What is the difference between servo and VSD?

Servo drives are used to drive servo motors. Frequency converters convert industrial frequency AC into current suitable for adjusting motor speed, which is used to drive motors.

Now some frequency converters can realize servo control, that is, they can drive servo motors, but servo drives and frequency converters are different.

1. Different definitions

Frequency converter is a device that uses the disconnection function of power semiconductor devices to convert the industrial frequency power supply to another frequency of electrical energy control. It can realise the functions of AC asynchronous motor such as soft start, frequency conversion speed regulation, improving operation accuracy, changing power factor and so on.

 Servo system is an automatic control system. It allows the position, location, state and other outputs of an object to follow any change in the input target (or given value). Mainly according to the requirements of the control command amplification power, transformation and regulation to make the output torque, speed and position control of the drive device is very flexible and convenient.

2. Different principles of operation

The speed regulation principle of the frequency converter is mainly affected by four factors, namely, the asynchronous motor speed n, the asynchronous motor frequency f, the motor rotation difference s, and the motor pole pair number p. The speed n is proportional to the frequency f. As long as the frequency f is changed, the speed of the motor can be changed.

When the frequency f changes in the range of 0-50Hz, the speed adjustment range of the motor is very wide. Frequency control is achieved by changing the frequency of the motor power supply.

It mainly adopts the AC-DC-AC method, which converts the AC power supply from industrial frequency to DC power supply through rectifier, and then converts the DC power supply to frequency. The voltage can be controlled by the AC power supply to provide the motor. The circuit of frequency converter is generally composed of four parts: rectifier, intermediate DC link, inverter and control. The rectifier part is a three-phase bridge uncontrolled rectifier, the inverter part is an IGBT three-phase bridge inverter with PWM waveform output, and the intermediate DC link is a filter. DC energy storage and buffer reactive power.

Servo system works on the basis of open-loop control of AC and DC motors, through the rotary encoder, resolver, etc. will be the speed and position signal feedback to the drive for closed-loop negative feedback PID regulation control. In addition, through the three closed-loop adjustment, the current closed loop within the drive greatly improves the motor output to follow the set value of the accuracy and time response characteristics.

Servo system is a dynamic following system, steady state balance is also dynamic balance.

3. Different performance and applications

　　(1) Different overload capacity

Servo drives generally have 3 times the overload capacity, can be used to overcome the inertial torque of inertial loads at the start of the moment, inverter generally allow overload 1.5 times.

　　(2) Precision control

Servo system control accuracy is much higher than the frequency converter, usually by the rotary encoder at the back of the motor shaft to ensure. The control accuracy of some servo systems even reaches 1:1000.

　　(3) Different applications

Frequency control and servo control are two types of control.

The former belongs to the transmission control field, the servo belongs to the field of motion control.

Frequency inverters apply to the general requirements of industrial applications which do not require high applications.

Servo is the pursuit of high precision, high performance, high response.

　(4) Acceleration and deceleration performance is different

Servo motor under no load from static state processing to 2000r/min, the use of time is not more than 20ms.

The acceleration time of the motor is related to the inertia of the motor shaft and the load. The larger the inertia, the longer the acceleration time.