246 Chapter 10
Forced Air Cooling. Resistors and components can be
operated at higher than rated wattage with forced air
cooling, Fig. 10-5. The volume of cooling air required
to keep the resistor temperature within limits can be
found with the equation
(10-5)where,
Volume of air is in cubic feet per minute,
'T is the permissible temperature rise in degrees F,
KW is the power dissipated inside the enclosure in
kilowatts.
Air density at high altitudes causes less heat to be
dissipated by convection so more forced air would be
required.
Pulse Operation. A resistor can usually be operated
with a higher power in the pulse mode than in a contin-
uous duty cycle. The actual increase allowed depends
on the type of resistor. Fig. 10-6 is the percent of contin-
uous duty rating for pulse operation for a wirewound
resistor. Fig. 10-7 is the percent of continuous duty
rating for pulse operation for typical NEMA duty
cycles. Fig. 10-8 shows the percent of continuous duty
rating for pulse operation of a 160 W vitreous enam-
eled resistor.
10.1.2 Combining Resistors
Resistors can be combined is series or parallel or
series/parallel.
Resistors in series. The total resistance of resistors
connected in series is the summation of the resistors.
(10-6)The total resistance is always greater than the largest
resistor.Figure 10-5. Percent of free air rating for a typical resistor
cooled by forced air. Courtesy of Ohmite Mfg. Co.
Volume of air^3170
'T=------------KW400350300250200150(^100) 0 500 1000 1500
Air velocity—ft/min
% rated watts
Figure 10-6. Effect of pulse operation on wirewound resis-
tors. Courtesy of Ohmite Mfg. Co.
Figure 10-7. Percent of continuous duty rating for pulse
operation of small and medium size vitreous enameled
resistors. Courtesy of Ohmite Mfg. Co.
Figure 10-8. Percent of continuous duty rating for pulse
operation of a 160 W vitreous enameled resistor. Courtesy
of Ohmite Mfg. Co.
15
30
45
75
70
75
15
15
15
15
10
5
OnOff
Seconds
0 100 200 300 400 500 600 700 800
% of rated load
1000
700
500
300
200
100
(^7050)
30
20
10
57
3
2
(^1) 1 2 5 710 20 50 100 200 500 1k 2k 5k 10k
Off time—s
On time—s
125%
200%
500%
1000%
10 W
50 W
1000
700
500
300
200
100
(^7050)
30
20
10
57
3
2
(^1) 1 2 5 710 20 50 100 200 500 1k 2k 5k 10k
Off time—s
On time—s
125%
150%
200%
500%
!000%
RT=R 1 ++R 2 }Rn