Rewiring the Lathe

If you recall, my Delta/Rockwell 46-499 lathe had its original motor replaced by a 1.5 hp Dayton motor.  While this is an upgrade over the original motor it is also problematic.  Unfortunately, half the time I turn the lathe on it throws the breaker.  Then I have to reset the breaker, cross my fingers and try again.  Why is this happening you ask?  Let’s find out.

Here’s the information of the motor plate…well sticker these days.   The sticker lists the horsepower, rpm, and some electrical information that is of interest to us.  This motor can operate on either 115 or 230 volts and will pull 20.4 or 10.2 amperes of current at the listed horsepower respectively.  The label also states that the motor has a Service Factor (SF) of 1.15 or 115% meaning that for brief periods it can put out more power and it will pull the higher amperage listed by SFA. The 115V circuit I run the lathe off of has a 20A breaker.  “Ah ha!” you say “20.4 is greater than 20 and that’s why it is tripping the breaker.”  Not quite.  If the motor is not heavily loaded it will only be generating a fraction of the listed horsepower. and therefore drawing less current.  So, it is unlikely in my application that I’d ever need 1.5HP at startup requiring less than 20.4A.

When motors start up they pull much higher amperage than is listed on the sticker.  This is called the Inrush Current.  Depending on conditions and motor the inrush current can be any where from 2-4 times ( or even higher than)  the listed current.  It does it just for just a few cycles of the waveform but it can be enough to throw the breaker.  Load on the motor, such as a heavy piece of wood on the lathe or the lathe set to a high speed, can increase the required current.

As an aside…most home 115V circuits have 15A or 20A breakers.  My 1.5hp motor routinely blew my 20A breaker.  So, if you see 115V tools or appliances saying they have more than 1.5 hp it is marketing BS.  Between Inrush Current and normal operating current is a period of higher motor current that lasts until the motor gets up to about 85%-90% of its operating speed called Starting Current.  It’s smaller than Inrush Current but lasts a little longer.  So what scummy marketing folks will do is use the Starting Current to calculate HP.  So, yes the motor might make this high hp but it is completely unusable and lasts only for a split second.  What for phrases like “Max developed hp” to tip you off.  Cheap tablesaws and vacuums used to be horrible about doing this.  When in doubt, if you want to compare motors look at the Amps.  For the same voltage, higher amps means higher power.

To solve this problem I have a couple options available to me.  I could replace the motor with a smaller one which Tim “The Toolman” Taylor would never approve of or convert my motor to run on 230V.  I’m lucky that I have 230V available in my garage or else I’d be forced to disappoint “The Toolman.”  “How is this done” you ask?  The sticker on the motor tells us.  If you look on the right side there are two diagrams.  One is labeled “Lo Volts” and the other “Hi Volts.”   This is a wiring diagram that tells you how to wire the motor to change the operating voltage.  Not all motors have the ability to operate on multiple voltages so if you don’t see one don’t be shocked.  The “Lo Volts” is for 115V and “Hi Volts” is for 230V.  The little ovals with a letter and number in them correspond to labeling on the wires in the motor.  As you can see, for 115V P1 is connected to one of the line wires and T2, T5, and T4 are all connected to the other line wire. P2, T8, and T3 are all tied together but do not connect to anything else.  Line is the electrical word for an AC Hot (current carrying)  or Neutral wire. Here’s what the wires look like when wired for 115V or “Lo Volts”.

To change the voltage T5 and T4 need to be connected to Line.  T8, T3, and T2 need to be tied together and P2 needs to be taped off to keep it from shorting against the case.  P1 remains connected to the other line wire.  With 115V one line wire has current while the other is neutral meaning it is there to provide a return path if needed.  On a 230V system both wires supply 115V that is 180 degrees out of phase.  Together they generate 230V.  A benefit of running 230V over 115V is that the current is halved due to Ohms law.  It says Voltage = Current x Resistance.  Since voltage is doubled current is halved.  Lower motor currents make for happier motors.

EDIT: As Nathan pointed out in the comments section, my use of Ohm’s Law ( and math) was faulty.  If it held, then the current would double instead of halving which is not what is happening here.  What actually occurs is that in a two voltage motor there are two separate windings.  When the motor is wired for 115V the two windings are wired in parallel and both get 115V.  The two windings are wired in series when the motor is configured for 230V resulting in both windings still receiving 115V.  By changing the way the windings are connected the impedance of the motor is either double (for 230V operation) or half (for 115V operation) that of a single winding.   As Nathan also points out,  Ohm’s Law can be written to include Power and the form that is most useful to us is Power = Voltage * Current.  For our motor, power is constant and this equation correctly shows that doubling the voltage halves the current.

Here are the wires connected for 230V.  Taping the wire nuts is good practice to keep them from loosening.  They were taped originally for 115V but I removed the tape before taking the picture.  I advise you to triple or quadruple check the wiring to make sure it is correct.  Nothing good will happen if it is incorrect.

You can’t plug a regular plug into a 230V receptacle for safety reasons and the code prohibits it because of this.  There are a variety of different 230V receptacles and plugs.  In my garage I have a NEMA L14-30 receptacle which requires the matching plug pictured below.  The top and bottom blades of the plug are hot, ground is on the right, and neutral is on the left.

The inside of the plug is wired as shown in the picture below.  The black and white wires are both hot and the green wire is ground.  I have no need for a neutral wire on my lathe so it is left unused.  Code says white wires should never be hot but since this is a for my lathe and I already had this wire I decided to use it.  The wire I used is 14/2 meaning it is 14guage and has two wires plus a ground.  14 gauge can carry up to 15 amps at 115V per wire so it fits my application.

The last connection I needed to make was to hook the wire running from the plug up to the switch.  It is a double pole single throw (DPST) switch meaning the one switch connects or disconnects two circuits simultaneously.  This was easily done by removing the wires connected to the old 115V plug and attaching the wires from the new plug.

So, there we go.  The lathe is now ready to operate on 230V which is does quite nicely.

As another aside you may see 110V, 115V, or 120V used when talking about the voltage of normal residential receptacles.  They all mean the same thing.  In some places the voltage is a little higher than others.  You’ll never notice the difference.  This carries over to 220V, 230V, or 240V receptacles too.  At my house I have a little over 120V and 240V but chose to stick with 115V and 230V in my post to match the motor.  sticker.

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2 Responses to Rewiring the Lathe

  1. Nathan Alday says:

    Awesome article as usual, though one thing is potentially confusing:

    “A benefit of running 230V over 115V is that the current is halved due to Ohms law. It says Voltage = Current x Resistance.”

    Ohm’s law does hold that V=IR, but that makes voltage and current proportional to each other in Ohmic circuits, not inversely proportional as you imply. The reason the current draw for the motor is halved at 230 V is due to the fact that Power = Voltage * Current. The power draw (horsepower, as you know) remains constant (for steady state operation) and so doubling the voltage halves the current draw.

    The same power load at a lower current actually implies that the resistance (or in this case, the impedance since it’s an AC electric motor) is increased.


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