16
Figure 20. Motor output horsepower vs. motor
phase current and supply voltage.
the output voltage across the resistor is also reduced,
which means that the o set 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 speci c design.
When sensing currents large enough to cause signi cant
heating of the sense resistor, the temperature coe cient
(tempco) of the resistor can introduce nonlinearity due
to the signal dependent temperature rise of the resistor.
The e ect increases as the resistor-to-ambient ther-mal
resistance increases. This e ect can be minimized by
reducing the thermal resistance of the current sensing
resistor or by using a resistor with a lower tempco. Lower-
ing the thermal resistance can be accomplished by repo-
sitioning 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 re-sistance of the leads
become a signi cant percentage of the total resistance.
This has two primary e ects on resistor accuracy. First,
the e ective 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 e ects 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 ter-
minals 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 HCPL-7840; this minimizes the loop area of the
connection and reduces the possibility of stray mag-
netic elds from interfering with the measured signal. If
the sense resistor is not located on the same PC board as
the HCPL-7840 circuit, a tightly twisted pair of wires can
accomplish the same thing.
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 a ect operation), and reasonable tolerance
(to maintain overall circuit accuracy). Choosing a par-
ticular value for the resistor is usually a compro-mise
between minimizing power dissipation and maximiz-
ing accu-racy. Smaller sense resistance decreases
power dissipation, while larger sense resistance
can improve circuit accuracy by utilizing the full input
range of the HCPL -7840.
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 horse-power, hp) and motor
drive supply voltage. The maximum value of the sense
re-sistor is determined by the current being measured
and the maxi-mum recommended input voltage of the
isolation ampli er. The maximum sense resistance can
be calculated by taking the maxi-mum 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 op-eration, 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 resistance 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
resistance 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,
MOTOR PHASE CURRENT – A (rms)
15
5
40
10 25 30
0
35
035
25
10
20
440 V
380 V
220 V
120 V
30
20
5
15
