76 Program featuresDC voltage control Overvoltage controlOvervoltage control of the intermediate DC link is typically needed when the motor isin generating mode. The motor can generate when it decelerates or when the loadoverhauls the motor shaft, causing the shaft to turn faster than the applied speed orfrequency. To prevent the DC voltage from exceeding the overvoltage control limit,the overvoltage controller automatically decreases the generating torque when thelimit is reached. The overvoltage controller also increases any programmeddeceleration times if the limit is reached; to achieve shorter deceleration times, abrake chopper and resistor may be required. Undervoltage control (power loss ride-through)If the incoming supply voltage is cut off, the drive will continue to operate by utilizingthe kinetic energy of the rotating motor. The drive will be fully operational as long asthe motor rotates and generates energy to the drive. The drive can continueoperation after the break if the main contactor (if present) remained closed.Note: Units equipped with a main contactor must be equipped with a hold circuit (e.g.UPS) to keep the contactor control circuit closed during a short supply break.1302603905201.6 4.8 8 11.2 14.4t (s)UDCfoutTMUDC= intermediate circuit voltage of the drive, fout = output frequency of the drive, TM = motor torqueLoss of supply voltage at nominal load (fout = 40 Hz). The intermediate circuit DC voltage drops to theminimum limit. The controller keeps the voltage steady as long as the mains is switched off. The drive runsthe motor in generator mode. The motor speed falls but the drive is operational as long as the motor hasenough kinetic energy.Umains204060804080120160UDC(V DC)fout(Hz)TM(Nm)
