Functions Required of Motor Protective Relays
Motor Protective Relay applications can be grouped by purpose into the
1. Protecting the motor itself (burnout protection)
2. Minimizing damage to the load connected to the motor
(In this case, you must select a Motor Protective Relay that is suitable
for the load rather than the motor.)
Motor Protective Relays have the following functions built in to provide
functions (1) and (2) above.
1. Overload elements (Overcurrent elements and time elements)
2. Open-phase elements
3. Reverse-phase elements
These functions are described in more detail below.
The motor will catch fire if it is
overloaded and overcurrent is supplied for an extended period of time.
This is why overload current must be instantly detected to shut down the
motor and protect it from damage.
Unfortunately, overcurrent around 500% the rated current is generally
present with inductive motors anywhere from several seconds to several
tens of seconds at startup as shown in Fig. 1. If the motor's overcurrent
element operates at that point, the motor will shut down immediately upon
startup and will not run. Therefore, Motor Protective Relays need to have
an overcurrent element that detects whether current exceeding the rated
value is being supplied to the motor as well as a time element that will
not operate with overcurrent present only during motor startup time, but
will operate if overcurrent is present beyond that time. In other words,
the time element is required to prevent faulty Motor Protective Relay
operation when the motor starts.
The time element is required for another very important reason. Fig. 2
shows the Iēt curve for the temperature characteristics of the motor. The
motor will operate normally and will not burn as long as it is operated in
the area below this curve. In the example, the motor can operate normally
even though overcurrent around 500% the rated current is supplied to the
motor for 40 seconds. If the current is halved to 250%, then we get the
and the motor will operate normally for up
to four times the time in the previous example, or 160 seconds. If we try
100% the rated current, then we get the following:
meaning the motor will last only 1,000
seconds according to the preceding formula. However, the formula is not
applicable in this case because the motor can in fact operate continuously
since 100% is the rated current. The curve then is an approximation rather
than a precise representation of Iēt.
This does not mean that the motor will burn up immediately with that much
overcurrent supplied to it. There is some built-in time flexibility to
allow the motor to withstand the overload resulting from the starting
current and startup time. To allow the motor to operate within those
parameters then, overcurrent should not cause the Motor Protective Relay
to operate immediately. The better choice would be to use activation time
characteristics, or the so-called inverse time-delay characteristics,
selected to operate the Motor Protective Relay quickly for large currents
and take longer to operate for low currents to ensure motor operation in
the area below the curve shown in Fig.2. This is precisely why the time
element is added to the overload element. It prevents unnecessary work
stoppages that would occur should the Motor Protective Relay operate
immediately when overcurrent is present.
This is not true for every application, however. In cases where the load
is fixed and current exceeding the rated value clearly is an abnormal
condition for the load, then failure to shut down the motor immediately
when overcurrent is supplied will damage the load and the damage may
spread through the system. The fastest possible overload element
activation time is always preferable for load protection applications.
Even here, a significant motor starting current will be supplied even in
these applications, so an overload element is needed that will not operate
for a specified period of time at startup and can operate instantly in
subsequent operation. These are generally referred to as instantaneous
The preceding section describes why overload
elements require both a current-detecting element and a time element, but
how do you select these current and time values?
The JEM 1357 standard (Inductive and Static Protective Relays for
Three-phase Inductive Motors) stipulates that the must operate value
should fall between 105% and 125% of the current SV and the majority of
Motor Protective Relay manufacturers conform to this standard. This value
is sufficient for any motors unless otherwise specified.
JEM 1357 also stipulates a maximum time value of 40 seconds for
overcurrent that is 600% of the current SV and 4 minutes for overcurrent
that is 200% of the current SV.
JIS B 8324 (Submersible Motor-Pumps for Deep Well) stipulates that
elements must operate within 5 seconds for current five times the total
load current to protect motors. There are various types of general Motor
Protective Relays with activation times ranging from several seconds to
several tens of seconds for overcurrents of 500% the rated value.
