From: "tedinoue" <etinoue@...>
Subject: query - 2HP PM DC motor controller - pointers please
I recently purchased a 2HP PM DC motor, hoping to build/buy a
controller so that I could use this on a small lathe. For those
interested, it's a Leeson motor, designated 120v 14A, 4800 RPM.
Model C42D340T7A. I plan to reduce the maximum speed then feed it
into my existing lathe drive system.
...
1) I've seen a lot of postings discussing 90v DC from 110v AC and
180v DC from 220v AC, but little to nothing on driving a motor rated
for 120v. I'd get plenty of power out if I ran it at 90v, so I'm ok
with a controller that does the basic 110AC -> 90DC conversion. Any
reason why this wouldn't be suggested?
How does one get 90VDC from 110VAC? By using a switching power supply, or
PWM? (In which case there is nothing magic about 90V, you could get any
amount of DC up to sqrt(2) * 110V minus a few volts for various losses.)
2) If I do run it at 90VDC, that's 3/4 of 120VDC, so would I simply
then treat the controller needs as 1.5HP / 90 VDC and 10.5Amps?
The current drawn by a DC motor is proportional to the torque that is
generated. The speed of a DC motor is proportional to the voltage across
the motor terminals minus the voltage drop through the resistance of the
motor windings and brushes. You can use the full torque (full current) at
any speed (voltage). One thing to watch out for is that the motor has
adequate cooling if running at low speed and high torque, because the heat
generated is proportional to the current, but the built-in fan won't provide
much cooling at low speeds.
I haven't seen mention of this elsewhere, so maybe it isn't a concern, but
if you operate a DC motor at full torque and stalled or nearly stalled (< 1
rev/sec), the current will go through a single winding for a long time, thus
all the heat is concentrated in that one winding, possibly leading to
overheating of that winding. If the motor is intended for servo
applications, this should be accounted for in the ratings, but otherwise,
you may want to limit the current to less than the rated current if the
motor will be operated in high torque and stalled or near-stalled
conditions.
Cheers,
- Jan
