1. A method for determining a rotary speed of an electrical machine, which is controlled in PWM operation, the method comprising: ascertaining the rotary speed from a tracking voltage, which is measured in a turnoff phase; estimating the rotary speed in the PWM phase with the aid of a mathematical model, wherein the rotary speed is ascertained in the PWM phase with the aid of a recursive function 
 ω i =ω i−1 +Δω, where Δω= f ( U k ,ω i−1 ); and controlling the electrical machine using a control signal, which has a PWM phase in which the electrical machine is periodically switched on and off using a prespecified pulse duty factor, and which has a measuring phase in which the electrical machine is switched off and the tracking voltage is measured.