Fig.1 Motor Startup Current
Fig. 2 Motor Temperature Characteristics
and Protection Curve
Summary of overload
The current element of the overload elements
will not operate at the rated current and is generally set to operate at
125% of the rated value. Set the time to several seconds or several tens
of seconds with an overcurrent of 500% of the rated value and set it
longer than the motor's startup time. With inverse time-delay
characteristics, the time characteristics are normally set for quick
activation with high currents and slow activation with low currents.
Some time elements operate with the same
time characteristics at startup and in subsequent operation and other time
elements have a time delay only at startup and operate instantly to
protect loads connected to the motor in subsequent operation. The latter
are generally referred to as instantaneous elements.
Open Phase Detection
The operation resulting from an open motor power line or a motor that was
originally supposed to operate with three-phase voltage, but is currently
operating with single-phase voltage because of loose connections, faulty
control switch connection, or an open wire in the motor is called
If the motor starts up under open-phase conditions, startup current will
be supplied indefinitely because a stopped inductive motor will not start
with a single-phase voltage. Therefore the overload element described
earlier can be used to detect this condition and prevent the motor from
burning up. Should an open-phase condition occur during normal operation
and result in single-phase operation, however, the three-phase inductive
motor will continue to operate as a single-phase inductive motor if the
load is light enough. Refer to Fig. 3. The three possible conditions are a
power-supply open phase in a star-connected motor, a power-supply open
phase in a delta-connected motor, and an internal delta open phase. Let's
see whether we can prevent the motor from burning up here simply by
inserting an overload element in the power supply line.
1. Open-phase in a Star-connected Motor
As shown in Fig. 3 (a), the power supply line current and the motor
winding current are the same no matter where the line is open. This means
the motor will not burn up because the overload element on the power
supply line will detect overcurrent if an open-phase condition occurs. The
overload element cannot detect overcurrent if it does not reach the
element when the motor load is light, but the motor will not burn up here
again because the current is too low and operation will continue with the
Fig. 3 Open-phase Current Distribution
2. External Open-phase with
a Delta-connected Motor
Let's see what happens with the situation shown in Fig. 3 (b). If we make
I the winding current during normal operation, then the power supply line
current is . If we
make In the rated current in the windings, then the rated current of the
power supply line will be
and the overload
element will monitor the equivalent winding current to determine whether
In < I by observing whether
In the open-phase condition shown in (b), however, the power supply line
current will be 3/2In when I = In. This means that 3/2 In <
, or 1.5 In < 1.732
In. Depending on the overload status of the motor then, the overload
element will not operate because the power line current will be below the
rated current even with overcurrent in the windings and the windings may
burn. Therefore a separate phase-detecting element must be installed in
such cases to prevent the motor from burning.
3. Internal Open-phase with a Delta-connected Motor
Let's look at the situation in Fig. 3 (c).
With I1 and I2, |I1| = |I2| just as in normal operation with a phase
difference of 120°. The V-phase power supply line current then is
again just as in
normal operation while the U- and W-phase currents are I1 and I2,
respectively. Since an overcurrent higher than that found in normal
operation is supplied to the windings when viewed from the standpoint of
the power supply line, the overload element will detect the overcurrent
and prevent the motor from burning. The wiring here then is essentially
the same as that in (1)
. The description so
far has been directed toward preventing the motor from burning.
Fig. 4 Vector Diagram for Open-phase Power Supply Line Current
An open phase is an abnormal condition. It
is not advisable to allow the motor to continue to run with an open-phase
condition even with a light load. The slightest increase in the load could
stop the motor from running. Short-circuiting or electrical shock
accidents could also occur if someone were to touch the outer covering of
a disconnected inductive line.
The fundamental principle of protection is
to detect and dispose of error conditions immediately and therefore
protection is not limited strictly to preventing the motor from burning.
The fundamental principle is always to detect open-phase conditions as
quickly as possible.
In other words, open-phase elements are generally installed in situations
like those shown in Fig. 3 (a) and (c) to speed up the detection process
because it takes a while for overload elements to detect open-phase
conditions with light loads rather than merely preventing motors from
There is one other thing to keep in mind about open-phase elements. The
U-, V-, and W-phase currents in Fig. 3 (a) and (b) are balanced during
normal operation, but the power supply line current of the open phase when
an open-phase condition occurs will drop straight to zero and single-phase
reciprocating current will be supplied to the other two phases. In this
case, the vector relationship before and after the open-phase occurrence
changes dramatically as shown in Fig. 4 (a).
