AMC8500 Datasheet, PDF (11/27 Pages) List of Unclassifed Manufacturers – TWO PHASE VARIABLE SPEED FAN MOTOR CONTROLLER

Preliminary Specification
aMC8500
Kick Start
Most DC motors exhibit a large difference between the voltage required to insure startup and minimum speed operation.
Figure 18 shows this difference as a hysteretic characteristic and is dependent upon bearing friction, lubrication,
temperature, and rotor inertia. With the majority of motor controllers presently available, the applied voltage for minimum
speed operation must be set equal to or greater than that required for startup. This limits the motors useful speed range from
about 35 to 100 percent even though most motors will operate down to 10 percent. In order to guarantee near minimum run
speed operation, the mechanical startup hysteresis must be overcome. This is accomplished in the aMC8500 by kick starting
the motor in a controlled manner where full power is initially applied for a for a prescribed time, and control then reverts back
to the level that is dictated by either the Speed Control or Minimum Speed Set input. This is graphically shown in Figures 1
and 2. With reliable starting guaranteed, the useful speed range is increased by approximately 25%, yielding lower speed
operation for reduced acoustic noise and extended motor life.
Figure 18- Typical DC Motor Startup and Run Characteristics
100
80
60
Kick Start Extended
Speed Range
Kick Start 100% Voltage
1.0 Second
40
20
0
0
Typical Useful Speed Range
20
40
60
80
%t_on, Motor Drives Percent On-Time (%)
Minimum Startup Speed
35% On-Time
Minimum Run Speed
10% On-Time
100
The Kick Start time, t_on(KS), is internally set to 1.0 second and is automatically activated whenever the motor is at rest and
commanded to run, or when it is under a command to run and one of the following events takes place:
1) Device comes out of power down mode
2) Rotor was locked and the obstruction has cleared
3) Device recovered from a thermal shutdown
4) Device comes out of an under voltage lockout condition
Current Limit
Abnormally high drive current conditions can occur if the motor is mechanically overloaded and may result in device and
the drive coil overheating. During motor overload, any reduction in rotational speed reduces the generated back
electromotive force, EMF, resulting in an corresponding increase in drive current. The most severe condition occurs when
the rotor is locked and there is no back EMF generated. Under this condition, the drive current is limited only by the
resistance total of the MOSFET switch and the driven coil. In order to protect the device and motor from abnormally high
currents, a programmable Current Limit Comparator is incorporated. The comparator indirectly senses the drive current and
when a maximum level is exceeded, the motor drive on-time is immediately terminated on a cycle-by-cycle basis of either the
internal oscillator or the digital control signal applied to the Speed Control Input.
The current limit threshold defaults to a peak current of 1.1 A with Pin 2 open. This level can be reduced by either applying
a bias voltage to Pin 2 or by connecting a single resistor from this pin to ground. Figures 11 and 12 show the current limit
behavior while Figure 32 illustrates two biasing methods. The Current Limit Set input is high impedance and in most
applications will require a 10 nF bypass capacitor to prevent false triggering due to noise pick up.
© Andigilog, Inc. 2006
- 11 -
www.andigilog.com
August 2006 - 70A04018