Electricity: motive power systems – Synchronous motor systems – Armature winding circuits
Reexamination Certificate
1999-03-01
2001-05-22
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Armature winding circuits
C318S434000
Reexamination Certificate
active
06236183
ABSTRACT:
The present invention relates to an angular position detector device for controlling a synchronous motor with permanent magnet excitation, the motor having a rotor without a damping cage, a stator provided with a set of windings, a frequency converter, and circuits for controlling the frequency converter.
Synchronous motors with permanent magnetic excitation are being used more and more in high speed applications such as molecular drag pumps, high speed spindles, and air compressors, where high-speed applications require, amongst other things, high power density per unit volume and low rotor losses.
To vary the speed of a synchronous motor with permanent magnet excitation, it is generally necessary to use a frequency converter (an inverter). The converter applies current that is variable both in amplitude and in frequency to the motor windings. Control is normally performed in a “closed” loop: an angular position sensor provides information about the real position of the rotor to the converter which then applies currents of appropriate phase and amplitude to the stator of the motor. As a result the motor is constrained to rotate in synchronous manner and a damping cage (which would give rise to additional rotor losses) is unnecessary.
Speed is generally regulated by means of two servo-control loops in cascade: an internal current loop servo-controls the currents in amplitude and in phase, thus making them proportional to motor torque, while a second loop servo-controls the speed of rotation. The outlet from the speed regulator is the reference value for the current regulator.
The power stage of the converter is generally constituted by a bridge comprising six electronic switches powered by a DC source and subjected to on/off control at high frequency using the pulse width modulation (PWM) technique.
Converter-and-motor assemblies of the above-described type require the use of Hall effect sensors, inductive sensors, resolvers, or optical sensors (absolute or incremental) for control purposes.
The present invention seeks to avoid the need to use one or more angular position detectors, and in particular it seeks to avoid using active Hall effect sensors which provide an on/off signal in static operation, but which are fragile and lack reliability, while posing problems of withstanding radiation, and to avoid using the use of “revolution pulse” type inductive sensors which provide one pulse per revolution, but which require angular position adjustment that can be difficult and expensive to achieve in practice, given that inspection can be performed only while rotating.
The present invention also seeks to make it possible to have angular position information available that is analogous to that which could be obtained using an inductive “revolution pulse” type sensor or using a Hall effect sensor, but without implementing sensors of those kinds.
According to the invention, those objects are achieved by an angular position detector device for controlling a synchronous motor with permanent magnet excitation, the motor having a rotor without a damping cage, a stator provided with a set of windings, a frequency converter, and circuits for controlling the frequency converter, the device being characterized in that it comprises a calculation circuit having measurement means for determining the voltage V
u
of a given phase U, first subtracter means for subtracting from the voltage V
u
as determined by said measurement means, a magnitude corresponding to the resistive voltage drop across the resistance of the winding under consideration carrying a current i
u
, integrator means for integrating the signal from the first subtracter means, second subtracter means for subtracting from the signal delivered by the integrator means a magnitude corresponding to the inductive voltage drop across the inductance of the winding under consideration carrying the current i
u
, and a comparator circuit for detecting the zero crossings of the rotor flux sinewave signal delivered by the second subtracter means, thereby delivering an information signal identifying the angular position of the rotor relative to the stator.
The device includes means for taking information concerning the magnitude of the current i
u
carried by the winding under consideration from the outlet of the converter or from the stator terminals of the motor.
The calculation circuit is essentially made from a small number of operational amplifiers.
According to the invention, the angular position of the rotor is thus identified by making use solely of the voltage and current information that is always available at the outlet from the converter or at the terminals of the motor itself, without it being necessary to implement special sensors for detecting angular position.
The calculation circuit enables a plurality of output signals to be delivered that are at phase offsets of 120 degrees.
The calculation circuit makes it possible to obtain a rotor flux signal which is the image of the flux generated by the rotor magnets and which presents phase and amplitude that are not disturbed by the action of the converter.
The detector device of the invention can be applied to an electric motor driving a rotary assembly mounted on active magnetic bearings.
When the active magnetic bearings are fitted with a system for automatically controlling unbalance, the information signal identifying the angular position of the rotor can be applied to said system for automatically controlling the unbalance of the active magnetic bearings.
The device of the invention can be applied to the electric motor of a molecular drag pump, of a compressor, or of a tool-carrying spindle rotating at speeds of the order of several tens of thousands of revolutions per minute (rpm).
In a particular aspect of the present invention, the detector device includes means for providing open loop control of the frequency converter on starting, and then for switching over from open loop control to closed loop control when said signal providing information about the angular position of the rotor is provided in stable manner by said calculation circuit.
REFERENCES:
patent: 4764711 (1988-08-01), Deller
patent: 4814677 (1989-03-01), Plunkett
patent: 5140243 (1992-08-01), Lyons et al.
patent: 5233275 (1993-08-01), Danino
patent: 5367234 (1994-11-01), Ditucci
patent: 5818192 (1998-10-01), Nozari
patent: 6005364 (1999-12-01), Acarnley
patent: 0 481 933 (1992-04-01), None
McDermott & Will & Emery
Ro Bentsu
Societe de Mecanique Magnetique
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