Variable Speed Motors - Which type suits the best?
Electric variable speed drive system consists as minimum of a VFD and an electric motor. So the question is: Which motor type suits the best for variable speed control?
As usual, the answer depends on multiple factors. Among those factors belong for example:
- Power and torque rating
- Nominal speed
- Speed range
- Load profile
- Dynamics
- Ease of control
- Fault behavior
- Loss capitalization
Power, torque and speed
Rated power, speed and torque are certainly some of the most relevant factors when it comes to motor type selection. Synchronous machines can be designed up to highest power (turbogenerators in power plants with unti power exceeding 1’000 MVA). On the other hand, asynchronous machine is often the best solution in the lower power range. We shall also acknowledge that the maximum output reachable with asynchronous machine is being challenged and some manufacturers claim to have solutions for up to approx. 80 MW.
Rated torque (not the power) is the quantity that most significantly determines the mass of the machine active parts [1]. Synchronous machines ideally cover the entire range from very low speed machines with high torque (typical example hydrogenerators) up to very high speed turbomachines.
Speed range
Some applications benefit from a wide speed range An example are various mills in the mining industry. A sub-critical rotor design is generally preferred as it does not restrict the minimum continuous speed [2].
Dynamic performance
In certain services the motor drives shall provide high dynamic operation. That means rapid change in torque, quick speed adjustment or even a fast speed reversal.
Ease of control
This category includes the complexity of the control algorithms for selected type of machine, amount of required input signals, sensors etc.
Fault behavior
Fault is a rare condition. Ideally it never appears but practically it cannot be completely excluded. Fault behavior of the machine impacts the VFD design (e.g. protection) as well as the load (e.g. transient torque). Fault behavior is also very relevant for various redundancy concepts and solutions.
Reliability and availability
For a successful service reliability and availability are at the centre. The motor shall be of robust design, provide certain fault tolerance, require minimum maintenance etc.
Efficiency and loss capitalization
The electricity prices are currently sky rocketing. Also in the long term electricity will not be cheap. Considering long lifetime of the motor (typically 20 years as an absolute minimum) every kW of reduced losses has a positive impact on the life cycle cost.
Investment cost
In the end the technical properties are often put in relation with the required capital investment. However, transparent evaluation shall consider both CapEx and OpEx to make the right decision.
Asynchronous or synchronous motor?
In medium voltage variable speed motor drive applications two traditional types of motors are extensively used:
- Squirrel cage asynchronous motor
- Electrically excited synchronous motor
How do these two workhorses compare? What are the strengths of each technology? How does the motor type selection affect the VFD design and dimensioning? Which type of motor suits the best for a specific application?
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References
[1] Power, speed and torque, https://mb-drive-services.com/power-speed-and-torque/
[2] Variable speed motors and critical speeds, https://mb-drive-services.com/variable-speed-motors-and-critical-speeds/
