DC Motors Vs AC induction (single or three phase)

[Neil WyattModerator @neilwyatt]

Posted by Andrew Johnston on 10/02/2016 22:51:38:

Posted by Neil Wyatt on 10/02/2016 20:27:06:

Variable torque at stall is one thing, but try achieving it at 20 rpm using an induction motor or a brushed DC one.

I don't know about brushed DC motors but I don't see what the difficulty is for induction motors? At least one of the inverter designs I was involved with could produce torque at zero speed. That's important for electric vehicle applications as it is the equivalent of slipping the clutch on a hill start.

Andrew

I KNOW!

Read what I am trying to say not what you think I am trying to say.

I agree that any fool can apply variable torque to a stationary motor using PWM or even a rheostat!

The challenge is maintaining constant rotational speed under variable torque.

What I am saying is that with a well-designed BLDC controller you can control the slow rotation of a slow motor like a servo-motor using vector control that effectively allows extra torque to be applied when needed. Unlike simply pumping up the PWM percentage on an ordinary motor, done well this won't cause the motor to speed up when the load is reduced.

You CAN'T do this with a standard brushed motor although I can accept you can do it with split winding motors (which is more like replacing the magnets of a BLDC with separate field windings than replacing the electronics with brushes).

Trivial but pertinent example – you can use a BLDC motor as a loudspeaker, try doing that with a brushed motor.

Neil

[AjohnwParticipant @ajohnw51620]

Actually Neil by looking at the voltage and current behaviour of a dc motor it is possible to maintain a constant speed with varying load but the usual way is to fit a tacho as it's more precise.

Also the basis of vector control of stems from this aspect and it's eventual use on 3phase ac motors

**LINK**

Forgot add that the same principle has been used in electric hand drills and related items for a long long time – even electric model loco's.

It sounds far more impressive when some one comes up with a nice technical name for it.

John

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Edited By Ajohnw on 11/02/2016 13:25:27

[Martin WParticipant @martinw]

Neil

I hope I have not missed the point but with a standard brushed DC motor with permanent magnets it is possible to maintain a relatively constant speed under varying torque conditions. One way of doing this is to measure the back emf of the motor by interrupting the power applied, off period during the pwm cycle, and comparing this to a preset value.The power applied, pwm duty cycle or DC voltage, can then be varied so that the power applied to the motor either increases or decreases and the measured back emf and hence motor speed remains constant under varying load conditions.

Cheers

Martin

[MWParticipant @mw27036]

There's also the cooling consideration, most totally enclosed DC motors i've seen never use fan cooling, yet they are dependent on the temperature in order to operate well, a cost cutting exercise or a design flaw?

Michael W

[MuzzerParticipant @muzzer]

Read what I am trying to say not what you think I am trying to say….

You CAN'T do this with a standard brushed motor

Neil

Neil

I suspect your point is that synchronous motors can be operated open loop and the speed control will be maintained – unless you exceed the breakover torque. That's true – and it isn't true with brushed motors, which is the point I think you are trying to make.

However, with the addition of a speed sensor, it's a simple matter of implementing a closed loop speed controller around a brushed motor. For instance, Curtis Instruments make a wide range of speed controllers for brushed motors for mobility scooters, golf karts etc.

In both cases to control the speed you require a motor controller of some form, the only question being whether you need a speed sensor. You certainly do for brushed but given that it's a pretty trivial addition relative to the rest of the controller, you can understand the confusion when you say it can't be done. I think that's where the frustration arises.

Murray

[MuzzerParticipant @muzzer]

404 error followed by double post…

Edited By Muzzer on 11/02/2016 15:18:52

[AjohnwParticipant @ajohnw51620]

Posted by Muzzer on 11/02/2016 15:17:52:

Read what I am trying to say not what you think I am trying to say….

You CAN'T do this with a standard brushed motor

Neil

Neil

I suspect your point is that synchronous motors can be operated open loop and the speed control will be maintained – unless you exceed the breakover torque. That's true – and it isn't true with brushed motors, which is the point I think you are trying to make.

Murray

The problem though is that the same is true of brushed motors. How some one might go about that can vary from simple as per electric drills etc and it seems electric model loco's or in more complicated fashions.

