U.S. patent application number 09/477753 was filed with the patent office on 2001-11-08 for electric motor and method for operating such a motor.
Invention is credited to ROTH-STIELOW, JORG, SCHMIDT, JOSEF.
Application Number | 20010038270 09/477753 |
Document ID | / |
Family ID | 26038040 |
Filed Date | 2001-11-08 |
United States Patent
Application |
20010038270 |
Kind Code |
A1 |
ROTH-STIELOW, JORG ; et
al. |
November 8, 2001 |
Electric motor and method for operating such a motor
Abstract
Conventionally converter-controlled electric motors are allowed
to operate in generational mode during braking and to convert the
electrical energy so produced into heat. Such motors may also
comprise electromagnetically actuated mechanical brakes. In the
present invention it is proposed to supply the electrical energy
produced by the motor when in a generational mode to an excitation
coil of the brake and there store it temporarily as magnetic energy
or convert it into heat.
Inventors: |
ROTH-STIELOW, JORG;
(BRETTEN, DE) ; SCHMIDT, JOSEF; (GRABEN-NEUDORF,
DE) |
Correspondence
Address: |
MARSHALL O'TOOLE GERSTEIN
MURRAY & BORUN
6300 SEARS TOWER
233 SOUTH WACKER DRIVE
CHICAGO
IL
606066402
|
Family ID: |
26038040 |
Appl. No.: |
09/477753 |
Filed: |
January 4, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09477753 |
Jan 4, 2000 |
|
|
|
PCT/EP98/04088 |
Jul 2, 1998 |
|
|
|
Current U.S.
Class: |
318/362 |
Current CPC
Class: |
H02P 3/04 20130101 |
Class at
Publication: |
318/362 |
International
Class: |
H02P 003/00; H02P
005/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 4, 1997 |
DE |
DE 19728711.5 |
Aug 8, 1997 |
DE |
DE 19734405.4 |
Claims
What is claimed is:
1. A method of operating a converter-controlled electric motor with
an electromagnetically actuated brake comprising an excitation coil
wherein the method comprises the step of supplying electrical
energy to the excitation coil of the brake during braking in a
generational mode of the motor for at least one of storage
temporarily as magnetic energy and conversion to thermal
energy.
2. A method as claimed in claim 1, wherein during braking in a
generational mode of the motor, the excitation coil is supplied
with a current that is larger than at least one of a current
supplied to release the brake and a current required to maintain
the brake in an off position.
3. A method as claimed in claim 1, wherein a thermal load on the
excitation coil is ascertained and a current supplied to the coil
is limited to a predetermined value when the thermal load exceeds a
predetermined level.
4. A method as claimed in claim 3, wherein the temperature of the
excitation coil is measured in order to ascertain the thermal
load.
5. A method as claimed in claim 3, wherein the thermal load on the
excitation coil is ascertained from monitoring the value of at
least one of the current and the voltage of the excitation coil, in
combination with at least one excitation-coil-specific
parameter.
6. A method as claimed in claim 3, wherein the
excitation-coil-specific parameter comprises its thermal time
constant.
7. A method as claimed in claim 3, wherein the thermal load on the
excitation coil is ascertained from a measurement of the ambient
temperature of the excitation coil.
8. A method as claimed in claim 3, wherein the thermal load on the
excitation coil is ascertained from a measurement of the
temperature at the electric motor.
9. An electric motor comprising an electromagnetically actuated
mechanical brake; an excitation coil forming part of the brake; a
converter; and a brake control means connected to the electric
motor and the excitation coil in such a way that during braking of
the electric motor in a generational mode of operation electrical
energy produced by the motor is supplied to the excitation coil for
at least one of temporary storage and conversion into heat.
10. A motor as claimed in claim 9, wherein the brake control means
is constructed so that during braking of the motor in a
generational mode, the excitation coil is supplied with a current
that is larger than at least one of a current required to release
the brake and a current required to maintain the brake in an off
position.
11. A motor as claimed in claim 9, comprising detection means to
ascertain a thermal load on the excitation coil, which detection
means are so adapted and connected to the brake control unit that a
thermal load on the excitation coil can be determined, in order
that when the thermal load exceeds a predetermined level a current
supplied to the coil can be limited to a predetermined value.
