ac motor instead of dc motor

hello.

is it possible to use an ac motor instead of 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.

Power is power, torque is torque. It doesn't matter if it comes from a DC motor, an AC motor, or a weight on a rope running over a pulley.

I see a lot of posts here that put the cart before the proverbial horse - they figure the motor is pushing power into the load and base ratings and load calculations on the motor nameplate. In fact, the load pulls power from the motor. If the load requires less power or torque than the motor rating or capacity then the motor only provides that power/torque to the load. If the load exceeds the motor's capacity then the motor will stall.

The last statement leades to Mustafa's comment. Different motors have different speed/torque characteristics. In general an AC motor has it's highest torque, called pull up torque, around 50% speed. A DC motor may have it's highest torque at zero speed. The weight on a rope will be capable of constant torque. For all of them at any given speed power is a function of torque and speed.

In customary US units hp = ft-lb_Torque x rpm / 5252

I have replace dc motors with ac motors in the past, main concern is torque and rpm. If rpm on dc motor is lower than same size ac motor, you have to increase hp number on ac motor in order to conserve torque. Also take into consideration if you use dc motor with field weakening.

As a general rule whenever updating or upgrading or changing any process or system one should begin by evaluating the current needs and proceed as if new. By replacing each component in kind will not assure an efficient system, only one that works about the same as what you had.

Formula HP = (T x RPM) / 5252 is a mechanical application. Applies to any mechanical system diesel, steam turbine, electrial motor and the rest of a near infinite list.

Motors take electrical energy and convert it to mechanical generally rotary. If I say all cases someoene will find an exception I do not know.

KNOW THY LOAD

You do not tell us what the load is.

BOTH Tom and Russ are right you MUST know the characteristics of your load both RPM and torque. Assuming NOT a centrifugal pump or blower then you can measure torque at the load input shaft. This will give you breakout torque ie what it takes to get it rolling. The actual torque when running may be less. Using breakout and fudging up by 10% is a good thing in my mind. Conveyers and "wheel driven" are generally constant torque loads wih constant load.

Machine tools can be decreasing torque

Centrifugal pumps and fans. They obey the affinity laws. Torque can increase with increasing RPM

Learn the demand of the load and then choose a motor to match it and allow another 10%.
Dan Bentler

Lots of very good advice above. I would only add one thing about DC motors. There is a lot of misunderstanding about the torque characteristics of DC motors usually describing them as very high torque compared with AC motors. With the exception of the series-wound DC traction motor, any ordinary DC motor develops torque in direct relationship to its armature current.

So, while it is true that many shunt field DC motors can develop high torque by themselves, the DC drive supplying the current is generally sized only 10% or 50% higher in amps than the motor nameplate armature amps. Therefore, as a drive-motor SYSTEM, somewhere between 10 and 50% shortterm overload is all that is available.

This matches the capabilities of AC motors on sensorless or flux vector drives.

Therefore, unless the DC drive is significantly oversized from common practice, you can trade out a DC motor for an AC motor at the same nameplate power and be ok.

But, most importantly, follow the advice above about finding the continuous and shortterm torque demands of the LOAD and size the new system according. It could very well be that you end up doing a better sizing job that was done originally with the DC system!

One more issue related to the DC to AC conversion comes to my mind.
An AC motor's generated torque is proportional to the "electromagnetical slip" present within its windings while a DC motor's one is, like posted by DickDV, proportional to the armature current.
Hence, a DC motor will provide full rated power/torque starting at zero speed while the same "size" AC machine will need to be actually rotating already in order to supply torque to the system.
This is where the careful analizing of the system's dynamic/inertia loading should determine if the conversion is possible and, in case the previous stement is found to be true, which should be the ratio between the replaced DC motor ratings and the proposed replacement AC ones.
In my experience, the required average increase in AC ratings when replacing a DC machine with an AC application is close to 33%.

I'm sorry, dmargineau, but much of what you say in your post is not true.

First, a DC motor develops constant torque (not power) down to zero speed. An AC induction motor with a good sensorless or flux vector drive can do the same thing. The shaft does not need to be turning to develop this torque. The stator field will be turning only enough to generate the required slip and torque but the shaft can be stationary.

Second, there is, therefore, no need to upsize an AC motor when comparing to a DC motor unless the DC motor-drive system is sized for more than 200% shortterm overload torque. And, those systems are rare.

Third, it is far easier to overspeed (or field-weaken) an AC motor than a DC motor so, if the high torque conditions occur only at start or low speeds and the motor's power train ratio can be changed (as in belt coupled loads), then by increasing the power train reduction ratio and compensating with overspeed, you can go above 200% shortterm torque (up to about 300%) without upsizing the motor.

"First, a DC motor develops constant torque (not power) down to zero speed"

I believe this is what I have stated.

" An AC induction motor with a good sensorless or flux vector drive can do the same thing."

I also believe this is slightly biassed by the VFD's manufacturer theoretical claims.

I have been part of quite a few DC to AC conversions (almost triple digits as we "speak") and unfortunately (financialy especially!) most of the conversions required a substantial increase of the AC motors' ratings even when using the latest AB PF 700-750 or ABB ACS 800 VFDs.

You will be puzzled to learn of how many applications were deemed to be strictly DC ONLY by the controls manufacturer's statics/dynamics theoretical surveys.
Maybe this is the reason most the major AC motor controls manufacturers are still majorly investing in DC controls development (AB PowerFlex DC, ABB DCS 550, etc.)
Most likely, hopefully somewhere in the near future, we will be able to switch HP for HP in all DC to AC conversions.
I am afraid we are not quite there yet.