The approximate efficiency of the 3-phase, 50 Hz, 4-pole induction motor using the following formula:

Efficiency = Pout / Pin

where

Pout is the output power or mechanical power developed by the motor
Pin is the input power or electrical power supplied to the motor.

First, we need to determine the synchronous speed of the motor.

The synchronous speed (Ns) of a 4-pole motor running at 50 Hz can be calculated using the formula:

Ns = (120 * f) / P

where f is the frequency of the supply voltage (50 Hz in this case) and P is the number of poles (4 in this case).

Ns = (120 * 50) / 4
Ns = 1500 rpm

Since the actual speed of the motor is 1350 rpm, we can calculate the slip (s) using the formula:

s = (Ns - N) / Ns

where N is the actual speed of the motor.

s = (1500 - 1350) / 1500
s = 0.1 or 10%

Now, we can calculate the output power or mechanical power developed by the motor using the formula:

Pout = (1 - s) * Pin

where Pin is the input power to the motor.

Assuming the input power to be 1000 watts, we get:

Pout = (1 - 0.1) * 1000 watts
Pout = 0.9 * 1000 watts
Pout = 900 watts

Therefore, the efficiency of the motor can be calculated as:

Efficiency = Pout / Pin
Efficiency = 900 watts / 1000 watts
Efficiency = 0.9 or 90%

So, the approximate efficiency of the 3-phase, 50 Hz, 4-pole induction motor running at 1350 rpm is 90%.