Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
1998-02-23
2001-06-05
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S768000
Reexamination Certificate
active
06242883
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to inverting or transmitting electrical power to a motor.
DESCRIPTION OF PRIOR ART
EMBODIMENT #1
The common and long-standing method to drive a three-phase AC motor is to apply three 120-degree-spaced AC voltages. This common method for a Variable-Voltage-Variable-Frequency (VVVF) drive from a DC energy source is illustrated in
FIG. 1
for the case of a system with a battery
10
, a three-phase DC-to-AC inverter
20
, and a three-phase delta-connected induction motor
40
.
The battery
10
is a DC energy source that supplies two output voltages on lines
12
and
14
. The output voltage on line
12
is positive X volts and the output voltage on line
14
is negative X volts, where X is a variable that can be any practical value. The battery also has a ground
16
and internal cells
18
that actually produce the voltage potential. The operation of batteries is well defined in the literature and therefore is not discussed in detail here.
The inverter
20
takes the DC voltages on lines
12
and
14
from battery
10
and produces three phases of AC voltage. The three phases are called V1, V2, and V3 and are made available on terminals
24
,
25
, and
26
, respectively. The three phases are produced by the inverter circuits
21
,
22
, and
23
, which are controlled by the logic circuit
30
. The voltage waveforms V1, V2, and V3 approximate sine waves with amplitude X and with phase shifts of
30
,
150
, and
270
degrees, respectively, for forward motor rotation, as detailed in Eqs. 1a through 3a. For reverse motor
40
rotation, the logic circuit
30
changes the sign of the phase angles for V1, V2, and V3 as detailed in Eqs. 1b through 3b. The accuracy to which these waveforms approximate the exact shape of a sine wave depends on the design of the inverter circuits
21
,
22
, and
23
, as well as the logic circuit
30
. For the purpose of discussion, we will assume that V1, V2, and V3 each has the exact shape as a sine wave. However, the accuracy of their wave shape is not critical to the present invention.
The following equations describe inverter output voltages for forward rotation, where w is the period of the sine wave in radians and t is the time variable:
V1=X sin(wt+30) (1a)
V2=X sin(wt+150) (2a)
V3=X sin(wt+270) (3a)
The following equations describe inverter output voltages for reverse rotation:
V1=X sin(wt−30) (1b)
V2=X sin(wt−150) (2b)
V3=X sin(wt−270) (3b)
The design and operation of the inverter circuits
21
,
22
, and
23
and of the logic circuit
30
are well documented in the prior art and therefore will not be described in detail here. For reference purposes, a typical inverter circuit is shown in FIG.
2
. Its power-conducting elements are transistor Q
1
50
, transistor Q
2
54
, diode D
1
52
, and diode D
2
56
. A set of silicon-controlled rectifiers (SCRs) could be used instead of the transistors. There are other peripheral circuit components which, for simplicity, are not shown. The logic circuit
30
could be composed of a microprocessor and/or other digital and analog circuit components. It typically produces a pulse-width-modulated (PWM) signal that controls when the transistors Q
1
50
and Q
2
54
are turned on. It is a simple matter for the logic circuit
30
to control the phase angle of each phase, as described in the prior literature. The design details of the inverter circuits
21
,
22
, and
23
as well as the design details of the control circuit
30
are not pertinent to the present invention.
The motor
40
produces a useful mechanical torque from the balanced three-phase voltages Va, Vb, and Vc across the stator coils
44
,
45
, and
46
, respectively. The three inverter
20
outputs V1, V2, and V3, on terminals
24
,
25
, and
26
, respectively, are connected to the three motor input terminals
41
,
42
, and
43
, respectively. The voltages on terminals
41
,
42
, and
43
then cause voltage waveforms Va, Vb, and Vc to be applied to coils
44
,
45
, and
46
, respectively, which make up the stator winding of the motor. Voltage waveforms Va, Vb, and Vc are defined in Eqs. 4a through 6a for forward motor rotation and in Eqs. 4b through 6b for reverse motor rotation. These voltages then cause currents to flow, which in turn produce magnetic flux, which in turn produces torque. The design and operation of an induction motor is well defined in the prior art and thus will not be discussed in detail here. The only critical point for our discussion is to note that, in Eqs. 4, 5, and 6, the voltage waveforms Va, Vb, and Vc are balanced. In other words, they have a 120-degree phase spacing between them and have the same amplitude. This is true for both forward and reverse cases and is a critical requirement for a three-phase motor to work properly.
The following equations describe motor coil voltages for forward rotation:
Va=V1−V2=1.73X sin(wt+0) (4a)
Vb=V2−V3=1.73X sin(wt+120) (5a)
Vc=V3−V1=1.73X sin(wt+240) (6a)
where V1, V2, and V3 are given by Eqs. 1a, 2a, and 3a, respectively.
The following equations describe motor coil voltages for reverse rotation:
Va=V1−V2=1.73X sin(wt+0) (4b)
Vb=V2−V3=1.73X sin(wt−120) (5b)
Vc=V3−V1=1.73X sin(wt−240) (6b)
where V1, V2, and V3 are given by Eqs. 1b, 2b, and 3b, respectively.
DESCRIPTION OF PRIOR ART—EMBODIMENT #2
The case of a VVVF drive for a three-phase motor with an single-phase AC energy source is shown in FIG.
3
. The system is comprised of an AC energy source
75
, a AC-to-DC converter
71
, a three-phase inverter
20
, and a three-phase delta connected motor
40
.
The AC energy source
75
is typical of a 240 volt 60 Hz single-phase source common in residences. Said energy source
75
is composed of two 120 volt 60 Hz AC energy sources
70
, supplies an AC voltage between lines
72
and
73
, and has a ground
16
. The shape of the voltage on line
72
is sinusoidal with respect to the voltage on line
73
, with any practical value for magnitude and any practical value for frequency. For illustration purposes, the magnitude of the voltage on line
72
with respect to the voltage on line
73
is x volts.
The AC-to-DC converter
71
produces a positive DC voltage on line
12
and a negative DC voltage on line
14
. The input to the converter
71
is the voltage between lines
72
and
73
. The input terminal
74
is considered the reference input to the converter
71
since the DC voltage on line
12
is positive with respect to terminal
74
and the DC voltage on line
14
is negative with respect to terminal
74
. For reference purposes, a typical AC-to-DC rectification circuit is shown in FIG.
4
. The values of capacitor
61
, capacitor
62
, inductor
64
and inductor
65
will need to be sufficient in order to supply near-constant DC voltages on lines
12
and
14
for three-hundred-sixty degrees so that adequate voltage is available for inverter
20
to produce output voltages V1, V2, and V3. There are many configurations for AC-to-DC converter circuits that are well defined in the literature and the design details of the converter
71
are not pertinent to the present invention and therefore will not be discussed in detail.
The inverter
20
is exactly the same as discussed previously in prior art example #1 so will not be discussed here.
The motor
40
is exactly the same as discussed previously in prior art example #1 so will not be discussed here.
The disadvantage of the common method illustrated for a VVVF drive is that it takes three inverter circuits, which are relatively expensive, to produce the balanced-voltage waveforms across the three stator coils of the motor.
DESCRIPTION OF PRIOR ART—EMBODIMENT #3
A method using conventional techniques to impl
Duda Rina I.
King & Schickli PLLC
Nappi Robert E.
LandOfFree
Variable speed three-phase motor drive with two inverter... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Variable speed three-phase motor drive with two inverter..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Variable speed three-phase motor drive with two inverter... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2459612