Difference between revisions of "Motor peak and continuous current limits"
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*[[MTC]] Motor thermal constant time | *[[MTC]] Motor thermal constant time | ||
*Peak current time limit ''T'', normally a fixed value of 1 or 2 seconds depending on drive model | *Peak current time limit ''T'', normally a fixed value of 1 or 2 seconds depending on drive model | ||
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[[File:Currentlimits graph.png|800px]] | [[File:Currentlimits graph.png|800px]] | ||
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In ideal case drive would output the demanded current always. However due to the fact that motor and drive generates more heat when current is higher, it is necessary sometimes to reduce output current to a safe level. | In ideal case drive would output the demanded current always. However due to the fact that motor and drive generates more heat when current is higher, it is necessary sometimes to reduce output current to a safe level. | ||
Revision as of 12:19, 22 July 2014
All GD motor drives feature a two stage current limit in order to protect motor and drive from overheating damage.
Descroption of operation
The basic parameters for current limiting are:
- MMC Motor maximum current a.k.a. motor peak current
- MCC Motor continuous current a.k.a. motor continuous current
- MTC Motor thermal constant time
- Peak current time limit T, normally a fixed value of 1 or 2 seconds depending on drive model
In ideal case drive would output the demanded current always. However due to the fact that motor and drive generates more heat when current is higher, it is necessary sometimes to reduce output current to a safe level.
- Drive outputs demanded current infinitely if demand is below MCC level
- Drive outputs demanded current if demand is below MMC and time limit T has not been exceeded
- Drive reduces T based on thermal time constant (MTC) if calculated motor temperature is already at maximum safe level
I2t motor thermal modeling
As mentioned earlier, continuous current limit may kick in also due to motor thermal limit. Drive's I2t protection continuously estimates motor temperature based on history of driven current and motor thermal time constant MTC value.
If motor is being driven long enough above continuous current limit, motor temperature will rise to a level where I2t will prevent drive from driving current above MCC.
MTC parameter sets the speed of motor temperature change. Larger motors have greater mass which heats up slowly giving them large thermal time constant while small motors heat up very quickly thus their MTC value is low.
Physical definition of time constant is the time where measured variable (in this case temperature) has risen to 63% of final settling value under constant driving.
