When a permanent magnet synchronous motor is rotated, it produces an electromotive force (EMF), usually called the back-EMF, which causes a sinusoidal AC voltage between the motor terminals. The back-EMF is linearly proportional to the rotational speed, so for example a motor with nominal speed of 2600 RPM will produce the nominal voltage (usually 500 V RMS line-to-line AC for Editron motors) when it is rotating at about 2600 RPM. At 1300 RPM, this motor would generate approx. 250 V, and so on.
The inverter converts DC power to AC power by PWM space vector modulation, and a minimum current control method is used to achieve the required torque and speed with the least current possible. The inverter controls the motor's speed by producing a voltage that causes the required current to produce torque that is either motoring or generating (4-quadrant drive principle).
The required voltage depends on the speed (= back-EMF). To be able to produce enough voltage, the DC link voltage of the inverter must be at least √2 × Uac (motor line-to-line voltage multiplied by the square root of 2), preferably higher.
There are several effects in lowering the DC link voltage.
Like stated previously, the AC voltage output by the inverter is roughly equal to the back-EMF of the electric machine. To achieve this, the DC voltage must be at least √2 × Uac. At higher speeds there might be a case when the DC voltage is not high enough in relation to the AC voltage. After that point the inverter uses some current output to weaken the magnetic flux in the machine so that the back EMF is reduced to a suitable level.
Because some of the current is now used for field weakening, the torque must be reduced. The following graph will show the relationship of power, torque and speed. The system goes to field weakening when √2 × Uac > Udc, in the figure below this point can be seen at the speed of "ω_base"
Operating in field weakening can be dangerous. If there is any fault condition like overtemperature or CAN timeout, the inverter will trip and stop modulating. As the inverter is stopped, it can no longer weaken the field and reduce the back-emf of the machine, so the full back-emf is rectified through the inverter's diodes to the DC link. This causes high current to the battery. If there is no battery in the system, a high DC voltage will occur.
A lower DC voltage also means that a higher DC current is needed to achieve the same power. Motor output power is
P = ω × T,
where ω is the angular speed in rad/s and T is torque in Nm. Also, in the electrical world AC power is defined as
Pac = √3 × pf × Uac × Iac,
where pf is power factor (usually between 0.85 and 0.95), Uac is AC voltage (line-to-line) and Iac is AC current. DC power can be defined simply
Pdc = Udc × Idc
where U is DC voltage and Idc is DC current. As the inverter basically converts DC power to AC power and vice versa, the Pac and Pdc have to be equal (minus the losses, which are now omitted for simplicity). The DC power formula shows that by lowering the DC voltage, the DC current will be higher.
In short:
Low DC voltage will limit the maximum available torque, power and speed.
Low DC voltage will increase the DC current.