Variable Speed D.C Motor Drive

[G.Wadham]

I would like to hear from anyone who has built a D.C. Speed Control based on the article by C.Eyer in H.S.M. May/June 1996 P50.
Problems with my unit are :- Excessive sparking at the P.M. motor brushes and erratic low speed control. Any experience and advice welcome.

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[Larry Cantwell]

I got disconnected before I could finish my original reply. Anyway, disassemble the motor to check the condition of the brushes as well as the condition of the commutator. Next, if you have a friend who has an oscilloscope, place the scope across the motor while operating at low speed. Look for excessive voltage spikes. This may indicate that snubbers are needed across the motor.

[G.Wadham]

Thanks for the suggestion. Have cleaned commutator and brushes but still checking electronic components. May change value of capacitor C1 and/or the trigger diac. I did try a capacitor in parallel with D.C. output but it soon failed.

[This message has been edited by G.Wadham (edited 05-24-2001).]

[Bob Beveridge]

[QUOTE]Originally posted by G.Wadham:
[B]I would like to hear from anyone who has built a D.C. Speed Control based on the article by C.Eyer in H.S.M. May/June 1996 P50.

My archives don't go back that far. If you have a way to get copy of the schematic and component values if not shown on schematic to me I should have a suggestion or 2. Fax # is (949) 650 7868.

[Bob Beveridge]

[QUOTE]Originally posted by G.Wadham:
[B]I would like to hear from anyone who has built a D.C. Speed Control based on the article by C.Eyer in H.S.M. May/June 1996 P50.
Apologies for excessively long post. Wadham's e-mail bouncing.
G. Wadham : subj DC motor control
1. Suggested procedure:

a. With device unplugged connect a wire from minus terminal of
bridge (B1) to minus terminal on motor. This bypasses SCR, motor
now will see full wave rectified DC.

b. Remove any load from motor. This test will run it at high speed.

c. If you have access to a 3 amp or larger variac connect it in
the AC line leading to this device. If not available go to step e.

d. Set variac for 0 volts out, apply power, bring voltage up on
variac. Observe sparking at brushes, both at low and high speed. go
to step g.

e. ( no variac method) Tie/clamp/bolt the motor down. It will start
with a significant jerk, can roll itself off the bench top.

f. Apply power, expect motor to run at high rpm. Observe brushes.

g. In either case, the SCR controller is not being used, the motor
is now running on the "most kind to the motor" power that this type
of motor can ever hope to see. Full wave rectified, unfiltered
power. This is not as kind to the motor as filtered DC power, but
that doesn't occurr in the real world.

h. Small sparks right under the brush where it touches the
commutator are normal. Arcs which extend from the brush part way
(or all the way) to the other brush are not normal. For a bit of
feel for it, check out the brush sparking on some of your brush
type tools- Drill motor, body grinder etc.

I just checked a motor very similar to yours which drives my
vertical mill. It is driven by a variac followed by a bridge,
identical to the test outlined above. I needed to make it go slow
one day, had a variac, haven't gotten around to making up an scr
controller yet. The brushes are clearly visable, In the dark at
full line voltage there is just a light "twinkling" right under
the brushes. Two drill motors I observed have more sparking. They
look as if the brush was sitting on top of a thin layer of white to
green sparks. This I would consider normal.

If your results are not good, please describe the sparking in some
detail, also length of brushes, estimated force with which the
brushes are held against the commutator, condition of commutator
surface.

Snubbers and capacitors across the brushes at the motor don't help on perminant magnet motors since the inductance is in the armature- you can't get the snubber across it.

Unstable low speed also points to brush/commutator prob. Running it for a few minutes at high speed may clean it up.

The speed control range is a readily solved problem once we get the
arcing under control. Bob.

[G.Wadham]

Thanks for the in depth reply to my query.
Today I tested the motor running on D.C without the S.C.R control. It ran fast but without previous heavy sparking. Next I re-assembled the control unit after reducing the value of capacitor C1 (in the H.S.M diag.)to .04 uF. The motor runs more smoothly with less sparking but the lowest speed is too high. I plan to increase the value of resistor R1.
I am sorry you could not reach me by e. mail
perhaps if you have any suggestions regarding
the speed range you could post them. I will have to put this project on hold for several
weeks so could check out the posting then.
Thanks- G.Wadham.