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System components
Figure 1
shows the main components. The incoming mains supply
passes through the rectifier which converts the alternating current
(ac) into pulsating direct current (dc). The smoothing section then
charges a capacitor up to the peak dc voltage. The power electronics
then converts the dc voltage into 3 phase ac voltage which drives the
motor, which in turn spins the load. This subject was dealt with in
more detail in an earlier article in this series.
Refer to the bibliography
for a list of prior publications.
Energy flow during a move
For a motor coupled to a load, there are four modes of operation:
• Motor drives the load in the forward direction
• Motor drives the load in the reverse direction
• Motor brakes the load in the forward direction
• Motor brakes the load in the reverse direction
These four modes are referred to as
Four Quadrant Control
and are
shown in
Figure 2
. The X direction depicts rotational velocity (posi-
tive and negative) and the Y direction depicts motor torque (positive
and negative).
• Quadrant 1 shows the load being driven (motoring) in the positive
direction
• Quadrant 2 shows the load being slowed (braking) in the reverse
direction
• Quadrant 3 shows the load being driven (motoring) in the nega-
tive direction
• Quadrant 4 shows the load being slowed (braking) while running
in the forward direction
In the motoring quadrants, the power flow is positive. Power is the
product of velocity and torque.
Quadrant 1
shows:
(positive velocity) X (positive torque) = positive power
Quadrant 3
shows:
(negative velocity) X (negative torque) = positive power
In other words, the flow of energy in Figure 1 is from left to right.
In the two braking quadrants, the power flow is negative. That is,
energy is extracted by the drive, from the load being braked. The
energy recovered is the kinetic energy of the rotation.
Quadrant 2
shows:
(negative velocity) X (positive torque) = negative power
Quadrant 4
shows:
(positive velocity) X (negative torque) = negative power
Flow of power
In the motoring mode, as noted above the flow of power in
Figure 1
is from left to right. Note that energy is the average power multiplied
by the measurement time interval.
When braking, the flow is reversed in
Figure 1
. Energy flows
from right to left in
Figure 1
. Thus the load is slowed by the motor
and the recovered energy flows in from right to left until it reaches
the smoothing section. The capacitor is then charged to a higher
voltage in an attempt to store the energy. At some point this voltage
will become dangerously high and the drive protection circuitry will
In this article the forces, velocities, inertias and power interact during a point to point move are studied.
Glyn Craig, Techlyn
DRIVES, MOTORS + SWITCHGEAR
Physics of a
Motion Control Move
Figure 1: System components.
Figure 2: Four quadrant drive principles.
Rectifier
Smoothing
Power Electronics
Motor
Load
Mains
Supply
Torque
Torque
Motor Fwd
Motor Rev
Brake Rev
Brake Fwd
Velocity
Velocity
Quadrant 1
Quadrant 4
Quadrant 2
Quadrant 3
Electricity+Control
September ‘16
14