Sizing a motor correctly is easy. It’s all about the load conditions. Starting and running. Those are the two operating points we have to know about in order to get it right. Most of the time the starting conditions are the most severe. When expressed as current, the starting current is generally expressed as a multiple of the running current. For an electric motor, this can be 5 to 7 times the amount of the running current.
There are only a few ways to start any load over 1000 Watts. Typically an electromagnetic contactor is used. Contactors are just a big switch with current carrying capability that will allow consistent operation for years of use. Central air conditioners have contactors to start the compressor. Larger loads, 3 horsepower and above, require contactors with large copper contact surfaces so that when the load is connected, the contact surfaces can operate reliably over thousands of operations. In addition contactors can be equipped with thermal overloads or current trip overloads to protect the motor from a stall condition that would cause the motor to pull in too much current and overheat.
As the laws of thermodynamics state, all transformations of energy result in losses. This is true for motors and semiconductors. So most of the circuit protection provided in motors and controls has to do with detecting heat and using the thermal information to protect the system and prevent failure. The problem is that heat, as work, has time as a component of its calculation. Instantaneous power is different from average or RMS power.
So it requires some detailed description of what we call ‘Duty Cycle’ in order to know how to deal with thermal conditions. These conditions are only partly described by starting and running conditions.
Semiconductor current ratings are a totally different matter as compared to electric motors. Power semiconductors are generally rated at 200% of continuous current and a slightly higher instantaneous current rating based on very short durations. Peak current ratings are typically for 1 minute and instantaneous current for 1/2 second. All of these conditions can be easily taken into account when specifying power devices with one caveat; power semiconductors have current rate limitation described as di/dt, the rate of rise of the incoming current. If that rate is exceeded the device fails, usually something spectacular.
So when matching any solid state based motor controller to an electric motor, the motors specifications and the load specs are crucial.
More details to come next week.