The difference between rating a solid state device and an electric motor can make all the difference in actual practice. Since we know that solid state electronics can only handle 200% operation for a limited period of time, proper drive or solid state starter ratings will have everything to do with reliable operation.
In most cases operating a fan or pump doesn’t require hard starting. There is usually plenty of time to get the fan blade or pump rotor started. Inrush current ratings can generally be limited by allowing the system to come up to speed gradually.
This makes sense because when calculating the torque required to accelerate a given load, the time allowed is in the denominator of the equation. This causes the torque required to increase arithmetically as the time required decreases. In the servo world, reducing the move time for a given application always has its limits. And it’s the same math for large loads, just bigger numbers.
Since the rate of current inrush that the power semiconductor can handle becomes the other constraint on the system, oversizing the drive electronics can help with high acceleration requirements should they come up. Torque is roughly analogous to current, so a demanding torque of acceleration means more current needs to be available to the motor.
Motor design can have a huge impact on current demand. The common motor windings will have defined current ratings for Full Load Amps (FLA) and starting current. Locked Rotor Amps is another value that is very important because it is unambiguous. Knowing that the motor is limited to a value for locked rotor means that you can rate the power electronics driving it for a fraction of a second and then shut down. This protects both the motor and electronics from damage, and since the power semiconductors react quickly, using the drive current limit settings is the best protection.
In a recent application the customer had a very high load requirement with changing load. Oversizing the drive began to cause the size and cost of the power electronics to go ‘out of sight’. When I asked the customer what the locked rotor current of the motor was, it turned out to be 105%.
The motor winding was similar to a Nema D torque curve which does not allow for very large overcurrents. These motors are worked very hard in heavy machinery applications, so they are already oversized for the mechanical work that has to be done. The magic in the situation was to know that the locked rotor current could be supplied easily for 1/2 second and that was a safe point for the drive to shut down. This allows “right sizing” the power electronics in any situation.