What type of asynchronous motor suits the best for VFD duty?
Asynchronous motor is sometimes called the work horse of the industry. And this nickname is well earned. It is because millions of such motors are utilized in different applications across the globe. They help the world turning. While most asynchronous motors are still fix-speed type, more and more are equipped with a variable frequency drive (VFD). So, what type of asynchronous motor suits the best for VFD duty?
First we start with a remark. The asynchronous machine is also called induction machine. These terms are normally considered as identical. In German the term ‘Asynchronmaschine’ is used. In English both terms ‘asynchronous machine’ and ‘induction machine’ can be used. Both terms describe the principle of the motor: the rotor spins slightly slower than the electromagnetic field on the stator (⇒ “asynchronous motor”). This difference is called slip. Due to the relative movement a voltage is induced into the rotor. Thus, the motor works on induction principle (⇒ “induction motor”).
We also use the terms ‘motor’ and ‘machine’ as almost identical. Machine may be more general, covering motors, generators or synchronous condensers, for example.
Types of asynchronous machines
There are multiple types of asynchronous machines depending on their design and way of operation. The main differences refer to the rotor design. In this section we mention some of the most common types.
Suitability of asynchronous motor designs for VFD duty
Let’s look at the most common asynchronous motor designs with respect to rotor winding and their suitability for VFD duty.
A) Squirrel cage asynchronous machine
This type of asynchronous machine has rotor bars connected with end rings to a structure called cage or in English squirrel cage. The construction is relatively simple and robust. However, there are few sub-types:
- Standard single cage
- Deep bar squirrel cage
- Double cage
Overall we can say that VFD driven motor works best with a standard squirrel cage, i.e. single cage with normal rotor bars. Deep bar squirrel cage utilizes the skin effect. Such cage is specially optimized to increase the rotor resistance while reducing the rotor leakage reactance at low speed (high slip). It results in higher starting torque and lower starting current compared to a normal squirrel cage design during DOL start. However, the price for the increased torque is higher nominal slip and consequently higher losses. That has considerable impact on the operation expenditure.
Double cage rotor has similar purpose like the deep bar slots. There is a squirrel cage used for starting (with high resistance) and another cage used in normal operation close to synchronous speed.
With VFD, the motor can provide full torque practically from zero speed. Thus, such method is superior with regards to maximizing the starting torque and minimizing the starting current. Special rotor cage is not desired (higher manufacturing cost, no benefits, path for harmonic currents etc).
B) Wound rotor asynchronous machine
The purpose of wound rotor design was historically the ability to connect the rotor via sliprings with an external resistor. By adding an external resistor the torque-speed curve can be adjusted. Thus, a speed control is possible. It is a simple method but very inefficient. The “slip power” is dissipated in the resistor in form of heat. Operating at 70% of the nominal speed would mean that approx. 30% of the nominal power is transformed into heat. No wonder that this method is very seldom used in these days. It is neither environmental friendly nor economical.
Wound rotor machine can be modified (retrofitted) for VFD operation. For example by removing the brushes and installing a short circuiting device attached to the sliprings or remove the sliprings completely. A detailed procedure is to be defined by the motor service unit (general inspection, cleaning, re-balancing of rotor, modification or replacement of bearings etc) and is out of scope of this post.
However, there is a very specific case where wound rotor type of asynchronous machine is used together with power semiconductor converters: doubly-fed induction machine (DFIM). Stator of this machine is directly connected to the supply of fix frequency (typically 50 Hz or 60 Hz grid) while the rotor is fed from a frequency converter. Such setup allows to control the speed of the machine without wasting the energy. The slip energy is not dissipated as heat but supplied back via the power converter in the rotor circuit. The DFIM concept allows to operate both sub-synchronous and super-synchronous, i.e. below and above the synchronous speed. Advantage is a smaller VFD that must be rated only for the slip power. Disadvantage is higher complexity of the machine (especially at very high power) and more difficult compliance with the grid code (if applicable).
Summary
Several designs of asynchronous machines exist with regards to the rotor construction and specifically rotor winding. Normal squirrel cage rotor is the simple, robust and cost effective. Special rotor cages aim to increase motor starting torque and/or reduce motor starting current during direct on-line starting. In VFD operation such special rotor designs lose their importance and may even be counter-productive. Inverter duty asynchronous machine works best with normal squirrel cage. Double cage or deep bar cage is not required. Also wound rotor is normally undesired. An exception is a doubly fed induction machine with VFD supplying the rotor of such machine. Such solutions were historically used e.g. in wind converters (low-voltage) and are still used in very high power range e.g. for variable speed pumped storage plants.
⇒ Squirrel cage rotor (normal single cage) is the all-rounder best suitable for supply from a VFD
⇒ Wound rotor machines not applicable for VFD duty with exception of doubly-fed induction machines (DFIM)
References
[1] Motor retrofit (series of articles), https://mb-drive-services.com/category/motor-retrofit/
