How do you implement slip compensation for VFDs in variable torque or constant torque applications?

1 What is slip and why does it matter?

Slip is the difference between the synchronous speed and the actual speed of an AC motor. Synchronous speed is the speed at which the magnetic field of the stator rotates, and it depends on the number of poles and the supply frequency. Actual speed is the speed at which the rotor rotates, and it depends on the load torque and the rotor resistance. Slip is usually expressed as a percentage of the synchronous speed, and it can vary from zero to several percent. Slip is important because it affects the torque and power output of the motor, as well as the power factor and efficiency of the system. Slip also causes a drop in the terminal voltage of the motor, which can lead to instability and overheating.

2 How does slip compensation work?

Slip compensation is a method of adjusting the output frequency and voltage of the VFD to compensate for the slip and maintain the desired speed of the motor. There are two main types of slip compensation: open-loop and closed-loop. Open-loop slip compensation uses a predefined slip curve or formula to estimate the slip based on the load current and the motor parameters. The VFD then calculates the required frequency and voltage to achieve the desired speed and applies them to the motor. Closed-loop slip compensation uses a speed sensor or encoder to measure the actual speed of the motor and compare it with the desired speed. The VFD then adjusts the frequency and voltage accordingly to minimize the speed error.

3 How to choose the right slip compensation method?

The choice of the slip compensation method depends on several factors, such as the type of application, the accuracy and stability requirements, the cost and complexity of the system, and the characteristics of the motor. Generally, open-loop slip compensation is simpler, cheaper, and more suitable for variable torque applications, such as fans and pumps, where the load torque varies with the square of the speed and the slip is relatively small and predictable. Closed-loop slip compensation is more accurate, stable, and suitable for constant torque applications, such as conveyors and hoists, where the load torque is independent of the speed and the slip can be large and variable.

4 How to set up open-loop slip compensation?

To set up open-loop slip compensation, you need to know the motor parameters, such as the rated voltage, current, frequency, speed, power, and slip. You also need to select a suitable slip curve or formula for your VFD. Some VFDs have built-in slip curves or formulas that you can choose from, while others allow you to enter your own values or coefficients. You can find these values or coefficients in the motor data sheet or by performing a no-load test on the motor. Once you have selected or entered the slip curve or formula, you need to enable the slip compensation function on your VFD and set the desired speed reference.

5 How to set up closed-loop slip compensation?

To set up closed-loop slip compensation, you need to install a speed sensor or encoder on the motor shaft and connect it to the VFD. You also need to configure the VFD to recognize the type and resolution of the speed sensor or encoder and to use it as a feedback source. You also need to set up the parameters for the speed control loop, such as the proportional, integral, and derivative gains, the speed reference, and the speed error limit. These parameters affect the responsiveness and stability of the speed control loop and should be tuned according to your application requirements.

6 What are the benefits and limitations of slip compensation?

Slip compensation can improve the performance and efficiency of your system by ensuring that the motor operates at the desired speed regardless of the load variations. It can also prevent voltage drops, instability, and overheating of the motor caused by excessive slip. Slip compensation can also extend the speed range and torque capability of your motor by allowing it to operate above or below the rated frequency and voltage. However, slip compensation also has some limitations, such as increased complexity and cost of the system, reduced power factor and efficiency at low speeds, and potential resonance and noise issues at high speeds.

7 Here’s what else to consider

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