Selecting VFD (Variable frequency drive) or ac drive Multi motor applications

There are few many applications where multiple motors are connected to a single VFD (variable frequency drive) or ac drive.

We would like to highlight few things before you chose a VFD for such kind of applications.

  1. Overload Protection
  2. Full load current of all the motors connected
  3. Output choke
  4. Disconnect connection for each motor.

 

  1. Overload protection

VFD has an inbuilt feature for overload condition, but when you connect multiple motors to a single VFD. It will be difficult for VFD to read individual current of each motor. So we will need to install Overload relay at the input of each and every motor. So in condition of overload of any motor, Overload relay will take care of that individual motor.

 

  1. Full load current of all the motors connected

While choosing a VFD for Multi motor application you should choose a VFD with current equal to or higher than the sum of all motors full load current.

For example you are using for motors

3.7kw (5hp) * 3 motors                 with 8.6 amps each

5.5kw (7.5hp)*1motor                   with 12.5 amps

 

So the total full load current of all the motors will be

=(8.6*3)+(12.5)

=25.8+12.5

=37.5 Amps

So while selecting a VFD you should select a drive with output current rating equal to or more than 37.5 Amps.

multi-motor-applications

 

  1. Output choke

Output choke is important to install at the output of VFD. Due to the parasitic capacitance of wiring and grounding of motor. You should use output choke. Manufacturer of each drive manufacturer can highlight more on this as there is variation in design of all manufacturers.

 

  1. Disconnect connection for each motor.

You should make provisions to disconnect each and every motor. You can put MCB or contactor at the input of each motor. So that when not needed you can put any motor off individually.

 

Apart from this there are few points which you will be not able to do in multi motor application.

 

  1. Direction will be same
  2. Speed will be same.
  3. Starting and stopping will be same

 

  1. Direction will be same

Direction of each and every motor will be same. You cannot change direction of a particular motor. All motors will run in either forward or reverse direction. (IF all phase are connected sequentially.

 

  1. Speed will be same.

Speed of each motor will be same, If nos. of poles of each motors are same. (Slip of motor may vary with design of each motor).  If they are different in nos. of Poles than Speed will in ratio of below formula

 

RPM = (120*Frequency) / (No. of Poles).

 

For example,

If All the are 4pole the than RPM of all motors will be 1500 RPM (Slip not considered)

And you set a frequency to 30 Hz in VFD

All your motors will run at 900 PRM

 

But if you are using a motor with 4 pole and other with 2 pole

And you set a frequency to 30 Hz in VFD

Your 4 pole motor will run at 900 RPM and

Your 2 pole motor will run at 1800 RPM

And speed will change accordingly.

 

  1. Starting and stopping will be same

All motors will start at same time and stop at same time. Ac drive will not be able to control start stop of a motor individually. You have to take care it by using MCB/ Contactor. Also the acceleration and deceleration will be same for each motor.

 

If you have any quires related to Multi motor applications please free to contact us we will be glad to help you on this application.

Recommended brands for Such Applications are

ABB, Crompton Greaves, Delta, Hitachi and FUJI.

For more information and details you can contact us on info@solims.com

We will be glad to help you from selecting VFD to Installation.

 

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