Bosch rexroth MCL, Project Planning Manual

Page: 140 / 206

9.23 Operation at or near motor standstill
In the motor is to be operated at standstill or close to standstill, special condi‐
tions apply. For means of simplification, this operation is indicated as stand‐
still operation in the following. The absolute standstill operation is marked by
the following aspects:
●
The duration of the standstill operation is longer than 10 % of the re‐
spective thermal time constant T
th
● Motor does not move
● Motor performs only very small strokes (≤ 2 * Τ
p
)
● Motor moves only at very low frequency (f ≤ 0.1 Hz)
Due to the 3-phase system, during standstill operation in one of the three
phases there is always an instantaneous value of the current, the amount of
which is higher than the permissible continuous current. Does the current
flow continuously, the motor will overheat and be thermally damaged. Also
refer to chapter 9.10 "Motor temperature monitoring" on page 107 ). The
peak value of the instantaneous current is equal with the amplitude of the si‐
nusoidal assumed phase current. Its value is higher by root 2 than the effec‐
tive value of the continuous current (I
N
). The power loss P
V
, created in the
coil, is calculated using
Δθ Temperature difference between operation temperature and
20 °C
α
Cu
Temperature coefficient of the specific resistance of copper =
0.0039 Ohm * m / mm²
Fig. 9-37: Power loss coil
For the nominal current, a double power loss occurs in the affected coil.
To avoid damaged winding in standstill operation, limit the current or the op‐
eration duration. A possible form of an additional motor protection is the limi‐
tation of the motor current to 87% of the nominal current.
132/197
Application and construction instructions
MCL Ironless Linear Motors
Bosch Rexroth AG R911330592_Edition 06