I have a 400-watt small DC motor, and I removed one magnet to see what would happen to the speed. The difference was minimal, however, the sparking increased a lot. What does that mean?

 

Welcome to AAC!
The effect of removing one magnet would depend on the motor construction. Is it a brushed or brushless motor? How many magnets did it have initially? How are they positioned/oriented relative to the rotor? What are the relative polarisations of the magnets? ...... ?

 

the sparking increased a lot. What does that mean?

You should replace it!
What was the point of the exercise?
You are operating with an un-balanced pole pair .

 

What was sparking? The brushes when running or the connection to and disconnection from the power?

 

Welcome to AAC!
The effect of removing one magnet would depend on the motor construction. Is it a brushed or brushless motor? How many magnets did it have initially? How are they positioned/oriented relative to the rotor? What are the relative polarisations of the magnets? ...... ?

Brushed DC 200v with 2 magnets, positioned on the sides of the case (as 99% of dc motor).

 

I removed one magnet to see what would happen to the speed. The difference was minimal

But how was the torque affected?

 

But how was the torque affected?

I don't know bcz the load is just a fan.
Anyway, the ideawas to simulate field weakening but it didn't go like that, instead it made the EMF coefficient smaller which require more current @ the same rpm (just my thaughts)