Overload Switch on Mini Lathe - Question

[Jerry Hall]

My club has been using a mini-lathe with what is stated to be a 3/4 HP DC motor. I believe it is rated at 5.5 amps. It was cutting out in moderately hard turning of a 8" size dry bowl blanks of medium hard woods. The manufacture replaced the 4 amp breaker with a 6 amp one since they could not find a 5.5 amp breaker. It has worked fine since.

Question: Does a 6 amp breaker in this case pose a risk for the motor over the long term? Is it a valid solution for a motor of this size?

Thanks!

Jerry

 

[Bill Boehme]

I think that it is OK. There are times when the current momentarily exceeds the nameplate rating such as during start up acceleration. I would agree that the 4A breaker was too small. There really is not any good way that you can predict motor life based on your use.

Also, the way that motors work is that when load current exceeds full load value (which is just another way of saying that the load torque exceeds the maximum torque that the motor can produce) the motor stalls. When the motor stalls the currently rapidly jumps to the LRC (locked rotor current or zero speed current) which is typically around five or six times the FLC (full load current). Of course when that happens, the 6A overload breaker will open immediately.

In the real world, you would not want the overload breaker to be the same as the FLC or else there would still be nuisance trips.

Hope that this helps.

 

[Jerry Hall]

Thanks, Bill!

Just the thoughtful info I wanted.

 

[jschnell1203]

Was this a variable speed lathe? We have problems with all of the variable speeds lathes turning bowls and even sometimes bottle stoppers stopping and having to reset them.

 

[Jerry Hall]

Yes, variable speed

but since the upgrade to a 6 amp breaker it has kept going while hogging the outside of an 8" dry olive crotch bowl blank with interrupted cut and a dry black locust bowl. Some little belt slipping and slow down, but really quite acceptable. Runs cool.