Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1999-09-21
2001-10-16
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S051000, C310S089000, C363S096000, C361S119000
Reexamination Certificate
active
06304013
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to electric motors and, more particularly, to a circuit for reducing voltage stresses, or surges, between a cable and an electric motor.
Voltage stresses and surges are generally recognized as undesirable for electric motors for a variety of reasons. For example, surges often present voltages to a motor having a high amplitude and steep wave front which can damage supply line insulation or damage a stator winding's first coil.
The steep wave front can lead to non-uniform distribution of the increased voltage through the winding. Generally the first coil, which is connected to the motor supply lead, takes the largest portion of the voltage. The non-uniform distribution of the voltage through the first coil can lead to premature motor failure.
Electric motors may include an inverter, such as a pulse width modulated (PWM) inverter, to supply a series of pulses to the motor. These pulses travel along the transmission line to the motor and can have steep wave fronts. Since the motor generally has a higher surge impedance than the transmission line, a reflection may occur which causes the voltage at the motor to increase by a ratio of 2 Z
m
/(Z
m
+Z
0
) where Z
m
is the surge impedance of the motor and Z
0
is the surge impedance of the transmission line. The motor impedance may be many times the surge impedance of the line.
Accordingly, it would be desirable to reduce, or eliminate, the reflection of the pulse along the transmission line.
BRIEF SUMMARY OF THE INVENTION
In an exemplary embodiment of the invention, a line termination network, integral with an electric motor, minimizes the reflection of pulses along a transmission line. The termination network includes a resistor and a capacitor connected in series across each phase of the motor.
For a three phase motor, the termination network includes three resistors and three capacitors, one of each connected in series, and the three capacitor and resistor series-connections are connected in a wye or delta connection configuration. A capacitor connected between each of two supply motor lines is initially uncharged and initially operates as a short circuit when exposed to the steep wave fronts of a pulse voltage. In these circumstances, the associated series-connected resistors have a total resistive value across the two supply motor lines equal to that of the high frequency surge impedance of the supply motor lines. When a peak pulse voltage or high amplitude voltage is transmitted, the charge across the capacitor becomes equal to that of the power source and the capacitor operates as an open circuit across the two supply lines. The capacitor filters and reduces the high amplitude pulse voltage within the circuit and effectively removes the associated series-connected resistors from the circuit. The values for the resistors are chosen such that a high frequency surge impedance approximately matches that of a cable connected to the electric motor.
The termination network is internal to the motor and is a component of a termination network assembly which is mounted in close proximity to a pair of motor end windings. Alternatively, the termination network assembly may be located near either or both of the end windings.
The termination network will reduce the peak voltage between a motor terminal and a transmission line connected to a pulse width modulated drive when high frequency surges, or spikes, are present. The reduced voltage allows motors with typical insulation systems to be used on adjustable speed drives without requiring special magnet wire or additional insulation systems. Further, the termination network will not adversely effect sinusoidal performance ratings of the motor on which it is used. In addition, since the termination network is integral with the motor, the termination network can be installed during assembly of the motor, thus lowering the cost of installation.
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Akers Jeffrey S.
Oldenkamp John L.
Armstrong Teasdale LLP
General Electric Company
Horton, Esq. Carl
Lam Thanh
Ramirez Nestor
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