U211B Datasheet, PDF (8/30 Pages) ATMEL Corporation – Phase Control IC with Overload Limitation for Tacho Applications

In each positive half-cycle, the circuit measures, via R₁₀, the load current on pin 14 as a potential
drop across R₈ and produces a current proportional to the voltage on pin 14. This current is
available on pin 15 and is integrated by C₉. If, following high-current amplitudes or a large phase
angle for current flow, the voltage on C₉ exceeds an internally set threshold of approximately
7.3 V (reference voltage pin 16), a latch is set and load limiting is turned on. A current source
(sink) controlled by the control voltage on pin 15 now draws current from pin 12 and lowers the
control voltage on pin 12 so that the phase angle α is increased to α_max.
The simultaneous reduction of the phase angle during which current flows causes firstly a reduc-
tion of the rotational speed of the motor which can even drop to zero if the angular momentum of
the motor is excessively large, and secondly a reduction of the potential on C₉ which in turn
reduces the influence of the current sink on pin 12. The control voltage can then increase again
and bring down the phase angle. This cycle of action sets up a “balanced condition” between the
“current integral” on pin 15 and the control voltage on pin 12.
Apart from the amplitude of the load current and the time during which current flows, the poten-
tial on pin 12 and hence the rotational speed also affects the function of load limiting. A current
proportional to the potential on pin 10 gives rise to a voltage drop across R₁₀, via pin 14, so that
the current measured on pin 14 is smaller than the actual current through R₈.
This means that higher rotational speeds and higher current amplitudes lead to the same current
integral. Therefore, at higher speeds, the power dissipation must be greater than that at lower
speeds before the internal threshold voltage on pin 15 is exceeded. The effect of speed on the
maximum power is determined by the resistor R₁₀ and can therefore be adjusted to suit each
individual application.
If, after load limiting has been turned on, the momentum of the load sinks below the “o-momen-
tum” set using R₁₀, V₁₅ will be reduced. V₁₂ can then increase again so that the phase angle is
reduced. A smaller phase angel corresponds to a larger momentum of the motor and hence the
motor runs up, as long as this is allowed by the load momentum. For an already rotating
machine, the effect of rotation on the measured “current integral” ensures that the power dissi-
pation is able to increase with the rotational speed. The result is a current-controlled
acceleration run-up which ends in a small peak of acceleration when the set point is reached.
The load limiting latch is simultaneously reset. Then the speed of the motor is under control
again and is capable of carrying its full load. The above mentioned peak of acceleration depends
upon the ripple of actual speed voltage. A large amount of ripple also leads to a large peak of
acceleration.
The measuring resistor R₈ should have a value which ensures that the amplitude of the voltage
across it does not exceed 600 mV.
8 U211B
4752B–INDCO–09/05