i asked about the effects of carrier frequancy on the pwm ac drives. And what happens if it increased or decreased.

 

The PWM carrier frequency is like a sample data system. The rule of thumb is that the frequency should be 10 times the highest frequency of interest.

For instance, if it is being used in a velocity loop where bandwidths of 50 Hertz on industrial machines is attainable, this says that the carrier
frequency should be at least 500 Hertz. If it is in a torque loop where vendors are achieving bandwidths up to 2,000 Hertz, this dictates a carrier frequency of 20 KiloHertz.

You may ask why we don't just make the frequency extremely high and forget about it. This is because the transition from on/off and off/on is where mnuch transistor heat is generated.
In a servo loop, the carrier frequency manifests itself as negative phase shift thereby reducing the phase margin and the stability. This is why most vendors go to 2,000 Hertz even though 500 Hertz may be acceptable from a sampled data point of view.

Regards,
Tom Bullock

 

Higher carrier frequency allows a better approximation to the sinusoidal form of the output current.

Higher carrier frequncies require smaller filter elements in output filters.

Carrier frequencies above the human ear's frequncy limit avoid noises due to carrier frequency.

On the other hand, losses increase with carrier frequency as each act of switching contributes it's share.

 

1) Lower frequency.

Acoustical noise from the motor.

More current ripple (Non-torque producing heating).

2)Higher Frequency

Above 16 kHz-no acoustic noise.

Possibly, more electrical noise.

Possibly, higher power switching losses in the power transistors.

Can implement higher current loop bandwidth (current loop bandwidth is limited to approx 1/10 switching frequency).

 

hello
the carrier frequency of an inverter depends to the distance between motor and inverter.when the distance is long we must increase the carrier frequency. but when we increase this frequucy the noise that produce by inverter also increase and me must take care for this problem.

 

PWM voltage control also generates additional motor loss. For iron machines, PWM motor core loss is supposed to dominate over copper one.

http://www.drbrushless.com/articles/ironloss

-Yuri