14
Figure 20. Motor Output Horsepower vs. Motor Phase Current and Supply
Voltage.
Current Sensing Resistors
The current sensing resistor should have low resistance (to
minimize power dissipation), low inductance (to minimize
di/dt induced voltage spikes which could adversely
aect operation), and reasonable tolerance (to maintain
overall circuit accuracy). Choosing a particular value for
the resistor is usually a compromise between minimiz-
ing power dissipation and maximizing accuracy. Smaller
sense resistance decreases power dissipation, while larger
sense resistance can improve circuit accuracy by utilizing
the full input range of the ACPL-C78A/C780/C784.
The rst step in selecting a sense resistor is determining
how much current the resistor will be sensing. The graph in
Figure 20 shows the RMS current in each phase of a three-
phase induction motor as a function of average motor
output power (in horsepower, hp) and motor drive supply
voltage. The maximum value of the sense resistor is deter-
mined by the current being measured and the maximum
recommended input voltage of the isolation amplier. The
maximum sense resistance can be calculated by taking
the maximum recommended input voltage and dividing
by the peak current that the sense resistor should see
during normal operation. For example, if a motor will have
a maximum RMS current of 10 A and can experience up
to 50% overloads during normal operation, then the peak
current is 21.1 A (=10 x 1.414 x 1.5). Assuming a maximum
input voltage of 200 mV, the maximum value of sense re-
sistance in this case would be about 10 mΩ.
The maximum average power dissipation in the sense
resistor can also be easily calculated by multiplying the
sense resistance times the square of the maximum RMS
current, which is about 1 W in the previous example. If the
power dissipation in the sense resistor is too high, the re-
sistance can be decreased below the maximum value to
decrease power dissipation. The minimum value of the
sense resistor is limited by precision and accuracy require-
ments of the design. As the resistance value is reduced,
the output voltage across the resistor is also reduced,
which means that the oset and noise, which are xed,
become a larger percentage of the signal amplitude. The
selected value of the sense resistor will fall somewhere
between the minimum and maximum values, depending
on the particular requirements of a specic design.
When sensing currents large enough to cause signicant
heating of the sense resistor, the temperature coecient
(tempco) of the resistor can introduce nonlinearity due to
the signal dependent temperature rise of the resistor. The
eect increases as the resistor-to-ambient thermal resis-
tance increases. This eect can be minimized by reducing
the thermal resistance of the current sensing resistor or
by using a resistor with a lower tempco. Lowering the
thermal resistance can be accomplished by reposition-
ing the current sensing resistor on the PC board, by using
larger PC board traces to carry away more heat, or by
using a heat sink.
For a two-terminal current sensing resistor, as the value
of resistance decreases, the resistance of the leads become
a signicant percentage of the total resistance. This has
two primary eects on resistor accuracy. First, the eective
resistance of the sense resistor can become dependent
on factors such as how long the leads are, how they are
bent, how far they are inserted into the board, and how far
solder wicks up the leads during assembly (these issues
will be discussed in more detail shortly). Second, the leads
are typically made from a material, such as copper, which
has a much higher tempco than the material from which
the resistive element itself is made, resulting in a higher
tempco overall.
Both of these eects are eliminated when a four-terminal
current sensing resistor is used. A four-terminal resistor has
two additional terminals that are Kelvin connected directly
across the resistive element itself; these two terminals are
used to monitor the voltage across the resistive element
while the other two terminals are used to carry the load
current. Because of the Kelvin connection, any voltage
drops across the leads carrying the load current should
have no impact on the measured voltage.
When laying out a PC board for the current sensing
resistors, a couple of points should be kept in mind. The
Kelvin connections to the resistor should be brought
together under the body of the resistor and then run very
close to each other to the input of the ACPL-C78A/C780/
C784; this minimizes the loop area of the connection and
reduces the possibility of stray magnetic elds from inter-
fering with the measured signal. If the sense resistor is not
located on the same PC board as the ACPL-C78A/C780/
C784 circuit, a tightly twisted pair of wires can accomplish
the same thing.
MOTOR PHASE CURRENT - A (rms)
15
5
40
10 25 30
0
35
0 35
25
10
20
440 V
380 V
220 V
120 V
30
20
5
15
MOTOR OUTPUT POWER - HORSEPOWER
