Braking Resistor

Hello, Hope that these might clarify the facts:

1) VFD FOC technology gets equivalent behaviour to inverse braking but in such a way that differential speed between stator and rotor magnetic fields is completely controlled. A lagged magnetic field is produced on stator windings applying voltage by means of PWM technique. As this magnetic field is lagged with respect to the magnetic field driven by the rotor, a force appears that makes the rotor speed to slow down .

2) In terms of voltages, these rotor and stator magnetic fields result in two differential voltages. The difference between them yields a voltage resultant which generates a current flowing back into the VFD. This energy returning path accounts for the dissipation of kinetic energy on the rotor axle. But, Where does the kinetic energy goes?

3) Braking resistors are just a junk box for energy dissipation. It is useful as long as it has got energy dissipation capability. They are passive elements, rough and basic elements. While the VFD is only an energy transducer, braking resistors are electric loads which must evacuate the mechanical energy produced on the motor axle (resistor powerdissipation ability>braking torque*angular speed)

4) This energy is evacuated by the operation of a transistor placed at the DC bus bar of the VFD, which switch on the path between bus bar and resistors when it is detected that energy is being sent back from the motor to the VFD.

3) The additional point to be clarified is why using this braking technology instead of using a magnetic field acting proportionally to axle speed or phase changing?? just for the same reason for which VFD are used instead of direct feeding of motor or static voltage-starters. As FOC theories allow treating AC motors as if they were DC control-prone motors, so does VFD braking. As long as theThyristor bridge is able to handle the currents, any braking curve may be obtained. Other technlogies as DC braking are cheaper but on its basic development it only allows pre-defined braking profiles like damped braking, with a counter-force proportional to speed, and it is usual that the mechanical brake have always to work at the end of the braking cycle. Best Regards. Ignacio Simón Yarza. Mech eng Electronic and automatism eng

"Bob" escribió en el mensaje news: snipped-for-privacy@4ax.com...

Quote: Actually, most VFDs do both. Dynamic Brak Fred

Read the post again boy, "motors controlled by a VFD"

Teddy Rubberford. Bizarre Bugger...

The AC/DC c Hello, can someone give me a explanation on how braking resistors work on AC motors controlled by a VFD?

Thanks, Dave

"Dave" escribió en el mensaje news:gJQNe.151327$wr.27141@clgrps12...

Dave

What do you mean by "how is the resistor used in the motor cct?"? Are you meaning "how is the resistor used in the motor circuit?"? If so, the explanation is given below, in the 4th point. Otherwise, may be I would need that you make the question clearer. Best regards. Ignacio Simón Yarza. Mech&electrical-electronics engineer.

"Dave" escribió en el mensaje news:u4nOe.234797$on1.229560@clgrps13...

The VFD must have a "chopper circuit" to feed the excess energy into the resistor. It basically senses the DC bus voltage, and as regenerative energy rises the DC bus, the chopper turns on to bleed that energy off by burning it into the resistor as heat. Once the DC bus level drops, the chopper turns off again.