We know intuitively that the vector in Fig. 3 (c) will change the way it
did in Fig. 4 (b) and that it is not as significant as that in Fig. 4 (a).
In fact, it will be harder to detect an open phase with the situation in
Fig. 3 (c) than with the situation in Fig. 3 (a) and (b).
Normally the condition shown in Fig. 3 (a) and (b) is referred to as open
phase and the condition shown in Fig. 3 (c) is referred to as internal
star-delta open phase. The most common detectable open phases are those
shown in (a) and (b), so special care must be taken when using star-delta
inductive motors (the most common of which are 1.5 kW or higher).
Fig. 5 Current-type Open-phase Element
There are other methods that use motor
voltages rather than current transformers to detect open phases as shown
in Fig. 5. This method cannot, however, detect an open phase on the motor
side because of where the connecting point for open-phase detection is
located. An open phase cannot be detected on the power supply side with
light load operation either because the motor terminal voltage will not
drop very much. For these reasons, the current method is the better choice
Open-phase Element Summary
An overcurrent element alone may not be
able to prevent an open phase from burning up a motor. An open-phase
element is needed as well.
An open phase does not generate overcurrent with light loads and
therefore overloads cannot be detected in that situation. An open-phase
element is required to quickly detect the abnormal condition.
Open phase generally means an open power supply line and special care
must be taken with internal star-delta open
phases because open phases cannot be detected in many cases.
Voltage-type elements are also available,
but current-type elements are far more practical.
If the phase sequence is reversed with three-phase inductive motors, the
rotating direction of the motor will be reversed. There are no
applications that allow motor rotation in just any direction and even the
briefest reversal may be fatal to the load. This is why we have a
reverse-phase element that will instantly detect when the phase sequence
of power supplied to the motor is reversed.
Fig. 6 Comparison between Voltage and Current-type Reverse-phase
Current- and voltage-type elements can be
used for reverse-phase detection as in open-phase detection. Refer to Fig.
6. If a reverse-phase element is installed prior to the magnet contactor
as shown in Fig. 6, a reverse phase can be detected before the motor is
started. This can prevent the motor from operating in reverse even briefly
as mentioned earlier. The fastest the current-type element can operate is
0.5 s, so some reverse operation is unavoidable. This makes the
voltage-type element more practical. The drawback to the voltage-type
element, however, is that there is one more connection to the Motor
Protective Relay, and a VT must be installed when using a high-voltage
motor, for example. The advantage of the current-type element is that it
can determine the phase sequence of current supplied to the motor
directly, but the drawback as mentioned earlier is that it requires a
little time before it can start detecting (the motor starts before the
reverse current is detected).
In both cases, the phase sequence is only detected in reverse-phase
detection at the connection point (power line position for voltage
detection and CT position for current detection). This means that special
attention must be paid to this point during installation.
It is extremely rare that the phase sequence would be reversed after the
motor was installed, and thus a reverse-phase element is often not
required. There are cases, however, when installing a reverse-phase
element would be best, such as in applications where connections are often
changed, such as for motors with mobile power supplies, or in applications
where connections are changed for maintenance.
Reverse-phase elements are not needed in
Voltage and current detection have
advantages and disadvantages. The advantages are that voltage detection
can detect reverse phase prior to startup and current detection can be
used to monitor the motor's current phase sequence directly.
Motor Protective Relay Characteristics
There are several problems that must be
addressed with using a motor and Motor Protective Relay. The following
section describes an open-phase voltage drop, motor current waveform
distortion, capacitor installation to improve the power factor, and
unbalanced motor currents.
