[0001] This invention relates to a control method for consuming in an inverter the regenerated
power from an induction motor that drives an elevator.
[0002] When braking is applied during operation of an elevator with an induction motor driven
by an inverter, the rotating speed of the motor is higher than the frequency of the
inverter, and regenerated power is formed in the motor. As this regenerated power
flows into the dc circuit of the inverter, a resistor is set in the dc circuit and
it is used to consume (absorb) the regenerated power.
[0003] Figure 3 shows the configuration in an example of the conventional regenerated power
consumption method. As shown in this figure, the principal circuit of the inverter
has converter (1) and inverter circuit (2). The 3-phase ac is converted to dc by converter
(1), and it is then converted to 3-phase ac by inverter circuit (2). In this case,
control is made by base driver (3) for induction motor control, so that the speed
of induction motor (4) can be controlled.
[0004] In the conventional power consumption method, a regenerated power consumption circuit
composed of resistor (5) and switch (6) formed of semiconductor elements in series
is inserted in parallel into the aforementioned principal circuit. On the other hand,
the dc voltage on the two ends of principal circuit capacitor (7) is detected by a
voltage detector (8). The signal from said voltage detector (8) is input to hysteresis
comparator (9). The magnitude of the dc voltage is the basis for ON/OFF control of
base driver (10) of said switch (6). In this way, the regenerated power formed in
the deceleration of induction motor (4) can be consumed by said resistor (5).
[0005] Figure 4 shows the voltage waveform of the principal circuit in the aforementioned
operation. As can be seen from this figure, with respect to the dc voltage of the
principal circuit, the switch ON level and switch OFF level of said regenerated power
consumption switch (6) are set at said comparator (9). As the dc voltage rises in
tandem with the regenerated power, the circuit is turned ON, the regenerated power
is thus consumed; then as the dc voltage falls, the circuit is turned OFF.
[0006] When the conventional regenerated power consumption method is used for an elevator,
as shown in Figure 5, the elevator has an induction motor (4) as the power source,
and has cage (12) and balance weight (13) loaded on winding drum (11). The velocity
pattern for acceleration, deceleration, and constant velocity is generated by a control
unit (14), and cage (12) may be stopped at any floor.
[0007] The maximum load of the elevator depends on the number of passengers in the cage,
etc. As there can be a significant number of passengers, there is a large regenerated
power in the case of deceleration.
[0008] In addition, the deceleration rate in the case of deceleration depends on the velocity
pattern, and the regenerated power varies depending on the passenger number and deceleration
rate.
[0009] Even in the case of constant velocity operation, when cage (12) is heavier than balance
weight (13) due to more passengers, regenerated power is formed when descending. On
the other hand, when cage (12) is lighter than balance weight (13), power is regenerated
when the elevator ascends. These regenerated power levels also vary as the passenger
number changes.
[0010] As explained above, the regenerated power of the elevator depends significantly on
the passenger number, and the operation status with deceleration or constant velocity.
Consequently, for hysteresis comparator (9), the switching frequency and the ON/OFF
ratio also depend significantly on the change in the regenerated power. In order to
realize reliable operation for switch (6), a switch which allows high-speed switching
operation up to several kHz must be used. In addition, it is difficult to design the
hysteresis width and ON/OFF operation level of hysteresis comparator (9) and to set
the resistance value of resistor (5). Also, the design of the circuit configuration
becomes complicated. This is a disadvantage.
[0011] The purpose of this invention is to solve the aforementioned problems of the conventional
methods by providing a regenerated power consumption method for elevators in which
a switch with a low switching speed can be used, and the circuit configuration for
the regenerated power consumption can be simplified.
[0012] In order to solve the aforementioned problems, this invention provides a regenerated
power consumption method in which the regenerated power generated when the elevator
is driven with an induction motor is consumed by a serial circuit comprising a resistor
and a switch and arranged in a dc circuit of the inverter. In this regenerated power
consumption method, there is a software configuration filter which performs filtering
treatment of the detected signal as the dc voltage of the aforementioned dc circuit
exceeds the ON/OFF level of the aforementioned switch; if the aforementioned dc voltage
rises or decreases, the aforementioned switch is ON/OFF controlled by the aforementioned
filter.
[0013] According to this invention with the aforementioned configuration, when ON/OFF control
of the switch is performed as the dc voltage of the inverter crosses the ON level
or OFF level of the switch, a filter treatment is performed for the dc voltage of
detection. In this way, even when the regenerated power varies, the switching speed
of the switch is limited by the frequency determined by the time constant of the aforementioned
filter; the switching speed needed by the switch can be set by the time constant of
the filter, and any type of switch element can be used. In addition, since the filter
has a software configuration, the setting can be changed easily, and the circuit design
may be realised by selecting the time constant of the filter and the resistance value.
[0014] An embodiment of the invention will now be described by way of example only and with
reference to the drawings, in which:
Figure 1 is a schematic diagram showing the configuration of an induction motor control
circuit according to this invention.
Figure 2 shows the filter operation waveform in the circuit of Fig. 1.
Figure 3 shows the configuration of a conventional circuit.
Figure 4 shows the voltage waveforms of the conventional circuit of Fig. 3.
Figure 5 shows the configuration of an elevator having an induction motor control
circuit.
