# driving a synchronous motor as a generator

Say I have a 100 kW synchronous motor in a building driving a pump/fan etc as part of an HVAC system.

1 - if I drive the shaft of the motor at 100 KW (from somewhere else), then the load on the motor will be zero and the motor will take no energy (to 1st order)?

2 - if I drive the shaft of the motor at 200 kW then it acts as a 100KW generator? This is what I understand from synchronous motors. If my electrical demand on that circuit is 100kW (from other things) then is my electricity drawn from the grid zero?

3 - If I have 100 kW other demand on this subsystem, and drive the synch motor shaft at 300kW (from elsewhere) then I have a 100kW 'extra' - does this get 'exported' to the grid? Am I missing something?

Russ

• There are many more questions about connecting a synchronous motor as a generator to the mains. The necessary excitation in the inductor, the synchronism between the generator and grid, etc ... In the case of powers, it is necessary to know whether the way in which the engine is built allows efficient operation as a generator, and if possible by example feeding the inductor. Nov 16, 2015 at 16:51
• You are totaly out of sphere. A synchronous machine is motor/generator. If it is rated for 100kW then it can act as 100kW generator or 100kW motor. I don't undersatnd your points (other people also, I guess). Nov 16, 2015 at 16:54

1. You don’t really drive the motor at 100 kW. You drive it with a prime mover that is capable of supplying mechanical power up to some limit. If the mechanical load is still connected to the shaft, the prime mover will be loaded to 100 kW. If the motor is to be connected to the electrical system, the prime mover speed and the motor/generator excitation would need to be adjusted so that it is synchronized with the system before connection. That can be done in a way that prevents the motor/generator from drawing power from or supplying power to the system.

2. If you wish to have the motor act as a generator, you can adjust the excitation to supply up to 100 kW. If the demand on your side of the meter is not more than 100 kW, you can adjust your generator to supply all of that demand and nothing would be drawn from the grid. The prime mover would need to supply 200 kW plus an additional 5% or so for generator losses.

3. If the motor is rated 100 kW, then any more than a 100 kW load as a generator would overload it. It would overheat, but more importantly, it would lose synchronization with the grid and potentially cause a lot of equipment failures. If your own loads use less that 100 kW you could export energy to the grid. If both the motor was rated higher than 100 kW and the prime mover was rated more than 200 kW, you could export energy with your stated 200 kW total load.

• Thanks Charles, that helps a great deal. In (1) I'm assuming a prime move connected to the shaft using some perfect 'clutch' so the prime mover can progressively take the load off the motor. In (2) what do you mean ' adjust the excitation to supply up to 100 kW'? What actually needs changing? If I use my prime move and clutch to take load off the motor then add more more, does this make it a generator or does somehting else need doing? Nov 17, 2015 at 13:13
• Theoretically, you could transfer the the load to a prime mover using a controlled clutch or coupling such as an electromagnetic coupling. The motor excitation would need to be controlled by a regulator that could keep it synchronized with the grid. I don't know how to describe how that would work through the transition. We would need to study the detailed performance of an automatic voltage regulator for a grid-connected synchronous generator.
– user80875
Nov 17, 2015 at 14:51