How a High Performance AC Motor Drive Controls a Machine Tool

A High Performance AC Motor Drive for a machine tool has to hold speed under a changing load, position a spindle accurately and respond quickly to a command. The V&T VA-SZ-TEST is built for that duty: it supports asynchronous and synchronous motors, provides torque, speed and position loops, and adds machine tool functions such as orientation and a second encoder input. The page lists a 400 V series from 3.7 to 75 kW and a sensor vector mode with a 1:5000 speed regulation range. Those figures describe a drive for a machine tool rather than a general-purpose fan or pump inverter.

Control for Machine Tools

The drive offers two control modes: sensorless vector control and sensor vector control. In the sensorless mode the page gives a starting torque of 180 percent of rated torque at 0.2 percent of rated speed, a speed regulation range of 1:200 and a steady speed precision of plus or minus 0.5 percent. In the sensor vector mode the figures improve to 200 percent starting torque at zero speed, a 1:5000 range and a precision of plus or minus 0.02 percent. The second mode needs an encoder, and the page lists incremental, resolver and sin/cos encoder support.

The machine tool functions go beyond a simple speed command. The drive supports orientation, a second encoder input and position control, which are the functions a spindle or a tool changer needs. It also provides speed, torque and position loops, current limiting, motor auto tune, inertia auto tune and deep flux-weakening control. Those functions let the drive match the motor and the load without a long manual setup. The buyer should confirm which functions the machine tool controller will use, because the wiring and the parameter set change with the mode.

Power Range and Hardware

The 400 V series covers models from 3.7 kW to 75 kW in the table, and the page also lists larger frames up to 160 kW in the dimension table. A brake chopper is built in as standard from 3.7 to 15 kW and is an option from 18.5 to 75 kW. The brake chopper action voltage is 650 to 750 V. The whole series supports a common DC bus, and every model uses an independent air duct so that the heat from the drive does not mix with the rest of the cabinet.

The hardware also covers the feedback and the pulse train. The drive accepts an incremental encoder, a resolver or a sin/cos encoder, and it supports pulse train input, an electronic gear ratio and pulse frequency division output. Those features are useful when the drive has to follow a master axis or a handwheel. The keypad is available in LED and LCD versions, and it can be used as a remote control box through a network cable. The buyer should choose the keypad and the encoder card with the machine design.

Communication and Protection

The drive has built-in Modbus communication and supports EtherCAT. The page gives a Modbus RTU distance of up to 500 metres. A machine tool cell often uses a fieldbus to coordinate several axes, so the communication choice should be made with the machine builder. The drive also offers a range of protection functions, including supply under-voltage, over-current, over-voltage, module protection, heat-sink over-temperature, drive and motor overload, output short-circuit to ground, encoder disconnection and communication fault. Those protections matter in a machine that runs unattended.

The efficiency figures are at least 93 percent for 0.75 to 7.5 kW, at least 95 percent for 11 to 45 kW and at least 98 percent for 55 kW and above. The environment section gives an ambient temperature of minus 10 to plus 40 C, with a 1 percent current derating for each degree up to 50 C, a humidity of 5 to 95 percent without condensation and an altitude of 0 to 4000 metres with a 1 percent derating per 100 metres above 1000 metres. Those limits should be checked against the cabinet and the site.

Installation and Ordering

The drive must be installed vertically in a well-ventilated electrical cabinet. The page does not allow a horizontal or another installation method, and it requires an environment free from direct sunlight, dust, corrosive gas, flammable gas, oil mist, steam and dripping water. The vibration limits are 3.5 m/s2 from 2 to 9 Hz, 10 m/s2 from 9 to 200 Hz and 15 m/s2 from 200 to 500 Hz. The installation dimensions and weights are listed per model, so the cabinet layout can be checked before the order.

V&T presents the VA-SZ-TEST as part of a range that covers standard and special-purpose VFDs, medium and high voltage inverters, servo drives, PLC and HMI, soft starters, reactors and filters, elevator products, EV controllers, oil-free compressors, PV inverters, energy storage and new materials. For a buyer, the practical step is to compare the control mode and the power range with the machine tool, then test the drive on the actual spindle before the order is placed.