Thread: Another VFD & 3 phase question

> Also, I posed a question before. Does anyone know if you can run a 220V DC permanent magnet motor on a 180V DC speed controller (assuming current rating is sufficient)??

Simon, you sure COULD run a 220VDC motor on a 180VDC controller. But.... is this "180V" controller designed for 120 or 240V AC input???

And, is the motor you have a permanent magnet motor, or is it shunt wound? DC motors usually come with armature voltages of 90 or 180V, and field voltages of 100 or 200V. Depends if your line voltage is 110 or 220V AC.

Most DC speed controllers can drive both PM and shunt wound motors. There are 120V controllers designed to hook into 100 or 110 or 120VAC home power supply, like in North America and Japan. Then there are 240V controllers designed to be used in countries tht have 200 or 220 or 240V AC home supplies, like Australia and Europe. And then there are controllers that can be switched to either 120 OR 240V input voltage, these are less common and cost more. And of course, you can use a step-down transformer to run a 120V controller on 240V.

The KBMM controller mentioned earlier can be bought for 120, for 220 and switchable by jumper for both. It is, like many DC controllers, a very old design. It is listed in my old 2001 Baldor catalog for 206 AU$ (240V version). Today you can buy it for much less than half that much on ebay. Remember it is a simple SCR controller, can only run forward. And if I remember well, it needs a plug-in resistor to match your motor. It has provision for a tacho though. A tach will keep the rpm constant as you load the motor. And it can drive a shunt wound or a PM motor. It has IR compensation too, something that can be set to prevent rpm drop when loaded without a tacho (within limits of course). You can download the manual from the Baldor site to get the idea. Also, the KBMM is just an open circuit board, you have to mount it inside a box. But it is aslo sold as KBMD, same unit but mounted inside a box (then it looks almost like a VFD). Chris

Hi Chris,

Once again thankyou for taking so much of your time in these replies. Is it a shunt wound of permanent magnet DC motor? Until recently I had never heard of a shunt wound motor! So it could be either, I'm still learning here! Is there a way of finding out like taking the armature out or something? I guess it would be too simple to look for magnets to decide if it's a PM DC motor? On the nameplate sticker under the heading field class it states "PM" does that mean permanent magnet, I don't know?

As for the controller, I don't have one for it yet, i'm still shopping/sniffing around. Quite frankly I'm still enjoying the learning curve ATM thanks to guys like you and others. I have since looked at the manual for the KBMM 240 and they state that it will happily run a motor of a higher voltage albeit at reduced RPM as Martin explained earlier. They will run on either 120 or 240V input to give 0 - 180 out. I do understand that they are a bare bones circuit board and will need an enclosure, that's OK and would be part of the fun to create. No reverse would be an issue though, I assumed they did both directions. That could possibly be a deal breaker for me, especially on the lathe.


Cheers,

Simon

Hi Simon,
As I understand it, shunt wound motors dont reverse if you change the polarity. Though I'm not sure that helps you yet?

Stuart

> Is it a shunt wound of permanent magnet DC motor?

If the stator is made out of magnets, then it is a PM motor. If the stator is a coil, very much alike a single phase motor stator, then it is shunt wound. Or if it has two pair of wires coming out, it is a shunt wound motor - but the reverse in not necessariloy true, if it only has two wires it could as well be a "universal motor" where the armature and the shunt winding are in series (universal motors can run either on DC or on AC, vacuum cleaners are a good example of universal motors).

> No reverse would be an issue though, I assumed they did both directions. That could possibly be a deal breaker for me, especially on the lathe.

You can always buy a reverser switch for between controller and motor. But you need to put a lid over the rocker lever, a lid that needs positive unlocking, to remind you that you kill the controller if you reverse before the motor is at standstill. The switch must be able to take the peak motor current, and be insulated for mains voltage. Chris

Hi Chris,
So you can reverse a shut wound motor without changing the brush timing? Or are there two sorts of shunt wound motor?

Stuart

OK, I'm learning more stuff here. The motor has 2 wires in only. I will have a closer look to see if it has a magnet stater. I assume if a screwdriver is attracted to the casing then it's a PM motor?

As for switching for reverse, if I were to convert say my lathe then I would definitely want to keep the existing lever for/rev control as I cannot imagine doing it any other way for such tasks as thread cutting and so I would use a contactor or relay switch arrangement for for/rev. However, how this would be wired so as not to switch the motor direction while still in motion, that would need to be looked at.

I would possibly be guided by how others have done that.

Cheers,

Simon

OK. I just ducked out to the shed to have another look at DC motor. it definitely has magnets inside it. Two to be exact. I'm enjoying the tech talk about these motors eventhough I'm probably getting ahead of myself WRT the use of the DC motor in the lathe. One machine at a time! Still I enjoy theoretical discussions on what can be done or what the best options are even if I'm nowhere near ready. I just had my offer declined on VFD for $129 delivered. Next supplier!

Simon

Originally Posted by Stustoys

Hi Chris,
So you can reverse a shut wound motor without changing the brush timing? Or are there two sorts of shunt wound motor?

Stuart

To reverse rotation you can EITHER reverse the armature polarity, OR the Field polarity. If you were to reverse both, the motor would not change direction.


