Brook motor problem.

Hi all, I am having problems with this motor, it's a 240v 3/4hp 1430 RPM motor removed from a myford. The problem is when switched on it gets up to speed and I get a yellow flash from the region of the centrifugal switch and the circuit breaker trips. I have connected the Live to terminals AZ and T with a link wire between the two. Neutral is connected to terminals Z and A with a link wire between the two terminals, is this right? The motor has been stood around for sometime so I cleaned out any swarf and debris from inside the case and inspected the wiring for chafing and damage none found.I have been assured by my boss that the motor was good when removed. I have disconnected the cable for clarity.

Mark P.

Edited By Mark P. on 15/08/2014 13:38:43

For a split phase motor, without capacitors, the Red and Black wires are the run winding. The yellow and blue wires are the starting winding and the centrifugal switch is connected in series with it. You can connect these windings in parallel either way, but you may need to reverse the connections to one winding if the motor runs in the wrong direction.

The additional red wire and the striped wire probably go to the centrifugal switch in which case your wiring is correct.

If it's the main Residual Current Device (RCD) serving all circuits which trips, then some kind of insulation breakdown from winding or switch to the frame is indicated. If on the other hand it's a Miniature Circuit Breaker (MCB) serving say a ring main which trips then an overload in indicated.

When the contacts of the switch break the back emf will result in a high voltage between them. This could be jumping across to the frame tripping the RCD.

You could try disconnecting the start winding and hand starting the motor by spinning the shaft. If the motor runs OK this confirms a problem with the start winding or switch.

Edited By MAX THE MILLER on 15/08/2014 14:20:19

Hello Mark,

Checking my own capacity start motor, Live goes to terminal A and Neutral to terminal AZ

There is probably a link between A and Z to couple in the start winding.

To reverse the motor swap the yellow and blue leads.

If it buzzes and refuses to start when coupled as I describe, provide a link between A and Z

What I think you have done is wire live and neutral across the cenrifugal switch, and providing a dead short when the switch is activated at speed.

I can't see terminal T in your picture and grovelling in the swarf behind my lathe I can't see T printed on my terminal board either.

Regards Brian.

Edited By Brian Wood on 15/08/2014 14:55:30

Edited By Brian Wood on 15/08/2014 14:58:55

This 'instructable' looks very good:

**LINK**

Neil

[QUOTE=Brian Wood]What I think you have done is wire live and neutral across the cenrifugal switch, and providing a dead short when the switch is activated at speed[/quote]

That can't be the case. The centrifugal switch is a BREAK switch designed to disconnect the start winding when the motor get up to speed leaving just the run winding connected. It's not a MAKE switch.

Sorry. Can't work how  to do a partial quote. This forum is different to every other one I use.

Edited By MAX THE MILLER on 15/08/2014 17:12:32

Edited By MAX THE MILLER on 15/08/2014 17:13:31

Hi Mark,
I agree with Max The Miller. If live and neutral were connected across the centrifugal switch the MCB would trip as soon as power was applied as there would be a dead short across the mains. Mark can you confirm what trips out ? Is it an MCB or the RCD ? My feeling is that the centrifugal switch is not breaking cleanly and flashing over to earth. If so there will probably black burn marks on the insulation near the contacts. My first step would be to CONFIRM that the wring is as we think. First remove your mains connections and links. Do the following resistance checks. You should get a reading of a few ohms between A and AZ. You should get a reading of a few ohms between Z and S. You should get almost zero resistance between S an T (This is the centrifugal switch if our assumptions are correct.) From A to Z you should get an infinite read. (Or at least 10 Meg ohms) Also check that there is an infinite (Or very high reading) from the motor frame and all of the terminals. (Preferably use a megger for this test.) If these tests give the expected results I would try starting the motor with an external switch connected in place of the centrifugal switch. Have this switch closed (In the on position) Apply the power and switch this switch off about a second after power is applied. (This simulates the centrifugal switch opening.)

Les.

Edited By Les Jones 1 on 15/08/2014 17:58:38

Hello Mark and Max,

Yes Max, you are of course right. I was swayed by Mark's description which suggested the short takes place when the motor is up to speed.

Mark.

I would remove the two link wires you put in and leave the supply coupled to AZ and A. It doesn't really matter which is to live being AC. As in my last response, motor reversal is done by swapping the yellow and blue leads.

Better luck this time Brian

Hit the quote button and delete the parts fo the message you don't need.

