You can use constant V/f up to rated voltage. For many motors, you can continue to increase frequency above rated frequency, but you should not increase the voltage. Increasing frequency will cause the motor to spin faster, but, if the voltage is held constant, the available torque will decline as frequency increases.
Note that the rotor never quite catches up with the frequency you supply to the motor. Your motor has a synchronous speed of 1500 rpm. If you supply it with 50 Hz, it will try to spin at 1500 rpm, but won't quite get there. This difference, 1500 rpm - actual speed, is called slip. Slip is proportional to torque.
As you increase the mechanical load on the motor, it will slow down a bit, but this will increase slip and increase torque until the torque matches the load torque.
The nameplate speed (1375 rpm) is the speed at which the motor will deliver its rated torque and power when supplied with the nameplate voltage. If you do not apply a mechanical load, the slip and torque will be less than nameplate rating, and the motor will spin somewhere between 1375 and 1500.
If you need to control the speed of the motor, you may need to look at a variable frequency drive (VFD). It may also be called an AC drive.
One other thing to consider is cooling. There are different types of motors. But if your motor has a fan built-in to it, as many do, it probably relies on that fan to keep the motor cool. If you run it at full torque, but low speed, the fan will not be able to keep the motor cool. For example, if the nameplate says TEFC, then the motor is totally enclosed fan cooled, and has an internal fan.
