Replacing DC drive

Hi, we are replacing an old Reliance DC drive with a Quantum 3 drive. The motor has a 240 vdc armature. The iso transformer for the drive is 480 vac 3 phase. We already have the drive here and we read in the manual that it is not recommended to use 480 vac on a drive for a 240 vdc motor. We were going to try it anyway, thinking that it has worked for over 20 years with a Reliance drive with 480 vac, but a tech from Quantum said he did not think there was any way it could have running like that for years. Has anyone seen 240 vdc motors operate from drives with 480 vac supply voltage? Also the transformer is delta wye and the transformer has a wire going from nuetral junction on wye side, going to armature. Why would it be wired like this? Is the wire from transformer to armature to give armature ground? We are going to order a step down transformer, but I would like to learn more about this. Thanks for any information.

An incoming 480 VAC 3 phases DC drive can drive a 240 VDC armature motor by setting the maximum armature voltage to 240 VDC, don´t know about field voltage of the motor but you have to set the field voltage of the drive to be the same as you field motor voltage.

Anyway It´s a waste, ideally you should use a 240 VAC 3 phases in order to drive a 240 VDC motor.

Well, the problem will be that your firing angle of the SCR/IGBT will be so low that you might get poor regulation.

The field regulators of drives typically want either an isolated single phase voltage, or at a minimum, voltage really close to the rated voltage of the field, for the same reason...regulation will be better with a longer firing angle.

Your logic is interesting...why compare the performance specifications of a 20 year old drive with a new drive?

And your transformer arrangement is unusual, for sure. Sounds like they are trying to hold something to a maximum voltage, or tie the phase somehow back to the source. I'll leave it to the even older-old timers to answer this one.

Thanks for the replies. I was just surprised when I was told a new drive could not do what a 20 year drive has been doing. I think the Quantum guy may have thought I was lying about the 480 vac drive used with a 240 vdc armature motor. He did not think it could be done because of current peaks.

Rtm

I worked with a lot of old reliance drives that had a jumper selectable transformer feeding the field supply. That is where a variety of field voltages were selectable, with 500vdc being the default for a 480volt input. I believe the armature voltage was set with small pin jumpers but I am not sure how that affected the armature regulation electrically.

If I were you, I would locate a manual for your new drive and study it carefully. Often, the distributor/salesperson will not be aware of all the features it may have. The user manual should give you a definitive yes or no answer.

EDIT: How hard would it be to change the output of the secondary of your isolation transformer to 240vac? If so, will it still handle the current inrush and continuous load of the new drive?

EDIT: I have seen quite a few DC drives in which one lead from a transformer is at the same potential (same terminal block sometimes or jumpered right to it) with one of the armature leads. Can't remember the details, but I don't think its that uncommon.

I'd also double and triple check that the reliance drive was actually a nominal 500VDC drive (normal for 480VAC input now). I'm thinking you might have had a 3 pulse drive, which returned one end of the motor armature to the neutral tap on the transformer, to give cheap regen action, which would be used with a 250 VDC motor.

Running a 250VDC Motor on a 500VDC drive will result in serious (even above and beyond normal DC Machine) efficiency loss.

The Quantum tech is correct, you do not want to run a 240 VDC motor from a 480 volt line if the drive is a 6 pulse unit, as is the Quantum. The average DC voltage may be 240 volts but the peak voltage can be up to the peak voltage of the 480 volt line (670 volts). The commutator bar to bar voltage will be excessive and will probably cause a flash over.

Ever notice that the commutator segments are much narrower on a 500 volt motor than on a 240 volt motor. The reason is to spread the voltage over more bars between adjacent brushes, thus reducing the bar to bar voltage.

Rdrast is correct, the old drive is a 3 pulse unit. This type of drive produced half wave rectified DC. The neutral point of the wye is the return path so the drive is really operating from 480/1.73 or 277 volts, which was fine for a 240 volt motor.

First off it is hard to beat a Reliance drive, even the 40-50 year old VS drives.

The manual for the Quantum states Page 47 Chater 8.4:

NOTE: 240VDC MOTORS

It is recommended that when operating 240vdc motors that your 3 phase Input AC Line voltage is also 240v. We do not recommend using a 480vac drive to power a 240vdc motor. Doing so results in high motor armature current peaks which can cause excessive motor heating and possible damage. In addition, the high voltage peaks delivered by the high AC line can potentially breakdown motor insulation which can result in both motor and drive damage. If your main power is 480vac, we would recommend the use of an Isolation Step Down Transformer from 480vac to 240vac.​

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Look closely at the transformer. It probably can be programmed to provide the actual 230 volt input for the drive.