A variable frequency drive develops heat during work and to avoid overheating ventilation is required
Variable frequency drives are common for controlling the electric motor speed in applications with fans, pumps, compressors, elevators, extruders etc.
Heat Loss from a Variable Frequency Drive
An amount of the power transferred through a variable frequency drive to the motor is lost as heat. The heat loss from a drive can be expressed as
Hloss = Pt (1 - ηd) (1)
Hloss = heat loss to the variable-frequency drive surroundings (kW)
Pt = electrical power through the variable-frequency drive (kW)
The heat loss expressed in imperial units
Hloss = Pt 3412 (1 - ηd) (1b)
Hloss = heat loss to the variable-frequency drive surroundings (btu/h)
Pt = power in to the frequency drive (kW)
To calculate maximum heat loss - the 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
Maximum ambient 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)
mair = mass flow of air (kg/s)
Hloss = heat loss to the frequency-drive surroundings (W)
cp = specific heat 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)
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, maximum ambient operating temperature 40 oC and outside temperature 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 at 40 oC.
The volume flow at the inlet (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