Neil

Hi Les, the RCD trips when the motor gets up to speed (and I assume that the capacitor is taken out of the circuit). I will do a resistance check tomorrow and see whats what. Also will wire it up without any links fitted if it checks out ok. If it is goosed I've not lost anything as it was free. By the way Les my dummy CR2032 batteries worked, I got a regulated 3.3V PSU from flebay and my DRO's are all systems go.

Mark P.

Hi Mark,
I have found that sometimes an RCD can trip due to a current of voltage spike when an inductive load is switched on or off. I have an old bench grinder that tripe the RCD every few weeks and my pillar drill used to do the same until I fitted a proper contactor type on/off switch. I do not fully understand the cause. I would expect a spark from the contacts on the centrifugal switch when it open but your description sounds like it is more than that.
I will wait for the results of tomorrow's tests.

Les.

Edited By MAX THE MILLER on 15/08/2014 22:31:16

I've tried quoting as suggested by Neil, but any text I type just becomes part of the quote.

--------------------------------

Mark P.

Does this motor actually have a capacitor. If so it will be external to the motor and very obvious.

Edited By MAX THE MILLER on 15/08/2014 22:35:10

Edited By MAX THE MILLER on 15/08/2014 22:36:45

To Chris Trice:

1. Depending on age of saw, call Makita and ask about a warranty repair?

2. Throw it out and buy a new one?

Makita tools are generally excellent equipment in my opinion, but after several years good service, if they fail, they are not valuable antique/vintage tools to be saved and restored, they are mass produced throwaways.

JD

According to my copy of 'Model Engineers Handbook' the red wire and the yellow wire go to live, the blue wire and the black wire go to neutral. To reverse direction then reverse the yellow and blue only. Of course earth the motor frame. The yellow and blue are the start winding in series with the centrifugal start switch.

I have had trouble like this. The problem started as an occasional trip and got worse and worse. If the flashover (to frame) has persisted for some time the mica insulation will track and it may be impossible to clean it up to remove embedded metal particles. Worth a try though. The alternative is the skip.

What you say is normally correct Roger, but this motor seems to have the centrifugal switch wired between the S and Z tags. Simply paralleling the two windings would leave the start winding permanently in circuit.

Hi Mark and others,
First Mark. From Roger's comments I think I guessed wrongly which connections were the centrifugal switch and which was the start winding. So in the tests I suggested I think you should get a reading of a few ohms between T and S and almost zero between S and Z (The opposite way round to my last post.)

Next Chris. If the cut off saw is a slow speed one with an HSS or carbide tipped blade then it will probably be an induction motor. If so a spark suppressor consisting of about a 100 ohm resistor in series with about a 100 nf capacitor (Of suitable rating for 250 volts AC) connected across the centrifugal switch contacts might help. You can get this type of suppressor with the resistor and capacitor encapsulated as a package. Another possibility is a transient suppressor (Sometimes called a metrosil I think.) connected across the start winding. (These are the devices use in surge suppressor sockets.) If the cutoff saw is the abrasive wheel type it will probably be a series (Also known as a universal) motor. If this is the case then the resistor/capacitor spark suppressor connected across the start switch might help. Also look for a build up of carbon dust around the brushes which might cause leakage to earth.

Finally Roger. Your tracking to earth theory is also my favourite one. As a test I would be tempted to see if the fault still occurred with the earth removed and the motor insulated from earth. !!!! I WOULD NOT SUGGEST THAT YOU SHOULD TRY THIS MARK !!!!!!

Les.

It's easy!

Mouse to just under or over the quote and a red line with a box with a cracked arrow in it should appear. Click the arrow and it will insert a return under (or before) the quote, giving you a plain line to type on.

Neil

Hello Les, I've had a look at the motor and this is what I find.

1. Between terminals S & T 15 ohms.

2. Between terminals A & AZ 4 ohms.

3. Between terminals A & AZ infinite.

4. Between terminals Z & S initial high reading, increasing for 2 minutes then infinite.

5. Between all terminals and casing infinite.

The capacitor is on terminals S & Z.

Terminal T goes to the centrifugal switch via a yellow wire.

Terminal AZ goes to the windings via a red wire.

Terminal S goes to the windings via a blue wire.

Terminal A goes to the windings via a black wire.

The other side of the switch goes to the windings via a yellow wire.

I assume that I will need a link from Live to terminal T to feed into the start winding,and a link to neutral to terminal S.

Mark P.