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Home » Motor protection » Motor Starting voltage protection
Motor Starting voltage protection
In many situations we deliberately reduce the starting voltage in order to better control the actual motor and machinery start, and there is rarely any legislation telling us how to achieve this. This could be by series-reactors, auto-transformers, or the popular, but expensive (virtual reduced voltage) star/delta start, so in some cases you can simply rely on the built-in series impedance of the supply cable.
With regards to the motor and application:
You must ensure that there is sufficient voltage at the motor to overcome the torque requirement from the driven apparatus. Other than that you can do pretty much as you please. Ensure that you do not design for such an extended start period that you overheat the motor windings (note the reduced cooling at low RPM), and the cable to the motor (due to the increased current).
With regards to the motor protection:
Make sure that the primary protection reacts sufficiently slowly to allow for your start-up period. Your back-up protection shall be able to protect against short circuit faults and earth faults. That is not the purpose of the motor protection on its own. The motor contactor is not necessarily rated to interrupt any potential short-circuit fault current.
With regards to the remainder of the installation:
As mentioned above, you need to limit the voltage drop such that you do not cause other equipment supplied from the same source to trip or malfunction.
If you do what 99% of all folk do, that is size a supply cable to provide no more that 5% voltage drop at the motor terminals when the motor is operating at nameplate full load current, you will be OK >90% of the time in my opinion.
If your motor has a long starting time (eg, a fan with relatively heavy impeller) - then you may need to do a quick study of the supply source impedance upstream to make sure you don't brown out the installation.
With regards to the motor and application:
You must ensure that there is sufficient voltage at the motor to overcome the torque requirement from the driven apparatus. Other than that you can do pretty much as you please. Ensure that you do not design for such an extended start period that you overheat the motor windings (note the reduced cooling at low RPM), and the cable to the motor (due to the increased current).
With regards to the motor protection:
Make sure that the primary protection reacts sufficiently slowly to allow for your start-up period. Your back-up protection shall be able to protect against short circuit faults and earth faults. That is not the purpose of the motor protection on its own. The motor contactor is not necessarily rated to interrupt any potential short-circuit fault current.
With regards to the remainder of the installation:
As mentioned above, you need to limit the voltage drop such that you do not cause other equipment supplied from the same source to trip or malfunction.
If you do what 99% of all folk do, that is size a supply cable to provide no more that 5% voltage drop at the motor terminals when the motor is operating at nameplate full load current, you will be OK >90% of the time in my opinion.
If your motor has a long starting time (eg, a fan with relatively heavy impeller) - then you may need to do a quick study of the supply source impedance upstream to make sure you don't brown out the installation.