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40 Unidrive M Regen Design Guide
Issue Number: 4
4 System design
4.1 Introduction
The sizing of a Regen system must take into account the following
factors:
1. AC power supply voltage variation
2. Motor rated current, rated voltage and power factor
3. Maximum required power and overload requirements
4. Heavy Duty / Normal Duty Regen drive ratings
In general, when designing a Regen system, equal Regen and motoring
drive rated currents will work correctly. However, care must be taken to
ensure that under worst case AC supply conditions the Regen drive is
able to supply / absorb all the required power including total system
losses.
If the Regen drive is unable to supply the full power required by the
motoring drive(s), the DC bus voltage will drop, and in severe cases may
lose synchronization with the AC power supply and trip. If the Regen
drive is unable to regenerate the full power from the motoring drive(s)
into the DC bus, then the Regen drive and motoring drive(s) will trip on
over-voltage.
4.1.1 Single Regen, single motoring drive
The following calculations can be carried out for either a single Regen
drive, motoring drive system or single Regen drive, multiple motoring
drive system.
Example
In the case of a 30 A (Normal Duty), M600-05400270 operating in
Regen mode from a 400 V supply, and a M600-05400270 driving a
400 V rated, 0.85 pf motor:
The rated power of the Regen drive is:
= 1.73 x 30 x 400
= 20.8 kW
The motoring drive can supply power:
= 1.73 x 30 x 400 x 0.85
= 17.7 kW
Drive losses:
2 x Unidrive M600-05400270 = 648 W
When the motoring drive is supplying rated current to the motor, the
Regen drive needs to provide 17.7 kW, plus drive losses = 18.348 kW.
The Regen drive can supply 20.8 kW at rated current, which is ample,
in this case.
Conversely, in some cases, a Regen drive of the same rating as the
motoring drive, may not be able to supply enough power, as the
following example shows:
Example
In the case of a 100 A (Heavy Duty), M600-07401000 operating in
Regen mode, and a M600-07401000 driving a 75 kW, 400 V, 0.95 pf
motor:
If the motoring drive is supplying 175 % maximum current, and the
Regen drive has its 380 V supply at the lower limits of -10 % (342 Vac),
then, with a Regen current limit of 175 %:
The Regen drive maximum available power is:
= 1.73 x 1.75 x 100 x 342
= 103.5 kW
The motoring drives maximum power is:
= 1.73 x 1.75 x 100 x 400 x 0.95
= 115 kW
Drive losses
2 x Unidrive M600-07401000 = 2.034 kW
The Regen drive is also required to supply the Regen and motoring drive
losses in this example 2.034 kW which brings the total power
requirement to 117.034 kW. However, this Regen drive is only capable of
supplying approximately 103.5 kW and therefore a drive of a larger
rating is required.
4.1.2 Multiple motoring drives
In multi-drive configurations, the Regen drive must be of a sufficient size
to supply the net peak power demanded by the combined load of all
motoring drives plus the combined losses, including its own losses.
Due to the effects of increased DC bus capacitance, there is a limit to the
number of motoring drives that can be supplied from a Regen drive. This
is true irrespective of the balance of power between the motoring drives
and the Regen drive.
The previous calculations can be used for the sizing of multiple motoring
drives also.
4.2 Power connections
The following section covers the power connections required for
Unidrive M Regen systems.
• For single Regen, single motoring systems, AC supply connections
are made to L1, L2 and L3 drive terminals and the drive’s internal
soft start circuit is used for power-up.
• The single Regen, multiple motoring and multiple Regen, multiple
motoring systems require an external charging circuit due to the
extra capacitance from the additional drives. No AC connections are
made to the Regen drive’s L1, L2 and L3 terminals. The external
charging circuit can consist of either the Unidrive M rectifier solution
or an external softstart resistor as shown in Figure 4-5, Figure 4-6
and Figure 4-7.
• For the Regen brake resistor replacement system, the motoring
drive’s internal soft start is used for power-up with no AC
connections to L1, L2, L3 on the Regen drive.
For control circuit connections refer to section 6.6 Control connections
on page 133.
If the Regen system is not a standard configuration or changes are
required to the following systems and set ups, contact the supplier of the
drive.
3 Rated current× Supply voltage×
3 Rated current× Motor voltage Power factor××
3 175 % Rated current×× Supply voltage×
3 175 % Rated current× Motor voltage×× Power factor×
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