What issues do you come across when a single VFD is used for multiple motors of different power rating?

There will be no problem

1.until the total rating of the connected motors exceed the maximum rating of the drive.

2. the motors must be started simultaneously and switched off simultaneously through the VFD only.

3. There must be no intermediate opeartion of any individual motor connected in the system.

 4. ramp up time and ramp down time must be considered seriously.

 

In concept, and without going into further technical analysis, you can connect several motors to the same VFD, as long as the total sum of the motors power does not exceed the the VFD rating, and as long as these motors, being connected to the same VFD are started and stopped together.

 

But, since the VFD is designed for one motor and all the readings and feedback you got shall be read for one motor, as for the current, the torque, the trip... I see no technical justification to use the VFD for more than one motor, this will not be professional and must be avoided.

Regards

Normally when a single VFD is connected with multiple motors, magnetic circuit identification pose some problem.

If the drive is running in closed loop control and suppose one motor trips because of overload there is equal chance that the whole drive will trip because of wrong magnetic circuit imbalance.

There will not be any problem if you run the VFD in simple V/f control with individual MPCB for each motor.

A VFD has the ability to create voltages that vary in frequency.
A VFD has an input rectifier (either4 or6 semiconductor diodes) which charge up a DC link capacitor.
Three pairs of semiconductor switches are also connected to the DC link capacitor.
These switches generate a pulse-width-modulated (PWM) voltage for each of the three-phases on the output.

Using a VFD with multiple motors of different power level is a very sensitive issue.

Selecting a perfect VFD for all the motors you have to cnsidier the folowing issues:

1. Motors operating charecteristics

2. Motors duty cycle

3. how many motors will start at the beggining and

4. how many motors are kept running durring operation

 

>>You can sum up all the motors power and then select a VFD for those using a common AC bus bar. But best thing is seggregate the converter part and the inverter part of the VFD. Use one converter (AC to DC) then use a common DC bus bar; then use a single motor module (actualy this is an inverter) for each motor.

>> If your motors are S1 type (continuous duty), then fixed torque type VFD is good enough. If these are S9 type motor then you must have to use variable torque type VFD.

>> while selecting a VFD for this type of complex operation get informed about the overload percentage and its sustained time duration (like160% overload for3 sec of140% overload for10 sec or125% ovrload for57 sec etc)

>> Its good you use same Power Factor and same efficiency class for all motors.

 

 

 

We suggest using VFD in scalar mode and use thermal overload relays at all motors as per their ratings.