DC motor increased speed

The application consists of a 100hp 500vdc 1750rpm motor(compound 300vdc field winding & series winding in-line with the armature)controlled by an older reliance flexpak 300 dc drive. There is a speed tach used for feedback. The motors measurements display 478vdc 33% of FLA at 1725rpm under Full-load. The customer is looking for the motor to go faster.
I have read the forum responses rearding field weakening to gain more speed. It appears this application is already at or very close to max speed with the 478dcv applied. I am guessing a new motor (higher rpm) and possibly new dc drive is whats needed. Any thoughts or input is greatly appreciated.

I think you could increase the speed somewhat by reducing the 300vdc field no more than 25 percent only after the Armature voltage is maxed out and maintained at that value. There is going to be an issue with the feedback tach since you want to maintain the Armature voltage at the Max amount. Once the armature goes above baseline speed, the feedback tach is going to reduce the Armature voltage output of the drive. It's not a simple change and probably requires a re-design of the drive.

Flexpak3000's had a module that could do field weakening for you, or you could change the drive pulley sizes to change the gearing if you are only running at 33% FLA this would increase the load without increasing the motor speed, but increase the loads speed.

First, check the Maximum Safe Speed listed on the motor nameplate. Do not attempt under any conditions to exceed that speed.

Second, since it seems that the load is not requiring anything near full torque at any speed, you can use the existing drive and simply reduce the field current across the speed range either by adjusting the field regulator, if the drive has one, or leaving the field supply at full voltage and inserting a resistor in series with the shunt field to weaken it.

The result will be a trade-off between torque and speed. Less torque and more speed. At full armature voltage, as the field is weakened, the speed will increase.

I would try a small reduction, maybe 5% in field current first, and see where full armature voltage takes you. If not enough extra speed, drop the field current another 5% and see where top speed is.

Staying under MSS, keep doing this until you get enough speed to meet the need. Same drive, same motor, just trade a little torque for a little speed.

Aren't DC motors curious machines?

alan_505 said:

Flexpak3000's had a module that could do field weakening for you, or you could change the drive pulley sizes to change the gearing if you are only running at 33% FLA this would increase the load without increasing the motor speed, but increase the loads speed.

Click to expand...

I have seen large HP applications like this where 33% amps at 'full speed' might sound like the drive has plenty of power for the application, but the OP needs to watch the Ia as the drive accelerates. Often times, depending on the nature of the load, the drive might need near 100% amps on the ramp up to full speed. So, there also might need to be a trade-off with accel rate, to limit the current.

A new drive might help, of course, if the present drive does not have the field weakening capability. Also, check the field-loss circuitry, and make sure that its not set to the field full amp rating...this is where some smarts inside the drive help.

Golden Nugget DC Motors, those curious beasts...

I used to support a bunch of those dusty 10 to 200HP Reliance Flexpak 3000? I think. They were trained together with a bunch of card rack (wire wrapped) based interfaces, using frequency as a velocity command through thuymbwheel switches at multiple locations...what a trip that was to troubleshoot with my $49 Sears meter!

Don't miss armature wear and resurfacing, brush maintenance, having to climb two flights of stairs to properly lock them out...They did seem to offer quite a nice inertia advantage even with field weakening (option card?). I do recall an issue with a Reliance DC Drive and it's field power supply not having a shutdown timeout...It did something to the shunt of a $1500 motor during a week long shutdown, and there was a revision to their add-on field weakening card.

They were pretty reliable when well maintained, but we did have issues with tach slippage or loss on the voltage versions. The newer FlexPak digital drives we installed with encoder feedback were much improved.