Soft starter heat dissipation calculation

Soft Starters use power devices in the form of thyristors or SCR's which generally have an efficiency of about 99%. Soft Starters may be configured to operate in a continuous or bypass mode. Continuous is when the SCR's are dissipating heat whist ever the motor is running. In bypass configuration the SCR's are only operating during starting and stopping, all other times the SCR's are bypassed using a bypass contactor. When the SCR's are bypassed they dissipate no heat, therefore minimising the ventilation required. The reduced heat dissipation is a common deciding factor for bypass configuration.

Calculating the heat dissipation of soft starter
The heat produced by the soft starter can be calculated from the motor full load current rating according to the following formula:
1. Continuous Duty P_LOSS = FLC x 4.5
2. Bypass Duty P_LOSS = (FLC x SC x 4.5 x t x N) / 3600

P_LOSS: Power dissipated by soft starter, in W (Watts)
FLC: Motor full load current, in A (Amps)
t: Starting time, in seconds
N: Number of starts per hour.
SC: Average starting current expressed as per unit of FLC

Ventilated Enclosure:
V = (3.1 x P) / T
Where:
V = Airflow required in m³/hour
P = Power dissipated in Watts
T = Temperature differential in °C (inside – outside)

Non Ventilated Enclosure:
A = P / (T x k)
Where:
A = Exposed surface area of cabinet in m²
P = Power dissipated in Watts
T = Temperature differential in °C (inside – outside)
k = Heat transmission constant (5 for Painted metal)

Example:
What is the expected heat dissipation of a 22kW motor of 42A FLC operating at 5 starts per hour at 15 seconds per start at a start overload 300% FLC.
Continuous Duty: P_LOSS = 42 x 4.5 = 189 W
Bypass Duty: P_LOSS = (42 x 3 x 4.5 x 15 x 5) / 3600 = 12 W