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Variable Frequency Drives - Generated Heat and Required Ventilation

A variable frequency drive develops heat during work and to avoid overheating ventilation is required

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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)

where

Hloss = heat loss to the variable-frequency drive surroundings (kW)

Pt = electrical power through the variable-frequency drive (kW)

ηd = variable-frequency drive efficiency

The heat loss expressed in imperial units

Hloss = Pt 3412 (1 - ηd)         (1b)

where

Hloss = heat loss to the variable-frequency drive surroundings (btu/h)

Pt = power in to the frequency drive (kW)

ηd = variable-frequency drive efficiency

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)

where

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)

where

ρair = density of air at the actual temperature (1.205 kg/m3 standard air)

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

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  • es: variador de frecuencia de trabajo sobrecalentamiento de calor de ventilación
  • de: Frequenzumrichter Lüftungswärme Überhitzung Arbeit
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