The 2-level converter topology in combination with pulse width modulation (PWM) is the standard converter for rotational speeds up to several thousand revolutions per minute (rpm). When the application needs rotational speeds up to several hundred thousand rpm or has extreme motor efficiency requirements then other converter topologies and modulations schemes must be considered. Normally pulse amplitude modulation (PAM) is the optimal choice. What are the advantages and disadvantages of these two modulation schemes? Here is a guide to help in making the best selection.
Besides the standard 2-level, DC-input converter there are other possible converter topologies, e.g. multilevel-converters, or current source input converters, that are mostly used in traction or high power industrial drives. For high-speed drive systems with rotational speeds above 100,000 rpm the two converter topologies and the corresponding PWM and PAM modulation schemes, as shown below, are used almost exclusively.
Converter topologies and current waveforms for standard 2-level converter with PWM (left) and PAM converter (right)
The choice between PAM and PWM is often not clear and is related to various factors:
- Motor typ (e.g. with air gap winding or slotted stator)?
- Control method (e.g. sensorless control)?
- How much loss is tolerated in which parts (e.g. minimize rotor losses or maximize total efficiency of motor and converter)?
The following table can be used as decision guideline:
|Converter losses||higher losses, especially with increasing PWM frequencies||low|
|Motor / rotor losses||low losses, especially at higher PWM frequencies||low to intermediate for motors with air gap winding, intermediate to high for slotted motors|
|Compatibility with slotted motors||very good||good|
|Compatibility with motors with air gap winding ||average (depending on the PWM frequency)||very good|
|Sensorless position detection||observer||zero crossing detection (ZCD)|
|Typical speed ranges (for a single pole pair)||0 to 200,000 rpm||5,000 to 1,000,000 rpm|
|Availability with Celeroton converters:|
High speed motors typically have low inductance values. If PWM is used, high switching frequencies are essential. If not, the so-called PWM losses lead to high motor losses – especially in the rotor where they are undesired. These losses can be reduced by increasing the switching frequency; however, this leads to increased converter switching losses.
With PAM, the converter is switched at fundamental frequency. The switching frequency of the buck converter input stage is defined through its passive components. Compared to PWM, the switching losses are decreased, however, the component count of the converter increases. A further drawback is the higher torque ripple at low speed operation and zero speed.
From 100,000 to 200,000 rpm (for a single pole pair) PAM typically leads to lower motor losses than PWM – but this depends strongly on the motor type and the PWM frequency. An exact boundary between methods cannot be given and has to be investigated case-by-case. In case of doubt, Celeroton readily gives support in the choice of the modulation scheme, for Celeroton motors and compressors, as well as for customer motors. Before the start of serial production, different modulation schemes can be tested experimentally with Celeroton converters.
The optimal modulation scheme depends on the motor type, the rotational speed and the loss optimization target:
- For air gap winding motors and systems requiring the highest rotational speeds and/or minimized total losses then a PAM topology and modulation method is selected
- For slotted motors, with intermediate to high rotational speeds and/or requirements for minimized rotor or motor losses then PWM is selected
Efficiency improvements or reduced motor temperature rises at high speeds are possible using the flexible Celeroton converters that provide both PAM and PWM modulation methods, and combining this with Celeroton’s expert know-how on the interaction of converter and motor.
Additional information on this topic:
- Sensorless Control
- Converter and motor loss calculation for PWM and PAM modulation
- Minimization of converter and motor losses