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66 Unidrive M Regen Design Guide
Issue Number: 4
4.4.6 Regen and motoring drive ratings - brake resistor replacement
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The Regen drive’s current limits are detailed in section 3.3 Ratings on page 17.
In general the Regen drive must be rated at a power greater than, or equal to, the maximum braking power.
Example:
• Two 30 kW motoring drives are each driving 30 kW motors. The load is such that only one drive is braking at a time.
If each motor supplies between 20 and 30 kW motoring, and the braking power varies from 0 to 30 kW, the maximum total braking power is 30 - 20 =
10 kW, which is what the Regen drive should be rated for.
In drive configurations where the motoring drive power rating is several times the expected braking power, it is necessary to consider the peak
braking power returned from the load.
Example:
• The motoring drive is a 75 kW Unidrive M. Motoring power is 75 kW. Steady state braking power is 20 kW.
From these figures, it may appear that a 22 kW Regen drive will provide sufficient braking power. However, dynamically the peak braking power could
be much greater. If the 75 kW drive current limits are set at 175 % for motoring and braking (default settings), the peak brake power could be:
3 x 157 A x 400 V x 175 % = 190.4 kW
This is much greater than the 22 kW Regen drive is able to return to the supply, hence a larger drive is required.
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If the Regen drive is not rated for the required braking power, then the drives will trip on DC bus over-voltage.
4.4.7 Isolating transformer and diode - brake resistor replacement
There are three main connection differences compared with normal operation.
• There are AC supply connections to both the Regen and motoring drives.
• The DC bus connection between the Regen and motoring drives is via the rectifier diodes shorting the line input connectors (L1, L2 and L3) and
connecting them to the –DC connection (as detailed in figure 4-5) ensures that power flows from the motoring drive to the regen drive only.
• The switching frequency filter inductors are replaced with an isolating transformer Trx with a leakage inductance 4%.
Isolating transformer Trx
This is a three phase transformer which provides isolation between the AC supply and the Regen drive. One isolating transformer can only supply
one Regen drive with the current rating equal to the Regen drive continuous current rating. The transformers leakage inductance forms the switching
frequency filter inductance. The optimum inductance value is specified in section 10.4.5 Switching frequency filter capacitors on page 299, any value
equaling or exceeding this by up to 100 % is acceptable. The required reactance is 4 % and a standard transformer has a reactance in the range 4 %
to 6 %. It should not be necessary to specify a special transformer for this purpose.
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A non isolating transformer should not be used under any circumstances.
It is not possible to use the Combi filter solution in a braking resistor replacement system.
When using the Regen drive as a braking resistor replacement, the Regen input must have an isolating transformer installed so that the
Regen drive input can float with respect to ground.
The Combi filter combines the switching frequency filter and EMC filter into one item. A significant part of an EMC filter are the capacitors
between line and ground.
The result of placing a Combi filter in circuit between the Regen drive and isolating transformer is that the ground connection to the Combi
filter prevents the Regen drive input from floating and damage to the system will occur.
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