ACS550 braking problem

GA although the braking time is programmable and not controlled by relays but usally IGBT's driving the bulk cap .overvoltage produced from decelerating to brake resistors at a controllabled manner 53 2E 4D 2E

i think maybe you're confusing AC with DC dynamic braking - the resistor is always connected and switched by the DC bus level as and when needed i.e. switches ON at regen and OFF at discharged level again by transistor (relay far too slow).

The +ive of DC is normally permanently connected to one end of the resistor whilst the other is connected to the braking transistor whose output is connected to -ive, thus discharging the DC at a faster rate than regen when needed in braking.

Actually, brake resistors can get very hot and are often rated at 140C rise and more, can even glow, as they take all the braking energy from the motor (less efficiency loss of course) and dump it across the resistor.

Anyway, just for information, not to criticise.

Thanks for the correction. What sort of centrifuge is this beast? Why chopped braking?

I guess most centrifuges are high stored energy so stopping always presents a problem.

The description of chopped braking is acuurate but can mislead. The motor is in 'regenerative' controlled braking situation, -ive load torque, the AC motor being excited turns into a generator feeding back to the DC Link of the VFD. This couls be regenerated back to the mains but this is complex and expensive, so for low duty (occasional stops are a small % of the cycle) its easy and lower cost to chop the DC acoross the resistor to dissipate the excess energy. This may mean that, for example, if the rectified mains gives 580VDC the brake switch may switch on at 750VDC and off at a level like 600VDC.

Hope this helps a little.

Check the de-acceleration time in the parameters of the drive. Maybe is is set too long?

The braking resistors should be connected via the braking transistor in the drive, so this will switch normally the +ive via the resistors to the -ive in the DC link when this goes higher. This is automatic in the drive.

If this is a 400V system and you need to stop quite quickly (100% torque), then the resistance should be about 40ohms and the power relative to the power absorbed. So, if using 195 ohm, reduce deceleration time until you get an overvoltage trip. This time is the limit for 195 ohms so 40 ohm will be 20% of this time or less.

I expect that the power rating for the resistor bank may need to be several kW continuous which you can overload for the short time operation but this needs estimating and calculating.

So it seems you need to check the braking circuit and transistor rating, and also change the resistor to get better braking, I have used this on washing machine applications similar to centrifuge.

You state 12.5kW motor but what/rating size is the VFD. Up to 11kW, this should have a braking circuit inside, above it is an option selected by current and thermal capacity.

Where and what terminals is this resistor connected to so it can function. The resistance is too high anyway (as per Florence info) but with no heat is not conducting. Is the VFD ramping down or freewheeling or, in anti regen mode where it extends the ramp to avoid regenerating (braking)? Parameter 2602 sets flux braking but I cannot see any function to select braking or not so i assume its automatic (normally is on most VFD).

If this is the case, either the ramptime is too long, or some anti-regen select is going on. So, please advise with more information, if you have a resistor, it should be connected to brake circuit terminals - maybe BRK+ and BRK-, so set your ramp time lower to effect braking. The resistance is way too high for effective braking so choose correct resistance and power according to your application (as advised by Florence again (GA)), seek advice from your supplier if in doubt as this needs thermal protection too (FIRE!!).