■ WITH RUSSELL KINCAID
In this column, I answer questions about all
aspects of electronics, including computer
hardware, software, circuits, electronic theory,
troubleshooting, and anything else of interest
to the hobbyist. Feel free to participate with
your questions, comments, or suggestions.
Send all questions and comments to:
Join us as we delve into the
basics of electronics as applied
to every day problems, like:
●✓Power Supply Filter Caps
✓Tool Setter Modification
I need your help on how
to know or estimate the
DC motor parameters if I
haven’t got the datasheet
for the motor.
Mohammed S. Salah
AThere are five basic motor
types that I know about:
shunt wound, series wound,
permanent magnet, and brushless
DC. Without taking the motor apart
to see how it is wired, you can make
some determination by measuring the
There is an IEEE paper: “A method
for torque-speed curve determination
for a DC compound motor without
loading the motor” by Zhaohui Zeng
and E. Rechie. The paper costs
■ FIGURE 1
money so I didn’t read it, but it no
doubt would be of interest to you.
The shunt wound motor may
have two or four terminals. Four
terminals will allow you to control the
motor speed and torque by varying
the field current. More current yields
lower speed and higher torque. The
motor is capable of high torque over
a wide range of speed (see Figure 1).
The series wound motor only has
two terminals and the only control is
the applied voltage. The motor has high
torque at low speed and can overspeed
if not loaded. Automotive starter motors
and most traction motors are series type.
The compound motor combines
the high starting torque of the series
motor with the high torque at high
speed, characteristic of the shunt motor.
The speed-torque curve will depend
on the amount of series winding. The
curve shown in Figure 1 is typical.
The permanent magnet motor
has a wound armature with brushes
but the field is provided by
permanent magnets. This is similar
to a shunt motor with constant field
current; the available torque will
linearly decrease as the speed
increases. The brushless DC motor is
a permanent magnet motor and has
the same speed-torque curve. It is
not shown separately in Figure 1.
In order to estimate the horsepower of a motor, you need to know
the current rating. You can measure
the diameter of the wire to the
armature and look up the current
rating in a handbook. The power
input (Pin) is Vin times Iin and the
horsepower input is Pin/746. The
horsepower increases with voltage,
and the limiting factors are how fast
the motor can spin without damage
and heat dissipation of I2R losses. A
rule of thumb is 1/2 watt dissipation
per square inch of motor surface.
QI’m looking for a
circuit to divide 40
MHz by seven (down
to 5.714 MHz) with a
4: 3 mark-to-space ratio output.
Can you help me with such?
AA fast counter is
needed for 40 MHz; I
chose the 74F161A, a
synchronous four bit
presettable binary counter (see