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MC3PHAC Monolithic Intelligent Motor Controller, Rev. 2

Freescale Semiconductor

Features

•

Regeneration Control — Regeneration is a process by which stored mechanical energy in the

motor and load is transferred back into the drive electronics, usually as a result of an aggressive

deceleration operation. In special cases where this process occurs frequently (for example,

elevator motor control systems), it is economical to incorporate special features in the motor drive

to allow this energy to be supplied back to the ac mains. However, for most low cost ac drives, this

energy is stored in the dc bus capacitor by increasing its voltage. If this process is left unchecked,

the dc bus voltage can rise to dangerous levels, which can destroy the bus capacitor or the

transistors in the power inverter.

The MC3PHAC incorporates two techniques to deal with regeneration before it becomes a

problem:

–

Resistive Braking — The DC_BUS pin is monitored at a 5.3 kHz frequency (4.0 kHz when the

PWM frequency is set to 15.9 kHz), and when the voltage reaches a certain threshold, the

RBRAKE pin is driven high. This signal can be used to control a resistive brake placed across

the dc bus capacitor, such that mechanical energy from the motor will be dissipated as heat in

the resistor versus being stored as voltage on the capacitor. In standalone mode, the DC_BUS

threshold required to assert the RBRAKE signal is fixed at 3.85 volts (110 percent of nominal)

where nominal is defined to be 3.5 volts. In PC master software mode, this threshold can be

set to any value between 0 volts (0 percent of nominal) and greater than 5 volts (143 percent

of nominal) and can be changed at any time.

–

Automatic Deceleration Control — When decelerating the motor, the MC3PHAC attempts to

use the specified acceleration value for deceleration as well. If the voltage on the DC_BUS pin

reaches a certain threshold, the MC3PHAC begins to moderate the deceleration as a function

of this voltage, as shown in Figure 7. The voltage range on the DC_BUS pin from when the

deceleration begins to decrease, to when it reaches 0, is 0.62 volts. In standalone mode, the

DC_BUS voltage where deceleration begins to decrease is fixed at 3.85 volts (110 percent of

nominal) where nominal is defined to be 3.5 volts. In PC master software mode, this threshold

can be set to any value between 0 volts (0 percent of nominal) and greater than 5 volts (143

percent of nominal) and can be changed at any time.

Figure 7. Deceleration as a Function of Bus Voltage

BUS VOLTAGE

ACCELERATION INPUT

BEGIN MODERATING DECEL

(LEVEL IS PROGRAMMABLE

IN PC MASTER SOFTWARE MODE)

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