DC drive specification help needed

[gravitar]

Hello! I've been asked to help in specifying a DC drive, and that isn't an area I'm familiar with.. I've only had to deal with low-HP drives that run on 120VAC mains. Anyway, I'm supposed to be using the A-B 1395 or 1397 family if possble. The application has been described as follows: Arm 152 Vdc / 32 A /3000 rpm Field : 10 A input : 380 V (is this enough information to select the drive?) Looking at the 1395 and 1397 brochures, it doesn't look like you can get a 150V armature voltage when you have 380VAC mains. Unless there's a parameter you can set to limit this voltage down to 150? If any of you that have experience with DC drives could give me their opinion, I'd be grateful!

[comeng]

sounds like someone has given you the partial details on a motor for example there is no mention of tacho feedback or braking If I were you I would go and look at the application get all the details and then speak to your local AB dealer particularly ask for somebody who understands DC drives because if the US is anything like the UK then finding somebody who knows DC now is rare

[P Daniil]

Before deciding on the drive, find out if the application is speed or position control. For speed regulation a four quadrant thyristor drive is most efficient as it can motor and break the load in both directions with minimal energy dissipation. Position control is trickier with DC motors as the number of current commutations per motor revolution and the size/footprint of the brushes limits the attainable position accuracy/resolution. Normally DC motors are specified in DC terms. So the 152 Vdc armature voltage can be derived from rectified 110 VAC and the field 380 V should refer to a dc quantity. What strikes me however with your specs is that the field excitation is high. Typically the field power is <5% of the armature and designed to operate from the same line, so maybe this is a specially wound motor designed for a specific type of operation. Normally DC motors are excited by a constant DC field/voltage (replaced by permanent magnets in small HPs) and controlled by adjusting the armature current. If there is still a nameplate on the motor can you provide its details?

[gravitar]

Thank you for the advice. I will ask if there's a nameplate on the motor that I can get a picture of. As far as the 380V, I *THINK* that's referring to 380VAC incoming line voltage. This installation is in Indonesia, perhaps that is the standard for industrial equipment there as opposed to 460VAC or 575VAC.

[comeng]

380VAC used to be a common European voltage Italy I think I remember getting motors from at this voltage. Working at the distance you are I would suggest it is even more important to find out how the drive is expected to work rather than just specifying any drive for the motor

[TERdON]

comeng: It used to be 220/380 V in Sweden as well, and in several other european countries as well, but it has been standardized on 230/400 instead. The difference really isn't that important - both voltages are within the error margin of the other, just around 5% off.

[comeng]

I work in an old factory where most of the transformers were tapped at 440V not 415 and the 380V motors did not like it needless to say the tranformers were stepped down to 415V and maybe at some point we will take the ones we can down to 400 but not soon i am betting

[paulengr]

Torque is also important. Most DC motors wouldn't go up to 3000 RPM unless you used field weakening (which drastically cuts the torque). Most DC drives can do field weakening but it's an option that you're probably going to need. In field weakening mode, you reduce the field voltage which increases the speed on the motor at the expense of torque loss. Field weakening is also a scary thing to deal with if it happens unintentionally with a drive that is not designed to detect and stop it...

[paulengr]

