# DC Drive

S

#### S. N. Bhagat

There are two identical separately excited DC motors ( 147KW, 460V, 705A) connected parallely have been coupled with one single load (Winch). The speed control is through Thyristor control using Allen Bradely Digital regulator 1395 in the armature. Field is adjusted and kept at certain value. Since the motors are connected in parallel, same output voltage is applied to both the motors.

Now while running the load at certain speed reference especially at high speed, it is observed that both the motors take different current. The difference of the current is as high as 300 A. Many of the times one goes into generating mode also.

It has been tried to trim the field to make both the motors share equal load, but it did not work.
1. Is there other method to control the load sharing of the two motors ?
2. What should be the maximum allowable load current difference ?
3. Will the difference of the two motors come down with increase of load?

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#### Jim Kline

If your motors are the compound (stabilized shunt) type, reversed polarity connection of the series winding can cause this issue.

[email protected]

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#### P.W.dalvi

1. Field of both the motors should be in series .
2. The control should be master/slave control.
3. Load current sharing must be within the range of must be within 10 %.
4.I dont think the difference will come down as the load increases.

Please let me know whether this has helped to slove your problem Best of luck. my id is [email protected]

regards pwd. 29/5

J

#### Johan Bengtsson

Is there any possibility at all that you can connect the motors in series instead?

Ok, I realise this will put completely different requrements on either the motors or the drive or both so it might not be possible for you do do that at all.

A DC motor is essentially a voltage=speed motor, ie twise the voltage = twice the speed. It is of course not really that easy but it is what the motor aims at. If the two motors would be exacly identical they would rotate with exacly the same speed when connected in parallel, sounds great?
Well, the bad news: they are not exactly identical, that means one is trying to run a little bit faster than the other.
Ok, you probably know this already and sice you have tried to adjust the field voltage you know how to change this.

The problem if of course that the motors are forced to run at the same speed by a mechanical link. And since they try to run at slightly different speed the one wanting to run fastest will take most of the load, and as you say, the slowest might even become a generator instead of a motor.

Another thing with the DC motor is that torque=current. If you could connect the motors in series they have the same current, and thereby the same torque. Since they are mecanically linked they will automatically have the same speed
and if they are similar they will have a similar voltage.

The problem is that you most certanly can't do this with what you have, I realize that.

Now possible solutions:
- One possible solution would be to have another voltage/current ratio on the motor or the drive and do as suggested above - this would however of course be quite expensive.
(By the way, could someone comment on if this would be legal?
just because it would work will not mean it won't break some electrical code somewhere)

- Another possible solution is to use the series winding on the motors if there is one since that one will make the motor drop more in speed when it gets loaded, but be careful to connect it
with the correct polarity since it will do it worse otherwise. The idea with the series winding is that it should icrease the field when the current increases and thereby make the motor
slower and stronger

- Next possibility is to have one drive for each motor and force one of them to output se same current as the forst one (check with drive manufacturer how this is done)

- Yet another is to have a circuit monitor the currents and control the field voltage of one of the motors within some narrow limits depending on difference in current. Preferably some PID controller I think.

Buf first of all you should check the suggestion of Jim Kline since it is quite possible it is that easy and that would not cost even a fraction of whatever I suggested.

/Johan Bengtsson

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M

#### Michael Griffin

I believe the conventional solution to this problem is to connect the motors in series, rather than in parallel.

With a normal DC motor (shunt wound or permanent magnet field), speed is proportional to voltage, and torque is proportional to current.
Since these motors are mechanically coupled together they will necessarily run at the same speed. However, when wired in parallel there is no reason why they would share the load equally as the two motors are unlikely to have identical load curves even if they are nominally the same. If the motors are connected in series though, they will always see the same current, and
therefore generate the same torque (approximately).

This subject has been addressed before on this list. You might search the archives for more details.

**********************
Michael Griffin
[email protected]
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K

#### Kelly, Rick

Hello...

My electrical apprenticeship was served at a printing plant. We had four production roto-gravure printing presses. Each press consisted of eight units and one folder. Each of these eight units were 6 feet deep, 10 foot wide and stood 35 feet tall. This press arrangement could produce a 48 page full color magazine or catalog at a rate of 50,000 per hour. Each of the eight units were driven by a common drive shaft. Connected to the drive shaft was a 250 HP DC motor powered by a very early thyristor drive.

We could, when required, close a series of clutches and connect two of the presses together. This then created one press of 16 units and one folder. This allowed us to produce a 96 page publication. Like your winch... we had
a 250 HP motor at one end and a 250 HP motor at the other end. Both motors were now connected to the same drive shaft and were controlled by two
separate DC drives.

The normal speed reference for the drives came from a motor powered rheostat. While connected in the 16 unit configuration the reference for
each of the drives came from a special load sharing device that watched the motor speeds (via a tach generator on each motor), the current being drawn by and the voltage being supplied to each motor. This device allowed us to decide what percentage of the total load was supplied by what motor and could even switch (what motor was the lead) back and forth on a time basis. It worked quite well and gave so no problems. The device was made by a British company that made printing press drive systems called 'Whitton-James". Maybe they can offer a similar solution for you as well. In any case I hope this is of some help to you.

Best Regards...

Rick Kelly

Chief Electrical Technician
Natural Cuts, Cheese Operations

[email protected]
(613) 537-8069 (V)
(613) 537-8044 (F)

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#### S. N. Bhagat

Dear Mr. JimKline

Thanks for your reply. In fact these are the old motors with no details and document. I have traced the winding connections with Brush (B1, B2..) and Interpoles ( IP1, IP2…). We have made the connection as below:

1. Terminal 1 & 7 shorted and positive supply connected
2. Terminal 2 & 8 shorted and positive supply connected
3. Terminal 3, 4 and 9,10 shorted together
4. Negative terminal connected to Terminal 5,6.
Kindly suggest if the connection made as above is correct. Please suggest if any alternative better connection can be made.

Thanks and regards,

S. N. Bhagat