Motor windings

[Mike Cunningham]

Recently I read that the windings on some Taiwanese and Chinese motors are not coated and therefore lack some protection. I further read that this protective coating can be accomplished by disassembly, dipping the windings in varnish and baking them. Anyone have knowledge of this?

 

[Bill Boehme]

No, that is not correct information. All motor, transformer, solenoid, and other such devices are wound with enameled copper wire that is insulated with a very thin varnish-like material. Formvar is a trade name of one of the insulating materials used. The iron laminations in the field winding and in the armature are separated from each other with a very thin insulating material to reduce circulating "eddy" currents in the iron that cause heating and reduce efficiency. The material used on the iron laminations is Glyptal varnish (made by GE) or a similar product. It would be a VERY BAD idea to take the whole armature or field assembly and dunk it into varnish, especially if it is not an insulating material designed specifically for the purpose. The primary reason that you should NOT do this is that the varnish layer produced by dipping acts as a thermal insulator that would inhibit heat transfer between the copper and iron and also to the air inside the motor which provides fluid cooling. The end result would be that the motor would run hotter. If you have a motor that looks like the innards have been dipped in something, then the manufacturer has taken this into account in the design of the motor.


I should mention that not all enameled wire uses the same insulation class. Different materials and thicknesses are used depending upon the application, temperatures, and operating voltage. One of the most stressful environments for a motor is when used with an inverter because the high frequency chopping of the supply voltage causes very high voltage spikes on the motor windings. Insulation that would be fine for ordinary use would begin to slowly fail as arc-through occurs between adjacent wires in the windings. After a while these microscopic fissures in the insulation become large craters with no insulation at all and slowly the wiring begins to short out between turns and at the same time run hotter and hotter which accelerates the process.

This may be TMI. My wife says when asked for the time, I tell how to build an atomic clock. OK, so the bottom line is: DON'T DO IT.

Bill

 

[Mike Cunningham]

Thanks

Thanks for the info Bill.

 

[Brian Hahn]

This may be TMI. My wife says when asked for the time, I tell how to build an atomic clock. OK, so the bottom line is: DON'T DO IT.

Maybe, but I appreciate the additional information that you usually provide.

 

[underdog]

I'm sure I can't go into the level of detail that you can in such areas, but I can certainly relate to what your wife says about you. My wife would probably say the same thing about me... 😱

Even so, (or perhaps, because of the above) I still appreciate the level of detail you bring....

 

[pencheff]

This may be TMI. My wife says when asked for the time, I tell how to build an atomic clock.

I love the various level of expertise I continue to find in this forum.

I was going to post this as a separate thread but I think it will fit here. What makes an electric motor reversible? What sets the RPM on a motor?

I have an old high speed grinder that I would love to turn into an MDF-wheeled- honing station, but to do that I'd need to reverse the motor direction and drop the speed considerably. Possible or impossible? If I can't mod the direction and speed this thing will make a nice boat anchor.

 

[Clem Wixted]

Greg,

One way to reverse the direction of a grinder, is to remove the base that the motor sits in, turn it around and put it back on. You might want to remove the wheel guards but at least you have reversed the direction.

I think the speed is built into the design but I have seen some (few) motors that can have the speed changed by changing the windings connections. I doubt that you have this on the old grinder but you could take the cover plate off where the connections are made and you might find a wiring diagram for this.

Clem

 

[Bill Boehme]

. . . . . . . What makes an electric motor reversible? What sets the RPM on a motor? . . . .

Greg,

I think that Clem gave the best and easiest answer to reversing a grinder motor. Sometimes, the mechanical approach is hard to beat. Solving the problem electrically may not always be easy, especially in the case of a grinder that normally does not have a start capacitor. Single-phase induction motors have two sets of windings (run and start) to create a pseudo poly-phase power input to the motor. This is necessary in order to get the motor turning and for the motor to know which direction to turn. Once the motor is turning, the second phase is not essential in keeping the motor rotating. In motors with a start capacitor, the capacitor and a "start" winding are in the circuit during start-up acceleration until a centrifugal switch kicks it out once the armature is near the running speed. In motors that do not have a start capacitor, such as some low end bench grinders, there may or may not be a centrifugal switch to kick out the start winding. You ought to be able to hear the centrifugal switch as the motor is coasting down to a stop if the motor has one.

