Effect of Motor Run Capacitor Size on Reel to Reel Performance

[Bo Deadly]

My understanding of motor run capacitors is that there is an optimal size for a paticular motor running at a particular speed. The capacitor charges and discharges in pace with the orientation of the rotor. If the motor run capacitor is a little too large, the extra current will just give the motor a little more torque. If it's too small, torque will be less. The motor will simply run faster or slower depending on the size of the cap.

So then how does motor run capacitor size affect the performance of R2R motors? From looking at a ReVox B77 schematic, the capstan has a speed controller. So it will apply more or less AC modulation to increase or decrease current to, and thus speed of, the capstan motor. But the reel motors are not modulated. They're just on or off. The supply motor uses an different tap on the PT that is 6VAC less than the take up motor. Presumably this is what creates the tension between the two reels. No doubt other machines are probably more sophisticated about tension control but this seems to be how it works in the B77. The reel-size button just switches the common tap for the two reel motors.

Ultimately I think this means that if the motor run capacitor of the supply motor is too big, the capstan motor will not need to work as hard to pull tape from the supply reel and tension between the supply reel and capstan will be lower than normal. If the supply motor run capacitor is too small, the capstan will have to work harder to pull tape from the reel and increase tension.

So is this understanding correct? I just got a B77 so I'm just trying to understand how it actually works.

 

[gyraf]

Motor run capacitors are used to trick the motor into thinking it is being driven with a 3-phase signal. Thus, the capacitance of it must be calculated so that the incoming AC voltage is delayed/shifted ca. 120 degrees at nominal load.

So it's not a matter of "more is better", but rather "matched is better"

Jakob E.

 

[Tubetec]

Yeah same type and voltage rating is important to how the motor runs , Normally 450V for run 250 Vfor start , Both value and esr can shift a considerable amount before it becomes apparent , a slow start could be a signal all is not well ,often there are snubber resistor or load sharing that are switched in temporairily , Ive only ever seen run caps in tape machine motors . The good news is new caps are cheap and plentyful . if the caps are out of spec swap em out ,a current clamp meter is handy for measuring ,and the motor should state current on load on the plate , with no load you should be at about 50% current . but it varies a lot . Meggering the windings to case is probably a good idea ,and measure interwinding insulation if you can too , but that may involve unhooking the windings from the terminal box .

 

[Bo Deadly]

I see. So if the motor run cap is a little small, that one pseudo winding will be a little under powered. So during certain parts of the stroke (1/3rd of the time?) it will not have as much power as the other windings. That sounds like a bad idea for a reel-to-reel.

Part of the reason I ask is because the B77 reel motors require a 4.3uF run cap (see pic). I have found the 3.5uF caps on Mouser but they only have 4uF. No 4.3uF. Supposedly there are a few specialty hi-fi repair shops that have 4.3uF but I'm about to buy stuff from mouser anyway. Do you think a 4uF run capacitor would be ok in place of 4.3uF?

Also the ones on Mouser are in an aluminum casing that is grounded through an M8 bolt. Others are frequently plastic housings. The old ones are in an aluminum casing and they have a ground symbol engraved on the end of the bolt. Do you think it matters if the run cap casing is grounded?

But more generally, I want to understand this. The thing I really like about the B77 is that it is relatively simple, easy to work on and there seems to be a following that can provide support and parts. I'm kind of excited about it actually. I've never had a reel-to-reel machine.

Anyway, here are some pics of the reel motor, an old (leaking) motor run capacitor and bits of schematic that are relevant:

I don't see anything being switched in temporarily during phase changes. The capstan has a speed controller that does modulate AC through it. But the reel motors appear to be switched in through triacs so they're either on or off.

I have completely disassembled the machine for refurb and I will be replacing bearings so the motors are going to be out at some point. I do not have a current meter but I have an oscilloscope. I would be interested in knowing how to measure windings and insulation and such things but I must confess it's not clear to me how to do that or what I would learn from that exactly. I assume this is just to test them and verify that they are ok?

 

[PRR]

> to trick the motor into thinking it is being driven with a 3-phase signal.

Historically 2-phase came first. If a single cylinder steam engine stops "dead center", it won't self-start. If you look at any railroad steam engine, there's two cranks. If the left crank is stopped straight-back, the right crank is stopped straight-up, so one or the other will be in the correct position to start turning. The early AC motor guys thought the same way. 2-phase.

In the B77 the reel motors are 2-phase. See the two coils drawn at right angles. As you say, since poly-phase power is awkward for small loads, we fake a second phase with a coil (pre-1960) or cap (post-1970).

