Simple Applications | International English |
5.6Current Limit and Protection Systems
The inverter must protect itself, the motor and system from overload and possible damage. Current limit now operates very rapidly, limiting the current and preventing a trip occurring.
Most inverters have several levels of current limiting:
Electronic | This is a very fast current limit which operates if there is a |
Trip. | short circuit (line to line or line to earth) on the output. It is a |
| fixed level trip and operates within a few microseconds. |
Overload | This is a fast limit, which operates within a few microseconds, |
Limit. | and removes some of the output pulses to limit the current and |
| protect the inverter. If this pulse dropping occurs during |
| overload, the operating condition will usually recover and the |
| motor continue to operate without tripping. |
Long Term | This is a slower limit, which allows an overload of at least 60 |
Overload limit. | seconds when the current lies above the motor limit, but below |
| the instantaneous limit values described above. |
Continuous | This is the level set as the maxiumum continuous motor |
Limit. | current. The inverter will control the current to this level after |
| the overloads described above have timed out. |
For further details refer to the Operating Instructions and Reference Manual.
5.7Other Protection Features
5.7.1I2t Protection
When the motor is running at low speed and high load, the built in cooling fan may not provide enough cooling and the motor may overheat. Parameters can be set to calculate the motor temperature, based on a motor model and operating history such that the inverter will take action to protect the motor under these conditions. Further information is given in the Reference Manual.
5.7.2PTC Resistor Protection
Many motors are available with a PTC (Positive Temperature Coefficient) resistor built into the windings. The resistance of the PTC rises rapidly at a particular temperature, and this change can be detected by the inverter. The input terminals of the inverter may be configured to accept a PTC signal and trip the inverter in the event of overheating.
5.7.3Overvoltage
If the inverter is connected to a high voltage, or if the internal voltage is forced high by energy from an external load, then the inverter will trip. Overvoltage usually occurs as a result of a braking or regenerative load. See section 8.2. If the supply voltage is too high the inverter may be damaged even if it trips.
5.7.4Internal Overtemperture
The inverter is protected from overheating. The heatsink temperature is monitored using a PTC and if the maximum temperature exceeded the inverter will trip.
24 | MICROMASTER Applications Handbook |