Blake Bailey
Technical Education Committee Member
Designmotors
Few things are less exciting in the motor world than a large synchronous motor that refuses to start—and all too often, the root cause is excitation. This article provides an overview of what synchronous motor excitation is, how it works, and the common issues you are likely to encounter.
What Excitation Is
All rotating electrical machines rely on electromagnetic fields to produce torque. Unlike induction motors, however, synchronous machines require an external power source to energize their rotor circuits. This power is delivered to the field windings (rotor poles) either through slip rings and brushes (brush-type excitation) or through a small, shaft mounted generator that supplies rectified DC (brushless excitation).
Regardless of the excitation method, the supplied current flows through the field windings to establish the rotor’s electromagnetic field. This field locks in step with the stator’s rotating magnetic field, allowing the machine to operate at true synchronous speed—without the slip inherent to induction motors. For example, a 12‑pole, 60 Hz synchronous motor will run at 600 rpm. Beyond enabling synchronous operation, external excitation also allows control of the motor’s reactive power (and therefore power factor (PF)) by adjusting the DC field strength.
Brush-Type Excitation
Brush-type systems use two slip rings and corresponding carbon brushes (see Figure 1) to deliver current to the main rotor field, and the field (rotor) voltage and current listed on the motor nameplate is required for the main field winding to achieve rated horsepower (HP), full-load amps (FLA) and PF.
While straightforward in concept, brush-type systems come with familiar challenges: sparking, carbon dust and ongoing maintenance—issues well known to anyone who has worked with wound rotor induction motors or DC machines. To make matters worse, many older systems rely on control cabinets (see Figure 2) that look like they were built by a mad scientist who forgot to include spare parts or a customer support phone number.
Thankfully, modern systems can achieve proper excitation application and functions such as voltage regulation and PF control through more compact, solid-state systems; and brush-type systems can generally source their DC power from any properly sized DC source available in the plant.
Brushless Excitation
Brushless systems rely on a relatively small, shaft mounted generator to supply the main motor’s field current. While so-called “AC-AC” exciters are applied for variable speed operation, the great majority of brushless exciters rely on “DC-AC” exciters. (See Figure 3.) These exciters include a stationary DC wound field (exciter stator), which induces current flow into a (typically) three-phase wound exciter rotor (armature), after which a rotating rectifier assembly (diode wheel) converts the AC out of the exciter rotor into the DC current the motor’s main field windings require.
The stationary DC field still requires a power supply, but the power level is far lower than what a brush-type system demands. And for brushless machines, the field voltage and current listed on the main nameplate correspond to the exciter stator levels needed to achieve rated HP, FLA and PF. These field voltages and currents should not be confused with those required by the main rotor field windings.
While free of the maintenance issues associated with carbon brush systems, brushless excitation carries with it the potential issues associated with another set of smaller windings, in addition to the operational issues that can arise via failure of any number of the components on the rotating rectifier assembly. Additionally, there is more space required for a brushless exciter than for a typical slip ring assembly. As with brush-type systems, DC power to brushless exciters can be supplied from any number of sources.
Diode Wheels
As mentioned, the rotating rectifier required for brushless excitation’s basic function is to rectify the AC current generated in the exciter rotor into DC current for direct supply to the main rotor poles. For synchronous generators, these assemblies are typically made up of a full wave bridge rectifier, consisting of properly sized diodes and heat sinks. For motor applications, however, diode wheel assemblies (see Figure 4) must also incorporate silicon controlled rectifiers (SCRs) to control the current flow and timing, field discharge resistors (FDRs) meant to protect the main rotor windings from overvoltage during acceleration, and some version of a control module/filter to ensure application of field current and removal of FDRs from the circuit. These components must seamlessly work together to provide reliable synchronization of the main machine.
Common Excitation Issues
Over the years, I’ve seen more issues with misunderstandings and improper setting of excitation systems than I can recall. Because synchronous motors require far more complex rotor power systems than induction motors, the potential for complications is much higher. Even with a fully functional excitation system, incorrect excitation settings can result in motors with anything from overheating field windings, excessive stator current, to PF and voltages far from rated. An understanding of the application and consultation of the motor’s vee curves for the setting of proper excitation for the desired load level are essential. (See Figure 5.)
Starting issues are also common and can arise from incorrect field power application timing, failure of diode wheel or starting cabinet components, or FDRs remaining in the field circuit after the motor reaches synchronous speed.
Final Thoughts
Understanding and troubleshooting excitation systems may not be exciting to everyone, but excitation is fundamental to the operation of some of the largest and most critical machines in industry. A solid understanding of how these systems work—and where they commonly fail—is an essential part of every motor professional’s toolkit and could lead you many places in an exciting motor career!
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