Operating instructions

Setpoint channel and closed-loop control
7.4 Vector speed/torque control with/without encoder
Drive converter cabinet units
Operating Instructions, 10/2008, A5E00288214A
273
Vector control without a speed sensor has the following characteristics at low frequencies:
● Closed-loop operation up to approx. 1 Hz output frequency
● Starting in closed-loop operation (directly after the drive has been energized) (induction
motors only)
Note
In this case, the speed setpoint upstream of the ramp-function generator must be greater
than the changeover speed in p1755.
Closed-loop operation up to approx. 1 Hz (settable via parameter p1755) and the ability to
start or reverse at 0 Hz directly in closed-loop operation (settable via parameter p1750)
result in the following benefits:
● No changeover required within closed-loop control (smooth operation, no dips in
frequency).
● Stationary speed–torque control up to approx. 1 Hz.
Note
When the motor is started or reversed in closed-loop control at 0 Hz, it is important to take
into account that a switchover is made from closed-loop to open-loop control automatically if
the system remains in the 0 Hz range for too long (> 2 s or > p1758, if p1758 > 2 s).
Permanent-magnet synchronous motors are always started and reversed in open-loop
operation. The changeover speeds are set to 10% or 5% of the rated motor speed.
Changeover is not subject to any time condition (p1758 is not evaluated). Prevailing load
torques (motor or regenerative) are adapted in open-loop operation, facilitating constant-
torque crossover to closed-loop operation even under high static loads. Whenever the pulses
are enabled, the rotor position is identified.
Closed-loop operation down to standstill for passive loads
By restricting to a passive load at the starting point, it is possible to maintain stationary
closed-loop operation down to zero frequency (standstill) without having to change over to
open-loop operation.
Parameter p1750.2 must be set to 1.
Closed-loop control without changeover is restricted to applications with passive load:
These include applications in which the load cannot produce a regenerative torque on
startup and the motor comes to a standstill when pulses are inhibited; for example, moments
of inertia, brakes, pumps, fans, centrifuges, extruders, etc.
Standstill of any duration is possible without holding current, only the motor magnetization
current is impressed.
The stationary regenerating operation at a frequency close to zero is not permissible.
It is also possible to select sensorless control for passive loads during commissioning by
setting p0500 = 2 (technology application = passive loads (for sensorless control down to f =
0)).