring
heavier wiring for a given wattage. DC is good in mobile applications, or
off grid cabins where pv is used for generation, where ac does not exist. In
ac applications, ac would have to be rectified to dc, and incurs a energy
loss (heat).