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Motors operating on non-sinusoidal power supplies will have many additional losses in addition to the normal losses caused by the fundamental wave. Mainly manifested in the increase of stator copper loss, rotor copper loss and iron loss, thus affecting the efficiency of the motor.
1. The harmonic current appearing in the stator winding due to stator copper loss increases I2R.When the skin effect is ignored, the stator copper loss under non-sinusoidal current is proportional to the square of the total effective value of the current. Through experiments, it was found that due to the existence of harmonic currents and the corresponding emergence of leakage flux, the degree of saturation of the magnetic circuit of leakage flux increases, so the excitation current increases, thereby increasing the fundamental wave component of the current.
2. When the rotor copper loss is at the harmonic frequency, the resistance of the stator winding can generally be considered to be constant.For the rotor of an asynchronous motor, its AC resistance is greatly increased due to the skin effect. This is especially true for cage rotors with deep grooves. For synchronous motors or reluctance motors under sine wave power supply, since the stator space harmonic magnetic potential is very small, the losses caused in the rotor surface windings are negligible.
When a synchronous motor is operated from a non-sinusoidal supply, the time-harmonic magnetic potential induces harmonic rotor currents, as in an asynchronous motor operating near its fundamental synchronous speed.
Both the 5th harmonic magnetic potential of reverse rotation and the 7th harmonic magnetic potential of forward rotation will induce a rotor current that is 6 times the fundamental frequency. When the fundamental frequency is 50Hz, the rotor current frequency is 300Hz. Similarly, the 11th and 13th harmonics induce a rotor current that is 12 times the fundamental frequency, that is, 600HZ.At these frequencies, the actual AC resistance of the rotor is much greater than the DC resistance. How much the rotor resistance actually increases depends on the conductor cross-section and the geometry of the rotor slots in which the conductors are arranged. For a usual copper conductor with an aspect ratio of about 4, the ratio of AC resistance to DC resistance at 50Hz is 1.56, at 300Hz the ratio is about 2.6; at 600Hz the ratio is about 3.7. At higher frequencies, this ratio increases proportionally to the square root of the frequency.Motors operating on non-sinusoidal power supplies will have many additional losses in addition to the normal losses caused by the fundamental wave. Mainly manifested in the increase of stator copper loss, rotor copper loss and iron loss, thus affecting the efficiency of the motor.
3. The harmonic current appearing in the stator winding due to stator copper loss increases I2R.
When the skin effect is ignored, the stator copper loss under non-sinusoidal current is proportional to the square of the total effective value of the current. Through experiments, it was found that due to the existence of harmonic currents and the corresponding emergence of leakage flux, the degree of saturation of the magnetic circuit of leakage flux increases, so the excitation current increases, thereby increasing the fundamental wave component of the current.
4. When the rotor copper loss is at the harmonic frequency, the resistance of the stator winding can generally be considered to be constant.For the rotor of an asynchronous motor, its AC resistance is greatly increased due to the skin effect. This is especially true for cage rotors with deep grooves. For synchronous motors or reluctance motors under sine wave power supply, since the stator space harmonic magnetic potential is very small, the losses caused in the rotor surface windings are negligible.
When a synchronous motor is operated from a non-sinusoidal supply, the time-harmonic magnetic potential induces harmonic rotor currents, as in an asynchronous motor operating near its fundamental synchronous speed.
Both the 5th harmonic magnetic potential of reverse rotation and the 7th harmonic magnetic potential of forward rotation will induce a rotor current that is 6 times the fundamental frequency. When the fundamental frequency is 50Hz, the rotor current frequency is 300Hz. Similarly, the 11th and 13th harmonics induce a rotor current that is 12 times the fundamental frequency, that is, 600HZ.At these frequencies, the actual AC resistance of the rotor is much greater than the DC resistance. How much the rotor resistance actually increases depends on the conductor cross-section and the geometry of the rotor slots in which the conductors are arranged. For a usual copper conductor with an aspect ratio of about 4, the ratio of AC resistance to DC resistance at 50Hz is 1.56, at 300Hz the ratio is about 2.6; at 600Hz the ratio is about 3.7. At higher frequencies, this ratio increases proportionally to the square root of the frequency.5. The core loss in the harmonic iron loss motor also increases due to the occurrence of harmonics in the power supply voltage; each harmonic of the stator current establishes a time harmonic magnetomotive force in the air gap.
The total magnetic potential at any point in the air gap is the combination of the fundamental wave and the time harmonic magnetic potential. For a three-phase 6-step voltage waveform, the peak magnetic density in the air gap is about 10% larger than the fundamental wave value, but the increase in iron loss caused by time harmonic flux is very small. The stray losses caused by the end leakage flux and the chute leakage flux will increase under the action of harmonic frequencies, which must be considered when using non-sinusoidal power supply: the end leakage effect in the stator and rotor windings All exist, mainly due to eddy current loss caused by leakage magnetic flux entering the end plate. Due to the change in the phase difference between the stator magnetic potential and the rotor magnetic potential, the chute leakage flux is generated in the chute structure, and its magnetic potential is largest at the end, causing losses in the stator and rotor cores and teeth.
