# replace dc with ac motor



## oldtimer

seee said:


> hello.
> 
> Is it possible to use an ac motor instead of A dc motor with same power?
> 
> i mean that can i use an 37kw ac motor instead of 37kw dc motor?
> 
> can the formula torque=(hp*5250)/rpm be used for dc and ac motor too?
> 
> best regards.


 
:001_huh::001_huh::001_huh::001_huh::001_huh:


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## denny3992

seee said:


> hello.
> 
> Is it possible to use an ac motor instead of A dc motor with same power?
> 
> i mean that can i use an 37kw ac motor instead of 37kw dc motor?
> 
> can the formula torque=(hp*5250)/rpm be used for dc and ac motor too?
> 
> best regards.


Im sure it would make a beautiful sound!


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## John Valdes

You can convert a DC application to an AC application. Yes.
You will need an AC drive (VFD) and a an AC motor that produces the same torque as the DC motor produces. Or more.
You have approximately a 50 HP DC motor. What is the RPM?
If you have an AC motor with the same amount of poles (poles equals RPM) as the DC motor you have, all you need is a VFD.
There are several other considerations, but if you can match the DC torque, you are in business.

Yes. The torque formula is exactly the same for both AC and DC motors. It is singly the most important consideration when putting an AC motor in place of a DC motor.


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## JRaef

denny3992 said:


> Im sure it would make a beautiful sound!


Took me a while to get the subtle joke in that...:no:


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## JRaef

John Valdes said:


> You can convert a DC application to an AC application. Yes.
> You will need an AC drive (VFD) and a an AC motor that produces the same torque as the DC motor produces. Or more.
> You have approximately a 50 HP DC motor. What is the RPM?
> If you have an AC motor with the same amount of poles (poles equals RPM) as the DC motor you have, all you need is a VFD.
> There are several other considerations, but if you can match the DC torque, you are in business.
> 
> Yes. The torque formula is exactly the same for both AC and DC motors. It is singly the most important consideration when putting an AC motor in place of a DC motor.


What he said.

The shorter (and maybe snarkier) version I was going to post first, then elaborate on as John ended up beating me to was:

*It depends.*

And it depends on what he said. Without knowing what your DC motor is being expected to do now, there is no way to know in advance if an AC motor of exactly the same HP size will be capable of doing the job.


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## John Valdes

JRaef said:


> What he said.
> 
> The shorter (and maybe snarkier) version I was going to post first, then elaborate on as John ended up beating me to was:
> 
> *It depends.*
> 
> And it depends on what he said. Without knowing what your DC motor is being expected to do now, there is no way to know in advance if an AC motor of exactly the same HP size will be capable of doing the job.


I sized an AC drive and motor when I first got into the selling/distributing end of this business. I really undersized the motor. I replaced an 800 rpm DC motor with an 1800 rpm AC motor. 
I thought my new career was lost. Turns out the original OEM way over engineered the application.
With a few mechanical adjustments (pulley/shiv) and a decent decel time, all was well. Lucky me. It was a 15 ft machine table.
I have never forgot that first sale. I never will.


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## denny3992

JRaef said:


> Took me a while to get the subtle joke in that...:no:


Actually i meant if you put a dc motor on an ac supply! But i see the joke now "ac/dc" the band....


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## greenman

No, unless it a Universal Electric Motor.
A universal electric motor is different from regular electric motors in that they can run on either alternating current or direct current supply. The basic difference between AC and DC power is the direction in which the current flows through the engine. DC (direct current) motors flow in only one direction, while AC can alternate direction without affecting the operation of the appliance to which it is supplying power.
But good luck on geting the hp you need!. Or the correct size motor.
Dc to Ac. Why switch? 
What type of motor do you want to switch.
*TYPES OF DC MOTORS*. There are three basic *types of dc motors*: (1) Series *motors*, (2) shunt *motors*, and (3) compound *motors*.
So you just can swap a dc motor out.
it,s dc for a reason.


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## varmit

There are several considerations to this change. Of course an appropriately sized VFD and AC motor would be required. The speed range that the motor operates is a big factor in sizing the motor, as typically an AC motor has less low speed torque than the same HP (kw) DC motor. Typically, if I replace a DC motor with an AC motor and VFD, I will upsize the AC motor by 50 percent. ( A 50 HP is changed to a 75 HP.) Also, starting torque is a factor. Modern VFDs can do some compensation for high torque starting, but usually at the cost of shortened motor life. If you have the KW per speed curve of the machine, it will make life easier.


