When an induction motor is used with VSD, the Base Frequency* is usually set to 50Hz. The Base Frequency is the point where the frequency Inverter outputs the Mains Voltage of 400V.
When the speed is reduced the frequency inverter reduces the output voltage proportionally to the required speed (and frequency). Voltage/frequency ratio 400V/50Hz = 8V/Hz is maintained.
If you set the required speed at 50% of the motor rated speed the Inverter outputs 25Hz and 200V to the motor.
This means that the motor will produce constant torque (and variable power) between 5Hz and 50Hz.
Please note that the motor performance at speeds under 5Hz depends on the frequency inverter more than on the motor.
If you operate the motor at speeds greater than the motor rated speed, the inverter increases the frequency accordingly but cannot output more than the mains voltage of 400V.
This means that Voltage/frequency ratio 400V/50Hz = 8V/Hz can no longer be maintained and the motor enters into Field Weakening mode.
The motor torque then decreases inversely proportional to the speed (because the flux in the motor decreases) and the power is constant.
87Hz Motor Technique offers an interesting solution for applications where Constant Torque is required above 50Hz (above the motor rated speed) as this Technique offers a relatively
constant flux up to 87Hz.
The Base Frequency of the inverter needs to be set to 87Hz. This means that the frequency at which the Inverter outputs the Mains Voltage of 400V is 87Hz.
Motors suitable for 87Hz Motor Technique must be wound for 230/400V 50Hz (not 400/690V 50Hz).
The motor is then connected in Delta (for 230V 50Hz) and 230V is the voltage the inverter outputs at 50Hz.
But if you operate the motor at speeds greater than the motor rated speed, the inverter increases the frequency accordingly and can output voltage higher then 230V until
the mains voltage of 400V is reached at 87Hz. This means that the motor Constant Torque range increases up to 87Hz.
The motor power at 87Hz is √3 greater than the rated motor power. The motor current at 87Hz is √3 greater than the rated motor current.
It should be noted that when using the 87Hz Motor Technique the frequency inverter should be increased to √3 x Motor Power Size.
3kW motor used for 87Hz Motor Technique: √3 x 3kW = 5.19kW .... 5.5kW inverter needs to be used.
The iron losses increase proportionally above the rated motor frequency and this is the reason that the motor torque must be reduced above the motor rated frequency.
Also please note that it is recommended that the 87Hz Motor Technique is used for up to 45kW or 55kW motors.
*'Base Frequency' is sometimes called 'Nominal Frequency' or 'Motor Frequency' (depending on the Inverter Manufacturer).