Understanding Airflow Stability Behavior with Incoming

Power Voltage Fluctuations

And Accuflow G2 Performance Evaluation

June 2009

By Franky Tanuwijaya and Fikri Muhammad

Introduction

Adjustable stable airflow is the fundamental for biosafety cabinet operation. Biosafety cabinets

must maintain airflows at designed parameters. To do this a speed controller is used to vary the

voltage to the blower motor. Increasing or decreasing the voltage to the motor directly

increases or decreases the motor speed or rpm. In turn the fan attached to the motor increases

or decreases the speed of the air moving within the cabinet. The most common speed

controller used in biosafety cabinets is a phase cutting electronic speed controller. These types

of controllers are commercially available at reasonable prices and due to their robustness are

practical to implement in various cabinet designs.

Maintaining stable balanced airflow is critical in biosafety cabinets to ensure Personnel, Product

and Environmental protection. One of the main factors influencing the stability of the airflow in

a biosafety cabinet is incoming power fluctuation. As the incoming voltage fluctuates the motor

voltage fluctuates resulting in changing airflow. Depending on the severity of the incoming

voltage fluctuations the airflow containment may be breached and safety compromised.

In this paper a study has been conducted to show the effect of incoming voltage fluctuations as

it impacts airflow stability in biosafety cabinets. We also identify the correlation between

measuring voltage with a standard averaging Digital Voltage Meter (DVM) and a True RMS

Digital Voltage Meter. For a more detailed explanation on the difference between True RMS

and AC rectified average measurement and its correlation to airflow, a technical paper titled

True RMS vs AC Average Rectified Multimeter Readings when a Phase Cutting Speed Control is

Used and its correlation with airflow”1 2is available from ESCO.

This paper also provides performance testing results of a newly developed ESCO Accuflow G2

AC speed controller. This controller maintains airflow stability with incoming line voltage

fluctuations by tracking the motor voltage.

Airflow Testing Experiment with Incoming Line Voltage

Fluctuations

An experimental test bench setup is used to simulate the effect incoming voltage fluctuations

has to blower voltage and airflow stability.

In this experiment the following equipment was used:

AC variable power supply to simulate incoming voltage fluctuation

Fluke 179 True RMS Digital Voltage Meter (DVM) and a standard Non True RMS (AC rectified

average) DVM UNI-T UT50C are used to analyze the correlation between True RMS and AC

rectified method in measuring motor speed

HoffMan Controls Model 706-123SB phase cutting speed control to provide baseline

information on how incoming voltage fluctuation impacts motor speed.

ESCO Accuflow G2 speed controller with output voltage stabilization to show the advantages of

utilizing a voltage stabilizing or tracking speed control.

ESCO Model LA2-4A1 Biosafety Cabinet

Shortridge Model ADM-870C Flow Hood to measure the inflow airflow.

The diagram below shows the experimental wiring setup.

1 True RMS vs AC Rectified Multimeter Readings when a Phase Cutting Speed Control is Used

2 True RMS Definition, application note 106, Linear Technology

The photos below show how the experiments were set up.

Experiment One shows the variance of inflow airflow as the incoming voltage is adjusted up

and down.

UNI-T

UT50C

Fluke

179 Phase

cutting

speed

control

AC Blower

Source

Variable

AC power

supply

Motor Voltage

The HoffMan Controls Model 706-123SB phase cutting speed control is used to provide

baseline information on how incoming voltage fluctuation impacts motor speed.

Initially, a nominal 230VAC incoming AC voltage is provided to the speed controller and the

respective Vavg, Vrms and Airflow readings are recorded for data analysis. Subsequently the

incoming AC source voltage setting is varied between ±20V in 5V increments from its nominal

of 230V. At each voltage setting the measurements are taken.

Experiment one results are shown below. The airflow deviation is between -22% (-0.117 m/s) to

+14 % (+0.076 m/s) from the nominal value.

Without Blower Voltage Tracking

Blower True RMS Blower Averaging Flow hood Inflow Inflow change Inflow change

Rating VAC Voltage (VAC) Voltage (VAC) reading (l/s) veloc (m/s) fr setpoint (m/s) fr setpoint (%)

+20V 250.0 151.1 139.7 176 0.6062 0.0758 14.29

+15V 245.0 147.5 134.8 172 0.5924 0.0620 11.69

+10V 240.0 143.9 129.9 167 0.5752 0.0448 8.44

+5V 235.0 140.1 125.0 161 0.5546 0.0241 4.55

Nominal 230.0 136.1 120.0 154 0.5304 NA NA

-5V 225.0 132.1 115.0 146 0.5029 -0.0276 -5.19

-10V 220.0 127.8 110.3 138 0.4753 -0.0551 -10.39

-15V 215.0 123.2 105.2 130 0.4478 -0.0827 -15.58

-20V 210.0 118.7 100.2 120 0.4133 -0.1171 -22.08

LA2-4A1, 230VAC/50 Hz With Hoffman Speed Control, Ambient: 25°C

Supply Voltage

The results indicate for each 1 V increase in incoming line voltage the inflow will increase

approximately +0.004 m/s and for each 1 V decrease in incoming line voltage the incoming

airflow will decrease approximately +0.006 m/s.

