UNADA PWM I/O
Today’s refrigeration industry is limited by the prevalence of stand-alone electrically-commutated motors that are capable of, at most two-speed operation. UNADA is transforming the industry with its standard third-wire control-wire motor model, which permits three distinct states depending on whether the motor is connected to nothing, phase or neutral inputs. UNADA’s motor brings the added advantage of being able to perform various additional tasks when power is applied, such as running slowly for the first minute or running at high speed in reverse for 180 seconds.
UNADA is responding to customer demands for system-control engineering. Our motors’ integrated circuitry delivers increased efficiency:
- On-board hardware that delivers start/stop signals to the motor, eliminating the need for a relay or other power-switching device.
- The ability to increase or decrease the motor speed in response to load demands (time of day, ambient temperature, compressor status, door openings, etc.)
- Easy monitoring of motor faults as indicated and relayed to the system control board.
Pulse width modulation (PWM) is a way of converting a direct current into an alternating current. In essence, it creates a variable signal using just an on/off switch. Visualize a room containing a heater connected to the wall switch. If you turn on the switch the heater will increase the temperature of the room. This is similar to a direct current. If instead of leaving the switch on, you turn it on for one minute and off for one minute (and then keep repeating the on-off cycles) you will heat the room half as much, as the heater would only be working half the time. This simulates a 50 percent signal and is virtually the same as running the heater continuously at half capacity. By changing the relative on and off times, you can achieve any heating fraction you desire.
The advantages of using a PWM input signal to control a variable speed motor are numerous:
- Easy to test and calibrate in a controlled laboratory environment. The PWM signal can be generated using a simple frequency generator, allowing any arbitrary duty cycle to be set.
- Cheap to implement, using a single signaling line to set the motor speed. This signaling line typically runs from the system controller, which can usually produce a PWM signal very easily.
- It can be optically isolated, which makes complying with safety and equipment isolation constraints easier.
- Uses common techniques and standard equipment, so design teams can easily implement and integrate the system with existing components.
There are trade-offs, however:
- PWM input signals may not be as precise as some serial data control and other options.
- Similarly, PWM input usually involves some “averaging” in the system, so the signal response may be less rapid response than with serial controlled systems.
- There needs to be some agreement about what speeds the PWM signal is describing; for instance, does 100 percent designate 3,000 rpm? 2,000 rpm? Or some other speed?
- PWM inputs require separate systems for the motor to communicate back to the system controller data such as faults, stoppages, actual running speed, etc.
Usually coupled with the PWM-in system, PWM-out is one way the motor can signal to the controlling system. This solution provides symmetry and provides isolation advantages by pairing the input’s optocoupler with another on the output component.
The PWM output is implemented as a pull-down interface, so the receiving equipment will need to contain a pull-up resistor to restore the signal to a positive voltage level, and the motor will pull the line down to signal.
The motor outputs an approximately 50 percent square wave, with one pulse per motor revolution.
An error is signaled by the motor pulling the line down continuously.
In the event of a transient error, the motor will attempt to self-correct, so the receiving equipment must be able to tolerate short intervals where the motor is not producing a square wave output.
- Three input wires (labeled 1,2,3)
- (1) Input to PWM system (50Hz..10kHz), >3V, >3mA
- (2) Open collector output. <30V, <10mA. 1 pulse / revolution. Optional ‘ALARM output’ (low when the motor stalled)
- (3) Common
- 100 percent PWM input is 2400RPM
- 10 percent PWM input is 240RPM
- Linear interpolation
- Less than 10 percent is off
Alternative mappings are available upon request.
- Customizing the motor model (Setting up the speeds and direction the motor will run at the factory)
- Testing the motor – The serial commands allow us to ask the motor to run at full torque so we can measure the output is within specification.
These commands also allow some interrogation of the motor, to check what speed it’s running, and what percentage of full rated torque.
The serial commands are pretty simple, for example, to ask the motor to run at 2000 rpm the command is *sspd2000 (sent at 300bps,8, N,1)
We also have an extension to the serial protocol which will allow multiple motors to be connected to the same communications line where each has its own address, and then the speed/direction can be set in a group command to all the motors, or individually.
The intent here is that by being incredibly simple, a system designer can add serial communications to the motor with the minimum overhead, and not require the complexity of Modbus, etc.