A.C. versus D.C. motor torque and horsepower

[mwuniseal]

Greetings,
I have a large material mixer that currently uses a 200hp D.C. motor. 1750 RPM 313 fla with a 300 volt field. We are installing an identical mixer with a 200 hp A.C. motor at 1780 rpm 480 volt 60 hz. I have been told that a 8 pole 800 rpm motor is able to produce about the same amount of work as the D.C. motor. My question is, are the two motors able to compete with each other (1750 RPM D.C. vs 1780 RPM A.C.)? The new 200 hp A.C. motor will be controlled by a VFD.
Thank you for any advice you may have.

 

[Besoeker]

Greetings,
I have a large material mixer that currently uses a 200hp D.C. motor. 1750 RPM 313 fla with a 300 volt field. We are installing an identical mixer with a 200 hp A.C. motor at 1780 rpm 480 volt 60 hz. I have been told that a 8 pole 800 rpm motor is able to produce about the same amount of work as the D.C. motor. My question is, are the two motors able to compete with each other (1750 RPM D.C. vs 1780 RPM A.C.)? The new 200 hp A.C. motor will be controlled by a VFD.
Thank you for any advice you may have.

200hp is 200hp.
And power is speed times torque.

 

[GoldDigger]

The starting torque versus speed curves may not be the same, especially when you add the VFD into the equation on one motor.
But the full load torque at operating speed should be the same if the HP rating and speed are the same.

 

[Besoeker]

The starting torque versus speed curves may not be the same, especially when you add the VFD into the equation on one motor.

At 200hp, the DC motor almost certainly has some sort of controller on it too.

 

[Jraef]

... I have been told that a 8 pole 800 rpm motor is able to produce about the same amount of work as the D.C. motor. ...

First off, a 8 pole AC induction motor would have a synchronous speed of 900RPM, not 800. So if you are looking at an 8 pole motor and the SLIP speed is 800RPM, that is a 12% slip motor, which is very high and might indicate a Design D torque / speed curve which could provide very high Locked Rotor Torque.

HP = tq. x RPM/5250, and ergo Tq. = HP x 5250/RPM. So if you have an 800RPM motor labeled as 200HP, then it will produce 1312.5 lb. ft. of torque. That is MORE THAN TWICE the torque that a 200HP 1750RPM DC motor will provide (600 lb. ft.). But "work" is, as was pointed out, HP and HP is HP is HP.

 

[Besoeker]

First off, a 8 pole AC induction motor would have a synchronous speed of 900RPM, not 800. So if you are looking at an 8 pole motor and the SLIP speed is 800RPM, that is a 12% slip motor, which is very high and might indicate a Design D torque / speed curve which could provide very high Locked Rotor Torque.

And maybe that's what the mixer application requires.

HP = tq. x RPM/5250, and ergo Tq. = HP x 5250/RPM. So if you have an 800RPM motor labeled as 200HP, then it will produce 1312.5 lb. ft. of torque. That is MORE THAN TWICE the torque that a 200HP 1750RPM DC motor will provide (600 lb. ft.).

But not at the same speed.

 

[Jraef]

First off, a 8 pole AC induction motor would have a synchronous speed of 900RPM, not 800. So if you are looking at an 8 pole motor and the SLIP speed is 800RPM, that is a 12% slip motor, which is very high and might indicate a Design D torque / speed curve which could provide very high Locked Rotor Torque.

HP = tq. x RPM/5250, and ergo Tq. = HP x 5250/RPM. So if you have an 800RPM motor labeled as 200HP, then it will produce 1312.5 lb. ft. of torque. That is MORE THAN TWICE the torque that a 200HP 1750RPM DC motor will provide (600 lb. ft.). But "work" is, as was pointed out, HP and HP is HP is HP.

had to take a phone call before I could finish.

The complex part of this is that a DC motor does not have a straight line torque curve. When we call a DC motor 200HP, that is its 200HP at it's maximum operating power point, which is typically 1/2 it's maximum speed at 1/2 its maximum torque. As you slow it down, the torque increases. In a VFD on an AC motor, as you slow it down, the torque remains constant. The difference is all about where you intend on using it. So if you want to use it at low speeds, you may indeed need either a larger AC motor, or one with a higher torque capability, i.e. a Design D high slip motor.

 

[Besoeker]

had to take a phone call before I could finish.

The complex part of this is that a DC motor does not have a straight line torque curve. When we call a DC motor 200HP, that is its 200HP at it's maximum operating power point, which is typically 1/2 it's maximum speed at 1/2 its maximum torque. As you slow it down, the torque increases.

You may be thinking of a series motor. The OP mentions a 300V field which indicates a shunt wound motor where the torque is proportional to the armature current.
It is a constant torque machine.

 

[Jraef]

You may be thinking of a series motor. The OP mentions a 300V field which indicates a shunt wound motor where the torque is proportional to the armature current.
It is a constant torque machine.

OK, another brain fart. That's two for me today. I need to step away from the keyboard... :slaphead:

 

[Besoeker]

OK, another brain fart. That's two for me today. I need to step away from the keyboard... :slaphead:

I'm lucky if I get away with so few!

