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

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.

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.

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.

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

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

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

Yes and no. You are answering a different question. The "synchronous" speed is what you would see if there were no net load (read my words above), including windage and bearing loss (not quite practically possible). The only way to make them go faster is to give them a negative load ie make them regenerate. The machine works in exactly the same way but now the torque is opposite and the motor generates electrical power.

The rated power is with some slip, like about 5-10% or so of base frequency. It's usually thermally limited.

The reason for the belt drive being more powerful is because that style of speed reduction actually increases torque so apart from losses in the drive power remains the same. Electronic speed control on either type of motor doesn't do that. Torque tends to be constant so actual power is reduced as the speed is slowed down. Some ac inverter drives model the heating effects of the motor so may actually drop the troque. This happens because the motors have a max current rating and running them more slowly doesn't mean that they can pass more current – less in practice because at some point they will overheat as the usual cooling fan is also running at a lower speed. Actually in principle it's better to speed them up but at some point the armature will burst due to centrifugal forces.

As Ketan of Arceuro mentioned it's hard to be sure about what the actual output power of variable speed machine tools really is. The ones he sells are rated on output power. Some may be rated on input power. Either might have power stated for a certain time limit. Some AC motors state continuous some don't.

John

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Edited By Ajohnw on 03/02/2016 17:56:07