How can we reduce the amount of energy consumed by a large number of electrical motors in a factory?
This is a mechanical problem, not a electrical one.  Any one motor is going to have the efficiency it has, so either use the mechanical power out of the motor more efficiently or use more efficient motors.

One thing you can do to reduce cost, which is not the same as reducing power usage, is to regulate your power factor.  Large industrial electricity customers are charged not just for the energy they consume, but also by the power factor.  This is because a low power factor causes more waste and stresses on equipment for the electric company for the same power you consume.

Most electric loads look inductive, and a bunch of motors certainly so.  There are various ways of raising your power factor.  For example, one old but tried and true scheme is to switch in banks of capacitors as needed to cancel the inductive part of your load.  In the old days there would be a meter and a bunch of big knife switches, and somebody who's job it was keep the system tweaked.  Nowadays this is done more automatically.

There are various options.  Since this is for a large commercial installation, you really should hire someone that specializes in this sort of thing.  Don't be penny-wise and pound-foolish.  Get the right professional to do this right.

Added:

You now tell us you are a intern that has been tasked to look into this and reduce overall power consumption by 1 to 1.5 MW.

Ug.  You have been put into a bad position and are in over your head unless there is someone more senior around you can work with.  If your job is to help out the senior person with the grunt tasks, great, dig in and learn a lot.  However, if the company is trying to cheap out by getting a intern to do a real experienced engineer's job, then you and they are headed for trouble and you need to get out of there and find a better internship.

Even at only $.10/kWh, 1 MW is $100/hour.  Tell them the solution is to hire a experience industrial energy consultant, and that you will help minimize the cost of this by providing assistance.  That will get the job done and will allow you to learn something.

No, I'm not going to suggest anything that doesn't include a proper professional looking into things because anything else is irresponsible and will be more costly in the long run.