When it's safe, open the main furnace panel. The blower motor sits near the base and next to the main air filter. For a pulley blower motor, adjust the length of the pulley to increase or decrease the speed. For other models, consult your furnace manufacturer's manual.
The blower motor resistor is responsible for controlling the blower motor speed. In the past, this was done by varying the voltage supplied to the blower motor. In newer applications, the blower motor resistor is actually a solid state module that sends a varied signal to the blower motor.
The power applied to the motor is controlled by varying the width of these applied pulses, which in turn varies the average voltage applied to the terminals of the motor. By modulating or changing the timing of these pulses, the speed of the motor can be controlled.
The pulley has a screw that, when twisted, changes the fan speed. Tightening this screw makes the fan rotate faster. Loosening it slows the rotations. The technician will refasten the pulley to its location via the attached screw set.
Capacitors control fan speed by regulating electric current flow, allowing for higher or lower speeds.
Most controllers control fan speed by changing voltage. Newer technologies using variable frequency drives or electrically commutated motors change fan speed using an analog voltage, which can offer energy savings and simpler management, but for this article, we'll focus on traditional variable speed fan control.
However, most systems use a single-speed fan. Even though the fan only ever runs at one speed, you can still adjust this speed to be faster or slower. Most blower fans have three different speed settings: high, medium and low.
To speed up a driven pulley (and achieve less torque) it should be smaller than the drive pulley. To slow down a driven pulley (and achieve more torque) it should be larger than the drive pulley.
Adjusting the pulley on the blower driver motor lets you increase or decrease the blower speed. Refer to your owner's manual to find out exactly what the pulley and other applicable parts look like. When increasing the blower speed, loosen the setscrew a little bit.
Speed control of AC and DC motors works by adjusting the motor's operation to change its rotational speed according to the application's needs. DC motors have a simpler design and are easy to control. The speed control is generally attained by adjusting the voltage or current applied to the motor.
RPM stands for revolutions per minute and refers to the speed at which the fan's blades rotate. CMM stands for cubic meters per minute and is a measure of the fan's airflow, or the amount of air it can move in a given time.
Most air motor applications will need a speed control valve to set the speed while the motor is running under load. The key to improve starting torque is to place the speed control in the motor outlet, or in a meter-out mode on reversible air motors.
MOTOR PULLEY ADJUSTMENT
Speed can be increased by closing or decreased by opening the adjustable motor sheave. Two groove variable pitch pulleys must be adjusted an equal number of turns open or closed. Any increase in speed represents a substantial increase in horsepower required by the unit.
If a smaller pulley turns a larger one, the larger one will turn slower, but with more power available at the shaft. If a bigger pulley turns a smaller one, the smaller one will turn much faster than the bigger one but with less power available at the shaft.
If the fan speed is too low, there won't be sufficient air volume to effectively cool your home. This will obviously impact your home comfort. It also results in more wear and tear on your AC system and higher energy costs since your AC will need to run for much longer to sufficiently cool your home.
If the speed is set to high or too low, the air volume will also be too high or too low for the system to function properly. Setting the fan speed too low will usually cause the evaporator coil to freeze. If the fan speed is too high, it can cause the AC condenser to short-cycle or turn on and off constantly.
Capacitor Control:
A variable capacitance in series (usually few capacitors connected together with some tapping corresponding to each step) is used in this regulator. As we turn the knob the capacitance changes and it varied the voltage available to the fan. This will change the fan speed.
The speed of a DC motor can be controlled by adjusting the voltage applied. This is because the speed and load torque of a DC motor is inversely proportional, and this translates with changes in drive voltage.