12. A motor as claimed in claim 11, wherein the detection means
comprise temperature sensors that measure the temperature at at
least one of the excitation coil and in the vicinity of the
excitation coil.
13. A motor as claimed in claim 11, wherein the detection means are
adapted to monitor at least one of the values of the current and
the voltage of the excitation coil whereby from these values, in
combination with at least one excitation-coil-specific parameter,
the thermal load can be determined.
14. A motor as claimed in claim 9, comprising a rectifier which
together with the motor comprises a unitary device.
15. A motor as claimed in claim 14, comprising a DC intermediate
circuit to which the brake control unit and the converter are
coupled and to which an output voltage of the rectifier is
supplied.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an electric motor,
comprising an electromagnetically actuated mechanical brake and to
a method of operating an electric motor with an electromagnetically
actuated mechanical brake.
DESCRIPTION OF THE PRIOR ART
[0002] Electric machines can be controlled by their rotational
speed. Electronic means are now available with which such machines
can be powered via converters by means of either alternating or
direct current supplies. A known method of improving the
controllability of such machines during braking, as is required in
many applications of such machines, is to operate the machine as a
generator and to convert the energy so produced during braking into
heat by way of a load resistance. However, the provision of such an
additional load resistance involves an increase in expense and
complexity of construction of the machine, and furthermore the heat
produced must be dissipated by means of additional extra
apparatus.
[0003] It is further known that such machines, when operated
predominantly as electric motors, can be provided with mechanical
brakes that can be released or raised by an electromagnet
arrangement. Thus, when current is supplied to the electric motor,
it is also supplied to the excitation coil of an
electromagnetically actuated mechanical brake, and when no current
is supplied to the motor, the brake operates to immobilize the
motor.
[0004] The object of the present invention is to provide an
electric motor and a method of operating an electric motor wherein
the braking process is substantially simplified with respect to the
prior art.
SUMMARY OF THE INVENTION
[0005] According to a first aspect of the present invention there
is provided a method of operating a converter-controlled electric
motor with an electromagnetically actuated brake comprising an
excitation coil wherein the method comprises the step of supplying
electrical energy to the excitation coil of the brake during
braking in a generational mode of the motor for at least one of
storage temporarily as magnetic energy and conversion to thermal
energy.
[0006] According to a second aspect of the present invention there
is provided an electric motor comprising an electromagnetically
actuated mechanical brake; an excitation coil forming part of the
brake; a converter; and a brake control means connected to the
electric motor and the excitation coil in such a way that during
braking of the electric motor in a generational mode of operation
electrical energy produced by the motor is supplied to the
excitation coil for at least one of temporary storage and
conversion into heat.
[0007] By virtue of the invention, the electrical energy produced
while the equipment is operating in a generational mode during
braking is converted into heat in an excitation coil of the
electromagnetically releasable mechanical brake. Conventionally,
during mechanical braking the excitation coil is not supplied with
any energy. However, the invention makes it possible to operate
without a braking resistance and furthermore to exploit the
inductivity function of the coil that is, its particular dynamic
action, which the coil exhibits in contrast to an additional ohmic
resistance such as is customarily employed.
[0008] Preferably, when the brake is applied strongly during
operation in a generational mode, a current is supplied to the
excitation coil that is considerably larger than that used to
release the brake or keep it raised in an off position. That is,
advantage is taken of electrical properties of the coil that in
normal operation, for the usual release of the brake, are not
exploited. However, the excitation coil can be supplied for a
certain period with a much larger current than is supplied during
normal operation to release the brake or to keep it in an off
position.
[0009] Preferably, the thermal load on the excitation coil is
ascertained and the current supplied to the coil is kept below a
predetermined value beyond which the thermal load would exceed a
predetermined temperature. It can thereby be ensured that no damage
is caused by overheating. Preferably when there is a risk of
thermal overloading, the current is limited to the maintenance
level for the excitation coil, i.e. the amount of current that
flows through the electric motor during maintained operation of the
electric motor and is tailored to the excitation coil.
[0010] In a preferred embodiment of the invention the thermal load
is ascertained by measuring the temperature of the excitation coil.