Open-phase Voltage Drop
(Power Supply Connection Precautions)
The voltage supplied to the Motor Protective
Relay and magnet contactor does not go to zero even if the V phase is open
(open wire) as shown in Fig. 7. Only about half the voltage between the U
and W lines (Vuw) is applied because Vuw is split by the X and Y windings
of the motor and the impedance of the magnet contactor excitation coil and
the Motor Protective Relay power supply circuit is significantly higher
than that of the X and Y windings. You can choose here to have the Motor
Protective Relay detect the open phase and operate with just half the
rated voltage to shut down the magnet contactor or have it reset
automatically because it cannot operate on half the voltage.
Protection will be disabled, however, if the magnet contactor power supply
and Motor Protective Relay power supply are out of phase. As shown in Fig.
7, for example, both the Motor Protective Relay and magnet contactor are
drawing power from the U and W phases, but if we make it so only the
magnet contactor is drawing power from the U and W phases, then the magnet
contactor will not reset despite an open V phase because the rated voltage
will be applied to the contactor and the Motor Protective Relay cannot
operate because only half the voltage is applied to it. Therefore you can
operate the Motor Protective Relay with half the voltage or reset the
magnet contactor with half the voltage through careful wiring. Care is
required either way.
Fig. 7 Open-phase Voltage Drop and
Phase-advance Capacitor Position
Note: An open phase will not drop the
voltage to half while the motor is operating with a light load because
near-normal voltage is inversely supplied from the motor side. In that
case, the protection expected from magnet contactor release voltage is not
Motor Current Waveform
Motor Current Waveform
for a Ventilation Fan
Current should be supplied to the motor with
the sine wave shown in the figure above, but highly distorted current
waveforms have been observed in some battered, old submersible motors that
are operating perfectly fine.
Motor Protective Relays are designed for sine wave input, so distortion of
the waveform will increase activation current value errors with overload
elements or cause open- and reverse-phase elements to operate improperly.
You could conclude from this that voltage-type fast activating
reverse-phase elements are preferable over current types because there is
generally less distortion in the voltage waveforms of motors. However,
manufacturers have conducted countless studies on the problem of waveform
distortion and there is very little difference between the two types
Capacitor Installation to
Improve the Power Factor
Install an advance-phase capacitor parallel to the motor as shown in Fig.
7 to improve the motor's power factor. Current supplied to a motor with a
light load is almost always reactive and susceptible to infusion of higher
harmonics. Installing a capacitor parallel to the motor will negate the
fundamental harmonic component leaving only higher harmonics. If the
harmonic current is significant, then the open-phase and reverse-phase
circuits designed for 50/60 Hz sine wave input tend to malfunction. This
rarely occurs because the harmonic current is generally low. It is safer,
however, to install the capacitor before the Motor Protective Relay as
shown in Fig. 7 to lower the odds of circuit malfunction.
If the capacitor is installed after the Motor Protective Relay, the motor
current appears to drop. The problem is that you have to anticipate that
drop when setting the overcurrent must-operate value for the Motor
Protective Relay. From that standpoint, you should apply no more than the
motor current to the Motor Protective Relay.
Structure of the Static Motor Protective
Relay (Example: OMRON's SE Series)
A current converter consists of a current
transformer that steps down motor current to a level the transistor
circuits can handle, a three-phase full-wave rectifier diode that
rectifies the secondary current from the current transformer, and a
resistor that converts the rectified current to a DC voltage.
This current converter in particular has been designed for use with a wide
range of motor currents. The taps in the current converter can be switched
to vary the resistance in three levels. You could, for example, output 21
V from the current converter by connecting a 600-Ω resistor for a motor
current of 80 A, a 1,200-Ω resistor for a motor current of 40 A, or a
2,400-Ω resistor for a motor current of
20 A. The output from the current collector is always 21 V, so the Motor
Protective Relay can be set to operate at 20 A, 40 A, or 80 A simply by
switching the taps using 21 V for activation.
(Note: The voltages and resistances are
The current converter can also be operated at 10 A by selecting the 20-A
activation tap and winding the motor power line twice around the converter
so 10 A looks like 20 A. Similarly, it can be operated at 5 A by winding
the power line around the current converter four times.
