HSD
6.9.4 Use and technical characteristics of the thermal alarm
The electrospindle is equipped with a bimetallic normally-closed switch, inserted in the electric coils
of the stator, which opens when a temperature that is harmful for the electric coils is reached; the
contact closes automatically when the temperature drops and returns to safe values.
The thermal alarm must be connected to the numerical control, which in turn must stop the
machining operations as quickly as possible and stop the rotation of the shaft of the electrospindle,
if the switch opens.
If the shaft blocks while the tool is still being pushed against the piece being worked, the
bearings of the spindle could break; if you wait too long before moving the tool away
from the piece and stopping the rotation, the stator could be burnt.
6.9.4.1 Technical characteristics of the bimetallic switch:
Power supply
Current
Switching cycles
Contact breaking time
Contact resistance (according to MIL R 5757)
Insulation voltage
48V DC MAX
1.6A MAX
10,000 cycles
< 1 ms
< 50 mΩ
2 kV
6.9.4.2 Motor Thermistor
In some versions cuold be inserted a PTC thermistor in the stator windings to monitor temperature.
Electrical resistance from the thermistor increases rapidly as it reaches trip temperature (normally
100°C or 130°C depending on the model).
The signal from the thermistor must be fed to a control device to stop the machine in order to
protect the electro-spindle from the effects of overheating.
Thermistor trip temperature depends on the model of electro-spindle. (See data sheet)
The thermistors used in these electro-spindles comply with DIN 44081-44082 standards.
Temperature – Resistance curve
according to DIN 44081/44082
Main specifications
Nominal trip temperature
TNF= from 50°C to 200°C
in steps of 10K or 5K
Characteristic values for
PTC thermistors
Resistance in
temperature range from
-20°C to TNF-20K
Resistance at TNF-5K
Resistance at TNF+5K
Resistance at TNF+15K
Resistance
20 to 250
550
1330
4000
Measurement
voltage
2.5 V
2.5 V
2.5 V
7.5 Vpulse
Figure 6.3
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