ADT inverter solution for CNC machine tool and milling machine

 

I. Introduction:

At present, the domestic CNC machine tool spindle mainly adopts DC servo system and some others adopt the servo system that uses AC speed regulation. The AC spindle servo systems are mainly imported and expensive. And the rest of the AC asynchronous spindle motor’s variable speed drives are also mostly imported due to the high requirements including high output torque at low frequency, high stable speed precision, fast response to start and stop. While most of the Chinese domestic VFDs are using V/F control in which the rotation speed precision is affected by the slip ratio and the load features and thus the motor actual running speed could not be precisely controlled. So V/F control AC drives are not suitable to CNC applications.

 

II ADT variable frequency drives characteristics

ADT AD300 series variable speed drives adopt the advanced sensor-less vector control technology. In open-loop vector control mode, it can output 180% starting torque at 0.25Hz. At the same time, it has very high steady speed precision which is as precise as ±0.02% in open-loop vector control mode. It offers very fast response in high speed spindle application, realizing fast start and stop of the main spindle especially when the spindle is running at lower frequency.

The AD300 advantages in CNC applications are:

1) Stronger cutting force. Under the same conditions, ADT drives offer 1.3~1.5 times of cutting force and cutting depth comparing to other Chinese normal brands of AC drives. This increase the machining efficiency and output.

2) Better dynamic performance in rapid acceleration and rapid deceleration. The Acc./Dec. of ADT drives is 1.35 times faster than other Chinese normal brands. This also means higher efficiency and higher output.

3) Better low speed cutting force and machining precision. AD300 series can output stable 150% rated torque at only 0.25Hz while most of other brands need at least 0.5Hz to output torque.

4) Extraordinary overload ability for instant impact. The precise current loop control enables the ADT inverters better resist the sudden impact from load or shock from power grid. It is not easy to trip off in these cases and make sure the machinery has more stable and continuous working.

5) ADT AC drives have higher power density and more compact size which is good for space saving in installation and gives better looking of the whole system.

6) Other important features: Under vector control mode, the ADT inverter can work up to 200Hz, and in vector type V/F control it can output 3200Hz.

iii AD300 application in CNC machines

System wiring diagram

 

 

Start and stop performance comparison:   

 

Inverter brand                     Start time         Stop time

One international brand        7 seconds         6.8seconds

ADT frequency inverter        5 seconds         3.2 seconds

Note: The above testing data is the time the spindle accelerates from 0 to 4000rpm and decelerate from 4000rpm to 0 speed. The motor running frequency is 200Hz and brake resistors(1800W, 50Ω) is connected. The starting speed can be faster but it has been adjusted to 5 seconds according to customer’s need.

The inverter model AD300-T47R5GB/011PB (motor 7.5kW) parameter setting is as follows.

 

ADT AC drives for CNC Lathe

i. industry overview:
 

NC (Numerical Control) is mechatronics products integrating mechanical, electrical, hydraulic, pneumatic, microelectronics and information technology. It features high precision, high efficiency and high automation and high flexibility etc. advantages in mechanical manufacturing equipment. The technical level of CNC machine tools and its percentage in national metal cutting machine tool total quantity is one of the important indicators to measure the national economic development and the overall level of industrial manufacturing.

ii. Technical requirements of VFD in CNC lathes:

1)  wide speed regulating range

There are different machining tools, different materials to be machined and the different machining requirements of specific parts. The CNC lathe should has wide speed regulating range to ensure optimum cutting performance under different conditions. At the same time, there are specific requirements for different speeds ranges. At high speed, the speed is required to be stable, and the maximum power of the spindle motor is provided as much as possible, that is, the constant power range should be wide; at low speed, a large torque output is required to meet well the heavy cutting’s claim.

2) High accuracy in speed control

In order to ensure the machining accuracy of the CNC lathe, the driving system is generally required to have a high accuracy in speed control. The dynamic deceleration caused by the load change is supposed to be small. If the dynamic speed reduction is large, it will seriously affect the accuracy of work piece processing.

3) Short acceleration and deceleration time

CNC lathes require short acceleration and deceleration time and fast response characteristics. The inverter has a short acceleration/deceleration time, which can improve the efficiency of machining work pieces on CNC lathes.

iii. the characteristics of the ADT frequency inverter in the application of CNC lathe:

1. The speed adjustment range is wide from 0 to 3200HZ, which satisfies the requirements of different work pieces for the rotation speed.

2) High output torque at low frequency satisfy the heavy cutting requirements of CNC lathe at low speed.

3) The vector control method is adopted to improve the response speed and accuracy of the lathe.

4) The acceleration and deceleration time is short, which improves the production efficiency of the workshop

5) The carrier frequency is adjustable from 1 to 16K, which reduces motor noise and improves the working environment for workers.

iv. the wiring diagram of AD300 on the CNC lathe:

V. Conclusion:

The spindle motor is driven by the AD300 high-performance vector inverter, which not only satisfies the machining requirements of the work piece, but also prolongs the service life of the tool. Because the frequency converter adopts vector control, it can smoothly output 150% torque even under low speed (low frequency) operation to meet the processing needs of different workpieces. It can completely replace the traditional rolling bearing spindle structure. The spindle structure with VFD is simple, compact, and achieve true step-less speed regulation. Different processing techniques (such as roughing machining and fine machining) require different speeds. At this time, the numerical control system can output different analog voltage signals to the inverter to achieve different speeds. And the start and stop signals are also controlled by the numerical control system, improving the automation of the lathe.