ELECTRIC MOTORS

Hi Everyone,

I was reading some of the replies to questions regarding noisy motors ect which I found very enlightening.

Like most small workshops,mine operated on single phase for years,three phase being out of the question due to cost,so I never had any experience of it except where I worked,that was all three phase of course.

All my motors were single phase and I thought,smooth running although I did hear that 3ph ones were much smoother and quieter.Never understood why this should be. A friend had a power press in his shop which he had converted to run on single ph but he said that the growling noise it made would put you off using it. Later,when he got 3ph he put back it,s own motor and said it ran like silk. I have a saw bench with a 2hp single ph motor,my friend has the same model 3ph,and the difference between the two is quite clear. Mine is subject to a certain amount of vibration which ends up at the blade,of course,his is rock steady. I put this down to pulses in the single ph motor along with poor support for the motor itself [bad design ]

Later on I had to get 3ph because I had bought a planer and spindle moulder,each with 4hp motors,not the kind of things you can run on single ph. 3PH motors seem to be more powerful,size for size than single ph ones,and they are of course much easier to reverse,very handy on the lathe ! They are also much easier to come by secondhand and they seem to go on for ever.

Can anyone out there shed a bit more light on this subject for the benefit of people like me? Also,I have two questions

1 What is the relationship between the frequency of the supply and the speed of a motor?

2 Why are electric motors on ships running on 600 v ?

In answer to Q1, the motor shaft speed is a function of frequency and the number of poles the motor has.....

The formula for the calculation is as below....

n = f (2 / p) 60

where

n = shaft rotation speed (rev/min, rpm)

f = is line frequency, in the UK, taken as 50 hz or for the benefit of the American members, 60 hz

p = number of poles

cant answer Q2, .....

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I didn't realise that they were, but:

Higher voltage means lower current for the same power ... and therefore thinner cables, and less cost & weight.

MichaelG.

Michael,

I ought to have known that!.....

Silly me....

I thought that might have been the reason,but I just was not sure. Makes you wonder why they dont do the same in workshops,could it be for safety reasons?

35 years since I had to worry about putting a computer on a ship. How time flies.

Anyway, to power a 240 VAC 50/60 Hz 15A single-phase minicomputer I was offered: 120V DC, 110V AC 60Hz, 230V AC 3-phase at 400Hz, and 450VAC 3-phase at 60Hz. Some cabins were plumbed for 220V AC at 50Hz. but 'no chance' in the compartment I was allocated. These were the ship's domestic supply. I remember 2300V being available for winches etc, but not the details.

The variety was a surprise to me, but apparently ships are a bit special. 120VDC was used by small motors such as those in the ventilation system; 110V/60Hz was utility power; 400Hz was for wireless, radar and other electronics; 450V 3-phase ran stuff like big fridges and air conditioning plant. A small number of 220V 50Hz sockets were available so people could plug their radios in. The other thing that struck me was the bomb-proof nature of the installation. Steel conduit and trays throughout, sockets about 4 times normal weight, everything stood-off from the bulkhead, lamps in wire-cages, and fuseboxes like a small bank vault.

In my time in the MN only one ship was DC and that was how it was generated all the others were 440v 3phase with no earth or neutral, the accommodation and lighting was supplied 120v/110v 2phase giving 240v/220v again no earth or neutral. Power generators were/are delta wound where as shore side they are star, the main switch boards had earth leakage monitors some were very simply a bulb off each phase to the ships hull.

Edited By Bob Brown 1 on 22/03/2017 17:59:38

Aircraft AC supply is 400Hz, which i have always understood is to minimise vibration.

I was told it was to allow transformer weight to be minimised.

Correct, it allows smaller magnetics all round.

Andrew

My thanks to everyone who replied to me. I never knew there was so much difference between ship and shore.

It was just something I noticed when travelling by ship [I like to look around while the others are getting pissed at the bar ] I did notice that the gear was a bit better than normal and well protected,but assumed that this was to cope with conditions at sea.

They say that you learn something new every day,and in my case,I sure do.

Very true, as frequency goes up, the physical size of a transformer comes down....just look at a switch mode power supply supplying say 400 watts from the mains at 12v, then compare that to a linear....? Much heavier and bulkier to boot...

I stand corrected, every day is a school day

Not only transformers, if you use 50 hz your generator can't do more than 3000 rpm, if you use 400 hz it can do 24000 rpm, which makes it very much smaller. Remember the days of the miner's strike and the 3 day week? Company I was working for made aircraft auxilliary power supplies, so the electrical guys fitted up some flourescent tubes to run off 400 hz. No idea how, some trickery with chokes/capacitors I dare say, but the lights whistled. Bit un-nerving till you got used to it

Not sure anyone has answered the 'why is 3phase smoother?' question.

The power into a single phase motor is the product of current and voltage at any particular instant. as I and V are two sine waves 90-degrees out of phase the power equals zero at four points in each cycle of the 50-Hz supply giving pulsed torque and creating vibration. (the graph is like a rectified 100Hz sine wave giving a very 'rough' sounding 200Hz)

A 3-phase motor has the three AC signals each 120 degrees out of phase with the others. If you have the patience to overlay the 100Hz power cycles of the three phases and add them up you will find the power and therefore the torque is constant regardless of the position of the rotor.

In Auckland a few years back there was a major cable fault, someone came up with the bright idea that, well we have a Naval base, surely they have some ships that can supply some power, I think it took about one phone call to find that the Navies ships generate AC power at 400Hz, so useless.

Ian S C

On a 2 pole 2850 rpm motor on 50Hz supply you get two pulses per rev on single phase, whereas on three phase you get six pulses. Even better on four pole motor which most lathes are fitted with. Like a single cylinder Norton single cylinder with vibrations on the handle bars compared to a Honda 4 smooth.John

Um, if I and V are 90 degrees out of phase there is precisely no energy consumed by the motor as it is equivalent to a perfect inductor! An induction motor is like a transformer, with no load V and I are nearly in quadrature, the difference only being enough to create enough real power (as opposed to VAR) to overcome winding, air and bearing losses. As the motor is loaded, I increases and swings more into phase with V so more real power is consumed. If the motor was 100% efficient, V and I would be exactly in phase. The torque variation, or equivalently the power variation, is at twice the drive frequency, or 100Hz in Europe, as the product of V and I drops to zero once every half cycle. |For more details see this link:

**LINK**

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... or perhaps even more appropriately, compared to a Kawasaki triple

[120° crank angle, two stroke] **LINK**

http://www.cycleworld.com/kawasaki-h1-500cc-mach-iii-triple-motorcycle-history-classics-remembered

MichaelG.

Spot welding transformers for robots have benefited greatly by raising the frequency. A 75kVA 50hz weld transformer used to be too big to mount on the end of a robot arm, physically it was about 2'x1'x1' and weighed a couple of hundred pounds or more. A 75kVA 1khz weld transformer is about 6"x6"x8" and weighs about 30lb including 2 diodes to give DC welding. Both types are water cooled. The diodes are 55mm diameter and about 5mm thick and must be rated at about 20,000A. The great reduction in size and weight means the transformer can be easily built into the weld gun on the end of the robot arm.

Mike