DC-bus connection - parallel operation of several autonomous brake choppers, without synchronisiation

Question:
Is it possible to operate several brake choppers in parallel in a DC-bus connection and what must be observed?

Answer:
Master-slave operation of brake choppers (synchronised):
If control is synchronised, it is possible to operate several brake choppers in parallel in a DC-bus connection. 

Parallel operation requires synchronisation via control signals or a communication link (master-slave operation). 

 

Device series types within synchronisation function (in general, sychronisation cannot be used across device series types):

• Servo-Inverter i950
• 9400 servo drives (implemented overload protection for brake chopper. The braking power is limited in case of imminent overload)
• 8200 vector inverters
  

Autonomous operation (not synchronised):
If synchronisation is not possible and the brake choppers are autonomously, i.e. independently of each other, controlled, safe operation can only be achieved under certain conditions.
The problem arising from parallel operation of several autonomously controlled brake choppers is that due to hardware tolerances and other influences the effective switching thresholds of the individual brake choppers are different. Even small differences may lead to an extremely unequal degree of utilisation of the individual brake choppers and the corresponding brake resistors. In the worst case, one brake resistor may be overheated and a risk of fire may occur while another brake resistor will almost be under no load and cold.

Conditions which allow parallel operation of autonomous brake choppers:

• Parallel operation is only required due to the high peak braking power required for a short time.
• The average continuous braking power required over one operating cycle is lower than the continuous brake chopper power (or brake resistor power) with the lowest tripping threshold. Extremely high braking power is only required for a short time.
• The average continous braking power required over one operating cycle is higher than the continuous brake chopper power (or brake resistor power), but during braking, high braking power (close to the selected peak braking power) is required for most of the time. In this case, due to the quick DC-voltage change, all brake choppers will always almost in parallel be switched on and off and without considerable delays.
  

Conditions which may cause overload situations:

• Parallel operation is due to the high continuous braking power required over a cycle and not to the peak braking power.
• The average continuous braking power required over one operating cycle is higher than the continuous power of only one brake chopper (or brake resistor) and braking is effected at comparatively low braking power, however, over a longer period of time.

Relationship of cause and effect:
The greater the difference between the effective switching thresholds of the brake choppers, the more pronounced will be the effect of an unequal brake chopper utilisation. The DC-bus capacity and, in particular, the height and the time characteristics of the required braking power are further factors of influence.
The problem will show if the first brake chopper with the lowest switching threshold is switched on. Then, two cases have to be distinguished:

• The current regenerative power is lower than or equal to the peak power of the activated brake chopper (P = U²/R): The DC voltage does not increase any further so that the second brake chopper with the higher switching threshold will not be activated.
• The current regenerative power is higher than the peak power of the activated brake chopper: The DC voltage continues to increase, but slowlier than before. Accordingly, the switching threshold of the next brake chopper will be reached later and the brake chopper will be switched on with delay. The highest 'delay' will occur, if the current regenerative power is only slightly higher than the peak power of the first brake chopper. A higher DC-bus capacity will also lead to a slower rise of the DC voltage and thus intensify this effect.

In braking phases as in the first case, only one brake chopper will be subject to thermal load (initially loaded). If a braking phase with high braking power follows, both brake choppers will be subject to the same load, but due to the initial load of the first brake chopper, the first brake chopper would sooner reach its limit.

Search terms: Multi-chopper operation, multi-brake chopper, brake chopper

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