Experiment Two is similar to experiment one with the following exceptions:

The Speed control was changed to an ESCO Accuflow G2 speed controller. This controller

maintains constant average blower voltage value regardless of incoming voltage fluctuations.

With Blower Voltage Tracking

Blower True RMS Blower Averaging Flow hood Inflow Inflow change Inflow change

Rating VAC Voltage (VAC) Voltage (VAC) reading (l/s) veloc (m/s) fr setpoint (m/s) fr setpoint (%)

+20V 250.0 142.5 113.4 154 0.5304 0.0000 0.00

+15V 245.0 142.1 113.4 154 0.5304 0.0000 0.00

+10V 240.0 141.3 113.4 154 0.5304 0.0000 0.00

+5V 235.0 140.5 113.4 154 0.5304 0.0000 0.00

Nominal 230.0 140.0 113.4 154 0.5304 NA NA

-5V 225.0 139.3 113.5 154 0.5304 0.0000 0.00

-10V 220.0 138.5 113.5 154 0.5304 0.0000 0.00

-15V 215.0 137.6 113.0 154 0.5304 0.0000 0.00

-20V 210.0 136.7 112.9 154 0.5304 0.0000 0.00

LA2-4A1, 230VAC/50Hz With Accuflow G2 Speed Control, Ambient: 25°C

Supply Voltage

Experiment Two results indicate the airflow deviations have been reduced significantly as

compared with results obtained in the first experiment. The airflow deviation result is found to

be negligible for ±20V AC line fluctuations.

The ESCO Accuflow G2 design uses an Average AC voltage measurement method to maintain

constant motor voltage regardless of incoming voltage fluctuations. This provides stable

airflow.

The chart below shows the blower voltage using a standard phase cutting speed controller and

an ESCO Accuflow G2 speed controller with blower output voltage tracking.

The chart below shows the inflow airflow using a standard phase cutting speed control and a

ESCO Accuflow G2 speed controller with blower output voltage tracking.

Both charts indicate that voltage and airflow are stabilized using the ESCO Accuflow G2 Speed

Control.

Accuflow G2 performance evaluation

The following sections show the ESCO Accuflow G2 speed controller performance evaluation at

±10% incoming line voltage fluctuations at multiple ambient temperature settings from 5°C to

35°C.

This experimental setup is similar to experiment two with the following exceptions:

An ESCO Model FC2-4A1 biosafety cabinet is used to provide additional information.

Environmental temperature settings are included in the test conditions.

The ambient temperature settings are 5°C, 15°C, 25°C and 35°C. Stabilization period of 3 hours

between set points is used to ensure temperature uniformity inside the environmental

chamber. At each temperature set point, after stabilization, airflow measurements are

conducted from nominal incoming voltage of 230 VAC, ± 10% in 5% steps.

Below are the inflow velocity deviation results at various AC main voltage (shown on the x axis)

and at various ambient temperature settings.

The inflow maximum velocity deviation is ±0.008 m/s which is within ±0.025 m/s limit per

NSF/ANSI Standard # 49 – 2002.

Airflow Deviation Overtemperature and Overvoltage

FC2-4A1 max 0.504 0.008

m/s nom 0.496

min 0.488 -0.008

Conclusion

In this technical paper it is shown that incoming voltage fluctuations change the output voltage

of a standard phase cutting speed control. This change in motor voltage directly impacts the

inflow airflow of the biosafety cabinet. Changes in incoming airflow can compromise the barrier

protection provided by biosafety cabinets.

This paper also shows that incoming voltage fluctuations do not affect the output motor

voltage of the ECSO Accuflow G2 speed controller even under changing ambient conditions.

The resulting airflow deviation is within the deviation limit of 0.025 m/s as per NSF/ANSI

Standard # 49 – 2002.

References

1. True RMS vs AC Average Rectified Multimeter Readings when a Phase Cutting Speed

Control is used, ESCO Technical Paper, 2009.

2. True RMS Definition, application note 106, Linear Technology

3. Airflow Deviation limit on Biosafety cabinets, NSF/ANSI Standard # 49 – 2002.