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## John Valdes

varmit said:


> There are several considerations to this change. Of course an appropriately sized VFD and AC motor would be required. The speed range that the motor operates is a big factor in sizing the motor, as typically an AC motor has less low speed torque than the same HP (kw) DC motor. Typically, if I replace a DC motor with an AC motor and VFD, I will upsize the AC motor by 50 percent. ( A 50 HP is changed to a 75 HP.) Also, starting torque is a factor. Modern VFDs can do some compensation for high torque starting, but usually at the cost of shortened motor life. If you have the KW per speed curve of the machine, it will make life easier.


I disagree, unless you up size 50% due to personal preference only. 
Torque is torque. I do agree there are other considerations. But if you can match the full load torque you can usually use the same HP, AC motor. 
A 1200 rpm DC motor and a 1200 rpm ac motor have the exact same torque rating. The case is the same with 2,4, and 8 pole motors.
I look to the the VFD for starting torque, not the motor. Most all VFD's today can provide that starting torque. Sometimes the addition of a feedback device will also facilitate the torque requirement and provide 100% full torque at zero speed. 
Up sizing a motor by 50% is overkill IMHO. Especially if you are quoting an application.


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## varmit

John Valdes said:


> I disagree, unless you up size 50% due to personal preference only.
> Torque is torque. I do agree there are other considerations. But if you can match the full load torque you can usually use the same HP, AC motor.
> A 1200 rpm DC motor and a 1200 rpm ac motor have the exact same torque rating. The case is the same with 2,4, and 8 pole motors.
> I look to the the VFD for starting torque, not the motor. Most all VFD's today can provide that starting torque. Sometimes the addition of a feedback device will also facilitate the torque requirement and provide 100% full torque at zero speed.
> Up sizing a motor by 50% is overkill IMHO. Especially if you are quoting an application.


I deal with a lot of antiquated equipment that runs continuously at 10 to 20 percent of nameplate RPM. Yes, torque is torque, but at lower speeds it usually requires some motor upsizing from DC to AC. This is especially true is the load is a reciprocating or cyclic machine. On most of the old junk equipment, where I do these upgrades, there is no existing documentation to determine actual mechanical loads, so a 50 percent upsize has become my recommended criteria. This is also the value recommended by two of the local drive dealers in my area. There have been a couple of cases where a 50 percent upsize was barely enough to allow low speed running without nuisance trips. It took some very precise tuning and adjusting to make these work.


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## JRaef

Just for S&G, I thought I might point out that the OP _never mentioned variable speed_. He simply asked if you could swap out a DC motor for an AC motor of the same HP. So to John's point, only if they are both the same SPEED rating will the torque will be the same. All the other issues fall under that catch-all category of "it depends".


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## John Valdes

varmit said:


> I deal with a lot of antiquated equipment that runs continuously at 10 to 20 percent of nameplate RPM. Yes, torque is torque, but at lower speeds it usually requires some motor upsizing from DC to AC. This is especially true is the load is a reciprocating or cyclic machine. On most of the old junk equipment, where I do these upgrades, there is no existing documentation to determine actual mechanical loads, so a 50 percent upsize has become my recommended criteria. This is also the value recommended by two of the local drive dealers in my area. There have been a couple of cases where a 50 percent upsize was barely enough to allow low speed running without nuisance trips. It took some very precise tuning and adjusting to make these work.


The post was a general overview and not applied to any particular application.
I do understand that looking at each individual application is the only way to go. 
When I hear drive manufacturers suggesting motor upgrades, they are usually looking at the increase in drive requirements to. Increase motor size you might/have to increase drive size. They like that. And some drive manufactures sell motors too. 
As long as everyone is quoting apples to apples I could care less if they upgraded 100%.


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## varmit

JRaef said:


> Just for S&G, I thought I might point out that the OP _never mentioned variable speed_. He simply asked if you could swap out a DC motor for an AC motor of the same HP. So to John's point, only if they are both the same SPEED rating will the torque will be the same. All the other issues fall under that catch-all category of "it depends".


I did the dangerous- I assumed at that HP, variable speed was desired.


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