Replace the value of Table 1 into the formula in Select External Heatsink by Thermal Resistance.
The thermal resistance of the external heatsink “EF(98) L: 150“ is 1.6 K/W, which is sufficient. Yaskawa recommends the external heatsink "EK (95) L: 150" to prevent overheat if there is a temporary temperature increase or a decrease in the cooling capacity of the external heatsink because of dust and unwanted material.