Actually they tend to be constant speed devices as the back emf aspect is self correcting, motor speed goes down, back emf drops, current goes up stable speed achieved again – if only it was really as simple as that. With super conductors it might be. There are also the various types shunt, series and a mix / compound motors. There is a noddy view on these here

**LINK**

A brushless or perm mag motor has to effectively be shunt

John

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[John HaineParticipant @johnhaine32865]

Just to add a couple of points. What causes much of the variation of speed of a wound or permanent field DC motor is the series resistance of the rotor, which drops some of the applied volts in addition to the back emf. When you load up the motor, the total of IR drop and back emf has to equal the supply voltage, so if I is bigger (as you're asking the motor to deliver power), back emf is smaller, so speed must go down. Quite a few speed controller compensate for this by electronically inserting a compensating negative resistance in series with the motor to cancel out the positive rotor resistance. The KBE controller I have on my little mill is a case in point – they supply a range of sense resistors that plug in in series with the motor depending on its current rating. Once you "cancel" the positive R the speed depends only on the applied voltage.

On induction motors, I have been surprised at how much torque the motor on my mill controlled by a VFD seems to deliver even at very low speed (i.e. low frequency). But thinking about it, the rotor sees the stator field rotating at mains frequency minus rotational frequency (i.e. the slip frequency). If you reduced the mains frequency say to the slip frequency but kept the peak current the same, the stator flux is the same, so the rotor would see exactly the same flux even when it's stationary, so will deliver the same torque. So I can see why induction motors can deliver high torque at low speeds with VFD control, and if you sensed the shaft position with an encoder you could "brake" the load, at least up to the point where the stator current gets too high for the VFD or winding.

[AjohnwParticipant @ajohnw51620]

A common way of getting more precise speed control with dc motors is to arrange for them to run at the required rpm at 180V with no load and drive them from 240v via certain circuitry. That way as the winding resistance causes problems when the load and current increase the extra voltage that is available can be used to maintain the correct back emf for the speed required.

This is usually applied to universal motors driven from AC but the same principle can be used with a dc drive. On AC the 180v is the rms value of the wave form, only part of the mains cycle is allowed to drive the motor. The same sort of thing would be true of DC drive.

In practice so that the control loop can be control fully from no load to full load they generally never see the full voltage.

John

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[John FieldingParticipant @johnfielding34086]

Posted by Ajohnw on 03/02/2016 15:02:40:

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

Yes you are quite correct for a single phase or 3-phase motor. In both types the rotating magnetic flux is generated in the stator winding(s) and the rotor is a series of shorted turns that generate a high circulating current when there is slip.

If the rotor managed to approach synchronous speed the rotor current falls away. Hence, maximum torque occurs when there is a large amount of slip, as when starting from zero speed. That is why an ac induction motor draws heavy current on starting. It is simply a rotary transformer with the primary being the stator winding and the secondary is the rotor windings.

If you load up the motor with a heavy load it behaves like any other transformer, high load equals high primary current and hence high secondary current. To generate torque there has to be some slip. That is why the rotor never gets to synchronous speed, it always runs below it. On 50 Hz with a 2-pole motor the synchronous speed is 3,000 rpm and a 4-pole motor is 1,500 rpm. If you had a 6-pole or 8-pole motor – and they do exist – then the speed would be less.

Using variable speed drives you can alter the frequency of the supply and hence obtain lower speeds, but this is a deep subject and not many people really understand the technique from what I have seen in print! A common misunderstanding is how low the speed can be safely reduced. Most reputable ac motor manufacturers frown on speeds less than 1/3rd of the normal 50 Hz or 60 Hz application because the cooling becomes a major concern and the voltage supplied to the stator winding has to be reduced to limit the primary current. The limitation of the primary (stator) current is the inductance of the windings.

At high frequency – read 50 Hz or 60 Hz – the main current limitation is performed by the reactance of the winding which is a linear law. If you double the frequency for the same applied voltage the current drops to half. However, if you halve the frequency the current doubles, unless you also reduce the supply voltage. So there is a point in the speed reduction curve where it is necessary to begin lowering the supply voltage to keep the primary current to a safe level. This is the basis of simple VSD electronics. By lowering the frequency and hence the supply voltage to get a very low speed then ultimately you run out of torque and the speed drops under heavy load. With a very low speed the internal fan cooling is not enough to keep the windings at a safe temperature and they can overheat.

 

Edited By John Fielding on 17/02/2016 14:23:10

[John HaineParticipant @johnhaine32865]

John F, if your comments are responding to mine, I specifically said "keeping the peak current the same" which is what the vfd does as it has current sensing. The stator voltage will of course have to reduce proportional to frequency. Yes, the problem is the cooling as the fan gets less effective.

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