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DC Motors Vs AC induction (single or three phase)

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MW03/02/2016 10:50:44
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Y'know i'm always up for a bit of chit chat but, can anyone tell me what is it exactly that makes AC induction motors more powerful than their DC counterparts?

Michael W

roy entwistle03/02/2016 11:02:57
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I would have thought that a 1 horse ac induction motor was the same as a 1 horse dc motor But its a long time since I was at tech

Roy

MW03/02/2016 11:04:53
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Yeah, maybe dc motors have had a bad time because theyre normally used for variable speed and AC ones are normally belt driven, there still seems to be a distinctive lack of grunt from a DC motor though, i mean i haven't seen an old english lathe use anything but single/three phase induction AC

MW03/02/2016 11:07:45
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I heard rumor that in the days before Induction motors all you had were big and beastly DC motors though, perhaps they were rather energy inefficient

Ian Parkin03/02/2016 11:16:05
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DC motors are far smaller than AC for the same output

But in general as Roy says a 750w DC is the same as a 750W AC.... 1 horse output

John Reese03/02/2016 11:22:13
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DC motors in powr tools run at very high RPMs. That allows a lot of power in a small motor. AC motors running on line current cannot exceed 3000 or 3600 RPM (synchronous speed) depending on whether you have 50 or 60 hz.

MW03/02/2016 11:35:42
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"DC motors are far smaller than AC for the same output"

Yep, it makes them ideally suited for mounting onto a mill because it can support the weight of it better.

I wonder if its more a question on the number of poles per motor that dictates both slower speed but more inertia.

MW03/02/2016 11:39:59
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i remember someone once saying that on the face of it, a basic motor rotated by magnetic energy appears to be a very feeble thing and a wonder it ever worked in the first place, but then theyve done an awful lot to make them better and better at what they can produce over history.

Edited By Michael Walters on 03/02/2016 11:40:21

Martin Kyte03/02/2016 11:51:03
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Back in the day if you wanted variable speed on large machines (10's of HP upwards) you generally went for DC motors with Ward Leonard control. You start off with a 3 phase machine as a prime mover driving a DC generator. The Generator supplied your DC motor on the machine and by controlling the current in the field windings you had variable speed. This could be done simply with a switched resistor bank. Upgrades when semiconductor control arrived usually consisted of upgrading the current control by replacing the resistors with 3 term controllers.

I can think of several other high power applications, Trains,Trams and Submarines to name but three.

Martin

Ajohnw03/02/2016 11:59:13
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if you get to the bottom of this subject neglecting things like permanent magnet motors you'll find that the sizes of an AC and a DC motor will be very similar if they run at the same speed and provide the same output power.

Essentially DC and universal motors can run at higher speeds so can be smaller for the same horse power output.

John

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roy entwistle03/02/2016 12:46:20
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John Reese Wrote "AC motors running on line current cannot exceed 3000 or 3600 RPM (synchronous speed) depending on whether you have 50 or 60 hz." I would suggest that this only applies to induction motors Brushed ac motors in hand drills etc certainly can exceed this speed

Roy

Muzzer03/02/2016 12:53:50
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Posted by Ajohnw on 03/02/2016 11:59:13:

if you get to the bottom of this subject neglecting things like permanent magnet motors you'll find that the sizes of an AC and a DC motor will be very similar if they run at the same speed and provide the same output power.

Essentially DC and universal motors can run at higher speeds so can be smaller for the same horse power output.

John

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Indeed true to a fair extent. Bottom line is that the torque generated in the gap between the rotor and stator is limited by the saturation flux of the steel laminations (regardless of how the field is generated) - and the active surface area of the rotor. Beyond that it's a question of how fast you can operate it (power = torque x speed).

The main difference between induction and permanent magnet motors of equivalent rating is the size of the rotor. Modern magnets inherently produce a strong field, whereas the rotor in an induction motor only generates torque as a result of the current induced in it by the rotor slipping against the rotating field. There's more material and gubbins in its rotor (which is basically a shorted winding) required to generate the same field. This is why modern electric and hybrid vehicles tend to use PM machines and bigger, lower tech (eg older US) vehicles tended to favour (favor?) induction machines.

