Have a look at the code below and let me know if you have any questions.

\documentclass{article}
\usepackage{nicefrac,amssymb}
\usepackage{mathtools} % Loads amsmath and also adds \mathrlap{}
\usepackage{relsize}
\newlength\matheqindent \setlength\matheqindent{3em} % Length name controlling the indentation
\begin{document}
\noindent%
\begin{align*}
\mathrlap{
\int \limits_{0}^{2\pi} \hat{B}_{\delta r \mu} \cdot \cos \left( \mu p \alpha - \mu \omega_{el} t \right) \cdot \hat{A}_{\nu} \cdot \sin \left( \nu p \alpha - \omega_{el} t \right) d\alpha
}\hspace{\matheqindent} \\
&= \hat{B}_{\delta r \mu} \cdot \hat{A}_{\nu} \cdot \int \limits_{0}^{2\pi} \cos \left( \mu p \alpha - \mu \omega_{el} t \right) \cdot \sin \left( \nu p \alpha - \omega_{el} t \right) d\alpha \\
&= \frac{\hat{B}_{\delta r \mu} \cdot \hat{A}_{\nu}}{2} \cdot \Bigg(
\begin{aligned}[t]
& \overbrace{\int \limits_{0}^{2\pi} \sin \left( p \alpha \left( \nu - \mu \right) - \omega_{el} t \left(1 + \mu \right) \right) d\alpha}^{\larger\textcircled{\smaller[2]1}} \\
& + \overbrace{\int \limits_{0}^{2\pi} \sin \left( p \alpha \left( \nu + \mu \right) - \omega_{el} t \left(1 - \mu \right) \right) d\alpha}^{\larger\textcircled{\smaller[2]2}} \Bigg) \\
\end{aligned} \\
\mathrlap{
\larger\textcircled{\smaller[2]1}:
}\hspace{\matheqindent} & \\
\mathrlap{
\int \limits_{0}^{2\pi} \sin \left( p \alpha \left( \nu - \mu \right) - \omega_{el} t \left(1 + \mu \right) \right) d\alpha
}\hspace{\matheqindent}\\
&= \frac{1}{p \cdot (\nu - \mu)} \cdot \Big[ -\cos \left( p \alpha (\nu - \mu) - \omega_{el} t (1 + \mu) \right) \Big]_{0}^{2\pi} \\
&= \frac{1}{p \cdot (\nu - \mu)} \cdot \Big(
\begin{aligned}[t]
& - \cos \big( 2\pi \overbrace{ p (\nu - \mu)}^{\in \, \mathbb{Z}} - \omega_{el} t (1 + \mu) \big) \\
& + \cos \big( - \omega_{el} t (1 + \mu) \big) \Big) \\
\end{aligned} \\
&= \frac{1}{p \cdot (\nu - \mu)} \cdot \Big(
\begin{aligned}[t]
& - \cos \big( - \omega_{el} t (1 + \mu) \big) \\
& + \cos \big( - \omega_{el} t (1 + \mu) \big) \Big)
\end{aligned} \\
&= 0.
\end{align*}
\end{document}
Update according to the comment. Equation without indentation and multi-lines (with one exception).
\documentclass{article}
\usepackage{nicefrac,amssymb}
\usepackage{mathtools} % Loads amsmath and also adds \mathrlap{}
\usepackage{relsize}
\begin{document}
\noindent%
\begin{align*}
&\int \limits_{0}^{2\pi} \hat{B}_{\delta r \mu} \cdot \cos \left( \mu p \alpha - \mu \omega_{el} t \right) \cdot \hat{A}_{\nu} \cdot \sin \left( \nu p \alpha - \omega_{el} t \right) d\alpha \\
&= \hat{B}_{\delta r \mu} \cdot \hat{A}_{\nu} \cdot \int \limits_{0}^{2\pi} \cos \left( \mu p \alpha - \mu \omega_{el} t \right) \cdot \sin \left( \nu p \alpha - \omega_{el} t \right) d\alpha \\
&= \frac{\hat{B}_{\delta r \mu} \cdot \hat{A}_{\nu}}{2} \cdot \Bigg(
\begin{aligned}[t]
& \overbrace{\int \limits_{0}^{2\pi} \sin \left( p \alpha \left( \nu - \mu \right) - \omega_{el} t \left(1 + \mu \right) \right) d\alpha}^{\larger\textcircled{\smaller[2]1}} \\
& + \overbrace{\int \limits_{0}^{2\pi} \sin \left( p \alpha \left( \nu + \mu \right) - \omega_{el} t \left(1 - \mu \right) \right) d\alpha}^{\larger\textcircled{\smaller[2]2}} \Bigg) \\
\end{aligned} \\
&{\larger\textcircled{\smaller[2]1}}:
\int \limits_{0}^{2\pi} \sin \left(p \alpha \left( \nu - \mu \right) - \omega_{el} t \left(1 + \mu \right) \right) d\alpha
\\
&= \frac{1}{p \cdot (\nu - \mu)} \cdot \Big[ -\cos \left( p \alpha (\nu - \mu) - \omega_{el} t (1 + \mu) \right) \Big]_{0}^{2\pi} \\
&= \frac{1}{p \cdot (\nu - \mu)} \cdot \Big(- \cos \big( 2\pi \overbrace{ p (\nu - \mu)}^{\in \, \mathbb{Z}} - \omega_{el} t (1 + \mu) \big) + \cos \big( - \omega_{el} t (1 + \mu) \big) \Big) \\
&= \frac{1}{p \cdot (\nu - \mu)} \cdot \Big(- \cos \big( - \omega_{el} t (1 + \mu) \big) + \cos \big( - \omega_{el} t (1 + \mu) \big) \Big) = 0.
\end{align*}
\end{document}
larger
andsmaller
commands?&
in front of your equals signs (but your equation will still be too long for a single line).