What do I need to know to calculate size of single phase motor?

[pgrbff]

About 20 years ago I bought an old table saw from a local school. I only had single phase at the time so I changed the motor and switchgear as it was 450V. I'm thinking of buying a 3 phase 50s bandsaw and have the same problem, but I can't remember how to work out the size of the replacement single phase motor. Are there likely to be any other problems?

 

[Andy-1960]

I would have thought the easiest way round the issue is to match the new motor to the one you take off. I.E the same h.p / kw rating and the same speed as the three phase one you take off

 

[Strima]

Why not use a VFD and keep the three phase motor?

 

[pc1966]

Interesting idea, I guess these days a VFD is probably cheaper than a new motor? Also you probably get soft-start and the speed control might prove useful.

 

[pgrbff]

Why not use a VFD and keep the three phase motor?

I am/was assuming changing the motor will be the most economical route at the moment.

 

[pgrbff]

I would have thought the easiest way round the issue is to match the new motor to the one you take off. I.E the same h.p / kw rating and the same speed as the three phase one you take off

This is all the info I have.

 

[Strima]

I am/was assuming changing the motor will be the most economical route at the moment.

VFDs are dirt cheap and will save the hassle of trying to find the right frame to fit your machine.

 

[SheffSimon]

This is all the info I have.

Can't read that, but how old is the motor? A VFD will put a lot more stress on the motor windings and an old motor with old insulation will not like it.

Note also that a VFD running from a single phase supply provides 230VAC 3 phase not 415, at 50Hz. Which is fine if your motor is wound and can be connected accordingly.

Edit...can read it now....you can connect in delta 250V so the VFD voltage level should work. I would be concerned about the winding insulation with VFD operation.

 

[pgrbff]

Can't read that, but how old is the motor? A VFD will put a lot more stress on the motor windings and an old motor with old insulation will not like it.

Note also that a VFD running from a single phase supply provides 230VAC 3 phase not 415, at 50Hz. Which is fine if your motor is wound and can be connected accordingly.

Edit...can read it now....you can connect in delta 250V so the VFD voltage level should work. I would be concerned about the winding insulation with VFD operation.From what I'm told the machine is 50s or 60's.

From what I have been told the machine could be 50's or 60's. Is it possible to estimate the HP or KW from the info on that plate?

 

[telectrix]

it's round about 1kW.

 

[Julie.]

It's around 1.5HP - if you are using the 260V line the "power" it will consume is around 260 x 5 x 1.73 = 2.25kW (not quite right it's actually VA, but let's keep it simple),

However not all this power is converted to output, so it is normal to take around 60% of this on newer motors and 50-55% on older motors (the "loss" is efficiency and power factor) - giving an output around 1.125kW (then divide by 0.746 to convert to hp) = 1.5HP

Difficulty to actually say as a more efficient motor could give up to 1.8hp output, one less down to 1hp.

This motor could be wired in delta for a 260V voltage, that should be ok for a VFD at 230V from a 230v single phase supply.

Try somewhere like Newton Tesla | Mitsubishi Electric Products, Motors & Gearboxes (newton-tesla.co.uk) if you speak to them they would advise if the motor is suitable.

 

[pgrbff]

It's around 1.5HP - if you are using the 260V line the "power" it will consume is around 260 x 5 x 1.73 = 2.25kW (not quite right it's actually VA, but let's keep it simple),

However not all this power is converted to output, so it is normal to take around 60% of this on newer motors and 50-55% on older motors (the "loss" is efficiency and power factor) - giving an output around 1.125kW (then divide by 0.746 to convert to hp) = 1.5HP

Difficulty to actually say as a more efficient motor could give up to 1.8hp output, one less down to 1hp.

This motor could be wired in delta for a 260V voltage, that should be ok for a VFD at 230V from a 230v single phase supply.

Try somewhere like Newton Tesla | Mitsubishi Electric Products, Motors & Gearboxes (newton-tesla.co.uk) if you speak to them they would advise if the motor is suitable.

Now I know why I got a E in A level physics. Thank you.

 

[telectrix]

i only got a B.... i spelt my name wrong on the exam paper.

 

[pgrbff]

Why is a VFD considered the best option? Apart from the possible difficulty of finding a single phase motor with similar mountings, would it not be simpler to put in a single phase 1.5kW? Is it just price?
I suppose other considerations are that it would be better to have a breaked motor or the bandsaw might continue turning for minutes when switched off and the possible need for soft start. I guess this is something a VFD allows?
I have no idea of quality but shouldn't I be able to buy a single phase motor for around £200? Surely a VFD and paying someone to wire it is going to cost more?

 

[Julie.]

Why is a VFD considered the best option? Apart from the possible difficulty of finding a single phase motor with similar mountings, would it not be simpler to put in a single phase 1.5kW? Is it just price?
I suppose other considerations are that it would be better to have a breaked motor or the bandsaw might continue turning for minutes when switched off and the possible need for soft start. I guess this is something a VFD allows?
I have no idea of quality but shouldn't I be able to buy a single phase motor for around £200? Surely a VFD and paying someone to wire it is going to cost more?

Well a 2kw vfd will cost around £55 off ebay or similar.

It can provide adjustable speed control without having to shift belts in pulleys etc.

It can also soft start to reduce the impact both mechanically and electrically plus a few other advantages.

One other advantage is that to a greater or lesser extent the existing switchgear can remain in place, often when moving to single phase most of the switchgear needs to change.

One other alternative is to use a rotary converter, this takes 230V input and produces a 415V three phase output, you can then use this for multiple machines which then don't require modifications.

My husband uses this to drive his lathe or milling machine, or surface grinder all of which are as they were designed, no mods