Method for maintenance of a frequency converter and software program realizing the same
The invention relates to a method for maintenance of a frequency converter for driving an electric motor of a transportation device, said frequency converter being supplied with AC mains power via a contactor, and including a rectifier circuit for providing a DC supply voltage, a capacitive intermediate circuit for leveling DC supply voltage, and an inverter circuit for providing power to the electric motor. In the method, after the transportation device has been vacant for a predetermined time period, said contactor is opened to disable power flow from mains supply to the frequency converter, and a test load is connected to the intermediate circuit, parallel to a capacity of the intermediate circuit. The DC supply voltage U dc (t) is detected, and the detected DC supply voltage U dc (t) is utilized for establishing a maintenance information indicating that a maintenance should be done on the frequency converter. Another aspect of the invention is a software program realizing the method when executed on a computer.
1. Method for maintenance of a frequency converter configured to drive an electric motor of a transportation device, said frequency converter being supplied with AC mains power via a contactor, and including a rectifier circuit for providing a DC supply voltage, a capacitive intermediate circuit for leveling DC supply voltage, and an inverter circuit for providing power to the electric motor, wherein said method comprises:
after the transportation device has been vacant for a predetermined time period, opening said contactor to disable power flow from mains supply to the frequency converter;
connecting a test load to the intermediate circuit, parallel to a capacity of the intermediate circuit;
detecting the DC supply voltage U dc (t); and
utilizing the detected DC supply voltage U dc (t) for establishing a maintenance information indicating that a maintenance should be done on the frequency converter, the maintenance information being established before a failure arises, wherein the test load is one of a braking chopper circuit, a machinery brake, and a motor winding, wherein the maintenance information is indicative of a certain kind and/or severity of problem of the frequency converter, wherein the established maintenance information is transferred or made accessible to a remote maintenance center or a mobile service unit or a local control unit of the transportation device, depending on the 25 kind and/or severity of problem indicated by the maintenance information.
2. The method of claim 1 , wherein connecting the test load includes activating a braking chopper circuit included in the frequency converter for dissipating electric energy.
3. The method of claim 2 , further comprising monitoring and/or storing patterns of the detected DC supply voltage U dc (t), and utilizing the monitored and/or stored DC supply voltage patterns for establishing the maintenance information, preferably by at least one of:
determining a trend in the monitored and/or stored DC supply voltage patterns, and establishing the maintenance information if the trend in the DC supply voltage patterns is judged to be abnormal,
comparing a difference between the monitored and/or stored DC supply voltage pattern and an initial voltage pattern of the particular frequency converter, and establishing the maintenance information if the difference is judged to have reached a predetermined threshold value; and
determining a rate of voltage change
dU
dc
(
t
)
dt
in the monitored and/or stored DC supply voltage pattern, and establishing the maintenance information if the rate of voltage change is judged to have reached a predetermined threshold value.
4. The method of claim 2 , further comprising:
estimating a capacitance of the intermediate circuit based on the detected DC supply voltage U dc (t); and
establishing the maintenance information if the estimated capacitance of the intermediate circuit is judged to have reached a predetermined threshold value.
5. The method of claim 1 , further comprising monitoring and/or storing patterns of the detected DC supply voltage U dc (t), and utilizing the monitored and/or stored DC supply voltage patterns for establishing the maintenance information, preferably by at least one of:
determining a trend in the monitored and/or stored DC supply voltage patterns, and establishing the maintenance information if the trend in the DC supply voltage patterns is judged to be abnormal,
comparing a difference between the monitored and/or stored DC supply voltage pattern and an initial voltage pattern of the particular frequency converter, and establishing the maintenance information if the difference is judged to have reached a predetermined threshold value; and
determining a rate of voltage change
dU
dc
(
t
)
dt
in the monitored and/or stored DC supply voltage pattern, and establishing the maintenance information if the rate of voltage change is judged to have reached a predetermined threshold value.
6. The method of claim 5 , further comprising:
estimating a capacitance of the intermediate circuit based on the detected DC supply voltage U dc (t); and
establishing the maintenance information if the estimated capacitance of the intermediate circuit is judged to have reached a predetermined threshold value.