Fig. 8 Internal Block Diagram of the SE-series Static Motor Protective
Relay (Inverse Time-delay Type)
1. These figures are plug-in terminal
numbers. The parentheses indicate panel-mounted terminal symbols.
2. Terminal 3 (W) does not have to be wired if the reverse element is OFF.
The current converter output is input to the Motor Protective Relay via
lines to terminal 7 (C+) and terminal 8 (C−). (The figures here are
plug-in terminal numbers and the parentheses indicate terminal numbers for
embedded models.) There the current is first split by the current scale SV
circuit and then input to the overcurrent detection circuit. The current
scale SV circuit is a simple potentiometer-type voltage splitter that can
convert current must operate values by varying the split rate. If the 20-A
tap is selected here, then the must operate value can be set between 8 and
20 A by turning the potentiometer knob. When overcurrent occurs, the
overcurrent detection circuit will detect it and drive the next time SV
circuit. This time SV circuit has the inverse time-delay characteristics
described earlier and its time characteristics are shown in Fig. 9.
Fig. 9 Overload Activation Time Characteristics (Reference values)
Time Changeover Switch: Ũ 1
Time Changeover Switch: Ũ
The time SV circuit also contains a
potentiometer. The operating time can be varied between 2 and 10 s by
turning the potentiometer know when current 600% of the current SV is
supplied. It also has a multiplier switch that can increase the operating
time by a factor of 4, that is between 8 and 40 seconds. One feature of
the time SV circuit then is the wide time selection range from 2 to 40 s.
When overcurrent flow continues beyond the
SV time settings, the output relay (X) is energized from the output
circuit through the OR circuit, the contact switches from X to c, and the
alarm, breaking, or other action associated with the contact is operated.
With an instantaneous Motor Protective Relay, the motor is considered to
have started when motor current exceeds the rating by at least 30% and the
start time circuit will begin operating. There is no output for a
prescribed period of time, however, regardless of the size of the input
current. The overcurrent detection circuit operates immediately and
generates an output, but the motor will not operate without the AND
output. After the startup time has elapsed, motor current ranging from the
rating to 50% of the rating will be supplied and the startup time circuit
output will continue without interruption.
The overcurrent detection circuit will operate as soon as overcurrent is
detected and the (X) relay will operate less than 0.5 s later.
Fig. 10 Internal block diagram of the SE
Static Motor Protection Relay (instantaneous type)
The box () on inverse
time-delay models indicate the following:
X Relay Operation
(Manual Reset Type)
SE: The (X) relay is a mechanically
self-holding relay that operates until it is manually reset even if the
power goes out.
The (X) relay remains locked even if the power goes out because it uses a
keep relay system. A power supply is required to reset the relay.
(Automatic Reset Type)
The relay will reset automatically if the power drops below the set value.
(An open phase will reduce the operation power supply output if the relay
is used in conjunction with an open-phase element. Therefore, use another
power supply other than the motor power supply for the U and V phases.
Reverse-phase elements cannot be
used for this reason.)
Fig. 11 Current Converter Output Waveform
Fig. 11 (a) shows the rectified output
waveform during normal operation, (b) shows the waveform with an open
phase, and (c) shows the waveform with an open phase in a star-delta
inductive motor. From this we know that the DC component is large, the AC
component is small, and the frequency component is high from at least the
6th harmonic during normal operation. We also know that the DC component
is small, the AC component is large, and the 2nd AC component harmonic is
the largest with an open phase. We can conclude then that an open phase
can be detected with a circuit structured to respond to the second
harmonic/DC component ratio or the AC component/DC component ratio.
Fig. 12 Structure of an Open-phase
The static Motor Protective Relay conforms
to this principle, so an open-phase has occurred when the ratio of the DC
component extracted by one filter and the second harmonic component
extracted by another filter from among the current converter outputs (more
precisely the split voltages) has been exceeded. As shown in Fig. 13, the
operating time of the relay with an open phase is approximately 1.5
seconds regardless of the current.
Fig. 13 Open-phase Operating Characteristics
It has been found, however, that the 2nd
harmonic component/DC component approximates an unbalanced rate function.
