Back-EMF and rectified voltage
When a permanent magnet motor is rotating, it will always generate a sinusoidal AC voltage to the motor phase connections. This voltage is also referred to as the back-EMF (electromotive force) of the motor. The frequency and amplitude of this voltage are linearly correlated to the RPM of the motor, and for standard Editron motors the generated back-EMF is approximately 500 V AC at the motor nominal speed. Consequently, the generated back-EMF at half of the nominal speed is 250 V AC, and so on.
When a rotating motor is connected to an EC-C1200 inverter (not running/controlling the motor), the generated AC voltage will freely pass through (rectified by) the inverter's IGBT module diodes, and charge the DC-link of the inverter. The resulting rectified DC voltage can be calculated by multiplying the motor AC voltage with the square root of two, or approx. 1,41. Because of this phenomenon, there is a limit to how fast a motor can be rotated when connected to a non-running inverter, as the DC-link voltage of the inverter must stay within the specified voltage range. Rotating the motor at a too high speed will impose a risk of catastrophic failure of the inverter and the motor.
Overspeed and field weakening
When a PM motor is controlled by an EC-C1200 inverter, the inverter can suppress the magnetic field of the motor to allow running the motor at speeds above of the nominal speed without the back-EMF of the motor becoming too high and raising the DC-link voltage accidentally. This feature is called field weakening, and it is only possible when the inverter is running and actively controlling the motor.
If the inverter trips and stops modulation while running at a high speed, the field weakening effect is nullified and the full back-EMF voltage will be present in the motor AC connections, which in turn might cause the DC-link voltage to rise too high and cause a failure. Because of this, the maximum overspeed which is safe for normal operation is usually approximately 1,2 to 1,4 times the nominal speed of the motor.
The mechanical maximum speed of the motors is usually twice the nominal speed, but this speed can only be achieved safely with some external method of limiting the maximum back-EMF and/or DC-link voltage in case of an inverter tripping. Such methods could be e.g. installing an AC isolator between the inverter and motor, which is then quickly disconnected in case the inverter trips, or installing an automatic electrical brake chopper to limit the maximum DC-voltage.
Example calculation
If a motor/generator has a nominal speed of 1500 RPM, and it is driven by a diesel engine at 2200 RPM, the generated back-EMF of the motor is approx. 733 V AC. When the motor is connected to an inverter, this voltage will cause an (uncontrolled) rectified DC-link voltage of 733 V * sqrt(2) = 1040 V DC if the inverter is not applying field weakening to the motor. If the inverter trips, for example while generating full power from the motor at full speed, the diesel engine will likely rush due to a sudden decrease in the load, resulting in the speed rising even higher than 2200 RPM, and leading to the DC voltage exceeding 1040 V. This would most probably lead to a short-circuit and instant destruction of the equipment, including but not limited to the generator, diesel engine, inverter(s), and any devices connected to the DC-link of the inverter. To run this system safely, a faster speed variant of the motor/generator is recommended.