The brush timing is another thing altogether. Motors that are designed to run equally well in either direction, have a symmetric timing. Motors that are designed to run mostly in a particular direction, have an asymmetric timing. Think of a petrol engine, where the ignition has an advance, the motor is also only good to run in one direction. And if the ignition is timed to fire symmetric at the top of the piston stroke, the motor may run both ways, but not very well. In the DC electric motor the timing is set at the factory. But is often very easy to change, by rotating the motor endplate that carries the brushes. This is why, when you take a DC motor apart, you should always mark with a punch line how the flange that carries the brushes was originally oriented. Some motors have a little notch or pin or similar to ease assembly and fix the timing, if you want to change timing you have to remove this index first with a file or similar. If the timing is offset a few degrees say in ccw direction, the motor will run smoother and have more power and be more efficient in cw direction. But it will run rough with less power and be less efficient and run with a sparking commutator in ccw direction. You cannot have it all.

Beware of taking apart modern high performance PM motors just to have a look, or just to clean. Without the proper jigs to keep the rotor centered, the rotor will be forcefully attracted to one magnet, and as you slide it out, you de-magnetise one magnet. The result can be a motor that after assembly has lost 1/3 of its power and is now essentially junk. The effect is worst with rare earth magnet motors such as DC servo motors which can have enormeous power at very small size and maybe a duty cycle of only 20%. In such motors, sometimes the magnets are only inserted and glued in place AFTER the rotor is assembled - it is not possible to take apart such motors without ruining them, they are not intended to be repaired. If you buy such a motor second hand, make sure the previous owner did not play with it - if an old motor looks shiny and polished like new, be wary it could have been taken apart for cleaning without proper tools and is now junk. Motors that look heavy and large for the rated power are less likely to have such powerful rare earth magnets.

Another thing to watch out for is, if you constantly overload a PM motor the magnets will over time loose some of the magnetism and the motor looses power. Chris

Hi Chris

Originally Posted by cba_melbourne

To reverse rotation you can EITHER reverse the armature polarity, OR the Field polarity. If you were to reverse both, the motor would not change direction.

Ok I think I'm getting it now. You cant reverse a shunt motor that only has two wires. You need one that has four(other than playing around inside)?

Originally Posted by cba_melbourne

The brush timing is another thing altogether.

I'd been told if you move the timing far enough it would reverse(but I've never tired it and have no idea how much sparking might go on while you were doing it)



Thank you

Stuart

> Ok I think I'm getting it now. You cant reverse a shunt motor that only has two wires. You need one that has four(other than playing around inside)?

yes correct. But it is very easy to modify a shunt wound motor to bring all four wires out. The easiest is then to simply reverse the field wires to switch direction.

> I'd been told if you move the timing far enough it would reverse(but I've never tired it and have no idea how much sparking might go on while you were doing it)

moving the timing by 180 degrees is exactly the same thing as revering the polarity of the armature (if it is a motor with only two brushes, there are many motors with 4 brushes and there some thinking may be necessary.....)

Chris

You certainly know your motors Chris! Also like I said earlier, I had not heard of Universal motors but it makes sense now. I have several of these without even knowing. A vacuum cleaner motor, washing machine and (don't laugh) a paper shredder motor. It had me baffled why they had more than 2 wires coming out.

As for not taking apart PM motors, thanks for the warning! I have a habit (have had all my life) of taking perfectly good stuff apart just to have a little look!

As for using PM DC motor on a lathe, what is the consensus with people with the for/rev control? Do they keep the lever on the apron or bye-pass with a switch on the headstock? The for/rev lever seems a little analogous to living on the seat of your pants if what you say is correct. Flicking it into rev while the motor still has forward motion can fry the DC speed controller. Is this because of the back emf of the motor would be directed into areas not designed to go?

Cheers,

Simon

> As for using PM DC motor on a lathe, what is the consensus with people with the for/rev control? Do they keep the lever on the apron or bye-pass with a switch on the headstock?
> The for/rev lever seems a little analogous to living on the seat of your pants if what you say is correct. Flicking it into rev while the motor still has forward motion can fry the DC speed controller.

I believe that most people are using free (or cheap surplus) treadmill components, and would simply accept the risk. Only those that already had a controller go up in smoke are likely to either make some sort of safety cover, or buy a very expensive reversible controller..... or much better simply upgrade to a VFD

> Is this because of the back emf of the motor would be directed into areas not designed to go?

There is no difference between a DC motor and a generator. When a DC motor coasts to a halt, it produces electricity. Controllers with a brake function connect the motor to a braking resistor, which transforms this electric power into heat, therby braking the motor. If you reverse the motor whilst it is still coasting.... think of jumpstarting a car and accidentally cross the jumpstart leads.

Morning Simon,
In Ueee's thread on Mars lathe D.C. motor conversion there is a description of my motor wiring setup, this more than adequately protects the motor from rapid direction change switching.
Regards,
Martin

Originally Posted by toolman49

Morning Simon,
In Ueee's thread on Mars lathe D.C. motor conversion there is a description of my motor wiring setup, this more than adequately protects the motor from rapid direction change switching.
Regards,
Martin

Cheers Martin!

I had been looking at that thread but I must have missed it.

Simon