Half Controlled Bridge Rectifier

[anu_rags]

Hello All,

 

I will like to understand better that how a Half Controlled Bridge Rectifier is better or worse than a Full Wave Diode rectifier in VSD? I have some understanding but would like you experts, to elaborate in bit detail.

 

Thanks in advance,

Anurag

[marke]

Hello Anurag

 

With a VSD, there are two major issues which can justify the used of a half controlled bridge as opposed to a full wave bridge.

 

First, when power is initially applied to the VSD, there needs to be a mechanism to limit the current that charges up the DC bus, otherwise, the DC bus capacitors look close to a short circuit while they are charging and there is a very high current on the supply and through the rectifiers. The charging current can damage the rectifiers, blow fuses and also damage the capacitors.

The common means of limiting the current, is to use a "soft charge" resistor which is in series with the DC bus for a period of time allowing the capacitors to charge before it is bridged out. The alternative means is to control the voltage with a half controlled bridge and cause the DC voltage to raise slowly.

The disadvantage of the soft charge resistor, are size, heat and reliability, and the need for a soft charge contactor to bridge it out.

The disadvantages of the controlled bridge are: complexity, high harmonics in the current in the supply and into the capacitors during the charging cycle, commonly affected by phase imbalances during the charging cycle.

 

The second reason for using a controlled bridge, is the requirement that the output voltage varies with frequency. It was common to use a simple waveform generator and a controlled bridge to get a voltage variation with frequency. The PAM topology was an example of this method. Today, it is easy to produce a waveform generator that can vary both voltage and frequency and so this technique is no longer common.

 

Best regards,

[anu_rags]

Hello Marke,

 

Manufacturers who employs HCB in their VSD claims that there is no limitation in the no. of times a VSD can tunred ON or OFF by a input contactor whereas for a Full wave Diode rectifier this may cause serious damage to the drive.

 

Also could HCB improve VSD performance in 'Power loss Ride through' conditions during voltage sag?

 

Regards,

Anurag

[marke]

Hello Anurag

 

If you switch on the input often, you are putting a strain on the soft charge resistor and contactor which can lead to a failure of these items. If the soft charge resistor is undersized, this can lead to a catastrophic failure of this resistor which could cause a failure of the input rectifiers as well. (the resistor can break down to ground)

 

So yes, the controlled half bridge does enable more frequent switching on the input.

 

Best regards,

[anu_rags]

thanks marke for your prompt reply. Can you pls clarify whether HCB can improve the VSD performance during a 'Power loss ride through' conditions on input voltage sag.

[marke]

Hello Anurag

 

There are two possible scenarios:

1. PAM modulated VSD where the output voltage is controlled by a half wave bridge rectifier
2. PWM where the output voltage is controlled by the waveform generator.

In the case where the output voltage is controlled by the bridge rectifier, and the output voltage is reduced due to the drive running at reduced speed when a voltage dip occurs, it is possible for the half wave bridge to phase forward and increase the output voltage relative to the input voltage and that way maintain a constant output voltage. This requires that the motor is operating at reduced speed and reduced voltage and that the voltage dip is small enough for the HWB to compensate.

 

In the case where the VSD used PWM techniques to control the output voltage, the HWB is providing full output voltage and will not add value to low voltage ride through. The normal technique is to use the regenerative technique of causing the motor to slow down fast enough to regenerate voltage and use this to keep the DC bus up. This can prevent a low voltage trip, but will result in a drop in motor speed.

 

Best regards,

[anu_rags]

Hello Marke,

 

But do the modern VSD employ PAM technique in their drives. To my understanding all the vector control drives are PWM drives. Not sure for the Direct Torque Control one?

 

Regards,

Anurag

[marke]

Hello Anurag

 

The norm is for low voltage VSDs to use PWM techniques. PAMwas common many years ago and I am sure that there will still be some around using this technique.

 

Best regards,