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Vector Drive Technology

Vector Technology in variable speed drives

What is vector drive?

A basic variable speed drive (VSD), perhaps it would be more technically correct to call it a variable frequency drive (VFD), uses digital techniques (pulse width modulation, PWM) to synthesise varying frequency sine waves to change the speed of the motor. If the motor is subject to a load torque, then the actual speed of the motor is very likely be less than the speed commanded by the VSD. The VSD will not be aware that there is any discrepancy between command and actual speed as there is no means to feed this information back to the drive. Early designs of VSD’s operated this way and generally the slower the speed of the drive the greater the discrepancy between commanded and actual speed.

As electronic technology developed and in particular the speed of microprocessors increased, then it became possible to integrate a feedback loop into the VSD. There are two common ways of achieving feedback in a modern VSD; either with or without an external sensor fitted to the motor. The former can be known as a closed loop vector controlled drive whereas the latter can be know as an open loop vector controlled drive. This last term is technically incorrect as the drive is not truly open loop and the term ‘sensorless’ vector control is a more accurate description of the VSD.

The term Vector Control relates to controlling the vector sum of the torque producing current, which is in phase with the voltage, and the magnetising current which lags by 90 degrees. The VSD is managing two currents in two control loops whilst monitoring speed, torque and magnetising current. Any change in these monitored conditions which deviate from the commanded speed or torque will be fed back into the control loops to be corrected.


What is the difference between sensor and sensorless vector drives?

Whilst commissioning a vector controlled VSD you would normally be required to enter basic data from the motors nameplate. This is because the drive will model the motors characteristics using it’s knowledge base and this information will be used in the control loop algorithms. Internally the drive will monitor the currents flowing and make any necessary corrections with reference to the commanded speed or torque and the model data. This is essentially how the senseless vector drive is operating.

A vector drive fitted with a sensor feedback in the form of a rotary incremental encoder, or sometimes an analogue resolver, has additional information giving actual speed of the motor. This provides a more accurate control of the motor and, in particular enables the motor to deliver full torque at zero speed, something a drive without the encoder feedback can not do reliably.


What applications require a vector drive with sensor feedback?

Whilst senseless vector control in modern well designed fast VSD’s minimises torque induced motor slip, it will still occur and there are some applications which would normally still need to have encoder feedback fitted. For example, winder applications need very close control of speed and/or torque. Errors in these applications can accumulate and become a major problem.

Lift, hoists and any application with a holding brake that can not be released until 100% torque is on the motor will need encoders fitting to the motors.

Any application that needs precision speed or position will need encoders. It may also be worth considering a servo motor system rather than an inverter system if accuracy is critical as servo systems are designed to control speed and position very accurately.


How does an inverter work?

An inverter when used in the context of motor speed control can also be known as a variable frequency drive (VFD). It essentially generates a varying frequency three phase AC voltage to effect a change in the speed of a motor. It achieves this by converting the incoming power supply into a DC voltage and then generating a three phase AC voltage from this DC supply. The development of electronics since the manufacture of the first semiconductors has seen the speed and processing power increase enormously which has made it possible to, not only digitally synthesise the required AC frequency for any given speed of the motor but to also analyse the motor current and rotor position.

Why is it called an inverter?

The term inverter only relates to the final part of the VFD's electronic architecture, the part that converts DC voltage to AC. There is no clear technical reason for the use of the term 'inverter' as it is generally believed to refer to the inversion of the early mechanical process of converting AC voltage to DC, sometimes referred to as an 'inverting converter'.

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