Induction motor power ratings

[Robin GrahamParticipant @robingraham42208]

[Robin GrahamParticipant @robingraham42208]

I don't need another drilling machine, but the good folk at Amazon have twigged that I sometimes buy tools so suggested that I might have a look at this;

**LINK**

It's a 13mm 1/2 HP drill press for 60 quid, so I had to have a look,the alternative being distasteful domestic chores. The second review of the machine seems pretty thorough – the guy (Geoff) obviously made a lot of measurements. He found :

'The steady power consumption, after about 7 minutes, was about 300W, with no drilling load, so this 350W drill is mostly producing heat, rather than drilling power, so it is little wonder the motor quickly gets so hot.'.

The implication seems to be that that the motor was dissipating most of its input power in heat. I have a dim recollection that induction motors run at about 30-40% of rated power under no load -if that's right, there's something wrong here!

Any elucidation?

Robin.

[Les Jones 1Participant @lesjones1]

How confiident are you that its WAS 300 watts and not 300 VA ? That makes a big difference to the reasoning.

Les.

[MuzzerParticipant @muzzer]

On this side of the world there are requirements for efficiency. Good summery here Std efficiency (IE1) motors should be about 70-80% at full load. Obviously they will be 0% at zero shaft load but even if the losses remained constant across the range, that would be 20-30% of the full power.

More likely that he didn't take account of the power factor. If you measure voltage and current and simply multiply them you might be overlooking the fact that the current will be substantially inductive (lagging) ie out of phase with the voltage. He might actually be seeing 300VA, not 300W. With a theoretically perfect inductive load, you could actually draw a large lagging current / reactive power but dissipate zero real power (aka heat).

Can only guess, as not much to go on.

Murray

[John OlsenParticipant @johnolsen79199]

Probably very skimpy on both iron and copper, and maybe also not enough turns on the winding. The latter can lead to the iron saturating. (magnetically) That leads to a higher magnetising current, most of which ends up as heat. The motor would quite likely behave better if the voltage could be reduced a bit. (Say by 10 to 20 percent or so)

The picture looks pretty much like the generic Chinese drill, I had one years back. It was Ok, didn't run too hot under normal use but the motor was not very grunty. Back then (last millenia!) I got mine for NZ$169, my Dad said "you would never get a drill cheaper", but later I saw them go down to as little as NZ69

John

[Bill Davies 2Participant @billdavies2]

I have a similar bench drill I bought some years back, similar to offerings from B&Q and Wickes. I've never noticed it running hot, but has about similar power to my similarly powered pistol drills (B&D, Bosch).

The table is a bit flimsy (someone recently suggested a scissor jack underneath, I've used blocks of wood). In steel, you have to progressively take the drill sizes up to the final size required, or it stalls. It does the job, but probably maxes out at about 1/2 inch or so diameter. Mine cost £40, back then.

Bill

[larry phelan 1Participant @larryphelan1]

That looks very similar to the Aldi drill which I have,comes under many different names/brands.

For the price,they are OK and they will drill 1/2" holes,just not straight off. I find it fine for small work up to about 8mm/10mm. Aldi/Lidli still sell them here from time to time for around 60 Euro,so at £60,it,s no great deal.

I have used mine on long runs a few times but I never noticed it running too hot,but then,my Chinese bandsaw does run hot on long runs,but the good book say,s it does that and it,s OK..

[Russell EberhardtParticipant @russelleberhardt48058]

Even if he was measuring VA rather than true power that seems rather high for such a small motor. Perhaps his example had stiff bearings. The current shouldn't change significantly with time so why did he quote 7 minutes?

Russell

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, maybe you need to checkout the duty cycle of the motor. if it is not rated as continuous, often shown as CONT on the motor plate, then it will need the appropriate rest time between jobs. Such a motor would reflect a cheaper price.

Regards Nick.

[Robert Atkinson 2Participant @robertatkinson2]

Remember that for heating, which is mainly resistive loss (I squared R), it is the VA that matters not the true power W.

Robert.

[MuzzerParticipant @muzzer]

No sorry, your logic is back to front.You will only dissipate heat ("real" power) if the impedance has a resistive element to it. Or put another way, if the VAs have a "real" component, in phase with the voltage.

Remember that we are talking about measured values here. If there is no mechanical shaft power involved, all of the real / resistive part of the VA is dissipated as heat.

In the example I have, you could have a large VA and zero power factor ie zero dissipation.

Murray

[Andrew TinsleyParticipant @andrewtinsley63637]

I have the same type of drill. I bought mine for £15 because the DIY Chain had gone bust. There were 4 left and I nipped home to get an engineer's square. Checked them all out and got one which was as near as could be, with vertical column at right angles to the base and a tight quill .

It has been a minor miracle. It will drill mild steel with a half inch drill, without any hassle. It only has the so called 300 watt motor (and I bet the O/P is a good bit less).

OK I got very lucky with this buy, almost in the "precision" class for a load of potential Chinese junk. The only thing that is wrong big time, is that the table deflects when doing heavy drilling. I use a scissor jack to stop this, but I wish I could find a real cure for the deflection. I don't know if it comes from the swivel table or the collar. One day I will come up with a good idea. But by then I shall probably have burnt out the motor.

