7 replies to this topic

[Carl]

Hello All

For the pasfew months I've been looking at building an inverter I'm now looking at the method of dealing with the regen from the motor.

There are two methods that I am aware of :

i) Dissipating the excess energy via a resitor
i) Feeding the energy back to the main supply line.

The question I have is what are the pro's and con's of the above methods.
Is it legal to send voltages down the main line as I'm sure this will cause distortion and possibly affect other equipment?

[marke]

Hello carl

The normal way to get rid of the regen energy is to use a brake chopper and power resistor. A more expensive and elegant solution is to use an "active front end" or regenerative inverter.

A brake chopper is easy to implement and requires a voltage sensing circuit and a large enough IGBT to switch the current through the resistor.

The active front end comprises an active recifier instead of a standard bridge. This active rectifier is essentially the same as the output stage of the inverter and the supply is connected to the output terminals. Some manufacturers use two equal inverters to create an active front end inverter. The software is different and the inverter is connected to the supply by a three phase reactor which helps to eliminate the switching noise etc.

My recommendation would be to build a sandard inverter first and use a brake chopper if required. Once you have that perfected, look at changing the input rectifier to an active rectifier.

The major disadvantage of the brake chopper and resistor is the power dissipated in the resistor.

The advantage of the active front end is the improved efficiency when braking is required, and the input current can be sinsoidal.

Best regards,

[Carl]

Thanks for the reply, yes first the brake resistor then the active front end, makes sense.

I not sure if it is worth putting an active front end on an inverter smaller than 2.2Kw ...from a cost point of view.

I am already battling, to getting away with using one uP. I'm desperately trying to keep the price down


Regards

Carl

[marke]

Hi Carl

Adding an active front end will significantly increase the cost and I would only consider this if you need sinusoidal supply current, or you have significant regen energy.

Best regards,

[kada]

Hi Carl

There is another solution how to dissipate energy from motor. It is called "High slip braking" or "AC brake".
From Yaskawa report:
HSB is achieved by modifying the volts/Hz pattern during deceleration to maintain enough motor slip so that load energy is absorbed in the rotor bars, stator windings and stator core of the motor. Current level is controlled to avoid excessive motor heating. Deceleration time is directly related to the actual load torque and inertia.
That's up to you.

[kada]

Hi Carl

There is another solution how to dissipate energy from motor. It is called "High slip braking" or "AC brake".
From Yaskawa report:
HSB is achieved by modifying the volts/Hz pattern during deceleration to maintain enough motor slip so that load energy is absorbed in the rotor bars, stator windings and stator core of the motor. Current level is controlled to avoid excessive motor heating. Deceleration time is directly related to the actual load torque and inertia.
That's up to you.

[marke]

Hello kada

Yes there are a number of variations on this theme that essentially put the excess energy back into the motor as heat. This is OK if the motor is running under light load conditions and it happens infrequently. If however, the motor is running close to it's rating, this will add to the thermal stress in the motor and will conribute to an early failure. Tread carefully!!

Best regards,

[Carl]

Thanks All
I've opted for the brake resistor for the moment, one of my goals is just to get something working and then play with all the different techniques.
The above solution does seem interestring but.... so little time so much to brake...pardon the pun

Carl