uselectric2403
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What effect does the 7 harmonics have on a 3 phase motor? Thank you for your help.
Harmonics
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winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
When discussing three phase motors, there are generally two types of harmonic to consider: 'spatial' harmonics relating to the shape of the magnetic field produced by the motor windings, and 'temporal' harmonics in the voltage applied to the motor terminals.
The 7th voltage (temporal) harmonic is a positive sequence harmonic. It is trying to make the motor spin in the normal direction, and thus adds a small amount of torque to the motor output. However the synchronous speed of the 7th harmonic is 7 times the normal operating speed of the motor, so with respect to the 7th harmonic, the motor is almost stalled, and thus this little bit of useable torque is produced very inefficiently. 7th harmonic will reduce the efficiency of the motor, increase motor heating, etc.
-Jon
The 7th voltage (temporal) harmonic is a positive sequence harmonic. It is trying to make the motor spin in the normal direction, and thus adds a small amount of torque to the motor output. However the synchronous speed of the 7th harmonic is 7 times the normal operating speed of the motor, so with respect to the 7th harmonic, the motor is almost stalled, and thus this little bit of useable torque is produced very inefficiently. 7th harmonic will reduce the efficiency of the motor, increase motor heating, etc.
-Jon
beanland
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- Location
- Vancouver, WA
Cause vs. Effect
Cause vs. Effect
Another point: induction motors draw current in a waveform rich in harmonics. Generally, 5th, 7th, 11th, and 13th harmonics dominate a 3-phase motor. So, the voltage being measured at motor terminals will have harmonic content both caused by the specific motor and from other similar sources on the electric system. But, harmonic currents are a normal unavoidable aspect of induction motors.
Cause vs. Effect
Another point: induction motors draw current in a waveform rich in harmonics. Generally, 5th, 7th, 11th, and 13th harmonics dominate a 3-phase motor. So, the voltage being measured at motor terminals will have harmonic content both caused by the specific motor and from other similar sources on the electric system. But, harmonic currents are a normal unavoidable aspect of induction motors.
The supply 6n ? 1 harmonics are typical for the input to an inverter fed motor.beanland said:
The output to the motor is generally PWM in the order of a few kHz.
Insulation problems are related to rate of rise of voltage (dv/dt) and overvoltage both as a consequence of fast switching IGBTs rather than fundamental supply frequency and its harmonics.
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