System, device, and method for over-current relays protecting motors
A system, method, and device for protecting an induction motor are disclosed. The exemplary system may have a module for determining the current drawn by the motor and a module for determining the state of the motor. The system may calculate a used thermal capacity based on a first formula when the motor is in an active state. The system also calculates the used thermal capacity based on a second formula when the motor is in an inactive state. When the used thermal capacity attains a threshold, the relay is tripped thus removing current to the motor and preventing motor from overheating. A method to derive thermal time constants from desired trip time limits (such as those defined by IEC standards or the thermal limit curves provided by motor manufacturers) is also presented. For example, the thermal time constants can then be used in tacking motor used thermal capacity throughout various motor states.
1. An over-current relay protecting an induction motor comprising:
a module for determining the current drawn by the motor;
a module for determining the state of the motor;
a module for calculating a used thermal capacity based on a first formula when the motor is in an active state, wherein the first formula for used thermal capacity is selected from the group consisting of
θ
n
=
M
n
2
×
Δ
t
T
h
+
θ
n
-
1
×
(
1
-
Δ
t
T
h
)
and
θ
n
=
M
n
2
(
1
-
ⅇ
-
Δ
t
T
h
)
+
θ
n
-
1
ⅇ
-
Δ
t
T
h
wherein n is a sample number, Δt is a sampling period, T h is a thermal time constant, θ n is the used thermal capacity at an n th sample, and M n is multiple of a pickup current of the relay at the n th sample;
a module for calculating the used thermal capacity based on a second formula when the motor is in an inactive state; and
a module for tripping the relay when the used thermal capacity attains a threshold.
2. An over-current relay protecting an induction motor comprising:
a module for determining the current drawn by the motor;
a module for determining the state of the motor;
a module for calculating a used thermal capacity based on a first formula when the motor is in an active state;
a module for calculating the used thermal capacity based on a second formula when the motor is in an inactive state, wherein the second formula for used thermal capacity is selected from the group consisting of
θ
n
=
θ
n
-
1
×
(
1
-
Δ
t
T
c
)
and
θ
n
=
θ
n
-
1
ⅇ
-
Δ
t
T
c
wherein n is a sample number, Δt is a sampling period, T c is a thermal time constant, and θ n is the used thermal capacity at an n th sample; and
a module for tripping the relay when the used thermal capacity attains a threshold.
3. The relay of claim 1 , further comprising:
a module for calculating the used thermal capacity based on a third formula when the motor is in an acceleration state.
4. The relay of claim 3 , wherein the third formula for used thermal capacity is selected from the group consisting of
θ
n
=
M
n
2
×
Δ
t
T
h
-
acc
+
θ
n
-
1
×
(
1
-
Δ
t
T
h
-
acc
)
and
θ
n
=
M
n
2
(
1
-
ⅇ
-
Δ
t
T
h
-
acc
)
+
θ
n
-
1
ⅇ
-
Δ
t
T
h
-
acc
wherein n is a sample number, Δt is a sampling period, T h-acc is an acceleration thermal time constant, θ n is the used thermal capacity at an n th sample, and M n is multiple of a pickup current of the relay at the n th sample.
5. The relay of claim 1 , further comprising:
a module for calculating the used thermal capacity based on a fourth formula when the motor is in a deceleration state.
6. The relay of claim 5 , wherein the fourth formula for used thermal capacity is selected from the group consisting of
θ
n
=
M
n
2
×
Δ
t
T
h
-
decel
+
θ
n
-
1
×
(
1
-
Δ
t
T
h
-
decel
)
and
θ
n
=
M
n
2
(
1
-
ⅇ
-
Δ
t
T
h
-
decel
)
+
θ
n
-
1
ⅇ
-
Δ
t
T
h
-
decel
wherein n is a sample number, Δt is a sampling period, T h-decel is a deceleration thermal time constant, θ n is the used thermal capacity at an n th sample, and M n is multiple of a pickup current of the relay at the n th sample.
7. The relay of claim 1 , wherein the relay complies with International Electrotechnical Commission standard 60947-4-2 and amendment 2 of International Electrotechnical Commission standard 60947-4-2.
8. A method for an over-current relay protecting an induction motor comprising the following actions:
determining the current drawn by the motor;
determining the state of the motor;
calculating a used thermal capacity based on a first formula when the motor is in an active state, wherein the first formula for used thermal capacity is selected from the group consisting of
θ
n
=
M
n
2
×
Δ
t
T
h
+
θ
n
-
1
×
(
1
-
Δ
t
T
h
)
and
θ
n
=
M
n
2
(
1
-
ⅇ
-
Δ
t
T
h
)
+
θ
n
-
1
ⅇ
-
Δ
t
T
h
wherein n is a sample number, Δt is a sampling period, T h is a thermal time constant, θ n is the used thermal capacity at an n th sample, and M n is multiple of a pickup current of the relay at the n th sample;
calculating the used thermal capacity based on a second formula when the motor is in an inactive state; and
tripping the relay when the used thermal capacity attains a threshold.
9. A method for an over-current relay protecting an induction motor comprising the following actions:
determining the current drawn by the motor;
determining the state of the motor;
calculating a used thermal capacity based on a first formula when the motor is in an active state;
calculating the used thermal capacity based on a second formula when the motor is in an inactive state, wherein the second formula for used thermal capacity is selected from the group consisting of
θ
n
=
θ
n
-
1
×
(
1
-
Δ
t
T
c
)
and
θ
n
=
θ
n
-
1
ⅇ
-
Δ
t
T
c
wherein n is a sample number, Δt is a sampling period, T c is a thermal time constant, and θ n is the used thermal capacity at an n th sample; and
tripping the relay when the used thermal capacity attains a threshold.
10. The method of claim 8 , further comprising the following actions:
calculating the used thermal capacity based on a third formula when the motor is in an acceleration state.
11. The method of claim 10 , wherein the third formula for used thermal capacity is selected from the group consisting of
θ
n
=
M
n
2
×
Δ
t
T
h
-
acc
+
θ
n
-
1
×
(
1
-
Δ
t
T
h
-
acc
)
and
θ
n
=
M
n
2
(
1
-
ⅇ
-
Δ
t
T
h
-
acc
)
+
θ
n
-
1
ⅇ
-
Δ
t
T
h
-
acc
wherein n is a sample number, Δt is a sampling period, T h-acc is an acceleration thermal time constant, θ n is the used thermal capacity at an n th sample, and M n is multiple of a pickup current of the relay at the n th sample.
12. The method of claim 10 , further comprising the following actions:
calculating the used thermal capacity based on a fourth formula when the motor is in a deceleration state.
13. The method of claim 12 , wherein the fourth formula for used thermal capacity is selected from the group consisting of
θ
n
=
M
n
2
×
Δ
t
T
h
-
decel
+
θ
n
-
1
×
(
1
-
Δ
t
T
h
-
decel
)
and
θ
n
=
M
n
2
(
1
-
ⅇ
Δ
t
T
h
-
decel
)
+
θ
n
-
1
ⅇ
-
Δ
t
T
h
-
decel
wherein n is a sample number, Δt is a sampling period, T h-decel is a deceleration thermal time constant, θ n is the used thermal capacity at an n th sample, and M n is multiple of a pickup current of the relay at the n th sample.
14. The method of claim 12 , wherein a standard IEC trip class curves is based on induction motor thermal model IEC standard 60947-4-2 and amendment 2 of IEC standard 60947-4-2.
15. The system of claim 1 , wherein the used thermal capacity tracks the thermal capacity throughout various motor states.