MECH 1500 Motor Controls

Method 1: Direct Control - Motor is directly wired to the PLC output module For small motors Current less than 2A Least common

Method 2: Indirect Control - Motor wired through a set of relay contacts that separate the PLC’s control electricity from the power electricity that drive the motor Most Common Consists of relays and overload protection device, which is really a Magnetic Motor Stater

If an overload occurs, the protection detects it and opens the contactor, cutting power to the motor Only a small current from the PLC is used to energize the coil, but the contacts can handle large current to the motor. This is how PLC’s can control large motors The same method applies to Hydraulic and Pneumatic motors with their solenoid valves

The same method applies to Hydraulic and Pneumatic motors with their solenoid valves

PLC Output Diagrams Electrically – Only shows the control wiring, not the power Fluidpower– Only shows the control wiring, not the power

Inputs can be controlled by outputs Normally used in the following applications: Sequence operations Simultaneous operation of more than one output Motor control programs

PB1 is Pressed, Q4. 0 turns on, M1 turns on. The N. O. contact Q4 PB1 is Pressed, Q4.0 turns on, M1 turns on. The N.O. contact Q4.0 turns on (true), turning on Q4.4, simultaneously the N.C. Contact for Q4.0 opens, turning off Q4.5

Seal-In Program Logic Design Used to perform memory functions, such as starting and stopping a motor Causes an Output Coil to stay on or seal-in after a button or other input device has initially turn the coil on

Memory Coil Instruction Operate like Output Coil instructions, but use an internal memory address instead of an output address Used to provide additional control logic to the PLC program Can be N.O. or N.C. contacts Denoted using a M for an address Siemens S7-300 Processors can have up to 2014 Memory Coil instructions that use addresses M0.0 – M255.7

Memory Coil Instruction Memory coils can be used to start and stop a machine.

Midline Coil Instruction Used for intermediate results for a portion of the network. Operates like an output coil

Midline Coil Instruction Helps reduce the amount of ladder logic needed for a project

Interlocking functions Cause an output to turn on or off only if certain inputs or outputs are in a certain state Used in: Sequencing – to keep one output from turning on before another output has turned on (On the 870 trainer when the clamp comes on before the spool is shot into the valve body) Safety – to keep outputs from turning on if certain safety conditions are not satisfied (Keeping a motor from trying to go forward and reverse at the same time, or keeping something from turning on until the right pressure has been reached)

Symbols IEC – International Electrotechnical Commission - Accepted mostly in Europe, but U.S. has accepted to enter into certain markets. JIC – Joint Industrial Council – Standards developed in the U.S. Need to know both