This means that the Motor Protective Relay is designed more for unbalanced
detection than for open-phase detection and it is set to operate when the
unbalanced factor reaches approximately 35%. Fig. 14 shows how the
unbalanced factor varies with the current value.
Fig. 14 Unbalanced Operation
The current plotted on the horizontal axis
represent the largest of the three phases. When the open-phase detection
circuit provides an output, the (X) relay will operate via the OR circuit
shown in Fig. 8.
Fig. 15 shows the schematic and Fig. 16 shows vector diagrams for a
reverse-phase detection circuit. Here, the 1.5 Vuv between d and e in
normal operation drop to 0 V with a reverse phase and basic reverse phase
detection is achieved when 0 V is applied to the transistor. The (X) relay
is operated by the output from the reverse-phase detection circuit via the
OR circuit. The U and V phases supply power to the
Motor Protective Relay, so you might have felt compelled to disconnect
terminal 3 (W) because you did not need a reverse-phase element. This
could have the opposite effect, because the Vuv can split between RP1 and
CC1 and split voltage applied to the transistor may cause it to operate.
It may not operate if the voltage input is low
enough, but you can never be sure. You may have to switch to a circuit
without a reverse-phase element in this case. This same Motor Protective
Relay can be used at 50 or 60 Hz and therefore Vde never really drops all
the way to zero even in normal operation. The difference in Vde between
normal and reverse-phase operation is significant enough, however, to
ensure stable operation.
Fig. 15 Structure of a Reverse-phase
Fig. 16 Vector Diagrams for a
Reverse-phase Detection Circuit
Fig. 17 (a) and (b) are examples of external connections. Terminal 3(W)
does not have to be connected if a reverse-phase element is not used.
The same phase should be input to the excitation coil of the magnet
contactor and power supply terminals (U and V) of the Motor Protective
Install an advance-phase capacitor on the power supply side after the
Make sure the polarities are correct when connecting the current
converter to the Motor Protective Relay. You do not have to be
particularly concerned about the current capacity of the connecting lines
because the current is normally only a few mA or several tens of mA at
most. By the same token, 600-V insulated lines may be used because the
voltage is normally several tens of volts or less and never exceeds 400 V
even with overcurrent. Environmental factors like noise are not a real
concern either, except that high current carrying lines should always be
kept as far away as possible.
Be sure to connect the correct phase to the voltage terminals (U, V,
and W). The phase sequence is not relevant when a reverse-phase element is
not required (reverse-phase element OFF) because only terminals U and V
The U, V, and W terminals should be wired before the magnet contactor
to enable reverse-phase detection before the motor starts.
Fig. 17 External Connections for a Star-delta Startup Motor
Fig. 17 (b) External connections for
no-voltage tripping with high-tension inductive motors
Unbalance Motor Current
The unbalance rate for motor current normally does not exceed several
percentage points, but an unbalance rate of 10% to 20% or more has been
observed in motors that have been in use for many years and motors that
are supplied power through V-wired transformers.
Refer to the simple unbalance rate calculation method in the reference
information on the following page. Use this method for
measurement. Delta internal phase detection may not be possible if the
unbalance rate exceeds 20%.
According to the law of symmetrical coordinates, the unbalance rate is
are the current of
the three phases and the vector operator is as follows:
This gives us a scale for expressing
three-phase current and the extent of voltage unbalance. These are
difficult calculations, however, and the following calculation table is a
simpler way to work out the unbalance rate.
Fig. 18 is a graph for determining the unbalance rate if you know the
absolute values for the three phase inputs.
If we have a three-phase AC input, for example, with a phase A current, IA,
of 50 A, a phase B current, IB, of 35 A, and a phase C current,
IC, of 45 A, then we can use current IA as a base
and divide the other phase currents by IA to find the unbalance
rate, as follows:
Check to see if arc B starting from the
point KB = 0.7 on the vertical axis on the right side intersects at point
P1 with arc C starting from the point KC = 0.9 on the vertical
axis on the left side. The point P1 is located on the circle representing
an unbalanced rate of 20%. In this case, then, we determined that the
unbalanced rate will be 20%.
