Technical solutions are described for diagnosing an input power supply providing power to a motor. A method includes: generating a sinusoidal stimulus signal; applying, using the input power supply, the sinusoidal stimulus signal to the motor; measuring a response to the sinusoidal stimulus signal; determining a degraded condition of the input power supply based on the response to the sinusoidal stimulus signal; and performing an action in response to determining the degraded condition of the input power supply.