M-3
© FANTECH
2016
CONTROLLERS, VSD, SENSORS & RUN-ON TIMERS
CONTROLLERS, VSD, SENSORS & RUN-ON TIMERS
TRIAC SPEED CONTROL
The triac is a type of semi-conductor device and is widely used to speed control
single-phase external rotor motors. It functions as a quick-acting switch which turns
on the current to the motor during each cycle of the alternating current waveform.
The power to the motor can be regulated depending on the point at which the current
pulse is received. If the current is switched on early in the cycle, the motor receives
maximum voltage and thus full power. The later the current is switched on, the smaller
the amount of voltage is released to the motor and thus the power to the motor
is lower.
As the voltage waveform supplied to the fan is no longer a sinusoidal shape,
magnetic oscillations occur in the fan motor. This is most noticeable at very low
speeds. This noise, which is loudest at 100 Hz, may spread throughout the duct
system if the motor is not properly vibration-suppressed.
The triac speed controllers are provided with radio interference suppression chokes to
reduce the radio interference produced on medium and short-wave bands.
The interference suppression is such that no special precautions have to be taken in
normal installations. On premises where there is very sensitive electronic equipment,
it may be advisable to use shielded cables and a separate supply or, alternatively,
use an auto-transformer control where available.
If more than one fan is connected to a triac-type controller, the amperage of all the fans
should not exceed 85% of the maximum rating of the controller. A speed ratio as low as
3:1 is possible.
VARIABLE SPEED DRIVES
(FREQUENCY INVERTERS)
External rotor motors can be speed-controlled using frequency inverters but only
if used with all pole effective sinusoidal filters. If using a sinusoidal filter no VSD EMC
shielding is needed after the sinusoidal filter.
See the special notes on page
O-2
for requirements.
SPEED CONTROL OF STANDARD FOOT AND FLANGE-MOUNTED MOTORS
We do not recommend standard motors being controlled by voltage reduction unless
they have specially designed high-resistance rotors and suitable cooling systems.
Experience has shown that trying to control the speed of a conventional motor
by voltage reduction is seldom satisfactory. On a standard motor, all changes in speed
when using a voltage reducing device, occur over a very narrow band of voltage
and it is therefore hard to achieve an acceptable accuracy of adjustment.
In addition, due to the level of heat generated, there is a risk of damage
to the windings and bearings.
Some motors with these features are available on request.
VARIABLE SPEED DRIVES
(FREQUENCY INVERTERS)
Standard three-phase motors can be speed-controlled satisfactorily using frequency
inverters which vary both the frequency and voltage of the electricity supply to the motor.
This system provides infinite variation of the fan speed, but the motors may become
noisy because the dynamics of the mechanical system may cause resonances or the
electrical noise may no longer be masked by the reduced air noise.
Shielded cable is normally required with frequency inverters.
Electro-Magnetic Compability (EMC) Compliance
These speed controllers normally require the electrical cables between the inverter
and fan motor to be shielded along their entire length to comply with regulations.
Inform our Sales Engineers if an Inverter is going to be used so it can be ensured that
all fan internal wiring is shielded to meet EMC requirements.
Additional care must be taken if the inverter unit is mounted remotely from the fan
being controlled.
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