• Created by Unknown User (henritur), last modified on 16.12.2014

Introduction


The basic function of a drive is to control the flow of energy from the grid to the process. Energy is supplied to the process through the motor’s axle. There are two different physical variables for the axle, torque and speed. To control the flow of energy, these quantities have to be controlled. The terms for the control are either speed control or torque control. Torque or speed control may be achieved with variable speed drives (VSD’s) usually in DC motors (also in AC motors) and variable frequency drives (VFD’s) in AC motors. When the drives operate in torque control mode, the speed is determined by the load and when operated in speed control, the torque is determined by the load. VSD’s can also be used to transform mechanical energy to machinery. DC motors are much simpler to control by using drives. There are almost infinite number of different types of drives, but the main functions are explained below.


DC motor drives

As mentioned in the main article, Electric motor, in a DC motor the magnetic field is created by the current through the field winding (coils) in the stator. This field is always at right angles to the field created by the armature winding or the permanent magnet. This condition, known as field orientation, is needed to generate maximum torque. The commutator and brush assembly ensures that the condition is maintained regardless of the rotors position. Once field orientation is active, the DC motor’s torque is easily controlled by altering the armature current and by keeping the magnetising current constant. The advantage of the DC drive is that speed and torque are controlled directly through armature current. By this definition any device which controls the armature current or the magnetic field is a drive.

The picture above is a simple DC drive circuit


Features of DC motor drives

  • Field orientation through mechanical commutator

  • Controlled variables are armature current and field current, measured directly from the motor

  • Torque control is direct, because of the field orientation


Speed control in a DC motor


Speed control means intentional variation to the drive speed to a value required to perform  a specific work process. Speed control differs from speed regulation where there is natural change in speed due to the change in load on the shaft. Speed control is either done manually by the operator or by means of some automatic control device.


Physics point of view this means that, because:


n = Eb/ (kb * Ø) and Vm = Eb + Rm * Ia


Where:


n = armature speed (rpm)

Eb = induced or counter electromotive force (emf) (V)

kb = counter emf equation constant

Ø = machine's total flux (Wb)

Vm = motor input voltage (V)

Rm = motor resistance (Ω)

Ia = armature current (A)


We get:

n = (Vm - Rm * Ia) / (kb * Ø)

n = kn * (Vm - Rm * Ia) / Ø

Where

kn = 1 / kb

This leads to conclusion that the speed of a DC motor can be altered by:

  1. Changing the input voltage of the motor

  2. Changing the resistance of the motor (also changing the current of the armature)

  3. And by changing the total flux

The two first methods affect the armature circuit and the third affects the magnetic field, therefore there are basically two control methods, armature control and magnetic field control.

Torque control in a DC motor

Changing the torque in a DC motor is relatively easy. Like in speed control also this differs from torque regulation. This is also dependant from the natural changes in the load of the shaft.

The physical torque equation is:

T = kb * Ia * Ø / (2π)


Where:


T = motor torque (Nm)

kb = counter emf equation constant

Ia = armature current (A)

Ø = machine's total flux (Wb)

kT = torque equation constant


We get:

T = kT * Ia * Ø

Where

kT = kb / (2π)


As seen above the torque of a DC motor can only be changed by altering the current of the armature or the magnetic flux.


Drives in AC motors

In an AC motor drive the speed is controlled by changing the frequency of the electrical supply to the motor. The 3-phase voltage from the national electric grid connected to a motor creates a rotating magnetic field as explained. The rotor of the electric motor will follow this rotating magnetic field, because it’s attached to the axle. An AC drive converts the frequency of the grid to another and thus controls the speed of the motor proportionally to the frequency.

This is done with (main parts of an AC drive):


Rectifier unit (diode):

The electric energy from the grid is fed to the drive through a rectifier unit. The rectifier unit turns the AC fed from the grid to constant DC. The rectifier unit can be uni- or bidirectional. When it’s unidirectional, the AC drive will run the motor by taking energy from the grid. If it’s bidirectional, the AC drive can also process the rotational energy and feed it back to the grid, therefore increasing energy efficiency.


DC circuit:

The DC circuit will store the electric energy from the rectifier for the inverter to use. In most cases, the energy is stored in high-powered capacitors.


Inverter unit:

The inverter unit takes the electrical energy from the DC circuit and supplies it to the motor by turning it back to AC. The inverter uses modulation techniques to create the needed AC voltage output for the motor. The frequency can be adjusted to match the need of the process via different modulation techniques. Basically the higher the frequency of the output voltage from the inverter is, the higher the speed of the motor.


A couple of different modulation techniques:

AC motor drives using frequency control by pulse width modulation (PWM)

Unlike a DC drive, the AC drives frequency control techniques uses parameters generated from outside of the motor as controlling variables, the source voltage and frequency. Both voltage and frequency references are fed into a modulator, an outside device usually in the inverter, which simulates an AC sine wave and feeds it to the motor’s stator windings (coils). This procedure is called pulse width modulation (PWM). It utilises the fact that there is a diode rectifier towards the grid and the intermediate DC voltage is kept constant. The inverter controls the motor by turning the constant intermediate DC to AC, in the form of a pulse wave modulation string, dictating both the voltage and frequency. With this technique, field orientation of the motor is not used. The frequency and voltage are the main control variables and are directly applied to the stator windings. The status of the rotor is ignored and therefore no metrics about the position or the speed of the rotor is fed back to the stator. That is why the torque can’t be controlled with any degree of accuracy. Also with this technique a modulator is used and it slows down the communication between the incoming voltage, frequency signals and the need for the motor to respond to the changes.

The picture above is the control loop of a AC drive with frequency control using PWM


Features of PWM controlled AC drives

  • Controlling variables are voltage and frequency

  • Simulation of variable AC sine wave using modulator

  • Flux provided with constant V/f ratio

  • Load dictates torque level


AC motor drives using flux vector control by PWM

To simulate the magnetic operating conditions of a DC motor,especially to perform the field orientation process, flux-vector PWM drives are used. The Flux-vector drive needs metrics about the spatial angular position of the rotor and of the flux inside the AC motor. With flux-vector drives, the field orientation condition is achieved by electronic means and artificially rather than with the mechanical commutator and brush assembly of the DC motor. Information about the rotor’s status is obtained by feeding back rotor’s speed and angular position relative to the stator field by using a pulse encoder. Drives which uses encoders is referred to as  “closed-loop drives”. In these kinds of drives the motor’s electrical characteristics are mathematically modelled with microprocessors to process the data. The electronic controller of a flux-vector drive simulates electrical quantities such as voltage, current and frequency, which are the controlled variables, and feeds these through a modulator to the AC motor. Torque in these kinds of drives is, therefore controlled indirectly.

The picture above is a control loop of AC drive with flux-vector control using PWM

Features

  • Field-oriented control - simulates DC drive

  • Motor electrical characteristics are simulated

  • Torque controlled indirectly


Could drives be used for all electric motors?

The answer is simple. Yes. In this question it is important to keep in mind that there are a lot of different definitions for the word drive. Not all drives are suitable for all electric motors, but every electric motor could have a drive. Some definitions even suggests that the motor is actually a part of a drive. In the parts above 1.5.1 and 5.1.2 the main functions which fulfill the definition of a drive are explained. The baseline is that whenever an electric motor requires some form of control, in either torque or speed, the control is achieved by the use of drives.


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