First of all it's a real shock hazard. The motor insulation probably isn't
designed to withstand 240VAC. There could be leakage to the case. If you DO
try it, please connect a safety ground wire to the motor case.
Second, not all thyristors are suitable for controlling motors. In
particular, triac-based incandescent "light dimmers" may not always turn
off between cycles of the AC line with inductive loads such as motors. The
result is that you could end up applying full AC line voltage to your motor.
Third, if you run the motor at the full rated torque you'll likely burn up
the motor due to I^2R heating. Let's say the motor delivers rated torque
when the shaft is spinning at 2/3 of the unloaded RPMs. That tells us that
the "back EMF" produced by the motor is 2/3 of the rated voltage, i.e. back
EMF = 8V = 2/3 of 12V. And let's say the current is 1A at full load.
Now, assuming that 12V _DC_ is supplied: The voltage drop through the
resistance of the windings and brushes is 12V - 8V = 4V. Thus, we can
determine the motor resistance is R=V/I=4V/1A=4 ohms. And the power
dissipated by the motor as heat is I^2R=1A^2*4 ohms=4W.
But what happens when the current is supplied in narrow pulses? Torque is
proportional to current. To get the same _average_ torque, you need a much
higher _peak_ torque - and corresponding peak current.
The pulses are approximately triangular (the edge of the sine wave). For
50Hz, I calculate the triangle width as .87 ms wide and 23 amps at its
peak. (You can verify that that gives an average current of 1A.)
The peak power is I^2R=(23A)^2*4ohms=2.13KW. The average power _during the
pulse_ (integrate the power) is 1/3 of that, 711W. But the duty cycle is
.87 ms/ 10ms so the overall average power dissipated in the motor is about
62W. Considering our hypothetical motor was designed to dissipate 4W, smoke
Bottom line, running a 12V motor with a thyristor controller from a 240V
line does not seem like a good idea. Likely to be harmful to the motor, the
controller - and you! :-)
On Mon, Feb 18, 2013 at 10:37 AM, oakite2000 <stackerjack@...>wrote:
> I want to run a 12Volt D.C. motor from the mains power.
> Has anyone got any idea what would happen if I connected the motor to my
> half wave thyristor power controller,( which is designed for 240VAC
> operation), and turned the output down to minimum?
> If I connect my 240 VAC electric drill to the controller, it doesn't start
> turning until I rotate the control pot. by about 30 degrees.
> I don't want to try it without checking first, in case it burns out either
> the controller, the motor, or both.
> Thanks Guys
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