Emco Compact 5 Motor Problem

I have a problem with the motor for my Emco Compact 5 CNC lathe. This machine has a 180V DC brushed motor of some 440W. The fuse on the drive board blew some months ago: replacement fuses also blew immediately power was applied. I took the motor to a repairer who said that they had fixed a small problem with it.

But it still blows fuses. I installed a new board from KB. With the motor on the bench it ran fine but, as soon as I installed it in the lathe, the fuse blew. I suspect that the problem is down to a short circuit in the motor when it is running (it measures open circuit between the wiring and casing when stationary).

What to do? I cannot find an equivalent DC motor in my web searches and Emco do not appear to support this machine any more (even if they did, I would need to mortgage the dog to be able to afford it). So, do I change to a 3-phase motor & VFD? Find a brushless DC motor of suitable power/speed/size? Convert to treadle power? What does the collective wisdom of the forum suggest?

Ta

Dave

Can you mount the motor on electrical isolators to see if that stops the fuse from blowing? That would confirm if there is a short to earth from the motor. If you put a low wattage incandescent lamp in series with the motor to restrict the current you will also find out if the fuse is blowing due to an internal short in the motor.

Martin

Just a thought, are you using slow blow fuses. The motor inrush current could easily blow a quick blow fuse even if there is nothing wrong

Martin, with the motor on the bench (i.e. isolated) it will run OK, which is why I suspect a short in the motor. Can you explain the incandescent lamp in more detail? I do not see how this would show a short in the motor.

Duncan, slow blow fuses used with the KB board (the standard fuse in a 13A UK plug suppling power to the driver board).

D

Hi Dave,
I could not find a schematic for your lathe on the web. The only information I found was a picture of the lathe that shows a switch that looks like a reversing switch. This will be located between the speed control board and the motor. I suggest you check the wiring between the speed controll board and the motor. Look for shorts to earth and check that the reversing switch works correctly with no shorts. If you have a schematic then posting it would be a great help. If not trace out the wiring between the speed controller and motor. If your tests sho no fault then connect a old style tungsten filiment lamp of about 60 to 100 watts in place of the motor. The speed control should change the brightness of the bulb.

Les.

Dave,

When you say you run the motor on the bench and all is well, are you saying you connect the motor direct to the kb board via the A- and A+ connections or via the existing wiring within the machine?

If the motor runs fine directly wired then that suggests a fault with the internal lathe wiring, as Les says, the wiring between the control board and motor....temporarily wiring in the test lamp in lieu of the motor will eliminate the motor.

The other thing is, does the fuse blow straight away, with speed pot at minimum?

The motor runs OK when connected directly to the A+ & A- connections. When the motor is installed, and wired through the machine, the plug fuse blows as soon as power is applied to the board (independent of pot setting). There is a clue here, as you point out. I have been concentrating on a short in the motor: there could be one in elsewhere in the wiring. I will ping some more of the wires with my meter (when I can get back in my workshop). The only differences between motor on bench (works) and installed (doesn't work) are that the motor is earthed and the drive cable is broken at a connector as it exits the cabinet when it is installed.

D

Then I suspect the fwd/reversing switch being faulty if the motor runs ok on the bench....try that test again with an earth to the motor casing...

John

Not normally a fwd/rev switch on the Emco 5 cnc

Emgee

Hmm.ok, if not the case, then we struggle without a cct or knowing what there is betwixt controller terms and motor...

John

The PM motor cct on the original controller went out from the PCB to a 3 pin socket on the back panel.

The positive went via a switch to an ammeter then through a choke to the skt, the negative direct from the board to skt. The motor was earthed by an external earth conductor between the main earth terminal and motor case.

If there is an earth fault on the motor when run on a bench all will seem OK if the earth is not connected, except it would be dangerous to touch the casing if you were not insulated from earth mass, 180v DC won't do you much good.

As soon as the motor is fitted to the machine, even without the supplementary earth bond, there will be an earth path from the casing where the motor plate is fitted, switch on and the voltage fault to earth ruptures the fuse.

A user close to me has the same problem on a F1 mill which has the same type of motor fitted.

Emgee

There is a three way switch on the Emco 5 CNC, it is not a forward/ reverse switch. It is the motor's manual mode / CNC mode /off switch.

The motor control board has three wires going to the motor. Motor +ve, motor-ve and ground. There is a large 33mH choke in the motor +ve line which controls the inrush current. The old Emco wiring, the choke and particularly the motor connector (both halves) on the back panel are all candidates for a short to earth. You should be able to check these out with a normal test meter.

I have circuit diagrams for the Emco 5 CNC lathe and also for the Emco F1 CNC mill. As well as the Motor control board. If anyone wants a copy, then send me a PM with your e-mail address.

Mike T

Mike, it is only on later model lathes where the rotary (contactor type) cnc/off/man switch is fitted, the model prior to that had a 3 position rocker with man/off/cnc function. Earlier than that was a white 2 position on/off rocker switch.

Got the parts for the conversion, thanks for the drawing.

Emgee

I am a happy boy again. Having given up on getting the DC motor to work, I spoke to Pete from Power Capacitors at the Ally Pally show (no connection, just a satisfied customer) and ordered a 3-phase motor & VFD. Bits arrived PDQ, but the motor came as a foot mount. The EMCO requires a face mount and Pete sent a replacement front cover out the next day. It was easy enough to change, but still no luck! The mounting holes would not line up...

I removed the cover again and, fortunately, four extra bosses are cast in at 45 degrees from the standard holes. They even have a dimple to guide a drill. Tap M5, put everything back together and it fits! VFD wired in and we have motion again.

DrDave

Good news you have it running, what output power is the new motor ?

Emgee

Hi Emgee

The new motor is 0.37kW (½ HP), compared with 0.44kW for the original. The big change is the motor speed, down from 4,000 rpm to 2,800 rpm (3,360 if overdriven to 60Hz). A small price to pay to get a working machine.

Thinks: I will need to make/fit a motor guard before I do any serious machining. That cooling fan looks vulnerable...

Regards

Dave

Dave,

Can you let me have the frame size of the motor please.

Joey,

Frame size is 63 and the mounting is a B14 face mount. I found that I needed to drill an extra set of mounting holes in the face mount (at 45° to the existing ones) in order to get the motor to fit. Fortunately the motor that I received (TEC) had bosses cast into the mount so this was a simple task.

Regards

Dave

The motor may well run up to 100 Hz to give you your max speed back? Lots of modern motors designed for VFD usage can be run at higher frequency I believe. On my big mill I have a very old Brooke motor that runs very happily at 75 Hz and that certainly wasn't designed for VFD!

John,

Thanks for the prod: I have just had a furkle around in TEC's online documentation. They recommend a maximum frequency of 75Hz for the motor that I have. That should give a motor speed of 4,200 rpm. Bingo!

Regards

Dave