difference between lead and lag compensator

difference between lead and lag compensator

difference between lead and lag compensator

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Difference between Lead Compensator and Lag Compensator

This page compares Lead Compensator Vs Lag Compensator and mentions difference between Lead Compensator and Lag Compensator. The circuit diagrams for Lead Compensator and Lag Compensator along with transfer functions are also mentioned.

Definition:
The compensator is a device used to improve performance specifications such as steady state and transient responses.

Lead Compensator

difference between lead and lag compensator
difference between lead and lag compensator

The figure-1 depicts circuit and transfer functions of Lead Compensator. It is similar to PD controller. Following are the features of Lead Compensator.
Here zero is closer to origin than the pole.
It improves transient response.
It increases the bandwidth.
(S/N)output < (S/N)input
It helps to increase error constant upto some extent.
and It allows to pass higher frequencies and attenuates lower frequencies.
Increases phase shift.

Effect of Phase Lead Compensation

  1. The velocity constant Kv increases.
  2. The slope of the magnitude plot reduces at the gain crossover frequency so that relative stability improves and error decrease due to error is directly proportional to the slope.
  3. Phase margin increases.
  4. Response become faster.

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Advantages of Phase Lead Compensation

Let us discuss some of the advantages of the phase lead compensation-

  1. Due to the presence of phase lead network the speed of the system increases because it shifts gain crossover frequency to a higher value.
  2. Due to the presence of phase lead compensation maximum overshoot of the system decreases.

Lag Compensator

difference between lead and lag compensator
difference between lead and lag compensator

The figure-2 depicts circuit and transfer functions of Lag Compensator. It is similar to PI controller. Following are the features of Lag Compensator.
Here pole is closer to origin than the zero.
It improves the steady state response.
and It increases error constant.
It decreases bandwidth.
also It reduces effect of noise.
It reduces stability margin.
It does not affect the transient response.
System becomes lesser stable.
It allows to pass lower frequencies and attenuates higher frequencies.
It decreases the phase shift.

Effect of Phase Lag Compensation

  1. Gain crossover frequency increases.
  2. Bandwidth decreases.
  3. Phase margin will be increase.
  4. Response will be slower before due to decreasing bandwidth, the rise time and the settling time become larger.

Advantages of Phase Lag Compensation

Let us discuss some of the advantages of phase lag compensation –

  1. Phase lag network allows low frequencies and high frequencies are attenuated.
  2. Due to the presence of phase lag compensation the steady state accuracy increases.

Lead Lag Compensator

difference between lead and lag compensator
difference between lead and lag compensator

The figure-3 depicts circuit and transfer functions of Lead-Lag Compensator. Following are the features of Lead-Lag Compensator.
• It improves both steady state and transient responses of the system.

Effects of Lag Lead Compensator

  • The lead characteristics of the cascaded compensator provide improvement in the overall bandwidth.
  • Also, due to the use of the lag network, the steady-state performance of the system gets improved.
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So, we can say, a lag lead network provides a quick response with good accuracy. The low-frequency gain of the system is increased with this network, thereby improving the steady-state response.

Hence, we use a combination of lag and lead network as a compensator to provide required compensation to the control system.

Advantages of Phase Lag Lead Compensation

Let us discuss some of the advantages of phase lag- lead compensation-

  1. Due to the presence of phase lag-lead network the speed of the system increases because it shifts gain crossover frequency to a higher value.
  2. Due to the presence of phase lag-lead network accuracy is improved.
difference between lead and lag compensator
difference between lead and lag compensator

Necessary of Compensation

  1. In order to obtain the desired performance of the system, we use compensating networks. Compensating networks are applied to the system in the form of feed forward path gain adjustment.
  2. Compensate a unstable system to make it stable.
  3. A compensating network is used to minimize overshoot.
  4. These compensating networks increase the steady state accuracy of the system. An important point to be noted here is that the increase in the steady state accuracy brings instability to the system.
  5. Compensating networks also introduces poles and zeros in the system thereby causes changes in the transfer function of the system. Due to this, performance specifications of the system change.

 

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