Can I change the RPM of a 3-phase DC motor by changing the oscillator?

I have access to these kinds of motors from old copiers -- they are great -- very quiet and decent power. Most have one or two oscillators on them (X1, X2) which I'm guessing provides a fixed RPM. One has a pin labeled "CLK" which I'm guessing would allow for an external timing signal for variable speed.

On the ones that have onboard oscillators, is it possible to change the speed by changing to an oscillator of a different frequency?

Also, on the ones that have two oscillators, there is a "H/L" pin. Does that select between High and Low speeds?

Documentation on these is hard to find as I think many were custom manufactured for their particular application. I know I could just experiment to figure stuff out, but I already burned out one doing that so I'd rather learn more about them than ruin more by learning on my own.


Try this:-

I just looked at the link you supplied and saw there is a PDF datasheet download. It appears that pin 7 controls the speed by a clock pulse according to the formula rpm = 60 f CLK / 50. The HI and LOW on pin 5 are for start and stop.

Thanks guys

Yankee: I don't know how I missed that. Your find confirms for me that some of the models allow variable RPM by accepting a pulse signal. However, many of the motors I've scavenged don't have a pin for that. Instead they seem to have an on-board oscillator which I think implies it's a fixed RPM motor.

Grumpy: I read through that twice and it's interesting but I don't think (at least, not with my limited knowledge) that it gets me closer to my answer.

Instead they seem to have an on-board oscillator which I think implies it's a fixed RPM motor.

That does not necessarily imply a fixed frequency oscillator. Any motor can have it's speed effected by the external load changes, main voltage changes, etc. What most brushless 3 phase motors have is a 'servo' type feedback control loop where the controller can sense the actual rpm speed of the motor and either increase or decrease the phase switching speed to match the desired 'setpoint' speed desired. There may also be special controller requirements for the initial start-up or shutdown of the motor which would also require switching frequency changes. The motor itself is but just one component of such a 'servo' loop.


In a copier or printer there are several reasons to vary the speed of a motor for the fuser / paper feed.

During initial rotation the fuser may spin at a lower speed to prevent hot spots and flat spots on the fuser rollers as well as uniform heating.

Another reason is to accomodate different paper stocks. Thicker media ( paper ) tends to dissipate more heat. By slowing the fuser / paper feed the paper is fed slowing causing more efficient fusing of the toner into the napth of the paper.

Most larger units have a "thick" paper mode that increases the fuser temp and slows paper feed. Your service manual should describe in the theory section what is involved and should provide a schematic of what the signal lines do. If you need a manual most can be found on the internet.

Well, one thing I do know is that these motors are designed to keep their speed constant despite frequent changes in the load. Copiers mostly have just one or in some cases (as copiertech mentioned) two operating speeds.

Of the five motors that I have here at home, I can tell you this:

The two motors that have a H/L pin have two oscillators
The two motors that do not have a H/L pin have one oscillator
The one motor that has a CLK pin has no oscillators at all

When I am at work tomorrow I will check out a few more motors.

I do have manuals for virtually every machine these motors came from, but unfortunately I didn't notate what motor came from what machine. In the future I shall try and write some information on them with a sharpie (model and what motor it is (fuser/main drive/etc.)

Some of the things to do on my list of spare parts is....

See how fast I can spin a laser motor. Normal is what 36 or 40K rpm? That sounds cool to see on my desk.

Another is to see what can be done with an LED array out of an older fax. The industry has pretty much abandoned led exposure for the drum but some older brother fax heads would be interesting to get a schematic for just to play with it.

You know, that's one component I haven't scavenged yet -- the laser unit. Yes that would be cool to run one of those opened up -- the motor that is, not the laser itself -- they say that isn't too good for your eyes :astonished: