Selection Of Servomotor Capacity; Load Inertia Ratio; Short Time Characteristics - Mitsubishi MELDAS HS Series Specifications And Instruction Manual

11-3 Selection of servomotor capacity

The following three elements are used to determine the servomotor capacity.

1. Load inertia ratio

2. Short time characteristics (acceleration/deceleration torque)
3. Continuous characteristics (continuous effective load torque)
Carry out appropriate measures, such as increasing the motor capacity, if any of the above conditions
is not fulfilled.

11-3-1 Load inertia ratio

Each servomotor has an appropriate load inertia ratio (load inertia/motor inertia). The control
becomes unstable when the load inertia ratio is too large, and parameter adjustment becomes difficult.
It becomes difficult to improve the surface precision in the feed axis, and the positioning time cannot
be shortened in the position axis because the settling time is longer.
If the load inertia ratio exceeds the recommended value in the servomotor list of specifications,
increase the motor capacity or change to a motor series with a large inertia. Note that the
recommended value for the load inertia ratio is strictly one guideline. This does not mean that
controlling a load with inertia exceeding the recommended value is impossible.
POINT

11-3-2 Short time characteristics

In addition to the rated output, the servomotor has an output range that can only be used for short
times such as acceleration/deceleration. This range is expressed at the maximum torque. The
maximum torque differs for each motor even at the same capacity, so confirm the torque in section
"2-2 Servomotor".
The maximum torque affects the acceleration/deceleration time constant that can be driven. The
linear acceleration/deceleration time constant ta can be approximated from the machine
specifications using expression (11-2). Determine the maximum motor torque required from this
expression, and select the motor capacity.
ta =
95.5 × (0.8 × T
Chapter 11 Selection
When selecting feed axis servomotors for NC unit machine tools, place
importance on the surface precision during machining. To do this, always
select a servomotor with a load inertia ratio within the recommended value.
Select the lowest value possible within that range.
) × N
(J + J
L M
(msec)
− T
)
MAX L
N : Motor reach speed
J : Motor shaft conversion load inertia
L
J : Motor inertia
M
T : Maximum motor torque
MAX
T : Motor shaft conversion load (friction, unbalance) torque (N· m)
L
................................................. (11-2)
11–6
(r/min)
–4 2
(× 10 kg· cm )
–4 2
(× 10 kg· cm )
(N· m)