Motor Replacement & Torque Problem

The power rating of the motor (kW in your case) is a short-hand way of expressing torque at a given speed. So if the speed of each 4kW motor is the same, the torque is the same. How the motors are wound internally has nothing to do with it.

Now in the case of these two specific motors, there is a very slight difference in speed and as you have correctly observed, the Siemens motor is a little faster, so technically, it is producing ever so much less torque than the ABB motor. Assuming he 4kW rating is essentially accurate in both, then the Siemens motor is a potential problem. But it appears you have correctly ascertained the required power at the shaft and it is less than what the Siemens motor will be able to deliver, so no problem.

Don't get caught up in the Star or Delta connection issue, it is a red herring here. That has only to do with how the motor was made, not with how it will perform at the rated voltage.

I totally agree to what JRaef said.

Here is just added few words for more clarification:

I understood, the main concern here is the Delta/Star configuration of motor A.

What actually "4 KW, 2850 RPM, 400/690 V (Delta/Star), 7.8/4.5 A" mean for motor A:

(1) If you connect the motor stator windings in Delta configuration and apply 400V to the terminals, the motor will produce maximum 4 KW power with full load current 7.8 A and its full load torque at 2850 RPM.

(2) If you connect the motor stator windings in Star configuration and apply 690V to the terminals, the motor will produce maximum 4 KW power with full load current 4.5 A and its full load torque at 2850 RPM.

(3) If you connect the motor stator windings in Star configuration and apply 400V to the terminals, the motor will produce maximum 4/3 = 1.33 KW power with full load current 4.5 A, the full load RPM will be slight less than 2850 RMP and its full load torque will be close to 1/3^rd of the rated torque.

In terms of motor output, both condition 1 and 2 are same but the motor is flexible to use for both 400V and 690V.

Because of the Delta/Star winding options, the motor A also can be used for Start/Delta starting and if it is started with Start/Delta starting, the condition 3 is applied first, then it moved to condition 1 after few second.

The motor B has only Start connection with 400V rating. So, if 400 V is supplied to its terminal, it will produce maximum 4 KW power and its full load torque at 2905 RPM. The full load current is 8 A.

- MS