Electrically reversing the motor requires swapping the phase of one winding with respect to the other. This means physically taking the two start winding leads and swapping one with the other while leaving the run winding leads as they are. You could also swap the run winding leads and leave the start winding leads alone, but just don't change both or else you are back to where you started. Sounds easy, but getting to the leads to separate them is not always so simple, especially if there is no start capacitor and practically impossible if there is no centrifugal switch.

Now for motor speed -- it is a function of the basic motor design -- it depends on the number of magnetic poles that the windings are configured to have. An ordinary bar magnet has two poles (north and south) -- the same goes for a two pole motor. That means that the armature that is in a two pole motor will turn half way around during one half of an AC cycle (1/120 th of a second) and another half turn during the next half cycle. Your AC power produces 3600 cycles per minute so the motor RPM should be close to that value. In actual practice, the motor turns a bit slower and under full load is more likely to be around 3500 RPM. The next most popular motor type has four poles. During each half cycle, the motor rotates 1/4 th of a turn. This means that the motor speed will typically be around 1750 RPM.

The more poles, the slower that the motor turns and also the smoother it runs. The bad news is that the cost of a motor goes up like the national debt for any more than four poles because of the complexity in winding them. It is increasingly difficult to have a motor with high horsepower output and more than four poles. Just as a reminder, motor power output (horsepower or kilowatts) is determined by the product of motor speed and torque output. And, motor current is directly a function of load torque. I mention this because the obvious implication from using a multipole motor is that the torque output must increase to maintain the same horsepower as a faster running motor. More torque means more current, which means fatter wire, but if there are more poles, there are more windings in the motor, but at some point there is no more room for more wire and that is why multipole motors cost more.

Bill

 

[Clem Wixted]

Great explanation, Bill

Bill,

Thanks for that excellent explanation of motor theory. It is the clearest, most succinct description I have ever heard.

Clem

 

[pencheff]

Motor theory

Like Clem said, that's exactly the "too much information" I was hoping Bill would provide 😀

The bench grinder in question is an asian import and the shafts have too much runout to use with regular grinding wheels. It literally shakes my entire 200+ pound bench. So the plan is to reverse the motor on the base to reverse the rotation, whip up a couple MDF honing wheels and just try the thing out. If it's too fast to use it goes to the recycler.

Thanks all for your assistance!

 

[Clem Wixted]

Another possibility.

... So the plan is to reverse the motor on the base to reverse the rotation, whip up a couple MDF honing wheels and just try the thing out. If it's too fast to use it goes to the recycler.

Thanks all for your assistance!

If it's too fast for honing, or has too much runout, maybe you could use it for buffing.

 

[Bill Boehme]

Just a thought

Just a thought if you want variable speed -- occasionally, you can pick up a variable frequency drive (VFD) for a fractional horsepower single-phase induction motor on eBay really cheap. As a general rule, VFD's are designed to operate with three-phase motors, but there are some that do operate with single-phase motors. When used with single-phase motors, the starting torque is extremely low. The applications where they are used would be for fans and blowers and sometimes for small low-flow pumps with essentially zero starting torque requirements (typically an ODP induction motor driving a squirrel cage blower in a HVAC system). For obvious reasons, the single-phase induction motor must not be of the type that has a starting capacitor nor a centrifugal switch (they may not get up to the necessary speed to kick out the centrifugal switch and that might lead to the start winding and/or start capacitor "letting the smoke out"). Some grinder motors may meet this requirement. One other thing to note about single-phase induction motors used in this way is that they have poor speed regulation; that is, the greater the load that you put on them, the slower they run. Also, the VFD must be of the type specifically for use with a single-phase motor.

Bill

 

[pencheff]

Just a thought if you want variable speed ..

What about a "Variac" or whatever those voltage controller things are called? I have one sitting on the shelf rated about 5 or 6 amps. Or is simply lowering the voltage to the motor a no-no?

 

[Bill Boehme]

You could use a Variac only if using a DC or universal motor in which case the speed is a function of the applied voltage. However in the case of an AC induction motor, the speed is a function of the frequency (thus the reason for variable frequency drives). Lowering the voltage on an AC induction motor would not make it slow down much, but it sure would make it overheat a lot.

I would not even recommend a Variac for controlling a DC or universal motor because of the power wasted in heating the Variac not to mention poor speed regulation. Since the advent of semiconductors, the Variac has become obsolete for most purposes. SCR's and similar devices are very efficient and effective in controlling the speed of DC and universal motors.

Bill

 

[pencheff]

Since the advent of semiconductors, the Variac has become obsolete for most purposes.

I do seem to have a knack for collecting boat anchors, don't I? 🙄

Thanks again for all the useful information. I very much appreciate it.