My background in drives started around the time SCRs were beginning to be used for DC but there was a lot of legacy kit, Ward-Leonard, mercury arc(haic), mag amps, thyratrons, that I got lumbered with.
A bit of a baptism by fire when you get sent to fix something you've never previously encountered - but you learn....very quickly....

 

[mwuniseal]

Next question is speed.

Next question is speed.

I'm lucky if I get away with so few!

My background in drives started around the time SCRs were beginning to be used for DC but there was a lot of legacy kit, Ward-Leonard, mercury arc(haic), mag amps, thyratrons, that I got lumbered with.
A bit of a baptism by fire when you get sent to fix something you've never previously encountered - but you learn....very quickly....

We typically run the mixer motor for most of the batch around 800-875 RPM. The D.C. drive regulates the speed via speed pot. So my next question is, what are the torque characteristics of the 4 pole 1780 RPM motor running at the same speed ( 800-875 RPM). The A.C. drive will regulate speed via speed pot.

 

[GeorgeB]

You'll get a more complete answer from jraef or besoeker, but the DC motor is likely blower cooled. A TEFC at 100% torque will likely need more cooling at half speed than provided by the shaft mounted fan.

Screws in blow molding (moulding for besoeker?) were classically DC and early changes to AC with vfd were less than successful. I'm unaware of the changes needed, but most now seem to be AC.

 

[Jraef]

We typically run the mixer motor for most of the batch around 800-875 RPM. The D.C. drive regulates the speed via speed pot. So my next question is, what are the torque characteristics of the 4 pole 1780 RPM motor running at the same speed ( 800-875 RPM). The A.C. drive will regulate speed via speed pot.

As Besoeker pointed out in correcting my brain fart, a shunt wound DC motor will be used as a constant torque machine, and since a VFD will provide constant torque from an AC motor, it will be the same. A good quality Sensorless Vector Drive will provide you with equivalent step-change torque resonse to the DC motor as well, although on a mixer, that is not likely to be necessary.

At half speed like that you could probably get away with a Scalar drive, although most will come with at least some form of SVC now anyway. The only issue is, make sure the VFD is rated as "Constant Torque" or with some mfrs, "Heavy Duty" because many mixers end up needing that. I see some vendors sell VFDs for mixers as being "Variable Torque" machines, which means they sell you a cheaper drive that is unable to supply higher current as the viscosity changes. Some mixer applications are like that, but if you are not absolutely sure, it is not a safe assumption in my experience. The DC drive and motor would not have had that option, it's unique to AC VFD marketing so given that, the safer bet is to assume constant torque.

Using an "Inverter Duty" AC motor is a good way to help insure that you will not have a cooling issue at half speed, plus it also aids in the survivability of the motor in the long haul.

 

[Besoeker]

You'll get a more complete answer from jraef or besoeker, but the DC motor is likely blower cooled. A TEFC at 100% torque will likely need more cooling at half speed than provided by the shaft mounted fan.

Screws in blow molding (moulding for besoeker?) were classically DC and early changes to AC with vfd were less than successful. I'm unaware of the changes needed, but most now seem to be AC.

We have supplied a lot of variable speed drives into the paper making industry. The paper machine uses a number of drives running at accurately controlled speeds in a close ratio to each other - paper is a fairly delicate medium particularly at the start of the process. The application is considered constant torque.

Sometimes the DC motors have a blower with the blower taking in air local to the process. Sometimes there is a ducted ventilation system and that has the advantage of being able to take air from a cooler area or from a chiller.

We have installations where we put in DC fed DC drives. Some of the upgrades to get, for example more power have been DC fed variable frequency drives. The DC distribution distribution to the suite of drive panels exists so it makes sense to use it. The AC motor is force air cooled in such cases to cope with the constant torque requirement.

 

[mwuniseal]

Thank you

Thank you

I came to the conclusion that our 200 hp DC motor is at 600 lb ft. torque. The AC motor will produce a constant torque at any given speed of 590 lb ft. torque. My examples are: At 60hz 200 hp at 1780 rpm =590 lb ft. At 30hz 100 hp at 890 rpm= 590 lb ft. At 15 hz 50hp at 445 rpm = 590 lb ft. As the frequency changes, hp changes proportionally, torque remains constant throughout. As long as the AC motor will produce that amount of torque, it will handle our application and be more efficient at the same time.
Thank you all for your help in this discussion.

 

[Besoeker]

I came to the conclusion that our 200 hp DC motor is at 600 lb ft. torque. The AC motor will produce a constant torque at any given speed of 590 lb ft. torque. My examples are: At 60hz 200 hp at 1780 rpm =590 lb ft. At 30hz 100 hp at 890 rpm= 590 lb ft. At 15 hz 50hp at 445 rpm = 590 lb ft. As the frequency changes, hp changes proportionally, torque remains constant throughout. As long as the AC motor will produce that amount of torque, it will handle our application and be more efficient at the same time.
Thank you all for your help in this discussion.

I agree with your figures.
Not so sure about your comment on efficiency though.
It depends on a few factors.
A simple consideration is the controller. The DC drive converts AC to controlled voltage to provide variable DC. It is a single conversion The AC drive first converts AC to DC then DC back to variable frequency AC. Two conversions are required.

If the mixer runs at light load, the DC motor could have an advantage. The armature current is low and losses associated with it are correspondingly low. The AC motor, even on no load will typically take 30% of full load current.