What is motoring mode of DC motor?

Answer 1

Read this

Part I:DC Motor principles pg 1

Part I: Principles of DC Drive Control

Learning Objectives:

* to learn the basic characteristics of DC motors and their control parameters

* to understand the various operating modes

DCmotors and their representation:

The basic principle of a DCmotor is the production of a torque as a result of the flux interaction

between a "field" produced on the STATOR (either produced by a permanent magnet, or a field

winding) and the current circulating in the "armature" windings on the ROTOR. In order to produce

a torque of constant sign, the armature winding loops are connected to a set of "brushes" which

commutate the current appropriately in each loop according to their geometric position. The

commutator is a MECHANICAL RECTIFIER.

Note that reversal of either the field current or the

armature current results in a torque in the opposite

direction. However, reversal of both fields does

not change the torque direction, hence it can be

used as a "universal motor" with DC or AC feed

if both windings are in series.

Basic Equations of a DC Machine:

Steady state conditions (assume all time

varying quantities have a constant average value)

( )

( )

( )

( )

( )

P T developed power

T K I I A B T electrical torque

V R I E R I K I armature voltage

E K I Counter emf or Back EMF

V R I fieldwinding

d e

e t f a load

a a a a a v f

v f

f f f

w

w

w

w

=

= = + +

= + = +

=

=

(The back emf assumes that the magnetizing characteristic, E(If) is linear)

Speed control:

we can extract:

v f

a a a

K I

V - R I

w = and one can control the speed with

1) Va "Voltage Control"

2) If "Field Control"

3) Ia (with If fixed) "Demand Torque"

In practice, for speeds less than the base speed (rated), the armature current and field currents

aremaintained at fixed values (hence constant torque operation), and the armature voltage controls

the speed. For speeds higher than the base speed, the armature voltage is maintained at rated value,

and the field current is varied to control the speed (note the hyperbolic characteristic). However,

N

+V

(free

wheeling

diode)

Armature

Current

(Rf, Lf)

(La, Ra)

Eo

Rfc

Field

Control

Resistor

Fig 1: DC Motor

Part I:DC Motor principles pg 2

this way the power developed Pd is maintained constant. This mode is referred to as "field weakening"

operation.

Case of Series Motor (or Universal Motor)

In this case the field winding is series with the armature winding, hence If and Ia are equal. This

leads to:

v a

a a f a

K I

V - (R + R )I

w =

One can either control the armature voltage (Voltage Control), or the armature current (Current

Control) which is a measure of the torque control.

Td , Pd

Torque

Power

constant torque constant power

speed

Ia

If

Fig 2: Control Modes

Torque

Power

constant torque constant power

speed

Td

Ia

Fig 3: DC Series Motor Control

Part I:DC Motor principles pg 3

OperatingModes of DCMotors:

Motoring:

The back emf E < Va

both Ia and If are positive. The motor

develops a torque to meet the load

torque.

Dynamic Breaking:

The voltage source is removed, and the armature is shorted.

The kinetic energy stored in the rotor of the motor is dissipated in

the armature resistance since the machine now works as a generator.

Note here that theoretically, since the armature voltage is

proportional to the speed, the motor would never stop... but in fact

because there is also friction and viscous friction (windage), the

motor will stop as soon as the speed is at a certain low value.

Regenerative Breaking:

The back emf E > Va , the machine acts as a generator, and

the armature current flows towards the source, hence energy stored in

the machine rotor is fed back to the source. Note however that this

will cause the machine to slow down usually until E=Va and then

revert to mode 1.

Plugging:

Plugging iswhen the field current is reversed, hence the back emf changes sign, and the equation

of the machine becomes:

a a a V = -E - R I hence

a

a

a R

I V E +

= which means a very high torque generated in

the opposite direction of rotation. Avery powerful breaking takes place. However it must be noted

that the armature has to be opened as soon as the motor reaches zero speed, otherwise it will start

rotating in the opposite direction.

N

+Va

Rfc

(La, Ra)

E

Ia

(Rf, Lf) +Vf

If

Fig 4: Motoring Forward

Tem

w

rated torque

V1

V2 V3 V2

Tload

FIG 5 torque speed

V2

N

+Va

Rfc

(La, Ra)

E

(Rf, Lf) +Vf

Fig 6: Dynamic Breaking

+

Ia

R

V2

N

+Va

Rfc

(La, Ra)

E

(Rf, Lf) +Vf

Fig 7: Regenerative Breaking

+

Ia

Part I:DC Motor principles pg 4

Two Transistor control of regenerative operation

When the main switch opens, the armature current I(a1) has to be dissipated through the

freewheeling diode.

Then if one closes switch T1, the

machine behaves as a generator with the

energy stored in its inertia. Therefore the

armature current I(a2)will start flowing and

follws I(1). After a certain time one opens

the switch T1, and the current I(a2) has to

be redirected via diode D2 back to the source

with I(2). This is because of the inductance

of the machine acts as an emf restoring the

flux stored in the magnetic field.

Then one closes T1 again and so on.

The chopping rate of switch T1 can be

set in order to control the average current

(Ia2), usually 1.5 times rated value. This is

possible only if the speed is fast enough to

provide terminal voltage. When the emf E

reaches E=Ra.I(rated), the switch T1 remains closed for

maximum breaking possible with the given emf.

Four Quadrant Operation:

Figure 8 summarizes the modes of operation in a four quadrant representation.

Torque

Speed

Forward

MOTORING

Forward

BREAKING

Reverse

BREAKING

Reverse

MOTORING

Fig 8: 4 quadrant operation

V2

N

+Va

(La, Ra)

E

Fig 7: Regenerative Control

+

T1

i2

i1

ia1

ia2

average

Ia