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dsPICDEM? MC1H 3-Phase High Voltage Power Module
1.2.9
Field Weakening
If the user is operating a brushless permanent magnet motor using field weakening by
employing phase advance, great care should be taken. If a FAULT trip occurs, firing
will stop, and the full back EMF magnitude, due to the motor's speed, will be present
on the output terminals. Should the peak of the back EMF be above the DC bus,
sudden uncontrolled motor braking will occur. The DC bus will rise in an uncontrolled
manner possibly causing damage to both power devices and the DC bus capacitors. A
speed greater than that which would produce a peak back EMF of greater than 450V,
with no field weakening, should not be used. This should adequately protect the unit. If
using the auxiliary DC input, the user should check the rating of the power supply and
adjust this speed accordingly or use a series blocking diode of suitable rating.
The same care should be taken with separately excited brushed DC motors if
employing field weakening at high speed. If the field current were to be increased in
error, a similar braking phenomenon may occur if the back EMF rises above the DC
bus. The effect is likely to be less severe as a DC over-voltage will occur tripping out
both the armature and field supply (assuming the field is not supplied separately). For
this reason, if using a separately excited DC motor it is recommended that both the
field and the armature are supplied from the unit.
1.3
CURRENT AND POWER LIMITATIONS
The maximum power and current capability of the system is dictated by the allowable
temperature rise of the different components. Establishing maximum limits is not
simple given the host of different ways the user may use the system. The voltage and
the nature of the electrical load used both affects the dissipation that occurs. In
determining the allowable limits for the power semiconductors, the following
assumptions have been made:
? Heat sink is at 70°C (worst case over temperature trip point)
? Thermal resistance of the insulating thermal pad is 4°C/W
Note that the maximum power of the system will always be the lower value due to the
AC input stage or the inverter output stage.
1.3.1
Inverter Output Current Limits
The inverter is capable of providing the full rated output of 2.5 A (RMS) within the
entire operating range (voltage, temperature and at up to 20 kHz PWM carrier
frequency) of the system. This includes being continuously stalled at such an
electrical angle that one of the motor phases is at the peak of the rated output (3.5A)
at just less than 100% duty cycle. This is a condition that causes high thermal loading
because one of the inverter switches has the peak worst case conduction and
switching loss continuously. Note that as far as the power devices are concerned,
operation at output frequencies of less than approximately 10 Hz are equivalent to
stall as far as peak device temperature is concerned because of low thermal
capacitance.
In a practical application, this condition of low output frequency/stall and high duty
cycle is unlikely to happen. With a motor correctly matched to the DC bus voltage, the
switch duty cycle at stall will be approximately 50% thus significantly reducing the
conduction loss in a particular switch. The complementary diode of the inverter phase
will also conduct for approximately 50% thus spreading the conduction loss between
two different power device packages. This in turn leads to a substantial reduction in
device temperature.
DS70096A-page 14
? 2003 Microchip Technology Inc.
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