Cars guys will sometimes say that the worst thing you can do is start your car. This is because after sitting still for any length of time, at the instant you turn the key, there is no lubrication in the engine. In the combustion engine the starter motor is turning the engine with electricity prior to ignition of the combustion process,so there is no significant torque load on the engine. When the engine is running, sudden changes in power are gated by the rate of change in the fuel and air mixture coming into the engine. So things happen quickly, but not that quickly.
In the mechatronics world, things are a bit more stressful. Starting an electric motor is definitely the worst thing you can do. In the case of a standard AC motor, all the electric power is coming into the motor and in 16 milliseconds, one AC waveform, the motor is trying to get to full speed, typically 1800 rpm. In large AC machines the starting process can last for almost 4 seconds during which time no significant work has been done. This is because the electrons moving into the wire have to magnetize the stator and the stator in turn magnetizes the bars in the rotor after which rotation can begin.
The electrical time constant of the winding is only part of the picture. The mechanical time constant is made up of the time required to overcome the inertia of the rotor. The inertia mass of a cast aluminum rotor with iron bars in it is significant and the rotor mass is increasing as the square of the radius, so it’s a big deal as you move into larger machines. None of this considers the load, which after all, is what the motor is there to deal with.
When you have a large diameter fan, as in a cooling tower, the starting problems are worse still. For a 50 horsepower motor with a 20 foot diameter fan blade attached to it, the starting stresses are enormous. This leads to an increase in all the structure and support for the system. Now, starting the motor involves huge inrush currents with the associate power consumption. So the cost of starting the load is also reflected in the cost of the gearbox, bearings and support structure. These factors have lead to the introduction of special motors and variable frequency drives that are designed specifically with single stage planetary gear reducers in the output of the motor and larger output shaft and bearing construction to allow direct mounting of fan blades. Check out the Baldor offering for HVAC.
The other side of the motion profile is stopping the load. Obviously for a fan load, nobody cares particularly. The fan blade can coast to a stop and if it takes 30 seconds or a minute, that’s fine. What happens when a robot is involved? It’s a totally different situation, stopping is everything, especially if there are human beings in proximity to the robot work envelope.
This will be the topic of next week’s post.