M300 Lathe - Single phase conversion


Wednesday, May 02, 2012

Rewiring a Harrison M300 (circa 1992) from Three Phase to Single Phase.

This instruction takes a standard 3-phase Harrison M300 and with the use of a TECO T-Verter inverter from Drives Direct ( www.drivesdirect.co.uk ) convert it to work with a standard household 240Vac supply.

Before starting I verified the circuit diagram, pasted on the electrical cabinet door, matched the actual wiring as I have seen it before when modifications have been made already and can lead to problems later.

I unwired all the contactors and zero volt circuitry, leaving behind everything connected to the terminal strips. I then checked out the foot brake, mushroom and change gear enclosure micro switches and all worked fine. My intention is to wire the e-stop and change gear enclosure switches in series to actuate the 240v incoming supply to the inverter, utilising one of the now redundant contactors to isolate the supply to the inverter.

Motor Re-wiring for DELTA operation

The first thing that must be done is to convert the motor to run from the inverter. To do this is it necessary to rewire the motor from a STAR configuration (found in most 3-phase motors) to a DELTA configuration needed by the inverter. This is a simple task and apart from having to drop the motor, which means removing the belts and braking system, and then re-fitting it all afterwards. The jumper plates in the motor are moved as per the following diagram:

STAR (used for 415Vac 3-phase supply)               DELTA (Used by inverter – 240Vac 3-phase)

This is how you should find the links                        This is how they should be set


Don’t forget to do the same to the suds pump motor!

Emergency STOP circuit

Unfortunately the panel transformer fitted to this model does not have primary windings for 240v, unlike the later models that have 240v right up to 440v. I tested the output with 240v connected to the 320v input but it only gave out around 60v which simply made the contactor chatter and could not pull it home properly. The solution was to fit a replacement transformer with 240v primary and 110v secondary at 50VA (probably overkill but the smallest panel transformer you seem to be able to come by.) I got the replacement from http://www.rselectricalsupplies.co.uk/ £13 ish..

The 110v output of the transformer is passed through all the emergency stop switches and on to the contactor. The transformer is supplied by the input 240v feed and is energised as soon as the (door mounted) isolation switch is set to ON.

Providing the mushroom and change gear enclosure micro switches are set correctly the contactor will be energised when the RUN button is pressed. The supply will then be enabled on the inverter. If the APRON LEVER is set to FWD or REV the power to the inverter cannot be enabled. The LEVER must be in the CENTRE, or OFF, position for the RUN button to pull in the contactor.

Normal Direction Control

The standard direction micro switches, operated by mechanical linkage from the apron mounted lever, are connected to the FWD/STOP and REV/STOP inputs on the inverter, using the N/O (Normally Open) contacts. The inverter has several run modes and with this micro switch arrangement the inverter will need to be set to the FWD/STOP, REV/STOP mode (which is standard on the TECO inverter). This means that in normal operation when the apron leaver is returned to the centre position the motor will STOP.

Finally a small additional control panel has been fitted to the back of the headstock containing a toggle switch to turn on and off JOG and a rotary (potentiometer) control for motor speed. These are wired into the inverter in the way recommended by the inverter manufacturer (remembering to enable the inverter for remote speed control).

Other Considerations

It is worth noting that if you are to run the lathe in a garage or shed then (as per IEE regulations) the supply to these places will be protected by an earth leakage detection device, such as an RCD or RCCB (combined leakage detector and circuit breaker). The input circuit of the inverter has noise filtering components that create a fair degree of voltage to the earth (earth leakage), thus tripping the device. This will happen regardless of there been a fault of not and I tested this with the TECO inverter and it did indeed trip the RCCB built into my (pretty standard) garage consumer unit. For this reason it will be necessary to connect the lathe to its own direct supply back to the main distribution board (non RCD side) in the house. It will also be necessary to use a shrouded mains connection, such as this:

This is a requirement as the circuitry of the machine can harbor residual current (such as when the motor is winding down). Using a normal domestic plug, with bare pins, could lead to electric shock if the plug was removed while the machine was running. I got mine from a local electrical suppler as well as the armored cable and additional MCB (Type C) for the house's distribution board, all for around £30.

Any questions to rupert AT astronoscope.eu (replace AT with @)





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