With this kind of load measurement particularly accurate results
can be expected. In an alternative embodiment of the invention,
which can also be employed as an adjunct to the first embodiment,
the current and/or voltage supplied to the excitation coil are/is
monitored and these values, in combination with parameters specific
to the excitation coil, in particular the thermal time constant of
the excitation coil, are processed in such a way that not only is
the momentary thermal load known but also, at any time, it can be
estimated how long the excitation coil can continue to be operated
with the present braking performance before the excitation current
must be reduced. By this means the braking behavior can be
optimized.
[0011] Preferably in addition to the thermal load on the excitation
coil, the ambient temperature is also measured. As a result, the
excitation coil is still more reliably protected from overloading.
The same applies to a measurement of the temperature at the
electric motor by means of a corresponding temperature sensor,
which is usually present in any case. Once the brake has been
mounted on the electric motor, along with its excitation coil, and
heat flow is occurring, the temperature of the electric motor also
provides a measure of the amount of heat that can still be
conducted to the excitation coil.
[0012] The various aspects of the present invention will now be
described by way of example with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a block diagram showing a method of operating an
electrical motor according to the present invention;
[0014] FIG. 2 is a circuit block diagram of a control unit shown in
FIG. 1;
[0015] FIG. 3 is a circuit block diagram of a triggering unit;
[0016] FIG. 4 shows in more detail part of the circuit block
diagram shown in FIG. 3,
[0017] FIG. 5 is a circuit block diagram of a second embodiment of
triggering unit;
[0018] FIG. 6 is a circuit block diagram of a third embodiment of
triggering unit; and
[0019] FIGS. 7 and 8 are diagrams showing two methods respectively
of deriving the thermal load on an excitation coil.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] In the following description, the same reference numerals
are used for identical components or parts with identical
functions.
[0021] In the block diagram of FIG. 1, the reference numeral 1
identifies an alternating-current mains supply, which is applied to
a rectifier 2. A rectified output voltage of the latter is applied
to a direct-current intermediate circuit 3. To the output terminals
of the DC intermediate circuit 3 are attached input connectors of a
brake control unit 4, in parallel with input connectors of a
converter 5.
[0022] Output terminals of the brake control unit 4 are connected
to an excitation coil 62 of an electromagnetically actuated
mechanical brake, which when supplied with current allows a motor
61 to run freely, and when the current is cut off brakes said
motor.
[0023] The motor 61 is driven in a manner known per se by the
converter 5.
[0024] The electric motor 61 is preferably combined with the
rectifier 2, the brake control unit 4, the excitation coil 62 and
the converter 5 to form a unitary device 8, preferably in a common
housing.
[0025] The brake control unit 4 comprises, as shown in FIG. 2, in a
first embodiment of the invention an electronic one-way valve 41,
which can be turned on and off by way of a triggering unit 42. This
one-way valve 41 is connected in series with the excitation coil 42
across the output terminals of the DC intermediate circuit 3. The
terminals of the excitation coil 62 are connected by way of a
recovery diode 7 disposed such that its polarity is reversed with
respect to the one-way valve.
[0026] In FIG. 2 the current flowing through the excitation coil 62
is designated I.sub.B and the voltage across the terminals of the
excitation coil 62 is designated U.sub.B. The mode of operation of
the arrangement is as follows.
[0027] The direct current required to raise the electromagnetically
actuated mechanical brake has the value I.sub.1. When this current
is flowing, a voltage U.sub.1 exists across the excitation coil
62.
[0028] When the current I.sub.1 is turned on, a period of time
t.sub.1 elapses before the brake has been completely raised. Once
it is in the raised position, the direct current needed to keep it
in that position is I.sub.2, which in general is smaller than or
equal to I.sub.1. In this state, the voltage drop across the
excitation coil 62 has the value U.sub.2.
[0029] From the circuit shown in FIG. 2 it can be seen that the
electronic one-way valve 41 can be switched on and off by the
triggering unit 42 in such a way that the entire output voltage
U.sub.Z of the DC intermediate circuit 3 can be applied to the
excitation coil 62. The modes of operation thus made possible are
as follows.
[0030] Operating Condition A: the motor 61 is not supplied with any
current from the converter 5. The excitation coil 62 is also
without current. In this operating condition the motor is firmly
braked.
[0031] Operating Condition B: at the beginning of motor operation,
i.e. when the converter 5 begins to supply current to the motor 61,
the brake control unit 4 supplies a direct current I.sub.B=I.sub.1
to the excitation coil 62 for a time period t.sub.1, in order to
raise the brake.