It just occurred to me that you need to look into one more thing: is a DC motor even appropriate? At one time any time that you involve speed or torque control, you were limited to two choices: hydraulic motors (or even hydraulic/mechanical variable speed drives), or DC motors. Especially now with flux vector drives, virtually all DC motor applications have been replaced with AC motor ones. The motors are cheaper, available off-the-shelf, much more rugged and reliable, and costs are much lower overall. With DC motors in contrast, you have to constantly do maintenance on them. For the DC motors in my plant, there only seem to be two conditions: something is failing internally, and something already has failed internally. Compared to costs of rebuilding an AC motor (if it's even worth it to rewind), figure about 4 times that for a DC motor of comparable size. And since the DC motor is already 4-10X more expensive, you WILL be spending a lot of money sending them to a motor shop. For a similar AC motor, I could buy 2-3 AC motors for every single DC motor rebuild job. Example: 15 HP DC motor usually runs $1200-$2000 per rebuild. And yes, this is a foundry...they don't seem to understand that liquid metal, metal dust, chipping hammers, water, oil, and motors are NOT good companions. The exceptions are: 1. AC motors can't run at extremely low creep speeds and don't have any braking capability at standstill. Some CNC applications require this. This still the world of DC. On the other hand, you may be better off with a servo motor or stepper motor which has the opposite characteristics of an AC motor...maximum torque at zero speed and gradually falling off as the speeds increase. These motors have the same rugged (extremely simple mechanical design) internals which make the AC motor desirable over DC. Servo motors are not nearly as torque and speed limited anymore either. Much larger sizes and speeds were developed in response to the needs of the robotics industry. 2. If you require very high startup torques (starting at full load), this pushes you into oversizing the AC motor to accomodate the weak startup characteristics of an AC motor. Usually, the motor is 150% to 200% of the normal required horsepower. Depending on the startup load requirements, this may push you into the DC world. In a long conveyor belt, you can overcome this by installing a mechanical clutch mechanism. The clutch allows the AC motor to get up to speed initially and after that point, everything runs as a normal AC drive application where it is only needed for speed control at rate.

[gravitar]

Thanks for all the advise! I went over to my A-B distributor to pick up some literature, and lucky enough one of the drives people was at the front counter, so I told her what I was asked to supply. It was as if she had been on MrPLC.com or something.. First thing she said was why don't I try to convert over to an AC motor.. Second thing was, what's the application :) She said that the 1395 was a poor choice, and the 1397 isn't a wonderful one, either. She suggested I take a look at what Reliance has to offer (in DC drives), so I grabbed one of their catalogs too. The first roadblock I'm noticing with ANY of these DC drive families is trying to get ~150V output when the input is 380VAC. Seems that when you choose an input voltage that high, an output voltage that low isn't an option.

[michael G]

Correct for a DOL Motor - incorrect for a PWM or Vector Drive system, talk to your drive supplier for more information. I have used Open loop vector control on a 3 km undergrould mining conveyor that would start fully loaded and with the tail bogged (not recommended but could do it).

[P Daniil]

When choosing a technology (DC, AC, DC brushless, AC servo etc) for an application, one of the things to check is what happens to the generated energy when the motor slows down the load. In the case of a four quadrant thyristor DC drive controlling a DC motor, this (re)generated energy flows back to the supply line (ie the area grid) which, because of its capacity, can absorb any practical amount and so break the load with theoretically unlimited torque. With the exception of ¨matrix¨ drives (still in R&D labs as far as I know) all other technologies will either use the internal DC link capacitors to absorb the energy and after that dump it to an external heat dissipating resistor. (It is interesting to note that hoist drives, due to safety concerns/regulations, inject DC current and effectively break the AC machine in DC mode). It is this inherent energy efficiency of the DC drive and motor which make them suitable for high power applications as the energy costs are significant. So, going back to your <10 KW application it may be a good idea to find out why a DC motor was used there in the first place. If there is minimal load breaking and mechanical refitting is not a problem, it should make economic sense to change to either an AC inverter drive and induction motor combination (speed control) or an AC servo drive and brushless motor combination (position control). If you have to keep this DC motor, the main drive specification is the current (32 A) required to turn the load at the given revs (3000 rpm, but normally a torque would be specified here as well). When driven under the specified conditions the motor will develop 152 V at the armature, so you are OK working off the 3x380 VAC line or a 3x200 VAC transformer secondary. Again what strikes me is the high field current. Maybe it is misquoted (1 A instead of 10 A). Any news on the motor nameplate? In this part of the world Control Techniques is a popular choice for DC drives.

[michael G]

Most AC drive suppliers now provide regen units that give unity powerfactor and full regeneration on the mains. eg. Control Techniques in their "Unidrive" product. Also in large unwind/rewind systems don't forget to look at common DC bus systems for AC drives. IMHO the higher initial costs of the AC drives systems (regen) outweighs the DC motor maintenance. MG