Three-phase induction motor in different reference frames

In order to understand, analyze and control the ac induction motor, a dynamic model is necessary. The significant step forward in the analysis of the three-phase induction motor was the reference frame theory. Using this theory, it is possible to transform the machine dynamic model to a different reference frame.

Figure 1.

The equations of the ACIM's (alternate current induction motor) is easy to deduce from the machine structure and circuits.
The stator equations are the following

$$ u_{sa}=R_si_{sa}+\frac{d\psi_{sa}}{dt}\\
u_{sb}=R_si_{sb}+\frac{d\psi_{sb}}{dt}\\
u_{sc}=R_si_{sc}+\frac{d\psi_{sc}}{dt}\tag{1}
$$
and the rotor equations are the following
$$ u_{ra}=R_ri_{ra}+\frac{d\psi_{ra}}{dt}\\
u_{rb}=R_ri_{rb}+\frac{d\psi_{rb}}{dt}\\
u_{rc}=R_ri_{rc}+\frac{d\psi_{rc}}{dt}\tag{2}
$$

Both the current and the voltage are time dependent. In the three-phase reference frame the motor windings are located at 120 degree. Applying the Clarke transformation on the three phase motor, more info in the previous blog, we can write the three-phase machine equations in two phase

$$ u_{s\alpha}=R_si_{s\alpha}+\frac{d\psi_{s\alpha}}{dt}\\
u_{s\beta}=R_si_{s\beta}+\frac{d\psi_{s\beta}}{dt}\tag{3}
$$
$$ u_{r\alpha}=R_ri_{r\alpha}+\frac{d\psi_{r\alpha}}{dt}\\
u_{r\beta}=R_ri_{r\beta}+\frac{d\psi_{r\beta}}{dt}\tag{4}
$$

The $3^{rd}$ and $4^{th}$ equations are based on two different reference frames, one is the stationary, fixed to the stator, the other is rotating with the rotor at electrical motor speed $\omega_r$. With respect of the $\alpha/\beta$ system, the ACIM can be equally describe by two other perpendicular windings. This transformation is applied to transfer the stator and rotor equation's to a common reference frame named $d/q$ (rotating reference frame). In the common reference frame it is easier to describe the dynamic model and behaviour of the ACIM.There are four reference frames widely used in ACIM analysis, namely he stationary, rotor, synchronous and arbitrary reference frame.

Stationary or stator fixed reference frame

In this case the reference frame speed is 0 and all the variables are referred to the stator fixed reference frame. With the following equation we can describe the current in the d/q reference frame.
$$
i_{rd} = i_{r\alpha}cos\theta-i_{r\beta}sin\theta\\
i_{rq} = i_{r\alpha}sin\theta+i_{r\beta}cos\theta\tag{5}
$$
The matrices of the transformation $T_{-\theta}$ and inverse transformation $T^{-1}_{-\theta}$ are the following
$$
T_{-\theta}=\begin{bmatrix}  cos\theta & -sin\theta \\ sin\theta & cos\theta \end{bmatrix}\tag{6}
$$
$$
T^{-1}_{-\theta}=\begin{bmatrix}  cos\theta & sin\theta \\ -sin\theta & cos\theta \end{bmatrix}\tag{7}
$$

The $6^{th}$ and $7^{th}$ equations also can apply on the voltage and flux, because of this we can transform the $3^{rd}$ and $4^{th}$ equations 

$$ T_{-\theta} \begin{bmatrix} u_{r\alpha} \\ u_{r\beta} \end{bmatrix}=T_{-\theta} R_r \begin{bmatrix} i_{r\alpha} \\ i_{r\beta} \end{bmatrix} +T_{-\theta} \frac{d}{dt} \begin{bmatrix} \psi_{r\alpha} \\ \psi_{r\beta} \end{bmatrix}\tag{9} $$
Because $$T^{-1}_{-\theta}T_{-\theta}=1\tag{10}$$we can multiplying the rotor flux vector, it remain unchanged.
$$\begin{bmatrix} u_{rd} \\ u_{rq} \end{bmatrix}=R_r \begin{bmatrix} i_{rd} \\ i_{rq} \end{bmatrix} +T_{-\theta} \frac{d}{dt}\left(T_{-\theta}^{-1}T_{-\theta} \begin{bmatrix} \psi_{r\alpha} \\ \psi_{r\beta} \end{bmatrix}\right)\\ \begin{bmatrix} u_{rd} \\ u_{rq} \end{bmatrix}=R_r \begin{bmatrix} i_{rd} \\ i_{rq} \end{bmatrix} +T_{-\theta} \frac{d}{dt}\left(T_{-\theta}^{-1} \begin{bmatrix} \psi_{rd} \\ \psi_{rq} \end{bmatrix}\right)\\ \begin{bmatrix} u_{rd} \\ u_{rq} \end{bmatrix}=R_r \begin{bmatrix} i_{rd} \\ i_{rq} \end{bmatrix} +T_{-\theta}T_{-\theta}^{-1} \frac{d}{dt} \begin{bmatrix} \psi_{rd} \\ \psi_{rq} \end{bmatrix}+T_{-\theta} \frac{d}{dt}\left(T_{-\theta}^{-1}\right)\begin{bmatrix} \psi_{rd} \\ \psi_{rq} \end{bmatrix}\tag{11}$$
Where
$$T_{-\theta} \frac{d}{dt}\left(T_{-\theta}^{-1}\right)=T_{-\theta} \left(\frac{d}{d\theta}T_{-\theta}^{-1}\right)\frac{d\theta}{dt}\\=\begin{bmatrix}  0 & cos\theta^2+sin\theta^2 \\ -cos\theta^2-sin\theta^2 & 0 \end{bmatrix}\frac{d\theta}{dt}=\begin{bmatrix}  0 & 1 \\ -1 & 0 \end{bmatrix}\omega_r\tag{12}$$
If we insert the $12^{th}$ equation into the $11^{th}$ equation finally we get the stator and rotor equations in the same, stator fixed reference frame.
$$
\begin{bmatrix} u_{sd} \\ u_{sq} \end{bmatrix}=R_r \begin{bmatrix} i_{sd} \\ i_{sq} \end{bmatrix} +\frac{d}{dt}\begin{bmatrix} \psi_{sd} \\ \psi_{sq} \end{bmatrix}\tag{13}
$$
$$\begin{bmatrix} u_{rd} \\ u_{rq} \end{bmatrix}=R_r \begin{bmatrix} i_{rd} \\ i_{rq} \end{bmatrix} +\frac{d}{dt}\begin{bmatrix} \psi_{rd} \\ \psi_{rq} \end{bmatrix}+\omega_r\begin{bmatrix} \psi_{rd} \\ -\psi_{rq} \end{bmatrix}\tag{14}$$

