Dual horsepower rating on DC motor 1

[trizik]

We have several DC motors in our plant that have nameplates similiar to the following:

HP: 15/20
Volts: 240
Amps: 56/75
RPM: 1150/2600
Cont. Duty
Field Volts: 240
Field Max Amps: 2.56
Field Amps: 1.82/1.22
Winding: SPEC. COMP.

We have these attached to DC drives that weaken the field above base speed and use them on machines that require maximum output. My question is when is the motor 15HP and when is it 20HP. I've heard two things.

1. The 15HP is at base speed and the 20HP is at max speed. But this does not coincide with the DC Motor chart that says that below base speed is constant torque and above base speed is constant power. At base speed the HP should be 15HP according to this answer but the chart says is should be 20HP.

2. The 15HP is at the nameplate armature voltage and the 20HP is at some higher voltage. If this is true I cannot find any information on what this other voltage is. Also if you do the calc for Ia=P*746/E*Eff. and use P=15,E=240,Eff.=.83 you come up with 56Amps (same as nameplate). But if you use the same equation and P=20,E=240,Eff.=.83 then you get 75Amps(same as nameplate). You should not get this because the 240V would not be the voltage that accompanies this HP.

Can someone shed some light on this HP mystery?
Thanks, Sam

 

[jbartos]

Suggestion: Visit

http://www.usmotors.com/PB630/gen_appl.pdf

etc. for more info

 

[jOmega]

Motor is rated at 20 HP at 2600 rpm and 15 HP at 1150 rpm.

Motor has a tapered HP curve.

The motor delivers approximately 68 ft-lbs at 15 HP at 1150 rpm and 40 ft-lbs of torque at 20 HP at 2600 rpm ...

Normally, when a dc motor is operated in the field weakend range (with armature voltage pinned at rated and armature amps pinned at rate)... the HP is constant while the torque drops off in inverse proportion to the increase in speed.

Your particular motor, from the nampelate data, shows that the armature amps are allowed to increase above the base speed of 1150 rpm -- from 56 to 75 amps. Thus, the motor HP is allowed to increase by 5 HP over the weakend field range from 1150 to 2600 rpm. In this way, the motor is able to source 40 ft-lbs of torque at 2600 rpm instead of 30 ft-lbs if the HP was to be held constant at 15 HP from 1150 to 2600 rpm.

It has to do with what the speed-torque-HP requirements are of the application load.

This motor is designed special for a particular application.

Hope this helps.

jOmega

 

[jbartos]

Suggestion to trizik (Mechanical) May 1, 2003 marked ///\\We have several DC motors in our plant that have nameplates similiar to the following:

HP: 15/20
Volts: 240
Amps: 56/75
RPM: 1150/2600
Cont. Duty
Field Volts: 240
Field Max Amps: 2.56
Field Amps: 1.82/1.22
Winding: SPEC. COMP.
///Visit

http://www.tpub.com/doeelecscience/electricalscience2102.htm

http://www.mech.uq.edu.au/courses/mech3760/chap28/s1.htm

for DC Motor theoretical background.\\We have these attached to DC drives that weaken the field above base speed and use them on machines that require maximum output. My question is when is the motor 15HP and when is it 20HP. I've heard two things.
///See above posting.\\1. The 15HP is at base speed and the 20HP is at max speed. But this does not coincide with the DC Motor chart that says that below base speed is constant torque and above base speed is constant power. At base speed the HP should be 15HP according to this answer but the chart says is should be 20HP.
///Also, the field current needs to be taken into consideration.\\2. The 15HP is at the nameplate armature voltage and the 20HP is at some higher voltage. If this is true I cannot find any information on what this other voltage is. Also if you do the calc for Ia=P*746/E*Eff. and use P=15,E=240,Eff.=.83 you come up with 56Amps (same as nameplate). But if you use the same equation and P=20,E=240,Eff.=.83 then you get 75Amps(same as nameplate). You should not get this because the 240V would not be the voltage that accompanies this HP.
///The field current needs to be taken intor consideration.\\Can someone shed some light on this HP mystery?
Thanks, Sam
///The nameplate includes field winding currents 1.82/1.22A that need to be properly aligned with the other data and motor operation.\\

 

[GusD]

Hello Trizic.
Jomega is right on track.
Trizic, you mention another voltage for the Higher horsepower,however;there is no other voltage.The motor is powered by a fixed 240V for both hpower rates.The change in Horsepower is due to a change in the armature current drawn,not by the change in the armature voltage.
Weakening the field acomplishes that.That is no Mistery voltage.

GusD