Why are DC motors used for low speed high torque applications 3

[rockman7892]

I have read some articles which mentioned DC motors and drives being used on several applications that required high torque at low speeds for starting torque or during operation. This may have been because the AC drive technology was not as advanced in the past and couldn't provide full torque at low speeds until recent. I understand now that modern AC VFD's are capable of producing full torque at low speeds.

My question is why DC VFD/Motors were used (and maybe sometimes still are used) for low speed high torque applications. What made them a better fit for providing high toque at low speeds as opposed to their AC counterparts? Does it have anything to do with the fact that you could vary the field intensity in the DC motors?

 

[DickDV]

Probably the best illustration of high torque and low speed is traction motors for railroad locomotives. Until about 15 years ago, DC motors were the only technology in use due to their ability to develop high torque at low speed. This is particularly true of series wound field DC motors.

About 15 years ago, AC traction appeared and also proved very capable of developing high torques at low speeds. They did this however with somewhat higher initial costs.

Today, the cost difference is shrinking. Anyone considering drive/motor systems that does life-cycle costing will almost always choose AC since the motor maintenance is significantly less and the efficiency is slightly better. In addition, where space is tight, the AC motor will be considerably smaller and, as a result, the dynamic performance will usually be better with the smaller lighter rotor.

In my experience, there are two exceptions where DC systems still rule. First is the very small drive-on-an-aluminum-plate and permanent magnet motor systems (0-3hp), and large four quadrant systems (500hp+) like steel rolling mills. In both cases, it is cost that makes the difference since performance has pretty much been matched either way.

 

[rockman7892]

What gives DC motors the abaility to develop high torque at low speeds?

I guess with newer AC technology this is acomplished with vector control, but what gives DC motors this ability, especially with series wound motors?

 

[waross]

You were close with "Vary the field". The speed of a DC motor is determined by the voltage applied to it.
A voltage output of a DC generator may be controlled by varying the field strength.
The old DC drives used a DC motor in series with a DC generator. The field of the motor would be full on. The voltage output of the generator and as a result, the speed of the motor would be controlled by varying the field strength of the generator. This arrangement could develop full torque (Full load current) indefinitely at any speed. A external cooling fan would be installed on the motor if the application called for extended operation at low speed.
Google Ward-Leonard.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

DC speed/field controls are much more simpler, precise and rugged and can be accomplished without use of electronics, even today, if you need to.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

The last large sale DC drive project I worked on had electronics in place of the amplidynes that were used for years. An amplidyne is a rotating DC amplifier with a gain of about 750.
The rest of the system was classic Ward-Leonard derived. The electronic amplidynes controled the fields of the exciting generators which in turn controlled the fields of the main generators. Field forcing was used for fast response.
This was a large dragline for excavating bitumen saturated sand in the Athabasca area of Northern Alberta.
As memory serves, the drives were sized as follows.
Main hoist drums;
8 x 1300 HP
Drag drums;
8 x 1050 HP
Swing 4 x 1300 HP
Walking;
4 x 1050 HP
The generators were driven by 4 x 3000 HP synchronous motors.
All the motors would never be loaded to maximum simultaneously.
The machine is now a museum piece.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

There is no universal truth. It changes with time and as technology develops.

When AC drives started to be used, they usually had a problem delivering rated torque below around 10 % of rated speed. The reason was that parasitic elements in the motor windings (resistance mainly) were more dominant at lower frequencies because the induced voltage vectors are proportional to speed while the parasitic (resistive) vectors are torque and temperature (winding's resistance changes with temperature) dependent.

As voltage vectors get smaller and resistive voltage drop gets higher, as it does at low speed and high torque, the earlier drives had a problem knowing what was happening in the motor. Adding an encoder helped, but the dynamics was still so bad that hoisting applications were still built with DC motors, where no such uncertainties are present.

Things have changed. Better algorithms and better motor designs, especially PM motors, make AC motors as crisp - or crisper - than any DC motor and torque at low speeds is also good.

Still, DC motors are being developed and built. ABB has a range of DC motors that is being used in harbour cranes, rolling mills and other demanding applications. In some cases, the DC motor's high torque/weight ratio is an advantage.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[DickDV]

Specific to the question of why series-wound DC motors develop such high torque at low speeds; I would suggest first to rockman7892 that a wiring diagram of such a motor be obtained. Second, notice that the field winding is in series with the armature winding so whatever current flows thru the armature also flows thru the field.

Third, avoiding lots of math here, the torque in the motor shaft is caused by the interaction of the magnetic flux from the armature and the magnetic flux from the field. The armature flux is directly proportional to the armature current and the field flux is also directly proportional to the field current. Since the field current is the armature current in this motor, it follows that the shaft torque is proportional to the square of the armature current. This is the key to understanding how the series-wound DC motor develops so much torque.

What helps even further is the fact that, as the motor slows down, it develops less reverse voltage so the armature current can be made to flow with very low voltages.

Conversely, as a series-wound DC motor speeds up, the internally generated reverse voltage bucks the applied voltage and the armature current begins to fall. The effect on the armature shaft torque is amplified due to the current-squared function so the torque drops off dramatically as the motor speeds up. So, at higher speeds, the motor is very wimpy for its size but, at lower speeds and especially at stall, it will dig in and comparatively roll the earth over trying to start a load. Perfect for starting a heavy train in cold weather or other very hard-starting loads.

Hope this provides an acceptable answer to your question.

 

[waross]

Series motors are great for starting engines but the massive current demanded by larger sizes make series motors unfeasible at higher ratings. Traction motors that i have worked on were shunt motors with full voltage applied to the field and a variable voltage applied to the armature.

it follows that the shaft torque is proportional to the square of the armature current.

This works for shunt wound motors also. The maximum current is dependent on the capacity of the supply, so with a fixed maximum current available, the shunt motor may develop as much torque as a supply limited series motor. The available torque of the shunt motor does not drop with increased speed.


Bill
--------------------
"Why not the best?"
Jimmy Carter