Fans & Blowers FAQs

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What is a tach output?

A tach output or “sensor output” as shown in the catalog indicates the speed
of the fan at different operating levels. Its purpose is to identify when the
fan drops below a certain RPM, and to identify a potential problem with airflow.
A tach output fan will always have at least 3 wire leads.

What is the part number for a tach output fan?

The tach output feature is bolded: 4715KL-04W-B39-P50.
The 9 indicates that it has a 3rd lead wire (sensor type) and the 50 indicates it
is a tachometer output.

What type of tach output does NMB offer?

The standard tach out feature is an open collector, 2 pulses per revolution,
square wave. In high volume, a custom TTL (5 volt) and isolated tach is available.

What is a locked rotor?

A locked rotor is also a type of “sensor output”; that measures when the fan
has completely stopped or “locked”. It sends a signal, frequently called an
alarm signal, at either high or low voltage when the rotor locks. If the fan
starts spinning again, the alarm signal condition will go away. This is known
as a non-latching locked rotor signal, which is standard with NMB fans.

What is the part number for a locked rotor?

The locked rotor feature is bolded: 4715KL-04W-B39-P00.
The 9 indicates a 3rd lead wire (sensor type) and the “00” indicates a locked rotor.

What’s the purpose of a temp sensing fan?

A temp sensing fan is designed to respond to changes in temperature within a
customers system. Basically, as the environment gets hotter, the fan works harder
to cool the system by delivering more air. A fan works harder by increasing its
RPM, which then increases airflow.

What are the different ways a temp sensing fan can be controlled or configured?

There are 3 basic options NMB offers: 1) a hub mounted thermistor, 2) a remote
third lead wire with a thermistor on the end, or 3) a remote 3rd lead with voltage
controlling the speed of the fan.

What is the part number for a temp-sensing fan with a remote lead wire?

A remote temp sensing fan requires a 3rd lead wire. The part number is a 4715KL-
04W-B37-P60. The 7 indicates a temp sensing
function and the P60 indicates it is remote and has a 3rd lead wire. This part
number is valid for both a thermistor controlled and voltage controlled.

What is the part number for a hub mounted temp sensing fan?

The hub mounted part number is 4715KL-04W-B37-P50.
The 7 indicates temp sensing, and the P50 indicates a hub mounted thermistor that
controls the speed according to temperature.

What information do we need from the customer to build a temp sensing fan?

Hub mounted thermistor:
Engineering needs to know the performance needed at a given temperature. This is
typically illustrated with a temperature versus RPM curve.

Remote temp sense thermistor controlled (3rd lead wire):

Engineering needs to know need the resistance (in Ohms) versus RPM. A curve is the
best way.

Remote temp sense voltage controlled:
Engineering needs to know the performance (RPM) at different voltages. The voltage
range we offer is from 0-6 volts. This is for the control lead only and should not be
confused with fan operating voltage.

What is the voltage range for DC fans?

It varies depending on the type of fan and voltage. Check the catalog for voltage
ranges.

What is the voltage range of our AC fans?

The voltage range is +1-10% of nominal.

Do AC fans have special control features (tach out, locked rotor, temp sense)?

AC fans do not come with the special control features. AC voltage pattern does not
allow same versatility as DC.

What is the standard lead wire length?

300 mm or 12 inches. For fan sizes 52 mm and below, standard lead wire length is
200 mm or 8 inches.

What is the relationship between RPM and airflow?

RPM and airflow are directly proportional. In other words, increasing RPM by 20%
also increases airflow by 20%.

What is the relationship between airflow and voltage?

Within normal operating range fan speed will increase with increasing supply voltage. Airflow will increase as a direct function of fan speed.

How do I calculate how much airflow I need?

Several pieces of information are needed:
The amount of heat to dissipate or “get rid of”, in watts;
The allowable temperature rise in Celsius or Fahrenheit; and
The derived factor of 1.76 for Celsius and 3.16 for Fahrenheit.

With that, the formulas are:
Airflow(CFM) =1.76 X Watts divided by allowable temperature rise in Celsius (Delta T).
Airflow(CFM) =3.16 X Watts divided by allowable temperature rise in Fahrenheit (Delta T.)

This is a general rule of thumb for calculating airflow needed. Other factors that also
need to be considered are altitude, hot spots, and humidity.

What can we do to lower the noise level in a fan?

1) Lower the RPM or the fan operating speed (this also lowers airflow.)



2) Check for obstacles in front of and behind the fan. If they exist, move the fan
farther away. In general, obstacles on the intake side of the fan negatively impact the
noise level more than on the exhaust side.

How do I determine my system’s static pressure?

This is difficult to do without an airflow chamber. Typical system impedance for
electronics equipment ranges between .05″ and .15″ of water.

If I put two fans next to each other (in parallel), how does that affect airflow?

This will have no affect on airflow, but will approximately double the pressure
building capabilities.

What is PWM speed control and how does it work?

PWM stands for Pulse Width Modulation. An alternating High/Low signal is fed into
the speed control lead of the fan. The duty cycle of this signal determines the speed
of the fan. Duty cycle is defined as the ratio of the time the signal is high as
compared to the time the signal is low.

What characteristics determine the lifetime of the fan?

In a well designed fan, the lifetime of the ball bearing is the limiting factor in the
lifetime of the fan. Bearing life is related to the size of the bearing, the speed of the
fan, the load of the fan, the grease used, and other factors.

Can I run my +48 Volt DC brushless fan with a – 48 Volts?

Yes. If the specifications for a DC brushless fan read red=+48V black=return, to run
this fan with a -48 volts, connect the -48 volt line to the black lead and the -48 volt
return to the red lead. With this configuration, the fan ‘sees’ an effective +48 volts
across the motor. Note any sensor circuitry (alarm signals, tach signals) will be
referenced to the black lead.

How much current can I run through the open collector tach signal?
…what about the open collector alarm signal??

We recommend a nominal current of 1 mA and a maximum of 10 mA through each
of these components.

What is the maximum voltage the open collector tach signal can be exposed to? …what
about the open collector alarm signal?

This maximum voltage varies according to fan size and input voltage. As a general
rule, do not exceed the maximum voltage rating of the fan on any of the sensor
leads.