Protection with thermal overload relay
Operating mode: When the Start button is pressed star contactor Q2 coil is energized. It closes and by means of normally open contact of Q1 the voltage is applied to main contactor Q1. Q1 closes and supplies the voltage to motor M1 in star connection. Q1 and Q2 are maintained via normally open contacts. Once main contactor Q1 is energized time ON delay timer T1 is supplied as well, this has two contacts one normally open (NO) and one normally closed (NC). After the set time elapsed , T1 timer opens the Q2 circuit by means of NC contact and by means of NO contact closes the circuit of Q3. Star contactor Q2 is opened and Q3 delta contactor coil is energized and the full voltage is applied to motor M1. At the same time the normally closed (NC) contact of Q3 cuts off the Q2 circuit and create an interlock, so we avoid simultaneous operation of Q2, Q3 while the motor is running.
A new start is possible only after pressing Stop button or an overload occur, in this case normally closed contact 97-98 of the motor protection relay TF1 is opened.
Calculation of fuses, contactors and thermal overload relay
Before starting any type of calculation it is important to respect the following rules:
Rule no. 1: Refer to the motor plate , there are found important information such as motor power, voltage, current drawn, the power factor, motor yield, etc; information that the manufacturer has obtained as a result of a large number of calculations and tests.
Rule No. 2: If you don't have the informations listed in Rule no.1, use the following formula for calculation, which is the active power of the motor:
P-active power [W]
U- voltage [V]
I- current drawn [A]
cosⱷ- power factor
We take for example a three phase motor which we know that has the power of 22 kW. So we have initial data:
Supply voltage being a three phase motor, or your supply voltage(460, 690 V)
Power factor in general = (0.6-0.95), so chose a mean of 0.8 unless we know the real value.
From equation (1) we obtain the full load current (FLC) of the motor :
Now for star contactor Q2 the current will be calculated as follow:
for thermal overload relay TF1, main Q1 and delta Q3 contactors:
For F1 fuse:
F2 and F3 fuses for control circuit will be 1[A]