Siemens MICROMASTER 411 - Manual (Page 118)

10 Engineering Information Issue 03/01
MICROMASTER 411 & COMBIMASTER 411 Operating Instructions
118 6SE6400-5CA00-0BP0
10.1.2 Fast Current Limit
Fast Current Limit (FCL) is a cycle-by-cycle hardware current limit built into the
inverter.
The current is rapidly reduced by pulse dropping, that is by turning off the
Insulated Gate Bipolar Transistors (IGBTs) on a pulse by pulse (cycle by cycle)
basis.
The normal current limit operation then takes over.
The FCL threshold is set slightly below the software overcurrent trip threshold and
reacts much quicker (i.e. in milliseconds), thus preventing spurious and unwanted
trips when sudden loads are applied or fast accelerations requested.
FCL is especially useful when working in open loop control to override unwanted
currents.
10.1.3 Positive Temperature Coefficient Resistor Use
Many motors are available with a Positive Temperature Coefficient (PTC) resistor
built into the windings.
The resistance of the resistor rises rapidly at a particular
temperature and this change can be detected by the inverter. If the resistor is
connected to the inverter terminals as shown in Figure 10-2 and the PTC input
enabled by setting parameter P0703=29, then if the resistance rises above 2 k
Ω ,
the inverter will trip and Fault Code F0085 displayed.
Most Motor Protection PTC resistors have a resistance of 200-300 ohms when cold
and this value rises rapidly at the ‘knee point’ to typically 10 k
Ω and greater. The
PTC input is set so that it will operate at 1 k Ω minimum, 1.5 k Ω nominal, and 2 k Ω
maximum. On this basis two or three PTCs may be connected in series when a
motor has more than one PTC built in, or if two or three motors are connected to
the inverter output and require individual protection.
1 2 3 4 5 6 7
Figure 10-2 PTC Resistor Connections