I am working in a plastic blow film extrusion plant. The machinery has 3 nos 15 HP DC motors.

We are planning to switch to AC motors. Need guidance regarding the switchover, motor to be used, and other associated implications. Any help will be highly appreciated. Thanks.

RS

 

A few comments.

1. Make sure the AC motors put out the same amount of torque as the DC motors can across the FULL range. This may mean using AC motors with more HP. Also look at startup and shutdown conditions where you may need to have far more torque than normal, but for only a brief period of time. DC motor/drive combinations can often handle this w/o much trouble, but maybe not with AC. Something to look at closely.

2. If you are using 4 quadrant drives currently, these regen back to the line if necessary. Standard AC drives cannot do this. You will either need an expensive AC drive, or brake resistors. This could be an issue.

3. Do you need to run at low RPM for extended periods of time? If so, you may need an external cooling fan on your motor. Or maybe not.

Fortunately the equipment is there, and hopefully running so you should be able to gather enough actual running data to figure these things out.

 

1. While not required, it is a good idea to buy motors rated for VFD use. They have better insulation designed to handle the "spikes" associated with VFD. They may also have insulated bearings to oppose shaft circulating currents.
2. You must tell Motor supplier the "turn-down" (speed range) you need to run - you may need a larger frame size or derated motor to run at low speeds.
3. Buy a larger hp AC motor than you would a DC - disreguard the VFD manuf. claims (- in order to get the torque you are used to with DC - use bigger AC motor & drive - for your app use 20 hp ac.
4. Use a Flux vector drive - your app will determine if you need tach feedback or not.
5. use an iso transformer - VFD's prefer a lower voltage on the input - examine if you need any type of regen or dynamic braking options to absorb energy trying to go back to the AC line

 

I assume this is a variable speed, constant torque application, if so you will have a number of issues to address in making this conversion, which may include but are not limited to:

Speed range Speed accuracy Starting, stopping or holding torque requirements

The speed range will dictate whether you'll need or can use a TEFC, TENV or TEBC motor. The speed accuracy will dictate whether you can use a V/Hz, open loop vector or closed loop vector drive. Additionally, the torque requirements will lead you to deal with V/Hz, open loop or closed loop vector selection as well as how to deal with any regenerative energy...

If you are unfamiliar with these details you should seek expertise from a drive supplier or system integrator.

Mike Adkins
Electro Mechanical Systems, Inc.

 

Hi, This is possible. The motor ratings depend on the duty cycle (loading) and also the speed control range required by the process.
For most applications you can use squirrel cage induction motors and a good vector controlled or DTC drives available in the market.
ABB site has useful technical guide to dimension a drive system.
Go to www.abb.com, in product guide choose motor & drives. (AC Drives Document and PC Tool Directory)

regards/VGS

 

You can very well switch over from DC to AC motors, Infact in our cement plant we use all AC variable frequency drive instead of traditional Dc drives used in many application except Kiln and cooler application.

What you have to look into important factors,
1)Select Direct torque Drives ,nowadays most of the drives are DTC only
2)How slow your Dc drive will be running,if its less that 10% of rated speed then ACVF are strugling to run,in that case we have to dodified the chute arrangement to make the speed more
3)it is standard practice the motor to be derated 10% for VVf application,but some manufacture claim that it is not required, hence it is better you can give both the motor and Drives to the same party, so there will be no dispute
That all the points came to my mind any more suggestion you can get directly from the manufacturer
with regards
P.Narendrakumar

 

We deal in Danfoss make AC drives for further details contact at [email protected], we are based in Delhi/Noida, India.

Y.K

 

Most drives can be configured for full torque at 0 speed or almost no torque at high speed