ROYAL
MOTOR PROTECTION

General features
The Royal A5 provides the measurement of the following A.C. parameters:
• Measurement of three currents
• Unbalancement between current measurement
• Measurement in true RMS value by sampling and successive approximations
• Memorization of peak and valley values for all the measurements
• Configuration of any .../5 A current transformer
• 2 alarm relays delivering common options of ROYAL series
• Alarm conditions defined by:
- Current value
- Current unbalance
- Phase inversion

Unbalancement

We will generally install at any three phase network an alarm system to control the value of the current flowing through each phase so that getting an alarm trip when any of these values exceeds a pre-set one.

This control might not be suitable for all cases since certain unfavorable operation conditions might not be detected.

Example: You want to protect a three phase motor rated at 100 A per phase; and let's suppose the two cases are unbalanced.

Case 1: IR = 130 A, IS = 131 A, IT = 129 A

Case 2: IR = 81 A, IS = 80 A, IT = 23 A

Our alarm system trips when any current is higher than 100 A, for the first case and we will have an alarm trip as the pre-set value has been exceeded. For the second case no alarm signal happens as all three currents are below 100 A.

Just analysing the second case, we can see that, although no alarm trip ocurred, a problem is present since a great unbalancement between phase currents is observed, which means that a winding of the motor is damaged, or phase voltages are not correct.

Similarly, if only unbalanced between phases are controlled for the above cases, an alarm trip would only appear for the second one, which is not acceptable, as rated currents are being exceeded in the first case.

We can therefore deduce that the ideal alarm system would consist of controlling both the current value and the unbalancement between phases.

For the calculation of the unbalancement, we firstly get the average value of the three currents:

Where IM stands for the average value of the three currents.

We then calculate the relative difference and the difference in absolute value for each current with respect to the average value.

Where ID1-D2-D3, stand for the % values of unbalance of the difference in absolute power for the current of each phase with respect to the average value.

The value that will be displayed and that will be considered for the alarm condition is the highest one among above three ones.

PHASE INVERSION
For such installations where several motors connected to the same network exist, a phase inversion effect could be occurring in the motor under control, so having balanced consumptions in absolute value being that the motor generating current throught one phase.

In order to control the above action, we will set the equipment with the option "YES" for the parameter 23IF or 33IF (whether using relay 1 or 2); so getting an alarm for a phase inversion event.

In case of alarm, both the energized relay signal and the letters U or I, that respectively indicates a trip due to unbalancement or phase inversion (placed at the frontal side) will remain lit on.

IM < 10 % (Average current < 10 % Rated current) As a motor can usually present a unbalancement effect in case of very low currents in comparison to the rated one, the alarm condition referred to the unbalancement will be automatically disabled while the average current measurement remains below 10% pf the pre-set rated current.