[0032] Operating Condition C: while the motor 61 is running in an
unbraked state, the brake control unit 4 sends through the
excitation coil 62 a direct current I.sub.B=I.sub.2 that is
required to maintain the electromagnetically actuated mechanical
brake in the raised position; this maintenance current I.sub.2 can
be smaller than the current I.sub.1. When the maintenance current
I.sub.2 s equal to the current I.sub.1 needed to raise the brake,
the previously described operating condition B is eliminated.
[0033] Operating Condition D: when the running motor 61 is to be
braked, i.e. switched to operate in a generational mode, the brake
control unit 4 sends the direct current I.sub.B=I.sub.2, which is
needed to keep the electromagnetically actuated mechanical brake in
the raised position, through the excitation coil 62 as long as the
power fed back from the converter 5 into the DC intermediate
circuit 3 is not larger than the power needed to keep the
electromagnetically actuated mechanical brake raised.
[0034] Operating Condition E: if the power fed back from the
converter 5 into the DC intermediate circuit 3 exceeds the power
needed to keep the electromagnetically actuated mechanical brake
raised, because the braking or generator performance has increased,
the brake control unit 4 conducts the entire power returned by the
electric motor 61 into the excitation coil 62. This current is
considerably larger than the above-mentioned values I.sub.1 and
I.sub.2.
[0035] Operating condition F: if the thermal load associated with
the supplied current exceeds a maximum permissible value for the
excitation coil 62, the brake control unit 4 reduces the current
I.sub.B supplied to the excitation coil 62 to the level required to
keep the electromagnetically actuated mechanical brake raised,
namely the direct current I.sub.B=I.sub.2.
[0036] In the following a preferred embodiment, a circuit for a
brake control unit 4 is described with reference to FIGS. 3 and
4.
[0037] As shown in FIG. 3, the trigger unit 42 comprises a signal
generator 421, the output of which is connected to an input of a
pulse-width modulator (PWM) 422. The output of the PWM 422 is
supplied to the input of an override unit 423, the output of which
is supplied to a control input of the electronic one-way valve 41,
which can thereby be turned on and off. The entire arrangement is
powered by way of the output terminals of the DC intermediate
circuit 3.
[0038] Triggering by way of the override unit 423 is achieved as
follows. In all operating conditions except Condition E described
above, the override unit 423 supplies the output signal of the PWM
422 to the electronic on/off one-way valve 41. The ratio of the
durations of "on" and "off", i.e. the duty factor .lambda. of the
PWM 422, in this case directly determines the mean direct current
U.sub.B supplied to the excitation coil 62. It follows that
U.sub.B=.lambda..multidot.U.sub.Z, where U.sub.Z is the
output-terminal voltage of the DC intermediate circuit 3. The
signal generator 421 generates the standard value for the voltage
U.sub.B.
[0039] In operating Condition E, when the power sent back from the
converter 5 during braking into the DC intermediate circuit 3
exceeds the power needed to keep the electromagnetically actuated
mechanical brake raised, the intermediate-circuit voltage U.sub.Z
begins to rise above the level of the rectified mains voltage.
During this process, if U.sub.Z exceeds a limiting value U.sub.3,
the on/off one-way valve 41 is switched into a conducting state by
the override unit 423, regardless of the standard voltage provided
to the pulse-width modulator 422.
[0040] The limiting value U.sub.3 is set such that on one hand it
is appreciably above the rectified mains voltage and on the other
hand appreciably below the highest voltage load that can be
sustained by the rectifier 2, the DC intermediate circuit 3, the
brake control unit 4, the rectifier 5, the motor 61 and the
excitation coil 62.
[0041] While the system is in the Operating Condition E, if a
signal .THETA..sub.B generated by a temperature sensor to represent
the thermal stress on the excitation coil 62, such as is explained
in greater detail below, exceeds the highest value that can be
sustained by said coil, namely .THETA..sub.Bmax, a transition to
the operating condition F occurs. In this case the override unit
423 again sends the output signal of the PWM 422 to the electronic
on/off one-way valve 41 such that current supplied to the
excitation coil 62 becomes I.sub.B=I.sub.2; that is, the current is
reduced to the level that the excitation coil 62 can withstand
during long-term operation of the electric motor 61.
