Electric motor ratings

Looking at getting my first mill, it's an older version of one which is still available today (the chester Lux)

The motor plate on the mill i'm hopefully buying says:

750W
240V
6.5A

The new version of this mill boasts about it's 1.5KW motor.

But, Volts x Amps = Watts... so 240 x 6.5 = 1560.... that'll be the 1.5KW rating the newer mills are sold as having.

So... where does the 750 watts rating of this motor come from?

One place is power factors as volts and amps aren't exactly in phase for inductive loads so power is not exactly I x V for a start.

Recently an example came up elsewhere of an (allegedly) 1000W amplifier whose rating on the case said 160W - in this case the rated power may be a calculated 'weighted average load' while the 100W is probably a very optimistic instantaneous peak value.

In short, there are power ratings and power ratings...

Neil

No idea, but might it be its continuous rating? Some motors are particularly inefficient and sellers quote input rather than output power. But 50% would be poor for an electric motor - but could be, if the duty cycle is low.

I see things like band saws, of similar capacity, having widely differing power ratings.

Same with compressor ratings. Good kit quotes free air delivered, cheap ones quote pump displacement. Not the same thing. You can guess which appears as the higher figure, numerically!

Buyer beware these days!

Hi Gazz,

Beware the can of worms that is kW ratings of motors. In an electrical machine, Watts are dissipated, and the amount is known as the Wattage of the motor , heater or machine. The rule of thumb for motors is that 1hp= 746 watts, but a motor that dissipates 746 watts will not produce 1hp at the shaft unless it is 100% efficient, now most motors can get into the high 80's, and even the low 90's of % eff, and this uncovers an interesting bit of kidology on the part of the manufacturers. Either they are over rating their motors output by claiming 100% efficiency, or they are marking the motors with less wattage than they actually dissipate. The only way you could sort this anomaly of marking is to test the motor running under load, and see what real world results you get! If someone marks a motor 1 Hp, then they are claiming a known power output at the shaft, but if they rate it as 746 Watts It cannot produce 1Hp because of friction, fan losses, and watts dissipated as heat rather than torque. Chinese motors are fairly inneficient, and also noisy because of poor layering in the windings, loose laminations, and even loose winding cores inside the body of the motor. Noise is also watts dissipated as something other than torque. Hope this answers your questions. The maths only produces a theoretical answer!

Phil

East Yorkshire

Edited By Phil Whitley on 08/01/2020 19:24:30

Thankyou all, i didn't consider the efficiency thing, i think only a resistive load like electric heating can be 100% efficient, where the heat produced is wanted (ignoring phase change units like air conditioners)

i remember the max power days with car stereos and car speakers...6 x 9 inch speakers claiming to be rated for say 800 watts...which was true... but a few milliseconds after the 800 watts went through them they self destructed... RMS power would be probably 50 watts at best.

So the modern milling machine motor is just using the ohms law calculations to get at the 1.5 KW they advertise.

Bit on the Sieg site about how some sellers quote input power which sounds a lot better than the output power that they quote.

Some confusion may arise because in SI units both electrical power and mechanical power are measured in watts. So a motor could be rated at 1500 W consumption but deliver 750 W at its shaft. So the new mill could "consume" 1500 VA (roughly, watts if it was a resistive load), have a lousy power factor so it actually absorbs 1000 W of real power, and an efficiency of 75% so it delivers 750 watts.

A 100w Marshall amp is more powerful than a Fender 100w because the knob can go to 11... fact

I thought the 750w related to the HP output so 0.75KW or 1hp of work, nothing to do with electrical power.

1 HP = 550lbs x 1 ft / 1 s, which after conversion to SI unit gives you 745,6998 Watts.

Ah yes ! the good old days of MAX POWER versus RMS !!! Noel

It may be that the motor as in this picture. Is power rated at 550w = 0.75hp. But takes 5.1A of power because of the losses discussed by some above.

About right at cos theta of 0.8 and motor efficiency of 60%

I take it the CR denotes ‘continuous rating’?

A very good reason for not being tight on the sizing for VFDs. A cheap 750W inverter might not manage to supply enough power for that motor (had it been a three phase example).

nothing to do with the above question......

BUT

where ever possible I always increase the size of the motor's HP or whatever by at least 50%...

(not nec for my 3phase machines as have 440v direct from the mains).......

Makes em last longer for little extra outlay.......

Thats unless it a Proper UK made motor.....do they excist any more......????

always buy the old fashioned Brit motors "Brookes" etc just for stock, if they are cheap.......

Well that is the motor I have taken off the Super 7 . To then fit a new when arrives, 1.1kw 1.5hp 3phase motor. Just looking at the manual for the inverter 167 pages. Ouch. Quite a lot of settings to program in.

I read the plate thus:

The motor is continuously rated "CR" to deliver 550W, probably out. (On an electric motor only the power output and running limitations are meaningful. A reputable motor maker like Parkinson is likely to respect this by putting output watts on the plate. Unfortunately, it's not unusual for sales persons and even some decent makers to quote Input Power instead. Rule of thumb electric motors are 80 to 90% efficient, so the difference may not matter much.

