--- title: "Open loop and closed loop V/f inverter-fed drive" subject: "Utilization of Electrical Energy" chapter: 3 key: "semester_7/utilization_of_electrical_energy:closed-loop-vf-inverter-drive" source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/closed-loop-vf-inverter-drive site: BCT Engineering --- # Open loop and closed loop V/f inverter-fed drive Utilization of Electrical Energy · Chapter 3: Control of Electric Drive - Asked in: 2072 Kartik - Video: [Voltage source fed Induction Motor Drive | Industrial Drives and Control |](https://www.youtube.com/watch?v=0TPjJBl3GCQ) — Mathspedia - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/closed-loop-vf-inverter-drive - Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-3 ## Question Discuss the operation of open loop V/F control of inverter fed induction machine drive. What are problem with it? How do you overcome this effect by closed loop operation, explain it with closed loop block diagram. ## Short answer Open-loop V/f control keeps flux constant but speed drops with load, can stall on fast changes, and has no current limit. Closed loop measures speed, a PI controller sets a limited slip frequency, and inverter frequency equals rotor speed plus slip, restoring speed and limiting torque and current. ## Answer #### Open-loop V/f control A voltage source (PWM) inverter feeds the induction motor. The speed command sets the inverter frequency $f$; a function generator sets the voltage so that $V/f$ is constant (with a small boost at low frequency for the $I_1R_1$ drop). The air-gap flux is then nearly constant and the motor runs close to $N_s = 120f/P$. ```text speed +------+ f* +----------+ V* +---------+ +----+ ref --->| ramp |--+-->| V/f |----->| PWM VSI |--->| IM | +------+ | | function | +---------+ +----+ | +----------+ ^ ^ +-----------------------+ | f* | dc link ac -> rectifier --------+ ``` - Simple, cheap, no speed sensor; good for fans, pumps, and many motors on one inverter. - A ramp (soft start) limits the rate of change of $f$ so that the slip stays small. #### Problems of open-loop operation 1. **Speed error with load**: the motor speed $N = N_s(1-s)$ falls as the load increases; there is no correction, so speed regulation is poor. 2. **Pull-out/instability**: if the frequency changes too fast, or a sudden load is applied, the slip exceeds the breakdown slip and the motor stalls or draws very high current. 3. **No current limit**: overcurrent during fast acceleration or braking. 4. **Low-speed weakness**: at low frequency the stator drop reduces flux and torque; a fixed boost may over- or under-flux the motor. 5. Poor dynamic response; the drive may oscillate at light load. #### Closed-loop operation (slip regulation) Speed is measured by a tachogenerator/encoder and compared with the reference. The speed error, through a PI controller, sets the **slip speed** $\omega_{sl}^*$, which is limited to a safe value. The inverter frequency is $$ \omega_e^* = \omega_r + \omega_{sl}^* $$ Since torque $\propto \omega_{sl}$ at constant flux, limiting the slip limits the torque and current. ```text w_r* + +----+ w_sl* +-----+ + w_e* +-----+ V* +-----+ ---->(O)->| PI |-------->|limit|-->(O)------>| V/f |----->| PWM |-> IM -| +----+ +-----+ ^ + +-----+ f* | VSI | | | | +-----+ | +-------------------- w_r ---+---------- tacho <--------------+ ``` - On a load increase, the speed falls, the error rises, and the slip and frequency increase until the speed is restored, so speed regulation is very good. - The slip limit keeps the motor below breakdown slip, giving fast acceleration and braking at the maximum allowed torque without stalling. - The voltage is taken from $\omega_e^*$ through the V/f function, so the flux stays constant. - A negative slip limit allows regenerative braking (with a suitable dc link). - An inner current loop may be added for direct current limiting. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does open-loop V/f control keep the voltage proportional to frequency? A. To keep the air-gap flux nearly constant B. To keep the slip equal to zero C. To keep the inverter dc link voltage constant D. To make the motor run above synchronous speed Answer: A. Flux is proportional to V/f, so a constant ratio holds flux (and hence torque capability) constant as speed changes. (Key point: Purpose of constant V/f) 2. What is the main speed problem of open-loop V/f control? A. The speed rises as load increases B. Speed falls as load increases (slip rises) with nothing to correct it C. The motor cannot run below base speed D. The inverter frequency drifts with temperature Answer: B. N = Ns(1 - s) falls when load increases, and with no feedback poor speed regulation results. (Key point: Main open-loop problem) 3. In closed-loop slip regulation, how is the inverter frequency obtained? A. w_e* = w_r - w_sl* B. w_e* = w_sl* only C. w_e* = w_r x w_sl* D. w_e* = w_r + w_sl*, where the PI controller sets the slip speed Answer: D. The measured rotor speed is added to the slip speed demanded by the PI controller to give the stator frequency. (Key point: Slip regulation relation) 4. Why is a limit placed on the slip speed command? A. Because torque is proportional to slip speed at constant flux, so limiting slip limits torque and current and keeps slip below breakdown B. To increase the inverter voltage C. To allow the motor to exceed breakdown slip D. To remove the need for a tachogenerator Answer: A. At constant flux T is proportional to ω_sl, so a slip limit protects against stalling and overcurrent while allowing fast acceleration. 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