I cannot find any info on the given model number, please check it.
Could be something as simple as a broken belt.
Does the motor make any noise at all?
The reason I ask is that the motor has a centrifugal switch on it that closes close to it's operating speed. It switches in the heating element and ensures there is air flow before the heating coil comes on.
If the motor is not turning the heater should not come on.
If it does there could be a problem with the centrifugal switch mechanism.
Usually though you here the motor buzzing as it tries to start.
Another possibility is a grounded element. Then the element comes on even if the centrifugal switch is open. Also the thermostats cannot regulate the heat so it blows the thermal fuse and that kills power to the motor.