Variable-Frequency Drives - Heat Loss and Required Ventilation
A variable-frequency drive develops heat loss during work. To avoid overheating it is often necessary to ventilate the location around the frequency-drive
Sponsored Links
Variable-frequency drives are common for controlling speed of electric motors in applications with fans, pumps, compressors, elevators, extruders etc.
Heat Loss from a Variable-Frequency Drive
A certain amount of the power transferred through the variable-frequency drive to the motor is lost as heat to the surroundings. The heat loss from a drive can be expressed as
Hloss = Pt (1 - ηd) (1)
where
Hloss = heat loss to the variable-frequency drive surroundings (W)
Pt = electrical power through the variable-frequency drive (W)
ηd = variable-frequency drive efficiency
The heat loss can alternatively be expressed in imperial units
Hloss = Pt 3,412 (1 - ηd) (1b)
where
Hloss = heat loss to the variable-frequency drive surroundings (btu/h)
Pt = power in to the frequency drive (W)
ηd = variable-frequency drive efficiency
For calculating the maximum heat loss - maximum power transmission through the variable-frequency drive must be used.
It is common that the heat loss from a frequency drive is in the range 2 - 6% of the KVA rating.
Necessary Ventilation for Cooling a Variable-Frequency Drive
The maximum surrounding temperature for a frequency-drive is approximately 40oC (104oF). Since frequency-drives often are physical protected in small cabinets or small rooms, ventilation - or even cooling - may be needed to avoid overheating.
The mass flow of air needed for transporting heat from the variable-frequency drive can be expressed as
mair = Hloss / cp (tout - tin) (2)
where
mair = mass flow of air (kg/s)
Hloss = heat loss to the frequency-drive surroundings (W)
cp = specific heat capacity of air (kJ/kg oC) (1.005 kJ/kg oC standard air)
tout = temperature of air out (oC)
tin = temperature of air in (oC)
Combined with (1), the mass flow (2) can be expressed as:
mair = Pt (1 - ηd) / cp (tout - tin) (2b)
The volume flow can be calculated by multiplying (2b) with the specific volume or inverted density:
qair = (1 / ρair) Pt (1 - ηd) / cp (tout - tin) (2c)
where
ρair = density of air at the actual temperature (kg/m3)
Example - Ventilation and Cooling of a Variable-Frequency Drive
The amount of air for cooling a cabinet mounted variable-frequency drive with maximum power of 100 kW and efficiency of 0.95, a maximum surrounding operating temperature of 40 oC and an outside temperature of 20 oC, can be expressed as (2b):
mair = (100 kW) (1 - 0.95) / (1.005 kJ/kg.oC) ((40 oC) - (20 oC))
= 0.25 kg/s
The volume and density of air depends on the temperature of the air. The density of air at 20oC is 1.205 kg/m3 and 1.127 kg/m3 for at at 40 oC.
The volume flow at the inlet of (20 oC):
qair = (1 / (1.205 kg/m3)) (0.25 kg/s)
= 0.208 m3/s
= 749 m3/h
The volume flow at the outlet (40 oC):
qair = (1 / (1.127 kg/m3)) (0.25 kg/s)
= 0.222 m3/s
= 799 m3/h
Sponsored Links
Related Topics
- Electrical - Amps and electrical wiring, AWG - wire gauge, electrical formulas, motors and units
Sponsored Links
Related Documents
- Asynchronous Motors - Electrical Data - Electrical motor data - nominal current, fuse, start ampere, contactor and circuit breaker of asynchronous motors
- Frequency and Speed of Electrical Motors - The velocity of 2, 4, 6 and 8 poles electrical motors at 50 Hz and 60 Hz
- Nema Starters - Nema size starters