6. The size of motor efficiency harmonic loss is obviously determined by the harmonic content of the applied voltage.
The harmonic component is large, the motor loss increases and the efficiency decreases. But most static inverters do not produce harmonics below the fifth order, and the amplitudes of higher harmonics are smaller. The voltage of this waveform does not seriously reduce the efficiency of the motor. Calculations and comparative tests on medium-capacity asynchronous motors show that their full-load effective current increases by approximately 4% compared to the fundamental wave value. If the skin effect is ignored, the copper loss of the motor is proportional to the square of the total effective current, and the harmonic copper loss is 8% of the fundamental loss. Considering that the rotor resistance can increase by an average of three times due to the skin effect, the harmonic copper loss of the motor should be 24% of the fundamental loss. If copper losses account for 50% of the total motor losses, harmonic copper losses increase the overall motor losses by 12%. The increase in iron loss is difficult to calculate because it is affected by the construction of the motor and the magnetic materials used.
7. If the high-order harmonic components in the stator voltage waveform are relatively low, like in the 6-step wave, the harmonic iron loss will not increase by more than 10%.If iron loss and stray loss account for 40% of the total loss of the motor, harmonic losses only account for 4% of the total loss of the motor. Friction loss and windage loss are not affected, so the total loss of the motor increases by less than 20%. If the efficiency of the motor is 90% when operating on a 50Hz sinusoidal power supply, the motor efficiency will only be reduced by 1% to 2% due to the presence of harmonics. If the harmonic component of the applied voltage waveform is significantly greater than the harmonic component of the 6-step wave, the harmonic loss of the motor will be greatly increased, and may be greater than the fundamental wave loss.Even with a 6-step wave power supply, a low leakage reactance reluctance motor may absorb a large harmonic current, thereby reducing the motor's efficiency by 5% or more. In this case, in order to operate satisfactorily, it is necessary to use a 12-step inverter or use a six-phase stator winding. The harmonic current and harmonic loss of the motor are actually independent of the load, so the size of the time harmonic loss can actually be determined by comparing the sinusoidal power supply and the non-sinusoidal power supply under no-load conditions. This is used to determine the approximate range of the efficiency drop of a certain type or structure of motor.8. The core loss in the harmonic iron loss motor also increases due to the occurrence of harmonics in the power supply voltage; each harmonic of the stator current establishes a time harmonic magnetomotive force in the air gap.
The total magnetic potential at any point in the air gap is the combination of the fundamental wave and the time harmonic magnetic potential. For a three-phase 6-step voltage waveform, the peak magnetic density in the air gap is about 10% larger than the fundamental wave value, but the increase in iron loss caused by time harmonic flux is very small. The stray losses caused by the end leakage flux and the chute leakage flux will increase under the action of harmonic frequencies, which must be considered when using non-sinusoidal power supply: the end leakage effect in the stator and rotor windings All exist, mainly due to eddy current loss caused by leakage magnetic flux entering the end plate. Due to the change in the phase difference between the stator magnetic potential and the rotor magnetic potential, the chute leakage flux is generated in the chute structure, and its magnetic potential is largest at the end, causing losses in the stator and rotor cores and teeth.
9. The size of motor efficiency harmonic loss is obviously determined by the harmonic content of the applied voltage.
The harmonic component is large, the motor loss increases and the efficiency decreases. But most static inverters do not produce harmonics below the fifth order, and the amplitudes of higher harmonics are smaller. The voltage of this waveform does not seriously reduce the efficiency of the motor. Calculations and comparative tests on medium-capacity asynchronous motors show that their full-load effective current increases by approximately 4% compared to the fundamental wave value. If the skin effect is ignored, the copper loss of the motor is proportional to the square of the total effective current, and the harmonic copper loss is 8% of the fundamental loss. Considering that the rotor resistance can increase by an average of three times due to the skin effect, the harmonic copper loss of the motor should be 24% of the fundamental loss. If copper losses account for 50% of the total motor losses, harmonic copper losses increase the overall motor losses by 12%. The increase in iron loss is difficult to calculate because it is affected by the construction of the motor and the magnetic materials used.
10. If the higher harmonic components in the stator voltage waveform are relatively low, like in the 6-step wave, the harmonic core loss will not increase by more than 10%.If iron loss and stray loss account for 40% of the total loss of the motor, harmonic losses only account for 4% of the total loss of the motor. Friction loss and windage loss are not affected, so the total loss of the motor increases by less than 20%. If the efficiency of the motor is 90% when operating on a 50Hz sinusoidal power supply, the motor efficiency will only be reduced by 1% to 2% due to the presence of harmonics. If the harmonic component of the applied voltage waveform is significantly greater than the harmonic component of the 6-step wave, the harmonic loss of the motor will be greatly increased, and may be greater than the fundamental wave loss.Even with a 6-step wave power supply, a low leakage reactance reluctance motor may absorb a large harmonic current, thereby reducing the motor's efficiency by 5% or more. In this case, in order to operate satisfactorily, it is necessary to use a 12-step inverter or use a six-phase stator winding. The harmonic current and harmonic loss of the motor are actually independent of the load, so the size of the time harmonic loss can actually be determined by comparing the sinusoidal power supply and the non-sinusoidal power supply under no-load conditions. This is used to determine the approximate range of the efficiency drop of a certain type or structure of motor.
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