Electricity + Control September 2016

DRIVES, MOTORS + SWITCHGEAR

Physics of a Motion Control Move

Glyn Craig, Techlyn

In this article the forces, velocities, inertias and power interact during a point to point move are studied.

Quadrant 2

Quadrant 1

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.

Torque

Brake Rev

Motor Fwd

Velocity

Velocity

Motor Rev

Brake Fwd

Torque

Quadrant 3

Quadrant 4

Figure 2: Four quadrant drive principles.

Energy flow during a move For a motor coupled to a load, there are four modes of operation:

Quadrant 1 shows: (positive velocity) X (positive torque) = positive power

• 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

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.

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

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 the motoring quadrants, the power flow is positive. Power is the product of velocity and torque.

Rectifier

Smoothing

Power Electronics

Motor

Load

Mains Supply

Figure 1: System components.

Electricity+Control September ‘16

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