Load inertia
From the formula for bandwidth, it is seen that bandwidth
changes inversely with total inertia. If the load inertia equals the
motor plus resolver inertia, the velocity loop bandwidth will be
half the values shown. If the load inertia is ten times the motor
plus resolver inertia, the bandwidths will be one eleventh these
values. Clearly
KVP
must be increased to compensate for
increased load inertia if bandwidth is to be maintained.
Typically, load inertia up to 3(motor + resolver) give acceptable
performance without further optimization.
The most common servo setup problem is adding large load
inertia without a corresponding increase in KVP.
The value of
KVP
to achieve a desired bandwidth can easily be
calculated as follows:
KVP
f
J
K
f
J
K
vc
TOT
T
vc
TOT
T
=
≈
2
3
2
7 26
π
*
*
.
*
*
Example
calculation
For example, to achieve 75 Hz bandwidth with an R32G motor
having 20 to 1 load inertia = 0.011 lb-in-sec
2
:
J
TOT
1
= 0.00055 + 0.011 = 0.01155 lb-in-sec
2
K
T
2
= 4.4 lb-in/amp
KVP
=
=
7 26 75
0 01155
4 4
143
.
*
*
.
.
.
MA950
4 - 3
1
Motor plus resolver inertia (0.00055 lb-in-sec
2
) for the R32G motor can be found in the
catalog.
2
K
T
can be found in the catalog as K
T
peak (4.4 lb-in/amp).
