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Engineering ToolBox > Variable-Frequency Drives - Heat Loss and Ventilation
Variable-frequency drives are common for controlling speed of electric motors in applications with fans, pumps, compressors, elevators, extruders etc.
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.
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 at 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)
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 40oC and an outside temperature of 20oC, can be expressed as (2b):
mair = (100 kW) (1 - 0.95) / (1.005 kJ/kg.oC) (40oC - 20oC)
= 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 40oC.
The volume flow at the inlet of (20oC):
qair = (1 / 1.205 kg/m3) 0.25
= 0.208 m3/s = 749 m3/h
The volume flow at the outlet (40oC):
qair = (1 / 1.127 kg/m3) 0.25
= 0.222 m3/s = 799 m3/h