[0042] The above situation is represented in FIG. 4 in the form of
a control circuit. Here the override unit 423 comprises a first
comparator, which sends out a positive digital output signal when
the voltage U.sub.Z at the output terminals of the DC intermediate
circuit 3 exceeds a predetermined voltage U.sub.3. This limiting
value U.sub.3 has already been defined above.
[0043] A second comparator is provided that compares an actual
temperature value .THETA..sub.B with a maximum permissible
temperature value .THETA..sub.Bmax and sends out a positive digital
output signal when the actual value exceeds the maximum value.
[0044] The value of the output signal of the first comparator is
sent to a non-inverting input of an AND gate, whereas the value of
the output signal of the second comparator is sent to an inverting
input of the same AND gate. The output of the AND gate is sent to
an input of an OR gate, the other input of which is connected to
the output of the PWM 422. The output of the OR gate is sent to the
control input of the electronic on/off one-way valve 41. As those
skilled in the art will see, this circuitry carries out the
procedure described above with reference to FIG. 3.
[0045] In the embodiment shown in FIG. 5, the voltage associated
with the current I.sub.B through the excitation coil 62 is again
supplied from the signal generator 421 to the PWM 422. This voltage
U.sub.B, associated with the current I.sub.B, has the value zero
when the system is in Operating Condition A, the value U.sub.1,
associated with current I.sub.1, when in Operating Condition B, and
the value U.sub.2, associated with current I.sub.2, while in
Operating Conditions C, D, E and F.
[0046] It is also possible to operate using the embodiment as shown
in FIG. 6, in which the current is regulated by reference to a
standard current. Here the signal generator 421 comprises a profile
generator 4211, the output signal of which is sent to a comparator,
the output of which is sent to the input of a regulator unit 4212
in the signal generator 421. The comparator receives from the
excitation coil 62 a signal proportional to the current I.sub.B, so
that the output signal of the comparator corresponds to the
difference or deviation between the output value or set point
derived from the profile generator 4211 and the
current-proportional value or actual value derived from the current
sensor.
[0047] In all the embodiments described herein, it is advantageous
for the thermal state of or the thermal stress on the excitation
coil 62 to be monitored. For this purpose, as indicated in FIG. 7,
a signal .THETA..sub.B can be obtained from an actual value p.sub.B
of the energy dissipated in the excitation coil 62 on the basis of
a thermal time constant .tau..sub.B of the excitation coil 62, with
addition of a value .THETA..sub.UBmax, which corresponds to the
maximal ambient temperature of the excitation coil 62. This value
.THETA..sub.B is then, as shown in FIG. 4, further processed in
order to protect the excitation coil 62 from overheating.
[0048] The actual momentary amount of energy dissipated in the
excitation coil 62, the quantity p.sub.B, is derived from the
current measured through or the voltage measured across the
excitation coil 62 and its ohmic resistance, or from the control
signal of an electronic on/off one-way valve 41, the (measured)
value of the voltage across the output terminals of the DC
intermediate circuit 3 and the measured value of the current
through the excitation coil 62 or its ohmic resistance.
Alternatively it is derived from the measured value of the voltage
across the excitation coil 62 and the measured value of the current
through the excitation coil 62. The resulting value of
.THETA..sub.B, as shown in FIG. 7, is then used at a later stage in
the circuitry as shown in FIG. 4.
[0049] The arrangement shown in FIG. 8 differs from that shown in
FIG. 7 in that the derivation is based not on a fixed predetermined
maximum ambient temperature .THETA..sub.UBmax for the excitation
coil 62, but rather on a temperature .THETA..sub.M that corresponds
to the temperature measured at the motor 6. This in turn is a
representation of the temperature at the electromagnetically
actuated mechanical brake or the excitation coil 62, because the
brake is attached to the motor 61 by way of a thermally conducting
contact area. This motor temperature .THETA..sub.M allows the
excitation coil 62 to be still better utilized, because during
ordinary operation the ambient temperature .THETA..sub.UB of the
excitation coil 62 is below the value .THETA..sub.UBmax which is
assumed above to be the maximum. The factor K.sub..THETA., by which
the motor temperature .THETA..sub.M is modified, as shown in FIG.
8, is so dimensioned that the quantity
K.sub..THETA..multidot..THETA..sub.M corresponds approximately to
the actual ambient temperature .THETA..sub.UB of the excitation
coil 62.
* * * * *