Muzzer03/02/2016 12:57:42
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Posted by roy entwistle on 03/02/2016 12:46:20:

John Reese Wrote "AC motors running on line current cannot exceed 3000 or 3600 RPM (synchronous speed) depending on whether you have 50 or 60 hz." I would suggest that this only applies to induction motors Brushed ac motors in hand drills etc certainly can exceed this speed

Roy

The minimum number of poles you can have is 2, which means at synchronous speed (ie with zero load), it will spin at these speeds. For a 4-pole machine it would be 1500/1800 rpm. The more pole pairs, the slower the synchronous speed. Of course, if you drive them with a VFD, the speed is simply limited by the mechanical and electrical parameters - back emf, burst speed etc.

John Haine03/02/2016 13:37:33
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It's an odd fact but true that the Tesla, one of the most advanced electric vehicles, uses an induction motor with a VFD,

**LINK**

One of the reasons for this I have heard is that it is not dependent on expensive rare earth elements; and also the motor is immune from demagnetisation over time which I believe can happen with PMs.

And another interesting point is that an induction motor is basically a transformer, and the flux in a transformer core is, ideally, zero.

Martin Kyte03/02/2016 13:52:59
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" the flux in a transformer core is, ideally, zero" ? WHAT

No field = No coupling = No transformer

Martin

roy entwistle03/02/2016 14:29:48
1716 forum posts

Muzzer I think you are confusing brushed universal motors with synchronous motors   Think of routers, engraving machines, etc All faster than 3000rpm

Roy

Edited By roy entwistle on 03/02/2016 14:43:14

Mike Poole03/02/2016 14:51:07
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DC was the motor of choice for variable or high speed applications until the arrival of semiconductors we could build a VFD with. With the addition of an encoder it is possible to get a basic squirrel cage motor to perform as a servo drive with full torque at zero rpm. The encoder is needed to give motor speed and rotor position so the current can be supplied at the right time to develop full torque at zero rpm. Encoder less drives cannot hold at zero rpm but low rpm can be achieved with good torque by some clever sensing and maths to estimate rotor position, the VFD drives that can do this usually incorporate the words flux and vector some where in their description. DC motors require specialist care and maintenance but the squirrel cage motor is virtually bulletproof with a minimum of maintenance.

A DC motor is much more complex to make than a squirrel cage motor and requires a fairly complex control, an AC motor and pulleys or gearbox is simple and effective for a machine tool drive, but as many have found, a VFD can make a machine very flexible and a pleasure to use.

Mike

JA03/02/2016 15:01:09
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Don't know but let's have a go at answering the original question.

I assume the power of a motor is based on the supply current and voltage and not measured on a brake. If so the delivered power would depend on the motor and gearing if necessary. Hazarding a guess I would think that an induction motor would be slightly more efficient than a brushed non-permanent magnetic field DC motor: both would have similar field and bearing losses but there would be losses at the commutator. If the motor required a gearbox that would add to the losses. There are other smaller losses like windage.

I have to admit that I last did any motor analysis at tech nearly fifty years ago.

JA

Ajohnw03/02/2016 15:02:40
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Actually I don't think that an AC motor will produce much torque at 1500, 3000 rpm as mentioned for 50Hz. That's why circa 1400 and 2,800 are more usual and 3000 rpm off hand grinders wild claims. There needs to be some slip in practice.

John

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Clive Foster03/02/2016 15:03:51
3630 forum posts
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In practice there is often a considerable difference between perceived and actual power. Actual power really only matters at full load which pretty much never happens in most peoples real world experience. A vacuum cleaner is probably the closest to full time, full power operation that most of us will ever see.

What we perceive as power is really the torque / load / rpm characteristic. When you increase the load on rotating machine the motor needs to generate more torque. For normal devices lacking relatively sophisticated controls the motor has to slow down to generate more torque. A motor which slows down more than you expect for any increase in load is seen as being less powerful, or in the vernacular, gutless. In contrast one that absorbs load increases with very little change in speed is often said to be torquey and will give the impression of being more powerful whatever the dyno may say. I suppose a proper analysis of work done over time might vindicate this impression.

Clive

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