Phase Converter Bogging Down when starting 2 speed motor in high speed

The title says it all. I have a 15" Harrison lathe with what is supposed to be a 5hp 2 speed motor. I can't get at the data plate with-out removing the motor from the lathe, so I am taking the seller at his word that it is a 5 hp motor. My Rotary Phase Converter is an older Ronk unit and is rated to start a 5hp motor and run up to 10hp in motors. The lathe starts just fine in low speed forward or low speed reverse. When I try to start the motor in high speed forward or reverse and do not have anything else running off the phase converter I get a fair amount of noise from the phase converter and it slows down - almost as if the load is too great. It will eventually spin the lathe motor and itself back up to speed, but it takes a few seconds. If I have another machine running when I start the lathe, there is less noise from the converter, but it still bogs down a little as it starts up the motor.

What is going on here? Are there any easy (read cheap) fixes?

Harrison Lathe

Andy,

I'm not familiar with the Harrison lathe but I think it is a geared lathe similar to the Colchester. I have a 13x36 Clausing Colchester 3hp /2speed motor. I can tell you from experience that starting the lathe in gear in Hi speed is the same as staring the motor with a load. I expect the rotary converter is just not able to provide the starting torque with the motor on Hi speed through the spindle gearing.

My lathe is on a autotransformer static converter. In order to start the lathe at both hi & lo speeds I have a relay that senses when the lathe is in Hi and automatically adds capacitance to the starting circuit for hi speed starting. A similar setup may be possible with a rotary but I really don't know since I've never built a rotary.

Joe

Joe H said:

Andy,

I'm not familiar with the Harrison lathe but I think it is a geared lathe similar to the Colchester. I have a 13x36 Clausing Colchester 3hp /2speed motor. I can tell you from experience that starting the lathe in gear in Hi speed is the same as staring the motor with a load. I expect the rotary converter is just not able to provide the starting torque with the motor on Hi speed through the spindle gearing.

My lathe is on a autotransformer static converter. In order to start the lathe at both hi & lo speeds I have a relay that senses when the lathe is in Hi and automatically adds capacitance to the starting circuit for hi speed starting. A similar setup my be possible with a rotary but I really don't know since I've never built a rotary.

Joe

Click to expand...

The Harrison lathe has a clutch, so the machine load on the motor is the same whether it is starting in low or high speed. I always have the motor running up to speed before engaging the clutch.

How is the relay you mention set up? How do you size the additional capacitor bank and where do you locate it?

I build RPC's and have noticed that the 2 speed motors takes a much larger converter than a single speed motor (for the high speed). I have found that 3:1 usually works OK (i.e, the converter needs to be 3X the hp of the load motor). I do not know why, but it may be related to the high speed option having about 1/2 the horse power of the low speed. One customer with a 7 1/2 hp 2-speed motor (7 1/2 hp @ 1800 rpm and 3 hp @ 3600 rpm) needed a 20 hp RPC to start satisfactorly in high speed, and two other customers with 10 hp 2 speed (10 hp and 5 hp) needed 30 hp converters.

This is very true.
Two speed motors are always tougher to start as previous commenters have stated.
I have built many RPC and static units and there is never a simple answer.
One friend even went so far as putting two seperate static convertors on his - he threw a switch for main power before turning the high speed handle on.

I have an idea, but want you to know I have not tried it.
What I have done is to add a timed relay and start cap to the high speed circut to assist the main convertor. This works.. but my idea is
To use a "Booster" relay/cap that grainger sells. This is a simple looking cylindrical unit that is used on A/C units.
I have used them for very simple ( The easiest ever seen) static phase convertors.
You can have a 1 HP start for 25 bucks and only need two wires !
So my thought was to add one to a high speed winding for grins and see what it does.
They boosters go to 5 HP as I recall

please ignore if this question is not do able. can you not start in low and while still rotating switch to high?

2 speed motors

Andy,

In addition to the gearing issue, the 2 speed motor normally found on lathes is a constant torque motor. Your 5hp 2 speed motor may actually be a 5hp 1800 rpm motor at hi speed and a 2.5 hp 900 rpm motor at low speed. The motor on my lathe is 3hp 1800rpm / 1.5 hp 900 rpm. The motors howard d mentioned are not constant torque motors. Look at the following typical comparison of the 2 speeds(all full load):

5 hp 1800rpm -- Power Factor 82% , Amps 13.4 , Torque 15.2 lb-ft

2.5 hp 900 rpm -- Power Factor 54.5% , Amps 11.6 , Torque 15.2 lb-ft

In my experience the Power Factor is a good indication of the relative ease of starting. Based on Power Factor the lo speed will be much easier to start than the Hi speed as you have found out.

My idea was along the same lines as Rich mentioned. Basically it would be a static converter circuit , start cap only, that would kick in (and kick out using a potential relay) only if the lathe was started at hi speed. The details are up to you. So far nobody has posted that they have actually set something like this up on a rotary.


Joe