Lathe motor replacement?

C

clowdis

Member
A question for those of you that have replaced the motor on your Chinese lathe. Did you replace it because it was kicking out the breaker or was there another reason. Mine keeps kicking out the breaker but I'm not sure it's a bad breaker or a bad motor. It doesn't seem to be drawing too many amps on start up, about 11 amps, so I thought I'd ask around before I did anything.
Thanks for any replys.
 
If you know, for a fact, that the motor is drawing less than the rating of the breaker, it would seem that either there is additional load on that circuit that you have not accounted for, or the breaker needs replacing.
 
starting peak draw vs running draw.
some breakers are slow trip to cover this.
having said that when i looked for one for my air compressor...no luck....

any working electricians out there.....
help
mike in co
 
I believe the breaker is 13 amps, so it's close but shouldn't be much of a problem. At least I don't think it should.
 
First, I am an Inside Wireman, and have been doing it for 16 years, for whatever that's worth. It's likely the breaker. A breaker is designed to hold at it's rated amps for a finite number of trips. At a class I took at SquareD on arc fault breakers last year, they told us that 11 is the magic number of nuisance trips you can still expect the breaker to continue to operate reliably. If your breaker has nuisance tripped a number of times, it's probably time to replace the breaker. It's likely a 15A breaker, as 13A is not an industry standard. And it's way cheaper to change the breaker and see if that works before dropping money on a new motor.
 
One thing that you probably already know, never change to a higher amp rated breaker, if you don't have wire of the proper size to match. A long time ago, an older friend of mine, had grown tired of a breaker tripping, so he bought one with a higher rating. Luckily I found out before he had made the change. He was a successful lawyer, but no electrician.
 
I am also faced with a problem with mine. The centrifigal switch (from start to run) on the shaft of my motor is cracked and continues to loosen at the most inoportune time. Anyone know where I can get a new one?

Jim
 
Clowdis

For motor loads the motor name plate will have the FLA or Full Load Amps listed for your voltage, either 120 / 208 / 240 / 480. This is the amperage we have to go by when configuring / calculating motor circuit protection. If no name plate is visible you can multiply 785w x total HP divided by Voltage (120 or 240) for your amperage. An additional 125% is added to the FLA for continuous duty (3 hours or more)

If your motor is drawing 11 amps and it's on a 13 amp breaker, the breaker is too small. As stated above, 13 amps is not an industry standard size breaker. Breakers can only be loaded to 80% for continuous load / duty or they'll start to trip out. If on a 15 amp breaker and no other loads are on that circuit, the breaker is most likly the culprit.

15 amp breakers can only be loaded to 12 amps continuous
20 amp breakers / 16 amps continuous

Hope this helps.
 
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Blair:

I replaced my Grizzly Precision Tool room lathe single phase motor with a NEMA Metric 3 phase motor. I found the same foot print and shaft size at Electric Motors in Wake Forest, NC . The Grizzly motor was 3 hp the replacement motor was 5 hp. I am using a VFD to create my 3 phase. It works great and I have never looked back.
Nat Lambeth
 
Nat,
I'm hooked to straight 240V 3 phase in my shop. I think what I'll do is try to find a 15 amp circuit breaker per Roscoe's recommendation and see if that cures to problem. Seems like the proper place to start anyway :)
 
Something else you might want to check for. I have run into this problem many times that voltages for motor and building do not match.

What is the electrical input to the building 208v or 240v 3phase (get a meter and check) , makes a big difference what is the voltage for the motor 208v or 240v they both need to be in sink. I have seen many mistakes when when hooking up motors to wrong voltage. They draw lots more current and eventually burn up motors and/or contactor switches. Many people "assume" 240v. If you have 3 transformers on pole you mostly likely have "wye" service and it is 208v, If you have 2 transformers on pole it is "delta" and is 240v. Again, use a meter and check.

Most 208v run up to 218v, however when the load picks up on the main line it, I have seen 208 drop to 195v. 208 motors can run +/- !0%, past that they don like it. Same goes for 240v motors +/_ 10%. I have seen this drop many times at my shop.

Some motors have a wiring diagram how to reconfigure internal hook up for 208v or 240v

Hope this helps.
 
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