Motor peak and continuous current limits
All GD motor drives feature a two stage current limit in order to protect motor and drive from overheating damage.
Description of operation
The basic parameters for current limiting are:
- Peak current limitMMC Motor maximum current a.k.a. motor peak current
- Continuous current limitMCC Motor continuous current a.k.a. motor continuous current
- Thermal time constantMTC 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 refuses to drive current above continuous level if I2 protection estimates motor temperature to be at maximum (see below).
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.
In no event the Product Information or parts hereof shall be regarded as guarantee of conditions or characteristics. The Product Information or any part thereof may also not be regarded as a warranty of any kind. No liability of any kind shall be assumed by Author with respect to Product Information or any use made by you thereof, nor shall Author indemnify you against or be liable for any third party claims with respect to such information or any use thereof.
As content of this Wiki may be edited by user community, Granite Devices Oy or it's affiliates do not take any responsibility of the contents of this Wiki. Use information at your own risk. However, Granite Devices staff attempts to review all changes made to this Wiki and keep information trustworthy.
Without written consent, Granite Devices' Products or Intellectual Property shall not be used in situations or installations where living beings, material property, or immaterial property could be harmed by the operation, features or failures of Product. Products may only be used in a way where hazards like moving parts, electric shock, laser radiation, or fire can't be realized even if the content of this Wiki would suggest otherwise.
