Multi-level inverter
Method of controlling a multi-level inverter having inputs connected to a gate drive unit controlling the inverter and an output connected to a load, the multi-level inverter capable of generating a PWM voltage signal having three or more modulation levels, the inverter powered by a voltage supply and comprising at least one neutral point (NP), the method comprising operating the multi-level inverter in a standard modulation pattern having three or more modulation levels when one or more parameters representative of neutral point stability each have a value within a first range indicative of a high neutral point stability, and operating the multi-level inverter with a two-level modulation pattern when said one or more parameters representative of neutral point stability each have a value within a second range indicative of a low neutral point stability, the first range separated from the second range by a threshold value.
1. Method of controlling a multi-level inverter having inputs connected to a drive unit controlling the inverter and an output connected to a load, the multi-level inverter capable of generating a PWM voltage signal having three or more modulation levels, the inverter powered by a voltage supply and comprising at least one neutral point, the method comprising operating the multi-level inverter in a standard modulation pattern having three or more modulation levels when one or more parameters representative of neutral point stability each have a value within a first range indicative of a high neutral point stability, and operating the multi-level inverter with a two-level modulation pattern when said one or more parameters representative of neutral point stability each have a value within a second range indicative of a low neutral point stability, the first range separated from the second range by a threshold value, wherein said threshold values of parameters representative of low neutral point stability include any one or more of:
ω min
Minimum motor stator
Threshold value having a value
frequency
between 0 to 40% of nominal
speed;
ε np _max
Allowed deviation of
Threshold value having a value
neutral point voltage
between 0 to 50 [%];
in %
PF min
Motor minimum power
Threshold value having a value
factor
between 0 to 0.7 [−];
I ph _min
Motor minimum phase
Threshold value having a
current
predetermined value depending on
the characteristics of the motor [A];
f pwm _min
Minimum PWM
Threshold value having a value
frequency
between 0 to 50 [kHz].
2. Method according to claim 1 , wherein the load is a multiphase electric motor, and the inverter comprises a multi-level modulation circuit for each phase leg, each multi-level modulation circuit having an output connected to a corresponding phase of the electric motor.
3. Method according to claim 1 , wherein the parameters representative of neutral point stability include any one or more of:
V np
Neutral point voltage;
ω
Motor stator frequency;
ε np
Deviation of neutral point voltage ( | V DC /2 − V NP | ) in %;
cos(φ)
Motor power factor = Real power/Apparent power;
I ph
Motor phase current;
f pwm
Modulation (PWM) frequency.
4. Method according to claim 1 , wherein the threshold value for each of the one or more parameters are selected as:
ω min
Minimum motor stator
Threshold value having a value
frequency
between 5% to 30% of nominal
speed,
ε np
Allowed deviation of
Threshold value having a value
neutral point voltage
between 5 to 30 [%],
in %
PF mi
Motor minimum power
Threshold value having a value
factor
between 0.1 to 0.5 [−],
I ph _min
Motor minimum phase
5% to 40% of Nominal current [A],
current
f pwm _min
Minimum PWM
Threshold value having a value
frequency
between 1 to 30 [kHz].
5. Method according to claim 4 , wherein the threshold value for each of the one or more parameters are selected as:
ω min
Minimum motor stator
Threshold value having a value
frequency
between 5% to 15% of nominal
speed,
ε np
Allowed deviation of
Threshold value having a value
neutral point voltage
between 10 to 20 [%],
in %
PF mi
Motor minimum power
Threshold value having a value
factor
between 0.1 to 0.3 [−],
I ph _min
Motor minimum phase
10% to 30% of Nominal current [A],
current
f pwm _min
Minimum PWM
Threshold value having a value
frequency
between 5 to 15 [kHz].
6. A drive for an electrical motor comprising a power electronics module and a gate drive unit connected to a controller and to the power electronics module to control the motor, the power electronics module comprising a multi-level inverter having at least one output connected to at least one phase of the motor, the multi-level inverter capable of generating a PWM voltage signal having three or more modulation levels, the inverter powered by a voltage supply and comprising at least one neutral point, wherein the controller is configured to control the gate drive unit to:
operate the multi-level inverter in a standard modulation pattern having three or more modulation levels when one or more parameters representative of neutral point stability each have a value within a first range indicative of a high neutral point stability, and to
operate the multi-level inverter with a two-level modulation pattern when said one or more parameters representative of neutral point stability each have a value within a second range indicative of a low neutral point stability,
the first range separated from the second range by a threshold value, wherein the threshold values are stored in, or computed from values stored in a non-transient memory of the drive and wherein an executable control algorithm is stored in a non-transient memory of the drive, the control algorithm executable by the controller to control the gate drive unit.
7. The drive of claim 6 , wherein the controller receives voltage (V DC , V NP ) and phase current (i u , i w , i v ) measurement inputs from a power supply and the motor, the power supply comprising a DC voltage supply.
8. The drive of claim 6 , wherein the motor is a multi-phase (U, V, W) motor and the inverter is a multi-phase inverter having one said output for each phase.
9. The drive of claim 6 , wherein the inverter is a diode clamped inverter.
10. The drive of claim 6 , wherein the inverter comprises a neutral point clamped (NPC) inverter circuit topology per phase leg.
11. The drive of claim 6 , wherein the inverter comprises a T-type three-level inverter circuit topology per phase leg.
12. The drive of claim 6 , wherein the threshold values are stored in, or computed from values stored in a non-transient memory of the drive and wherein an executable control algorithm is stored in a non-transient memory of the drive, the control algorithm executable by the controller to control the gate drive unit.
13. The drive of claim 6 , wherein the threshold values representative of low neutral point stability include any one or more of:
ω min
Minimum motor stator
Threshold value having a value
frequency
between 0 to 40% of nominal
speed;
ε np _max
Allowed deviation of
Threshold value having a value
neutral point voltage
between 0 to 50 [%];
in %
PF min
Motor minimum power
Threshold value having a value
factor
between 0 to 0.7 [−];
I ph _min
Motor minimum phase
Threshold value having a
current
predetermined value depending on
the characteristics of the motor [A];
f pwm _min
Minimum PWM
Threshold value having a value
frequency
between 0 to 50 [kHz].
14. The drive of claim 6 , wherein the parameters representative of neutral point stability include any one or more of:
V np
Neutral point voltage;
ω
Motor stator frequency;
ε np
Deviation of neutral point voltage ( | V DC /2 − V NP | ) in %;
cos(φ)
Motor power factor = Real power/Apparent power;
I ph
Motor phase current;
f pwm
Modulation (PWM) frequency.