Normally we will calculate the mechanical time constant of the motor using the formula J/B. But while i searching the formula related to mechanical time constant of Permanent magnet synchronous motor/ generator (PMSM/PMSG) I got the formula as follows
Tm= R(T)* J/Ke(T)* Kt(T)
R(T)- Motor resistance in Ohm
J- Moment of inertia in Kgm^2
Kt(T)- Torque function in Nm/Amp
Ke(T)- Back emf in Volt./rad/sec.
There is no provision to include Friction coefficient (B)
So i am in confusion with which formula is suitable to calculate the same. I would like to bring to your kind notice that, I am using Field oriented Control of PMSG.
I believe there is a distinction between
mechanical time constant and electromechanical time constant.
Mechanical time constant depends only on J and B
I found the answer for my question. Please correct me if it is wrong.
Since I am using FOC control in PMSG, the mechanism seems to be like servo motor control (Encoder Output is taken). In dynamic response to the applied voltage command, a servomotor responds both electrically and mechanically. Hence to determine the response one should combine the torque equation and voltage equation.
The voltage equation is
V(t)=Ldi(t)/dt+R(T)i(t)+Ke(T)*w(t)
Ke(T)- Back emf constant @ Temperature T
V(t)- Voltage: R(T) is the terminal resistance @ T
i(t) is the current
w(t) is the speed in rad/sec
The torque equation becomes
Kt(T)i(t)=Jdw(t)/dt+Bw(t)+Tf
Tf-Friction Torque Kt(T)- Torque constant
J B are Inertia and damping effects
Here B is more or less zero. While combine these two equation we will end up with the mechanical time constant as Tm= R(T)* J/Ke(T)* Kt(T).
But while we we control PMSG through bridge rectifier cum buck converter the control concept dislike the servomotor mechanism. Hence the time constant will be Tm=J/B.
Thanks for reading. Please correct me if i am wrong.
With thanks and Regards
Rajin M. Linus
Yes you are right. If you are considering in addition the voltage equation you compute the electromechanical time constant. Otherwise, mechanical time constant is based on J and B.
Linus another way of doing parameter estimation is to use the Optimisation Toolbox / Blockset in Simulink. Do you have it?
Then you can capture actual waveforms input them into MATLAB and use them in Simulink with the optimisation blockset. You can then set the parameter ranges and set the transient response envelopes and the optimisation Blockset will optimise the parameters until the desired transient response is achieved.
This works well and is accurate and fast. However, it has a bit of a learning curve but works well once you are used to it.
damping, aka rotation damping is the mechanical rotational equivalent of resistance. It restricts rotational freedom.
The bearings, additional magnetic drag, windage, fluid etc... All contribute to additional loading and loading which is related to velocity. Sometimes it is directly proportional, sometimes it isnt (airflow as a drag affect is a square law iirc)
How does it affect the current? Well with increased rotor velocity the viscose drag increases and so to maintain a given operating point more current is needed to offset this loss of torque.
In electrical circuit L provides drop proortinal to current derivative while resistive drop is proportional to current alone. In result time constant is L/R
similary in mechanical equation torque drop is promotional speed derivative times J while B provides such drop only as proportional to speed.
therefore emecanucal time const has to be j/B
Dear Sir,
I am working on a Quasi Impedance Souce inverter(qZSI) with PMSM using FOC
In most of the research papers, SVPWM techniques are used for PMSM and other motors after inverter transformation in both VSI FED and ZSI fed motors...
For ZSI these are the commonly used PWM techniques;
(i) Simple Boost PWM (SBPWM)
(ii) Maximum Boost PWM (PWM)
(iii) Constant Boost PWM (PWM
(iv) Modified SVPWM (MSVPWM)
Instead of the SVPWM technique can I use the Simple Boost PWM (SBPWM) ?
As I understand matters all these techniques are a variation of a theme. The question is why do you need this for your project and what is trying to be achieved by boosting it?
What one does depends on the problem trying to be solved which is not clear? Are you trying to reduce harmonic content in the input are you trying to get better performance from the PMSM etc?
Dear Sir,
I am working on a "variation of the DC link voltage control of the quasi Z source inverter fed to PMSM. Here voltage has to be boosted and vary with the speed and torque requirment and also maintain minimum THD.
So While doing in real-time implementation using dSPACE controller with the Simple Boost PWM, can we get the Same voltage, Power by as produced by SVPWM? ( Hardaware part still not yet tried sir, need start).
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