DC Motors Vs AC induction (single or three phase)

All BL is Neil is a dc motor that replaces the com with electronics. It's as simple as that.

John

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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

The EV's I worked on Andrew did the same with a DC motor.3 tonne GVW, van, single decker bus / coach and a taxi. They all did a real 50mph under normal road conditions not just on the flat, no clutch no gearbox and not a lot of gearing down to the drive either.

Some were also working on ac drives but it was a tough / inefficient method using the semiconductors that were available in the mid 70's so never went anywhere. I'd be more inclined to say impossible really at these power levels with any sensible levels of efficiency. Transistors 1/2 the size of a house brick also had problems competing with thyristors in a number of respects.

The bloke that initially developed the motor could actually pick it up and carry it around - No one else I know could and I expect he used it as a form of weight training.

The starting torque problems of AC motors that I am aware of mostly relate to single phase drive. Hence machine tools mostly using 3.

And as to brushless I can't really see them competing with what sep x can do if needed as the field on them is fixed.

Some of the claims for all sorts of things are crazy. I can remember reading about room temperature super conductors in Nature donkeys years ago. I just can't understand why they haven't hit the market.

mmmm I just wondered what would happen if the magnets were moved further out mechanically.

John

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Edited By Ajohnw on 10/02/2016 23:37:47

Edited By Ajohnw on 10/02/2016 23:39:17

I am sitting in a car manufacturing body shop and nearly every squirrel cage induction motor has an inverter of some description. All the hoists, high speed shuttles, turtables, trunnion units and lift tables are fitted with encoders on the motor for speed and position feedback, they all position to a stop and in the case of the hoist applications hold the full load at zero rpm before the brake is applied, the motor is also in full control before the brake is released. Positioning accuracy for these applications is set at a few 10ths of a millimeter although the drives can do better. The inverters for this work are made by SEW eurodrive and are Movidrive B units. The 1100 robots use BLDC motors with resolver feedback, Not a DC motor to be found in the place any more.

Mike

Is it?

How can you oppose the forces from different windings using a brushed motor?

N.

Edited By Neil Wyatt on 11/02/2016 08:26:18

Read my comment carefully. Variable torque at stall is trivial. Variable torque at constant low speed is not.

The level of field excitation plays a huge part in how dc motors behave - including allowing hill hold in the case I mentioned. LOL Downhill hold has more limitations but vehicles generally have brakes but in other case the regen can be varied.

AC inverter drive are constant torque from zero speed with a catch that we all know about however there are various type of inverter rated motors.

John

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Is it me or have some posts been removed from this thread ???

Martin

Wrong Neil. They are driven is a loosely similar fashion to stepping motors. In practice they are perm mag motors where the com has been replaced with electronics. In fact in theory some one could take a perm mag motor and convert it to brushless but they would have to take the drive off the case, a so called out runner.

When they could be found servo motors for some application could be driven from rectified mains. Due to them being so over built for reliable long term operation.

John

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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

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

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

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

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

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

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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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.

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

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.