Motor Speed Control

When I retired about 10 years ago we had many motor speed controls that worked very well ranging from less than 1 HP to 300 HP. Some were for DC motors and some were for 3 phase AC motors.

Were you thinking of AC or DC motors ?

The AC motor speed controlers used a very odd waveform and sometimes the motors would produce a sound of say 1000 Hz in frequency. The speed controlers were microprocessed based and we had to set several parameters depending on the motor and type of service.

They have probably gotten better in the last 10 years.

DC. This will be for an old reel-to-reel tape recorder that's been in storage for decades. All the rubber drive belts have perished and replacements are unobtainable. It has 3 speeds: 3 inches per second,

7.5 IPS and 15. I believe it's a 24V motor but will have to check. If the idea is feasible, I have a couple of other R-Rs I'd like to get working again as well.

One of the other tape recorders I have used a 240V motor. Not sure whether AC or DC motors are more suited to PWM control.

Thanks, John; very interesting. What do you mean by takeup reel motor ran stalled? And what is it that controls the speed of the tape - the capstan/pinch-wheel motor or the relevant reel motor?

The take-up motor is set to a speed (RPM) such that it maintains a tension on the tape on the capstan/pinch-wheel. This is to assure that the tape spools onto the take-up reel instead of unspooling on the floor. The result is that the motor is always being held back from its desired speed (i.e., stalled) by the tape.

The capstan and pinch-wheel controls the linear tape speed across the heads. The take-up motor maintains tension to keep the tape spooling onto the take-up reel, and the feed motor maintains a small back tension to prevent the feed reel from unspooling onto the floor.

Very clear explanation, John; many thanks indeed for that. The two main decks I'd like to get working again are both Ferrograph ones, so basically top the range of non-studio decks. Built like tanks. I'm guessing each must weigh 120lbs! They had issues at the time with rubber components disintegrating. Kind of damaged their reputation by the end of the 1970s. A real shame, as they were exceptionally high quality in every other respect.

Thanks, Betrand. That pretty much ties in with what I'm seeing when it's powered up. Looks like all of them will need a pretty thorough service to get them back into spec. A lot can go wrong in 40 years!

Nothing to do with what you're after, but you might find something of eventual use here:

There might be a problem if this causes mechanical vibration in the motor (maybe be audible). This vibration might affect the tape speed and be audible in the result.

Thanks, Jeff. Some promo stuff I've not seen before in among that lot.

In these machines they use a heavy flywheel on the end of the capstan roller, so that shouldn't be an issue.

A BLDC motor does not really need a tach feedback. The speed controller performs the commutation, so it already knows how fast the motor is spinning.

Some R/C hobby BLDC controllers have a governor mode, where they keep the motor speed constant regardless of torque.

The electronics is not complicated at all. It is essentially a microcontroller and six MOSFETs.

Very true.

It's another option - albeit perhaps a last one on grounds of complexity.

Or maybe an Arduino.

I dunno what "state memory" is, but the rest of that paragraph had already unhappily occurred to me.

That

By which I mean the prospect of several hundred feet of tape spewing out on a sudden stop like a f****ng tagliatelle factory on overtime.

Can I just get some clarification on one point here. The two spools are not speed controlled as such and just spin or drag (as the case may be) at the same speed regardless of the tape speed selected? So it's only the capstan motor that needs precise control speed? That seems to be implication of what's been posted here so far and it would make things much simpler if there was only one motor's speed to worry about.

When the machine is in "play" or "record" mode, I mean; not during FF or rewind.

The speed of the tape depends not only on the spool RPM but also how much tape is present, since that changes the effective diameter. If the spools have different amounts of tape on them (normal) they'll have to move at different RPMs to have the same linear tape speed.

If I understand this correctly, one doesn't need to worry about that aspect, because it will 'just happen automatically' on play and record.

Ah, I misunderstood the question. The reels can be run at a "constant speed" which is known to be slower[*]/faster enough than the capstain, but with sufficiently low torque that the capstain can override them.

[*] or unpowered, using only drag

I would consider this to be run at constant *torque* mode, not constant

*speed* mode, since you don't care how fast the spools are moving, just how much drag or tension they're creating for the tape.

If you *forced* the reels to run at a set speed, the tape would break.

Well yeah, I knew that. I was oversimplifying.