Filter systems for reducing bearing current in high-frequency motor control systems
A motor controller and methods for operating a motor are described herein. The motor controller includes a high-frequency (HF) inverter having a direct current (DC) input stage and an alternating current (AC) output stage, the HF inverter operable at a switching frequency to generate a three-phase output having a fundamental frequency. The motor controller also includes a differential mode (DM) filter coupled in series with the output stage of the HF inverter and having a resonant frequency less than 10% of the switching frequency, the DM filter configured to reduce harmonic components of the three-phase output and generate a first filtered output. The motor controller further includes a common mode (CM) filter coupled in series with the DM filter, the CM filter configured to filter the first filtered output to generate a second filtered output having a reduced CM voltage to operate the electric motor with a reduced bearing current.
1. A motor controller for an electric motor, the motor controller comprising:
a three-phase high-frequency (HF) inverter having a direct current (DC) input stage and an alternating current (AC) output stage, the HF inverter operable at a switching frequency to generate a three-phase output having a fundamental frequency;
a differential mode (DM) filter coupled in series with the AC output stage of the HF inverter and having a resonant frequency less than 10% of the switching frequency, the DM filter configured to reduce harmonic components of the three-phase output and generate a first filtered output;
a common mode (CM) filter coupled in series with the DM filter, the CM filter configured to filter the first filtered output to generate a second filtered output having a reduced CM voltage to operate the electric motor with a reduced bearing current; and
a high-frequency filter coupled between the HF inverter and the DM filter, the high-frequency filter configured to produce a frequency filtered output having a reduced line-to-line voltage spike.
2. The motor controller of claim 1 , wherein the switching frequency of the HF inverter is greater than about 100 kHz.
3. The motor controller of claim 1 , wherein the fundamental frequency of three-phase output is below about 1 kHz.
4. The motor controller of claim 1 , wherein the resonant frequency of the DM filter is at least twice the fundamental frequency of the three-phase output.
5. The motor controller of claim 1 , wherein the CM filter has a resonant frequency less than 20% of switching frequency of the HF inverter.
6. The motor controller of claim 1 , wherein the DM filter comprises a respective inductor coupled in series with each output phase of the HF inverter, and a respective leg coupled between each pair of output phases, each leg comprising a capacitor in series with a resistor.
7. The motor controller of claim 1 , wherein the DM filter comprises a respective inductor coupled in series with each output phase of the HF inverter, a respective damping resistor coupled in parallel with each inductor, and a respective capacitor coupled between each pair of output phases.
8. The motor controller of claim 1 , wherein the CM filter comprises a respective inductor choke coupled in series with each output phase from the DM filter, and a respective leg coupled between an output of each inductor choke and a ground, each leg comprising a capacitor.
9. The motor controller of claim 8 , wherein the CM filter further comprises a resistor coupled in series between each leg and the ground.
10. The motor controller of claim 1 , wherein the high-frequency filter has a resonant frequency greater than ten times the switching frequency of the HF inverter.
11. The motor controller of claim 10 , wherein the high-frequency filter comprises a respective inductor coupled in series with each output phase of the HF inverter, and a respective capacitor coupled between each pair of output phases.
12. An electric motor comprising:
a rotor;
a stator;
at least one bearing disposed between the rotor and the stator; and
a motor controller coupled to the stator and configured to provide power the motor, the motor controller comprising:
a three-phase high-frequency (HF) inverter having a direct current (DC) input stage and an alternating current (AC) output stage, the HF inverter operable at a switching frequency to generate a three-phase output having a fundamental frequency;
a differential mode (DM) filter coupled in series with the AC output stage of the HF inverter and having a resonant frequency less than 10% of the switching frequency, the DM filter configured to reduce harmonic components of the three-phase output and generate a first filtered output;
a common mode (CM) filter coupled in series with the DM filter, the CM filter configured to filter the first filtered output to generate a second filtered output having a reduced CM voltage at the stator to energize the stator with a reduced bearing current through the at least one bearing; and
a high-frequency filter coupled between the HF inverter and the DM filter, the high-frequency filter configured to produce a frequency filtered output having a reduced line-to-line voltage spike.
13. The electric motor of claim 12 , wherein the switching frequency of the HF inverter is greater than about 100 kHz and the fundamental frequency of the three-phase output is below about 1 kHz.
14. The electric motor of claim 12 , wherein the resonant frequency of the DM filter is at least twice the fundamental frequency of the three-phase output.
15. The electric motor of claim 12 , wherein the CM filter has a resonant frequency less than 20% of switching frequency of the HF inverter.
16. The electric motor of claim 12 , wherein the DM filter comprises a respective inductor coupled in series with each output phase of the HF inverter, and a respective leg coupled between each pair of output phases, each leg comprising a capacitor in series with a resistor.
17. The electric motor of claim 12 , wherein the CM filter comprises a respective inductor choke coupled in series with each output phase from the DM filter, and a respective leg coupled between an output of each inductor choke and a ground, each leg comprising a capacitor.
18. A method of operating an electric motor including a rotor, a stator, and at least one bearing disposed between the rotor and the stator, the method comprising:
receiving, at a high-frequency (HF) inverter, a DC input signal;
operating the HF inverter at a switching frequency to generate a three-phase output having a fundamental frequency;
filtering, by a high-frequency filter, the three-phase output to produce a frequency-filtered output having a reduced line-to-line voltage spoke;
filtering, by a differential-mode (DM) filter having a resonant frequency less than 10% of the switching frequency, the frequency-filtered output to produce a first filtered output having reduced harmonic components;
filtering, by a common-mode (CM) filter, the first filtered output to produce a second filtered output having a reduced CM voltage; and
energizing the stator using the second filtered output.