Getting Started: Motor controls

Getting Started is a semi-ordered rambling of things I learned from absolutely great mentors. I can offer but a shadow of their wisdom, but I hope someone can learn from it!

Motor controls are fairly tricky, compared to most things I have covered so far. We are going to have a few more symbols.

Figure 1: The basic components. Note that all relay components are in the same box.

Depending on your location, the exact versions of these will vary. Still, the basics should be the same. Let us start with a simple example.

Figure 2: On/off control for a three-phase motor.

While this might seem difficult at first, consider it from left to right. Power coming in goes to the disconnect switch, then goes straight into fuses F1, F2 and F3. If coil M is energized, it is allowed through to the motor, passing through the overload. F4 and F5 allow power to a transformer, which goes only engages coil M when the overload is not detected and the ON pushbutton is engaged.

As you can see, the majority of your logic is going to be in the bottom right. Things you would probably see in a real environment...

1. Hardwired emergency stop

2. Variable frequency drive (or at least a soft-start)

3. Motor management relay

4. Forward / reverse run contacts

If you use an E-STOP, which you are probably legally mandated to, you really should have fail-safe wiring. Make sure that any wire coming loose will engage the system to stop. Think about what wiring you would need to do that...