Correct Single Phase Electric Motor for Saw Bench

[Charles1948]

Can a knowledgeable Spark help please? I want to convert a 3 phase Wadkin saw bench to single phase, the blade is 10" in diameter. I reckon that I will need a 2.2kw, 3 HP motor with a speed of 3000 Rpm. However this is as far as my knowledge goes and my motor supplier wants to know whether I require a motor with a permanent capacitor or one with a capacitor start with the choice of EDA or TDA. please can you advice which is the best to go for? I realise also that I will have to look into pulley and driveshaft compatibility. Thanks C

 

[JACKC]

Instead of spending money on another motor, have you considered purchasing an inverter to give you the 3 phase from a single phase supply?

 

[Charles1948]

Hi Jack.

Thanks for your reply, yes I have considered an inverter but feel that single phase is less dangerous.

Regards C

 

[jj4091]

Whether you use a permanent capacitor or a switched one will depend on what you use to start/run the saw with.

 

[Spark123]

It will still take your fingers off regardless of how many phases!
A correctly installed 3 phase is no more dangerous than a single phase.

 

[Charles1948]

JJ - Am I right in thinking that a capacitor start motor only requires a simple on/off switch and a permanent capacitor motor requires a switch that incorporates a starting device?

Spark - The key is a correct installation, If I was to make a mistake surely the consequences would be worse with 3 phase.
C

 

[Spark123]

Nope, they will both dangerous and can quite easily bite you.
The capacitance start will have a centrifugal switch which disconnect the capacitor from the circuit once the motor is running. A single phase motor needs to be a fair bit bigger to get the same horse power rating than a 3 phase.

 

[Charles1948]

Thanks for the input everyone, I am coming round to thinking that an inverter would be the better option as the saw has new 3 phase switchgear fitted, which is presumably safe. I am not sure whether an inverter is the cheaper option though (with a 3HP motor costing around £100 at Machine Mart), but it would certainly avoid the hassle with pulley, shaft and motor sizes, also the motor bracket.

Are there any specific problems with inverters and can you recommend a specific make?

 

[JACKC]

We use Mitsubishi inverters at work, they are easy to set up and the manual is user friendly although you do need a bit of knowledge to understand what is what
http://www.mitsubishi-automation.com/products/inverters_fr-s500.html. These are what we use.

 

[briwire]

You can get a 2.2kW Allen Bradley Powerflex 4 from RS for about £250.

Easy to set up and safe. The cost of a single phase motor would probably cost more.

 

[Charles1948]

Much obliged thanks Jack and Briwire. They are not as expensive as I had thought so I will definately be going down this route. By the way the motor on the saw is a 2 HP 380/420 50 cycles 3 phase job, not one of the 240/440 more modern motors, are inverters still suitable for these older motors?

C