Rotor fixed reference frame

In this case the common reference is attached to the rotor and it is rotating with the electrical speed $\omega_r$. In this case the transformation matrices are the following,

$$
T_{-\gamma}=\begin{bmatrix}  cos\gamma & sin\gamma \\ -sin\gamma & cos\gamma \end{bmatrix}\tag{15}
$$
$$
T^{-1}_{-\gamma}=\begin{bmatrix}  cos\gamma & -sin\gamma \\ sin\gamma & cos\gamma \end{bmatrix}\tag{16}
$$

Applying the same calculation as in the stator fixed reference frame we will get the following equation,

$$
\begin{bmatrix} u_{sd} \\ u_{sq} \end{bmatrix}=R_r \begin{bmatrix} i_{sd} \\ i_{sq} \end{bmatrix} +\frac{d}{dt}\begin{bmatrix} \psi_{sd} \\ \psi_{sq} \end{bmatrix}+\omega_r\begin{bmatrix} -\psi_{sd} \\ \psi_{sq} \end{bmatrix}\tag{17}
$$
$$\begin{bmatrix} u_{rd} \\ u_{rq} \end{bmatrix}=R_r \begin{bmatrix} i_{rd} \\ i_{rq} \end{bmatrix} +\frac{d}{dt}\begin{bmatrix} \psi_{rd} \\ \psi_{rq} \end{bmatrix}\tag{18}$$

Arbitrary rotating reference frame

By replacing the $-\theta$ with $\gamma-\theta$ we can obtain the rotor variables and rotor equation in arbitrary reference frame. In this case the transformation matrices are the following

$$T_{\gamma-\theta}=\begin{bmatrix} cos\left(\gamma-\theta\right) & sin\left(\gamma-\theta\right) \\ -sin\left(\gamma-\theta\right) & cos\left(\gamma-\theta\right) \end{bmatrix}\tag{19} $$
$$ T^{-1}_{\gamma-\theta}=\begin{bmatrix} cos\left(\gamma-\theta\right) & -sin\left(\gamma-\theta\right) \\ sin\left(\gamma-\theta\right) & cos\left(\gamma-\theta\right) \end{bmatrix}\tag{20}$$
Applying the transformation matrices on the $3^{rd}$ and $4^{th}$ equations we will end up with the following equation in the arbitrary reference frame
$$
\begin{bmatrix} u_{sd} \\ u_{sq} \end{bmatrix}=R_r \begin{bmatrix} i_{sd} \\ i_{sq} \end{bmatrix} +\frac{d}{dt}\begin{bmatrix} \psi_{sd} \\ \psi_{sq} \end{bmatrix}+\omega_\gamma\begin{bmatrix} -\psi_{sd} \\ \psi_{sq} \end{bmatrix}\tag{21}
$$
$$\begin{bmatrix} u_{rd} \\ u_{rq} \end{bmatrix}=R_r \begin{bmatrix} i_{rd} \\ i_{rq} \end{bmatrix} +\frac{d}{dt}\begin{bmatrix} \psi_{rd} \\ \psi_{rq} \end{bmatrix}+\left(\omega_\gamma-\omega_r\right)\begin{bmatrix} -\psi_{rd} \\ \psi_{rq} \end{bmatrix}\tag{22}$$
The stator and rotor fixed reference frame is just a particular case of the arbitrary rotating reference frame.  In the stator fixed reference frame the $\omega_\gamma$ is set to zero yield. If the reference frame is rotating with $\omega_r$ the equations of the rotor reference frame are obtained.