7. The method of claim 1 , further comprising:
estimating a capacitance of the intermediate circuit based on the detected DC supply voltage U dc (t); and
establishing the maintenance information if the estimated capacitance of the intermediate circuit is judged to have reached a predetermined threshold value.
8. The method of claim 7 , further comprising detecting a test load current i test (t) flowing in the test load, wherein the capacitance C of the intermediate circuit is estimated e. g. based on the equation
i
test
(
t
)
=
C
dU
dc
(
t
)
dt
.
9. The method of claim 7 , wherein, if the test load is the braking chopper circuit, the capacitance C of the intermediate circuit is estimated e. g, based on the equation
U
dc
(
t
)
R
BC
=
C
dU
dc
(
t
)
dt
,
where R BC is the resistance of the braking chopper.
10. The method of claim 9 , further comprising detecting temperature information on the test load, and determining resistance of the braking chopper R BC based on the detected temperature information.
11. The method of claim 7 , further comprising:
establishing a rate of change of the intermediate circuit capacitance based on estimated capacitances of the intermediate circuit; and
establishing the maintenance information if the established rate of change of the intermediate circuit capacitance is judged to have reached a predetermined threshold value.
12. The method of claim 7 , wherein the threshold value is set to be a predetermined fraction or percentage of an initial value or a nominal value of the capacitance of the intermediate circuit.
13. The method of claim 7 , wherein the threshold value is chosen such that the maintenance information is established before the failure arises.
14. The method of claim 7 , wherein if the test load is a motor winding or a machinery brake, the method further comprises:
controlling the frequency converter so that current can flow through the test load;
detecting the DC supply voltage at a time t=t0 Ud(t);
detecting the current i(s) of the test load; and estimating the capacitance C 1 of the intermediate circuit based on the transformation, where s is the complex variable used for Laplace transformed functions, i(s) is the current of the test load, C is the capacitance of the intermediate capacitor, L is either the inductance of the machinery brake or the main inductance of the motor, and R is either the resistance of the machinery brake or the resistance of the motor winding.
15. The method of claim 7 , wherein if the test load is the machinery brake, the capacitance of the intermediate circuit is estimated based on the equation
U
dc
(
t
0
)
R
MB
2
-
4
L
MB
C
(
e
(
-
R
MB
2
-
4
L
MB
C
-
R
MB
2
L
MB
)
t
-
e
(
R
MB
2
-
4
L
MB
C
-
R
MB
2
L
MB
)
t
)
=
C
dU
dc
(
t
)
dt
where U dc (t 0 ) is the voltage of the DC circuit at the moment when the test load is connected, R MB is the resistance of the machinery brake, and L MB is the inductance of the machinery brake, or, if the test load is the motor winding, the capacitance of the intermediate circuit is estimated based on the equation
U
dc
(
t
0
)
R
MW
2
-
4
L
MW
C
(
e
(
-
R
MW
2
-
4
L
MW
C
-
R
MW
2
L
MW
)
t
-
e
(
R
MW
2
-
4
L
MW
C
-
R
MW
2
L
MW
)
t
)
=
C
dU
dc
(
t
)
dt
where R MW is the resistance of the motor winding and L MW is the inductance of the motor winding.
16. The method of claim 7 , wherein the comparing and establishing steps are executed at a remote monitoring unit or data analysis platform which preferably is located in a cloud computing system.
17. The method of claim 7 , wherein the determining and estimating steps are executed at a local control unit of the transportation device.
18. The method of claim 1 , wherein the transportation device is selected from one of an elevator, an escalator, a moving walkway, a cablecar, a railway locomotive, a railcar, a roller coaster, a conveyor, a crane, a positioning unit, and combined systems of a plurality of single units of the same.
19. The method of claim 1 , wherein establishing the maintenance information includes defining a maintenance action and/or scheduling the maintenance action or another maintenance action.
20. A software program realizing the method according to claim 1 when executed on a computer, wherein the computer is preferably a distributed computing system part of which being located in a cloud computing system.