My brother in law has just inherited an old wood turning lathe. I am little bemused as the motors, (he has a spare), are rated at 1/4 HP but at 2.6A. 1 HP = 750W, so 1/4 HP = 187.5W. At 230V = 810mA, not 2.6A? They're a definitely 1/4 and NOT 3/4 HP, am I making a stupid mistake? RES
The 1/4 HP is the mechanical output, the 2.6A the electrical input. You didn't really expect the motor to be 100% efficient and to run at unity power factor did you?
Sorry Today, missed your reply as I thought no one was going to answer. Anyway, appreciate efficiency and cos theta, but by a factor of 3 seems massively high/inefficient to me, especially at such low power. Pf should be around 0.8? at worst, and an efficiency of less than 50%? I certainly hope not! RES
What type of motor is it and is it 3 phase or single – your posting suggests single? Also, if single, is it 240 volt? Some early machines used reluctance motors and they were particularly inefficient (then- much better now), with very low Pf’s. If it’s induction and single phase, with separate start winding (has it got a capacitor fitted?), then perhaps nameplate amp’s includes that taken by the start winding (don’t know, just a suggestion). Has it got a BS number on the nameplate as that may give a clue?
Londoner Thanks for reply. I have never heard of a reluctance motor, at least I don't think I have? I will have another look later today. FYI, they are single phase, with just 13amp plugs fitted. Probably very, very old. When I get more info will let you know. RES
Motor is made by Hoover! Type 71993A317D. BSS170/39. Means nothing to me, looks a bit big to fit a domestic vacuum, could be out of a very old washing machine? Any way, all I was really querying was the amount of current needed for such a low HP rating. RES
The amperage is correct. I have a Motor Full Load Current Chart, and it lists motor ratings from 0.25 HP upto 100 HP! The 0.25 HP states a kW rating of 0.19, a 0.8A draw on 380V 3 phase, 0.7A on 415V, 0.5A on 500V and 2.8A on 230V single phase so your plate listing a current draw of 2.6A is about right.
do you not mean a replusion motor rather than reluctance motor He said it was an old lathe, so it's unlikely to be a repulsion motor, which needs electronics to make it run. It's powering a lathe. So it'll be a bog standard squirrel cage induction motor because there's no point in using a more expensive motor.
can remember working on repulsion motors in late 70's albeit it 3 phase and no electronics were involved
RES: Reluctance motor is simpler than an induction motor. Stator is the same but rotor is simply a shaped laminated iron assembly. 3 phase versions in particular were very common in machine drives years ago. With modern electronics, they can make a good drive (stepper motors in control systems for example are similar and very large versions are now appearing in specialist drive applications). However, sinewave’s response suggests yours is an induction motor and of correct current (fractional HP single phase induction motors do tend to be inefficient and also tend to draw a relatively large magnetising current component and have a relatively low PF).
chocolateblock, forgot to mention, older repulsion motors needed brushes for commutation. Modern ones (brushless) tend to have solid state switches embedded in the armature windings. They are not uncommon in specialist industrial drive applications.
Thank you all for your excellent and informative answers. This is how this forum is supposed to work. You are a credit to your profession. RES
and if i remember correctly, by altering the position of the brush box this adjusted the speed of the motor, i vaguely remember this type of motor in a dairy and it was used to allow the filling lines to speed up or slow down
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