The steam engine can gain speed gradually because the intake valves run on the crank. The electric motor "steam pulses" are actually timed a steady 3600rpm by the town generator. So a simple motor gives "no" torque unless it is flip-started to synchronous speed. Moreover a reel motor never comes up to sync speed. There are tricks, including this cap, to cover RPM lower than sync. In a sense this reel motor's cap is a "starting" cap, but not to give a BIG starting jerk and never cut-out because it never gets to max speed.

The significant advantage of 3-phase is reduction in line cost. There is some overlap between phases which allows less copper. Moot here. Another potential advantage is smoother running when wound with more poles off the added phase. I suspect that is why the B77 Capstan motor is drawn 3 coils at 120deg, a 3-phase. However there may be wonky stuff inside the box to the right of the motor, as well as the cap to the left, so it may be more complicated.

There is almost no study-material on motor caps. I think they all come from the pencils of just a few guys. 4.0 may work, but is near a 10% difference. Your local heating supply shop has all sizes of caps.

 

[radardoug]

Theres a guy in Australia who sells Revox parts, he's bound to have the right caps. Its possible to tune the value of the motor cap, but generally the manufacturer will specify the right value.

 

[Tubetec]

4.5 might suit .
If the guy in the shop has a box of 4.5uf .you could ask to select one ,your bound to find something closer the mark .
I think thats whats known as permanent start cap config , its the orientation of the start winding in conjunction with the cap that gives the motor the initial push in the right direction to start .

Theres a few common faults with motors , one is a winding shorted to frame , another is interwinding insulation breakdown and sometimes  winding burnt open circuit . Say to test your revox motor , you'd need to disconnect the two windings check for shorts to frame, and between the two coils ,if you have a megger its great,but even a multimeter will give a good indication ,you want to see megohms between the motor housing and either winding , and also megohms between the windings,when disconnected from the terminal block of course .Aside from visual inspection of the windings for burn or over heating thats really all there is you need to know to work out if electrically a synchronous ac  motor is ok .Capacitance value and esr tests  reveal  99% of faulty capacitors right off ,but we have seen the odd cap that passes value and esr tests but fails for another reason ,probably leakage . Sometimes a cap fault is very obvious ,bulged out ,puffed out or broken seals can or terminals other times it could look good as new but have almost no capacitance ,

Oh yes golden rule of motor breaking and repair ,
punchmark the housing ,the drive end cover and the non drive end plate ,two dots for drive end one dot for non drive end , that way when you go to reassemble everything lines up right ,exactly as it was , on bigger motors with terminal boxes be sure to punchmark the orientation of the terminal box cable exit relative to the frame ,again so everything goes back as it was . Suppose you reassemble a motor only to find its running in the opposite direction you need it to ,its easy ,you just swap the start winding leads on the terminal block , bam she kicks off opposite direction ,run winding remains the same .

 

[Tubetec]

Oh yeah just wanted to mention safety ,
always make a good ground connection from the frame of the motor if your testing it on the bench that way if there is a winding shorted to frame or even bad insulation , the ELCB will trip it out in microseconds . I never make the assumption its safe to touch the frame of a motor while its energised in anycase ,but if say you forgot to connect an earth and you did have damaged insulation you could very easily find yourself the path of least resistance between mains energy and ground . Also motors that arent bolted down will give a kickback when energised ,best clamp it up in vice or tie it down first.

 

[Bo Deadly]

Thanks Tubetec. Although I thought about hooking one up to the variac, I don't think I'm going to mess with them and risk damaging something.

I did some basic passive measurements on the windings. My meter can measure 10M+ but it could not measure any continuity between windings or the frame. DCR resistances and capacitance looked unremarkable. Spinning the motors while hooked up to the scope looked unremarkable.

Thanks to radardoug too. I believe you mean Nagravox who has listings on Ebay. I just ordered the correct run caps and a bunch of other stuff. Now I just have to wait a month and a half for it to get here from the other side of the world.

 

[Bo Deadly]

I just learned something new about this.

The Sony TC399 capstan motor run capacitor is actually different depending on the wall AC frequency:

Of course this makes sense. If the mains frequency is 50 Hz vs 60 Hz the stroke of the motor is a little longer and so the cap needs to be a little larger. In this case quite a bit larger at 2uF instead of 1.5uF.

So perhaps the size is not so critical after all. The ReVox B77 uses the odd 4.3uF for reel motors. Maybe they hedged and selected a capacitor value in between what is optimal for both frequencies in which case something a little lower for 60Hz is OK and something a little higher for 50Hz is ok. Meaning a 4uF would be fine for the US. Maybe even better actually.