The relationship between power-in (whether expressed in amps or watts) and power-out is limited mainly by how hot the motor is allowed to get. Depending on age, older motors were rated to run at between 80 and 120C because their insulation failed above that temperature. Modern insulation is rather better, perhaps allowing 160-180C or even as high as 220C. As the power output of an electric motor is limited by heat, it's possible to find the same motor with different plate values. The higher output motor will be time limited, look for look for IEC Duty Ratings labelled S1 to S10, or perhaps Service Factor (a measure of permissible overloads.) My grinding wheel is rated 400W out S3, which means 'Series of identical duty cycles each a constant load for a period, followed by a rest period. Thermal equilibrium is not reached during the cycle.' On my grinder this means 400W out for about 15 minute, them about 15 minutes rest, ie a 50% duty cycle. One of the differences between a hobby lathe and a professional machine is the motor. The professional motor is likely to be a hefty monster capable of delivering full output round the clock, whereas the hobby can only be run in bursts. Not a problem unless the owner tries to use his hobby lathe for heavy production work, in which case he's likely to cook the motor.

The 5.1A probably indicates the absolute maximum current the motor should be allowed to draw. It's allowed briefly when starting or for short bursts during operation, and can be used to size fuses and contact breakers etc. It's not a target.

The other information is that this is a 240v 50Hz AC single-phase motor spinning at 1425rpm. Not ideal for a machine tool, because single phase motors are somewhat unreliable, vibrate, can't be speed controlled, and don't like continuous stop-start operation. I prefer DC, Brushless or 3-phase motors. That said, Myfords have used motors like this Parkinson for 80 years and most users find them OK. Ditto Hoover!

Generally speaking motor plates aren't very reliable, ranging from accurate and informative through useful guidance down to outright lies. Motors recycled from domestic equipment may not be lablelled at all - what they're capable of is a secret between the manufacturer and his motor supplier. Take all motor plates with a pinch of salt!

Fortunately, for most practical purposes a motor of about the right physical size and voltage will do most amateur jobs OK. In case of doubt just make sure the motor doesn't get too hot - smoke is bad!

Dave

This is a scope trace of a Crompton motor on a Myford ML7 starting up.The motor is a 1/2 HP single phase,the current on the plate is 3.8 Amps.The scope shows a steady current of 3.9 Amps, after the initial peak start current of nearly 50 amps.There is no load apart from turning the chuck.

If a 1hp motor happened to consume 1.5kW at full load then either it would be incredibly noisy (from the fans) and hot or it would be a smoking puddle on the floor after a short while. Similarly, describing a 1.5kVA motor as a 1.5kW motor would be a false trades description. The motor may actually be a 2hp motor...

Time to knock up a test rig and maybe, rent a lawyer!

Edited By Mark Rand on 09/01/2020 21:10:59

Let's untangle some of the folklore from the technology here.

Motor as above is plated for 550 watts OUTPUT, ie mechanical power delivered at the motor shaft. It absorbs 5.1 amps times 230 volts = 1.17KVA to achieve this NOT kilowatts. KVA is a measurement of reactive power, not to be confused with real power measured in KW i.e Kilowatts.

This tells us something about the efficiency and the power factor imposed on the supply. Estimate max mechanical efficiency is say 70%, then power factor at full load is approx. 0.67. Power factor of a single phase motor will drop quite noticeably at part load, may get down to 0.3 while the measured line current (now highly reactive) drops only slightly.

Motor will run slightly faster than the quoted full load speed of 1425 rpm at part load, but not much. Estimate 1470 RPM at no load. Motor will never attain synchronous speed (1500 rpm) - it can't - but this slip factor is largely what controls the power factor the motor presents to the supply. This is with the motor running at service speed (duty point on torque/speed curve) in equilibrium with its applied mechanical load. This motor is continuously rated - which might be what the CR stands for - but it would have to be marked as such if not rated for continuous duty .

This motor will take multiples of its 5.1 amps quoted full load current briefly while starting. A typical figure for a three phase motor is that it will take about 7 times full load current in the first few milliseconds after it starts, for how long this continues depends on the Inertia of the motor and its load. But you'd better let it spin up the full speed pretty quickly or outgepouf und smoken mit ze blitzen. Maurice's 'scope traces show this effect and quantifies it nicely.

A single phase motor is even worse to start electrically, so budget on say 9 times full load current as starting inrush, get it over and done with as soon as possible. What makes single phase motors so treacherous is that this starting inrush is likely to be limited as much by the upstream supply impedance as the characteristics of the motor. In other words the supply wiring and contacts are often the limiting factor and are getting hot during the inrush event.

Whether non European manufacturers actually conform to the same standards is anyone's guess - the machine could only be (legitimately) CE marked if they do.

HTH Simon

Reading reading reading . If you could have seen the size of the capacitor banks we fitted to Kirkstall forge. they were huge to correct the power factor back to 0.99 as 1 is not possible. The high inrush currents at start up on machinery meant we had to, years ago fit HRC fuses. More recently it was circuit breakers of the D type to stop them tripping on start up. That is why the motor control circuits used to have oil filled dashpots, so the pull on the dashpots was controlled to stop the overloads tripping. An interesting anecdote was when we did lighting installations in workshops where turning machines were used. The rows of lighting had to be on different phases so that the stroboscopic effect did not see the rotation of the machines appear to be stood still. From an ex Electrician with experience & not much reading. Too busy earning a large crust. With pork dripping on. Regards to all of you & a very prosperous new year. If you are retired like me then enjoy. Steve. 

Edited By Steviegtr on 10/01/2020 02:01:39

I agree with most of your post, but not the bit about KVA.

The quantity VA is the apparent power, input current times input voltage. The 'K' is simply a multiplier of 1000. It consists of two components. First, real power, measured in W and second, reactive power measured in VARs (volt-amps reactive). The quantities are related using the equation for a right-angled triangle, where VA is the hypotenuse.

Andrew