[0015] Figure 1 shows the configuration of an embodiment of this invention. The configuration
shown in this figure differs from that shown in Figures 3 and 5 in the following respects:
the detected signal of voltage detector (8) is converted to a digital signal by an
A/D converter (15). The converted value is filter-treated by a filter (16A) in a control
unit (16) which comprises a CPU. The result of the filter treatment is taken as the
ON/OFF control signal for base driver (10).
[0016] Filter (16A) has a CPU software configuration. With respect to the sampling data
of dc voltage V
dc of the principal circuit of the inverter, the detected data are modified according
to the following formula: (Formula 1)
where,
V
dc(n): modified data
V
s: sampled input
V
dc(n - 1): data of last round
T: time constant of filter
dt: sampling time interval
That is, suppose the sampling treatment time dt is 10 ms, and the filter time constant
T is 30 ms, (1 + T/dt) becomes 4. Now, as 1/4 of the difference between current sample
V
S and last-round data V
dc(n - 1) is added/subtracted, the modified data V
dc(n) is delayed by the time constant T of the filter. Since the modified data V
dc(n) with the aforementioned delay is used to control ON/OFF switching of base driver
(1), the timing is delayed, and the switching frequency of switch (6) is limited.
[0017] For example, when dc voltage V
dc of the principal circuit of the inverter rises above the ON line, and a prescribed
time has passed after the OFF operation (the filter's time constant), switching is
made from OFF to ON, and the switching frequency is decreased. In this case, due to
the ON operation of switch (6), the regenerated power is consumed by resistor (5).
[0018] On the other hand, switching of filter (16A) from OFF to ON may be carried out in
any of the following cases:
(1) When troubles occur, such as insufficient voltage, overvoltage, overcurrent, etc.
(2) When dc voltage Vdc drops below the OFF level, and after a prescribed time (filter time constant) from
the ON operation.
(3) When a stop command is generated and the inverter is shutoff.
[0019] For above items (1) and (3), the operation is realized as controller (16) handles
the output frequency and operation state of the inverter and the state of the input
sequence.
[0020] For the regenerated power consumption operation with the aforementioned configuration,
as can be seen in Figure 2 from the filter output waveform with respect to dc voltage
V
dc, the maximum switching frequency is determined by the times T
H, T
L corresponding to the filter time constant.
[0021] As shown in Figure 2, when dc voltage V
dc first falls below the OFF level set at filter (16A) (at time t₁) and then exceeds
the ON level at time t₂, since the time interval between t₁ and t₂ is shorter than
a prescribed time T
H, the output of filter (16A) does not produce an ON operation, and the OFF operation
is continued.
[0022] When dc voltage V
dc exceeds the ON level at time t₃, since the time interval between t₁ and t₃ exceeds
the prescribed time T
H, the filter output performs the ON operation.
[0023] Similarly, when dc voltage V
dc falls below the OFF level at time t₄, since the time interval is shorter than a prescribed
time T
L (which may be identical to or different from T
H), there is no OFF operation; the OFF operation takes place at time t₅.
[0024] Also, when dc voltage V
dc exceeds the ON level at time t₆, as the time interval from t₅ to t₆ is shorter than
a prescribed time T
H, there is no ON operation, while the ON operation takes place at time t₈.
[0025] Consequently, for filter (16A), the switching output limited by the frequency determined
by times T
H, T
L set in the software is generated, and the maximum switching frequency for the regenerated
power consumption can be controlled by times T
H, T
L. For example, when times T
H and T
L are set at 30 ms, the maximum switching frequency is 33.33 Hz, and the regenerated
power consumption can be performed by using switch (6) with a switching speed appropriate
to this frequency.
[0026] As described above, the control of the regenerated power consumption is not limited
to the case of deceleration of the elevator, it may also be performed corresponding
to the variation in the regenerated power generated due to differences in the passenger
number and the ascending descending state in the constant speed mode. For the configuration
of the equipment, the time constant of the filter can be set appropriately by setting
the software, and filter (16A) itself can also be simplified by using the software
configuration.
[0027] In the aforementioned example, switch (6) and resistor (5) for the regenerated power
consumption are arranged as a single circuit. However, it is also acceptable to use
a configuration of multiple circuits with a selective switch operation. In this scheme,
even in the case when the variation amplitude of the regenerated power is abnormally
large and the switching operation of the maximum switching frequency determined by
the time constant of filter (16A) is inadequate, the aforementioned multiple circuits
can perform ON operation in a parallel way when an overvoltage for dc voltage V
dc is predicted; hence, overvoltage can be prevented. On the other hand, when an insufficient
voltage is predicted, the insufficient voltage can be prevented by the ON/OFF control
of one circuit.
[0028] As explained in the above, according to the described embodiment, the regenerated
power consumption is performed by detecting the dc voltage of the inverter, with the
dc voltage of detection filter-treated by a filter with a software configuration;
the maximum switching frequency of the switch is set by the time constant of the filter,
the filter time constant is set corresponding to the switching speed of the switch,
so as to ensure the regenerated power consumption. In addition, in the circuit design
and setting, it is only necessary to change the time constant of the filter, which
can be realized by making a simple change in the software. In addition, as far as
the circuit configuration is concerned, the conventional hysteresis comparator is
not needed, and only the function of the elevator control equipment is used.