Similarly, KA = 1.0, KB = 1.3, and KC =
1.0 when IA = 50 A, IB = 65 A, and IC =
50 A. The intersecting point for these arcs is P2, and P2 is close enough
to the circle representing an unbalanced rate of 20% that we can conclude
the unbalanced rate here too is 20%. The intersecting points for arcs
emanating from any KC and KB
combination falls on the 20% unbalanced rate circle and all have an
unbalanced rate of 20%. We know from this that there are an infinite
number of combinations with the same unbalance rate.
Similarly, D1 through D8 represent all combinations with an unbalance rate
If we consider star-delta P1XY then XY = IA,
P1X = IC, and P1Y= IB. In
other words, star-delta P1XY yields a vector diagram for IA,
IB, and IC.
Fig. 18 Calculating Table for the
Unbalanced Rate of Three-phase Current and Voltage
There are all kinds of motor circuit failures and protective devices
designed for specific purposes must be used to protect against such
failures. The following describes various motor accident details and
There are a variety of protective devices, including 3E relays, thermal
relays, and motor circuit breakers, and they are summarized below. All the
protective devices for motors have specific functions.
Unfortunately none of these functions can reach their full potential
unless they are used properly. This is why you must select the proper
device for the type of protection you require.
Inductive Motor Protection
Variation in the types of inductive motors available today leads to many
types of failures.
The following table lists appropriate
protective devices for these motors.
Purpose of Protection and Applicable Protective
Relays for Inductive Motors
Protection by 3E Relay
As discussed above, there are various motor protective equipment and
devices available. The 3E Relay is provided with three features to protect
motors: protection from overload, open phase, and reverse phase. These
three features of the 3E Relay are discussed next.
The overload protection feature of the 3E Relay protects the motor from
overload when it operates normally. With this feature alone, the motor can
be protected most of the time. To ensure effective protection, it is
necessary to set correctly the "must operate voltage" and operate time of
the 3E Relay.
The PC characteristics curve of the 3E Relay, (which is used as a
yardstick when designing a motor protection system) can be drawn in the
Protective Coordination of Motors
When designing a motor system, adequate considerations must be given to
whether the motors and their circuits can be effectively coordinated with
other equipment and devices. It is important that each motor operates in
protective coordination (PC) with the devices listed in the following
Can it switch sufficient
Can it withstand
Molded case circuit
Can the device interrupt
short circuit current?
Will the motor
malfunction when subjected to rush current?
Branch circuit wiring
Can it withstand the
short-circuit current until the MCCB operates?
Can it withstand the
overload current until the MCCB, thermal relay, or 3E Relay operates?
Thermal relay or 3E relay
Can it protect the
motor, when the motor is overloaded or constrained?
Can it protect the
motor, when the motor is overloaded or constrained?
Creating a PC Characteristic
As examples, correct and incorrect PC
characteristic curves are shown below.
Note: The current value at the end of the
load line must be lower than this value.
An open-phase condition refers to single-phase motor operation that
results from a broken motor power line or loose power line connections,
faulty switch contact, or a broken wire in a motor. When it occurs, the
phase current increases markedly compared to the increase in motor line
current, the winding temperature exceeds the
permissible limit, and the motor will burn up in some cases. Open-phase
protection is needed here rather than overload protection. The figure
shows open-phase accidents and current fluctuations. The point of the
figure is to show that the increase in phase current is relatively higher
than the increase in line current in cases 2, 3, and 5 in the figure and
that line current overload detection cannot always detect failures.
There is no need to set the activation values on the user end because
open-phase detection sensitivity is fixed. Examples are provided here,
however, because these failures are so difficult.
The motor rotating direction will be reversed if the phase sequence is
reversed. Reverse-phase elements will detect the phase sequence of the
power supply and will lock up the motor during startup if the phases are
reversed. Phase sequence detection is no longer needed once the elements
are properly installed. Although one side effect of the elements is motor
protection, the main purpose is to protect the
Open-phase Accidents and
Solid-state Relay enables choice of
three operating functions (over-current, open-phase, and
Solid-state Relay provides three
operating functions in a compact package.