The standard IEC 61800-9-2 stats the folliwing (Section D.4.2, Additional losses due to frequency converter voltage drop): "Often, frequency converters (CDM) cannot provide full (rated) fundamental motor voltage, which is required to maintain full flux at rated speed. In this case, the losses in the motor will be higher due to an increased motor current."
What is the reason of that? Why the rated voltage cannot be supported in this case?
Because of the DC voltage that is lower than the required Vdc needed to apply the Vsingle phase max voltage : Vmax = 230 *sqrt(2) = 325 V
when you rectify the 400V using a 6 Diodes rectifier you will not get the 650 V = Vmax*2
Thus, the speed at which you start defluxing in vector control will be lower than the IM nominal speed.
Lotfi Baghli, thank you for the answer. However when a space-vector PWM is used we need only 230* sqrt(2)* sqrt(3) = 564 V of the DC-link voltage to provide 230 V of the phase motor voltage, right? So, is that information mentioned in the standard correct only in the case of the simple carrier-based sinusoidal PWM?
Louelson Costa
, thank you. I think that may be useful in the case of induction motors of a large rated power. However, I study motors of a low rated power (0.55-5.5 kW). CSC cannot be used in this case. I just want to know what is the real limitation of maximum motor voltage when a traditional two-stage voltage-source frequency conveter with the 6 diodes rectifier and with the space-vector PWM for which the theoretical maximum modulation index (2*V maximum phase/V DC) is 2/sqrt(3) is used.
I don't understand the issue, if your PWM can't supply quite enough voltage, then use a motor designed for whatever voltage it can easily produce. It is only a question of varying the number of turns and the gauge of the wire used.So if a motor designed for 440V isn't optimum, what about one designed for 380V, and so on?
To Tony Maine. Yes, I think if one designs a new motor that is dedicated to be supplied exclusively by frequency converters (FC) it is a right decision. However, the majority of motors that are used with FC are still either usual DOL induction motors or motors that are dedicated both for grid supply and for converter supply. They have the rated voltage about 400 V. So, I want to calculate correctly the maximum available voltage in the case of using the DOL motors with FCs, in order to make calculation of energy consumption and other performances of variable speed electric drives with a higher accuray.
Vadim, the more accurate you want to be, the more accurate your input data will need to be. This means you will need to know the operating characteristics of your motor and drive unit to that accuracy, and this means choosing a specific design of motor and drive unit. Now an induction motor torque speed curve is a quartic, with zero torque at infinity, rising to a maximum near the drive frequency, dropping to zero at the drive frequency, reversing sign (making it an induction generator) with a maximum near the drive frequency, then tailing off to infinity. The difference between the drive frequency and motor speed is the slip. The slip has a peak torque dependent on the ratio between the rotor inductance and resistance - the lower the rotor resistance the closer the peak torque is to zero slip. This also means the starting torque is less. So make sure your system can start the motor and accelerate it up to maximum torque without stalling. Best wishes with your research!
Dear Dr. Maine. I agree, that in many cases it may be not necessary to take into account the limitation. It can still affect significantly the motor efficiency in the case of defluxing. Thank you for your suggestions!
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