PWM powered AC gear motors

[kwwrightjr]

I have several of new Trane AC fan coil units that use the old Comm 5 boards. Both the chill water and hot water valves are operated by 24v 50/60hz motors that are operated by PWM. I'm new to this and just got introduced to the Rover service tool to troubleshoot it. How is it that these AC motors can be powered by PWM. I was under the impression that only a DC motor can be powerd by PWM.

Looks like the days of troubleshooting with a meter and guages are over.

[orion242]

Do you have a part # on the actuators?

How many wires are going to the actuators?

[ctrlguy]

PWM in this context is simply a control signal based on time. The position of the actuator varies with the pulse width. The pulse width is the length of time that voltage (AC or DC, depending on the device) is placed on the input terminal. A typical range would be something like 0-25 seconds. A pulse of 12.5 seconds would drive the actuator to 50%. Pulses can also be quite a bit shorter. A triac is the usual output type for PWM motor control. You can test one by simply touching power to the input terminal (you'll want to check the documentation on which side of the transformer is input voltage).

[kwwrightjr]

I'll get the numbers the the unit, board and motors today.

[kwwrightjr]

seconds or milliseconds?

[orion242]

ditto with Crtlguy

Most likely what you have is an actuator that takes a PWM input on a signal wire and the supply voltage is constant 24vac or whatever.

What I can tell you, it not likely to milliseconds. The stuff I have dealt with uses a .259 - 3.59 second PWM signal which can be measured with any meter fairly easily. Without a part number and knowing what the PWM signal is exactly, I'm a bit hesitant to give you troubleshooting advice. PWM is not that hard to work with, you just need to know what you have. The actuator could be looking for a positive pulse, negative pulse, either, and no telling how long or short.

[kwwrightjr]

How do you see a PWM signal with a meter? All I see is the 25.6 volts.

[orion242]

You should be able to see it pulsing 24v to the input wire. It may have to be measured from the 24v common and the input or the 24v hot and the input.

Without a part# on that actuator, I can only guess. Most of these pulse slow enough to measure it with a standard voltmeter.