DC motor base speed off of nameplate

[ctolbert]

Type SK DC motor from Westinghouse 1950's vintage.
230VDC 131L frame 125 hp, 400 amps. No field information. Speed is 400 - 1200 rpm (Not 400/1200 rpm like I see on other types of DC motor.

Question 1 -- Is base speed 400 rpm or 1200 rpm? The seperate DC drive (looks like MG set) has no information at all. No documentation. We are looking to convert to AC motor and drive....but base speed determines torque and sizing.

Question 2 -- There are no field leads...There are S1, S2, A1, and A2...Does this make it Series wound?

Thanks,
Carl

 

[aolalde]

ctolbert:

Since the motor terminals are only A1, A2, S1 and S2, then you have a Series motor.

The speed regulation is provided by the load or armature current. The higher the load, the stronger the field and the armature speed will drop with the load. Based on that characteristic of a series motor, your base speed should be 400 RPM and the maximum safe operating speed is 1200 rpm. Note that to increase the speed the load must be reduced.
This type of motor (Series) can not be operated under no load since the field approaches zero and the speed will increase to extremes were the armature will be destroyed by the centrifugal forces.

 

[DickDV]

It is important to note that this motor was intended for m-g set operation. Based on that, it is likely a shunt wound DC motor with the field internally connectec. The field and the armature operate on the same voltage since the m-g set only produces one voltage. This is old stuff, remember!

I would take the 400-1200rpm statement to be the operating range of the motor at full load, the 400rpm being the slowest speed for thermal stability with a shaft fan. The 1200rpm would be base speed since there is no way to weaken the field independently.

The S1 and S2 leads are likely a small series field winding that can be added into the armature circuit for extra heavy load torque.

Given the 1200rpm base speed and the 125hp, your available torque should be around 547 ft-lbs. I would expect that to be with the S leads hooked up. Less torque if the S leads are left disconnected.

Conversion to AC is the right way to go. Putting this DC motor on a modern DC drive will result in excessive motor heat requiring as much as 30% derate. This is due to the AC content of the rectified DC--the motor was designed for smooth DC from the generator set.

 

[ctolbert]

OK, after recovering from the superbowl...I went back out and looked at this thing.

1.) There are field leads, but they were cut off during dis-assembly...and pulled all the way back into the casing. The Resistance is 55 ohms. Digging through the gunk on the mg-set I found the seperate excitation voltage of 230 vdc. (But like DickDV said...it was a safe assumtion of 230vdc.)

2.) The motor has a 3.25 inch shaft....I would think a larger torque req would need a larger shaft...

3.) They changed the motor speed with a gearbox and did no motor speed changes.

Carl

 

[Bung]

I thought (digging furiously in the memory banks) that fundamentally, a dc motor speed was voltage dependant, and the output torque was current dependant (F=BILsin(theta) stuff). Of course, they are interdependent to an extent, and the nature of the load torque - speed curve changes the V-I relationship, but all things being equal the fundamental physics of V => speed and I => torque will hold.

Bung
Life is non-linear...

 

[DickDV]

If you are going to change the power train when you convert to AC, figure on using a four-pole 1800rpm motor and pick the speed reduction ratio so your motor is turning at 80Hz (2400rpm) when the machine is at maximum speed.

If you do this, you probably can use a 125hp motor and heavy-duty rated 125hp drive (150% overload for 1 minute).

If you are keeping the old power train, I would size it based on torque and probably use an 8-pole motor running overspeed to 1200rpm.

 

[aolalde]

Bung

The speed of a DC motor is function of the armature Voltage (Va)and the magnetic pole field intensity (Phi, B=Phi/Area).

n=(Va-Ia*Ra)/k*Phi

if Phi=> inf, n => 0
if Phi => 0 , n => inf.

The magnetic flux per pole is function of the winding turns(T), the Field current (If) and the magnetic circuit reluctance (R).
Phi = AT/R were AT = If*T

For a DC SERIES motor, the field AT are directly proportional to the armature current(Ia), then the speed is directly affected by the armature current.