Replacement of Existing DC Motor With VFDs

Without knowing the duty of the motors it's hard to give a judgement.

You cannot replace a motor with a vfd. The vfd is what changes the frequency of an ac motor thus changing its rotational velocity (rpm). They may say that the turn-down ratio is 4 to one. However, for a TEFC motor you will generally need at least 30hz to allow the fan to cool the motor properly.

Building on the other answers, a DC motor is generally more expensive than a fairly standard (i.e., non-synchronous, non-wound rotor) AC motor of the same Hp. Also, the DC motor requires brushes (and commutator) and associated maintenance. Also, it is typically a bigger motor (iirc).

A DC motor requires a DC supply and a DC motor starter, usually using resistors.

It is an engineering problem to figure out which combination (i.e., VFD and AC motor vs. DC motor, DC starter, DC supply) is most appropriate in all respects.

In new rolling mill construction, AC motors were making inroads 15 years ago (when I was last involved). (Well, actually before that, I worked for a big steel company where, in many respects, new technology was suspect.)

Could I ask which stands used AC drives? I honestly can't see a cogging mill using them. To watch one near grind to a halt when the billet hits it and then the motor gets it's shoulder in to the job and then accelerates is fantastic. I couldn't see it being done with a AC drive.

Sorry, I guess I mis-stated my answer--I should have said that AC motors were making inroads into being used for table drives for rolling mills, not the actual mill stands.

Cheers, I'd got all sorts of ideas running through my mind as to the AC drive that could power a mill.

I've been lucky to see the River Don mill in full action, it's something I'll never forget.

Many moon ago, when I worked in a very large steel mill as a mechanical engineer, for huge stress situations (i.e wide flange mill) a very large fly wheel was turned by a 15,000 H.P GE motor.

I suspect you meant to say, but was it an AC or a DC motor?

It was an AC motor. Sorry

It all depends on application. If I remember starting torque values of DC Motor can not be achieved in AC (even with VFD) except in certain algorithms where they may reach nearby.

You need to chekc the Torque, power and current charactreistics for any combinations to choose. It is not recommended to go for AC motor unless you are clear of the same. Also I am npot clear why you require to have VF, which is generally used to reduce the speed and smoothen the starting current without compriomising the staring troque.

I am afraid intention is not inmprove the torque which is not at all recommended by changingf rom DC to AC motor

The latest plant installed by our client has now VFDs instead of DC motors.the process is the same i.e for Extruder machine in aluminum film foil plant.

Question: does your client want to replace the DC MOTORS & DC CONTROLS EQUIPMENT, replacing it with NEW AC motors with VFD's??

yes; my client is intrested to go for Vfd if saving on Vfd is proved to him

an immediate saving is the conversation of AC to DC as I guess you have an AC power supply.. unless you are generating DC, and the cost savings in the maintenance of all that old DC equipment, and its getting harder to find rewind shops that can repair DC motors.

In many instances you can replace DC drive systems with AC drive systems.

DC Regen drives have an advantage over AC VFD systems that typically have little capability to regenerate significant amounts ow power without a fancy regenerative front end, at which point DC is very attractive. I still prefer DC machines in applications on large Paper Machine Center Winders @ 300 + Hp.

ABB has good white paper "3adw000059.pdf" on AC vs DC drive systems. AC is not always better.

As far as the energy consumption is concerned, a variable speed motor and its drive will consume about the same energy. AC or DC motor makes little difference. The variable speed control is where most of the energy is saved if your process is variable.

The other differences have been explained by the other contributors.

The process is variable.

I've had some experience with VFDs coupled with circulating water pumps in Hydronic systems. They worked superbly for me. They are multiple times cheaper than then DC systems and have got all the kind of protection a squirrel cage motor needs. Connected to a BMS system you can monitor almost any data. And you can control your motor in many various ways. But in answer to the question do you really need it and are you ready for the extra expense if your DC system is working ok. Well it offers a few advantages. It is actually maintenance free compared to the brushes of a DC motor that need to be replaced sometimes. It offers many different ways of controlling your motor speed and torque and monitoring it. It saves you lot of space and saves lot of energy compared to the heat loss on the resistive control system of DC motors. Actually the energy loss on VFDs is minimal . I hope I made my point. But you need consultation regarding the suitability to your particular application.

Thanks for the detailed answer.my client has made up is mind to go for Vfd.will it be possible for you to guide me that how far it is advisable to change exixting 3 phase 22 kw and above standard run Ac induction motors with Vfd in term of annaul energy saving thanks

Resistive control on DC and power loss?

That has been obsolete for 25 years now, especially at 300kw level. DC is all Thyristor (or SCR) controled these days. The AC drive control challenge has been to make it have the flexibilty the DC control has always had.