Delta Tau GEO MACRO DRIVE User Manual download pdf (Page 39)

Geo MACRO Drive User and Reference Manual

Connections 27

Shunt Regulation

When the motor is used to slow the moving load, this is called regenerative deceleration. Under this

operation, the motor is acting as a generator consuming energy from the load while passing the energy

into the DC Bus storage capacitors. Left unchecked, the DC Bus voltage can raise high enough to

damage the drive. For this reason there are protection mechanisms built into the Geo Drive product such

as shunt regulation and over-voltage protection.

The shunt regulator monitors the DC Bus voltage. If this voltage rises above a present threshold (Regen

Turn On Voltage), the Geo Drive will turn on a power device intended to place the externally mounted

regen resistor across the bus to dump the excessive energy. The power device keeps the regen resistor

connected across the bus until the bus voltage is sensed to be below the Regen Turn Off voltage at which

time the power device removes the resistor connection.

Minimum Resistance Value

The regen resistor selection requires that the resistance value of the selected resistor will not allow more

current to flow through the Geo Drive’s power device than specified.

Maximum Resistance Value

The maximum resistor value that will be acceptable in an application is one that will not let the bus

voltage reach the drive’s stated over voltage specification during the deceleration ramp time. The

following equations defining energy transfer can be used to determine the maximum resistance value.

Energy Transfer Equations

Regen, or shunt, regulation analysis requires study of the energy transferred during the deceleration

profile. The basic philosophy can be described as follows:

• The motor and load have stored kinetic energy while in motion.

• The drive removes this energy during deceleration by transferring to the DC bus.

• There are losses during this transfer, both mechanical and electrical, which can be significant in some

systems.

• The DC bus capacitors can store some energy.

• The remaining energy, if any, is transferred to the regen resistor.

Kinetic Energy

The first step is to ascertain the amount of kinetic energy in the moving system, both the motor rotor and

the load it is driving. In metric (SI) units, the kinetic energy of a rotating mass is:

where:

is the kinetic energy in joules, or watt-seconds (J, W-s)

J is the rotary moment of inertia in kilogram-meter

(kg-m

)

ω is the angular velocity of the inertia in radians per second (1/s)

If the values are not in these units, first convert them. For example, if the speed is in revolutions per

minute (rpm), multiply this value by 2π/60 to convert to radians per second.

When English mechanical units are used, there are additional conversion factors must be included to get

the energy result to come out in joules. For example, if the rotary moment of inertia J is expressed in lb-

ft-sec

, the following equation should be used:

J678.0

If the rotary moment of inertia J is expressed in lb-in-sec

, the following equation should be used:

J0565.0

In standard metric (SI) units, the kinetic energy of a linearly moving mass is:

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Delta Tau GEO MACRO DRIVE User Manual Page 39

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