I wish I had purchased the other three now. A cheap way of getting motors, NVR switches and reasonable pulleys!

Andrew.

[Robert Atkinson 2Participant @robertatkinson2]

Posted by Muzzer on 07/09/2018 15:28:43:

No sorry, your logic is back to front.You will only dissipate heat ("real" power) if the impedance has a resistive element to it. Or put another way, if the VAs have a "real" component, in phase with the voltage.

Remember that we are talking about measured values here. If there is no mechanical shaft power involved, all of the real / resistive part of the VA is dissipated as heat.

In the example I have, you could have a large VA and zero power factor ie zero dissipation.

Murray

You are considering a perfect inductance, which the motor is not. It has resistance and X resistance will dissipate A squared times X Watts of heat (real energy) where A is the measured current in "VA". The phase with respect to the supply voltage is irrelevant because the voltage developed cross the resistance is of course inphase wth the current. This is why the power companies ar concerned about VA and power factor the out of phase current causes heating in their equipment without doing any work. If you connect a 50mH choke across the mains it will draw about 15A but dissipate no power (if it is a perfect inductor). A real choke has resistance so it will get warm and that is wasted energy. For a motor the primary losses are resistive (unless it has really poor iron and bearings) so it's current that counts for heating.

Robert.

 

 

Edited By Robert Atkinson 2 on 07/09/2018 17:51:58

[john fletcher 1Participant @johnfletcher1]

There was an article in Model engineer several years ago about cheap drilling machine table deflection. I followed the instructions and basically it was an adjustable leg in front of the table. In my case I used a length of 12mm square section 300mm long secured to the table, it slid up and down as required. A bracket was fixed to the machine base with a square hole, such that the 12 x 12 could pass through the bracket and could be locked at the appropriate height of the table. Later I was fortunate to buy a Fobco at a good price, never the less the cheap Taiwan machine severed me well, wort and all, especially after having just a B&D pistol drill for many years. John

[Robin GrahamParticipant @robingraham42208]

Belated thanks for replies – I've always had a bit of blind spot about the VA vs Watts thing, but now I understand. I think. It's a shame that the guy who reviewed the machine didn't give more details about his measuring methods – he obviously went to town on it – so we can't draw any definite conclusions.

I'm still wondering about all all this though. There was a post in another place from someone with a 550W drill press which he felt was under powered for cutting (wooden) plugs. I had a look at the plate on my Fobco Star motor and it's 250W. It'll drill half inch straight into steel, no pilot, without drawing breath. It soars through teak with a 20mm plug cutter.

What variation is there in the efficiency of induction motors I wonder?

Robin.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Robin Graham on 15/09/2018 23:19:47:

Belated thanks for replies – I've always had a bit of blind spot about the VA vs Watts thing, but now I understand. I think. It's a shame that the guy who reviewed the machine didn't give more details about his measuring methods – he obviously went to town on it – so we can't draw any definite conclusions.

I'm still wondering about all all this though. There was a post in another place from someone with a 550W drill press which he felt was under powered for cutting (wooden) plugs. I had a look at the plate on my Fobco Star motor and it's 250W. It'll drill half inch straight into steel, no pilot, without drawing breath. It soars through teak with a 20mm plug cutter.

What variation is there in the efficiency of induction motors I wonder?

Robin.

The power of an electric motor as given on the label is best seen as a hint rather than a fact.

• It's not always clear if the wattage quoted is the input or the output power.
• Power output, efficiency and torque all depend on the load and can't be expressed as a single magic number: you need to see the full specification. And as already discussed, with AC motors there's a hard to understand difference between Volt-Amps and Watts.
• Is the motor rated to deliver power continuously or intermittently? Motors rated for continuous operation are heavy and expensive. As many appliances are only run intermittently it would be daft to fit them with motors rated for continuous operation. There's little harm in overrunning a motor in short bursts provided it doesn't get too hot. But a mistake to fit a 500W motor intended to cool off for 40 minutes after 10 minutes at full power to a machine that's on all day.

Most of the time only designers need to worry about sizing motors and they have access to all the data. Unfortunately a home workshop is a major exception because what's done in them varies so much. The motor may not be matched to the condition of service. If you expect a 500W hobby pillar drill to work as hard as a 500W professional pillar drill, you are cruising for a bruising. Both motors will perform equally well drilling a few holes with sharp drills. But the hobby motor needs time to cool down and if you keep going it will overheat. If a few windings are shorted the motor's power output will be reduced permanently. Also, compared with continuously rated motors, intermittent types are less tolerant of abuse; a heavy handed operator is much more likely to burn-out a hobby machine than a professional equivalent.

When fitting a new motor or, adapting an old one to a machine, it's as important to match conditions of service as it is to match the power rating. This means understanding how heavily the machine will be used in practice. Are you a heavy user or a light one? Worst case, a light user who occasionally binges heavily, which might be the hall-mark of the true model engineer.

Once that's decided some motor plates are helpful, many aren't. When buying a drilling machine it's often possible to tell the difference by looking at the price. A very cheap new machine is going to have limited stamina, and an expensive new machine is likely to be the real McCoy. Unfortunately, there's a whole bunch of stuff in the middle. Buying an ex-industrial machine may be the answer, but bear in mind it may need an expensive new motor.

Dave

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