23
Normal Operation
Section 4
Operation
Before starting the unit, read these operating instructions.
The AC motor is usually started first. In this way, the
motor will accelerate to, and run at, its design speed under
no load condition. This is the easiest and most efficient
method. With the motor running and the clutch not
energized, a slight torque may still be applied to the output
shaft. This is normal, resulting from bearing friction and
windage acting on the clutch output assembly. With no
load, the torque may even start and accelerate the output
shaft. The driven machine friction is usually enough to
hold the output stalled.
To start the clutch, the controller is energized by the Start
pushbutton. The speed setting potentiometer can then be
set for the desired output speed, which causes excitation
to the clutch coil. Coil current produces the magnetic field,
resulting in eddy-currents being generated in the clutch.
The resulting torque accelerates the output shaft.
As output shaft speed increases, the tachometer
generator output signal also increases. This signal is fed
back to the controller and it is used to regulate the current
to the clutch coil. Modulation of coil current adjusts output
torque to hold shaft speed constant under varying load
conditions.
To maintain a set speed, the transmitted torque must
match the requirements of the load. Load increases and
decreases during operation will cause slight, momentary
speed changes. Such changes are detected by the
controller, resulting in adjustment of clutch coil current to
change the torque to the level required for returning the
output shaft to the set speed selected by the operator.
An alternative to speed control, as described above, is
torque control. When the remote electronic controller is
designed for torque or tension purposes, the clutch coil
excitation is regulated to control output torque instead of
speed. With this type of controller, the output shaft speed
varies with the load being driven.
Operation is normal under the following conditions: motor
load current is not more than full load nameplate rating;
the speed or torque output is being controlled properly;
the thermal rating is not exceeded; and the noise or
vibration levels have not increased. Although continuous
motor current should not exceed nameplate rating,
momentary overloads may be applied during acceleration
and sudden load changes. If the machine duty cycle
requires frequent overloads or frequent motor stops,
starts or reversals, the application should be reviewed,
and specific limits determined. Consult the factory.
Under normal speed control, the controller is constantly
regulating clutch coil current to maintain set speed.
Motor current and clutch coil excitation will vary in
proportion to the amount of regulation required. This is
normal; do not expect to measure a constant clutch
voltage or current. The constant parameter should be
speed or tachometer generator voltage.
Air discharge temperature from the clutch is an important
consideration during operation. Air discharge temperature
depends on inlet air temperature and operating
parameters of the clutch. Operation below the minimum
operating speed, as stamped on the nameplate, or
running above rated load, will cause excessive heating,
resulting in a thermal overload. Operation under these
conditions should be avoided. Supply an adequate
amount of clean, dry cooling air to the unit. Hot air
discharge from any adjacent unit or from this unit must not
be allowed to re-enter as cooling air.
Operating Limitations
The clutch, like any other machine, has certain limitations.
With speed control, it is possible to select any speed
between 0 and 100% of the maximum rated speed.
Because of standard controller and tachometer generator
characteristics, it is not normally possible to regulate
speed below 50 rpm of the output shaft.
Torque can also be varied over the full torque range of the
clutch. Eddy-current clutch capacity is much higher than
the motor rating and can easily overload the motor if not
limited. Remember that the AC motor used with the eddy-
current clutch also has overload considerations.
Cooling capacity places another limit on the clutch. Do not
operate it continuously below minimum rated speed
stamped on the nameplate or air discharge temperature
may become excessive.
Increased noise and vibration are indications of
mechanical problems. The operating and maintenance
personnel should be familiar with the normal noise and
vibration levels. When an increase is detected, shut down
the unit and correct the problem.
Note:
There are no brakes on the standard unit. If clutch
power is removed by turning off or stopping the controller,
or lowering the speed setting, the clutch simply coasts to
a stop or to the lower speed. The rate at which it
decelerates is a function of internal and external inertia
and friction.
CAUTION:
When power is removed from the unit,
causing the motor to stop, clutch coil excitation must
also be removed, or excessive coil heating and
possible damage may result.