IE5 Synchronous Reluctance MTPA: Maximum Torque-Per-Ampere Control and Efficiency

The synchronous reluctance motor (SynRM), one of the strongest representatives of the IE5 (Ultra Premium) efficiency class, stands out by offering high efficiency and high torque density without using magnets. But unlocking this motor's real potential requires the right drive control. A SynRM cannot run directly from the grid like an induction motor; it must always be driven by a variable frequency drive (VFD), and the control strategy the drive applies directly sets the relationship between the torque the motor delivers and the current it draws. This is where MTPA (Maximum Torque Per Ampere) control comes in. MTPA is the drive optimizing the d-q current angle so that it draws the lowest possible current at every torque level. In this article we cover the MTPA logic, d-q current angle optimization, how copper loss falls, its effect on efficiency and torque density, drive setup and correct drive-motor matching, with HEM Motor's engineering approach.

How Does a Synchronous Reluctance Motor Produce Torque?

The rotor of a synchronous reluctance motor has neither magnets nor windings; it consists of flux barriers designed to create directions where magnetic flux flows easily (the d axis) and with difficulty (the q axis). The motor produces torque from the tendency of the stator's rotating magnetic field to pull the rotor into the "lowest reluctance" position. This reluctance torque depends on the inductance difference between the d and q axes (Ld - Lq) and on how the current is split between these two axes.

This split is the heart of MTPA control. The stator current is a vector placed at an angle in the d-q plane. This angle sets how much of the current is used for magnetizing (d axis) and how much for producing torque (q axis). At the wrong angle, the motor draws more current than necessary to produce the same torque; this raises copper loss, heats the motor and lowers efficiency. At the right angle, the same torque is achieved with the least current.

• Magnet-free construction: no rare-earth magnets; low cost and supply risk, no demagnetization concern.
• High efficiency: almost no copper loss in the rotor; IE5 class efficiency is attainable.
• Drive mandatory: no direct grid start; the control strategy sets performance.
• Low rotor temperature: because rotor loss is low, the rotor stays cool and bearing life extends.

What Is MTPA and How Does d-q Current Angle Optimization Work?

MTPA means "maximum torque per ampere" and, as the name implies, its aim is to produce the desired torque with the smallest possible stator current. Knowing the motor parameters (Ld, Lq and how they change with saturation), the drive calculates the optimum current angle at every torque demand and places the current vector accordingly. This angle typically starts around 45 degrees and shifts as load and magnetic saturation increase.

The values in the table are conceptual; the real optimum angle depends on the motor's design and saturation behaviour. The key principle is this: for a given torque there is a single optimum angle that minimizes the current, and MTPA control tries to hit this angle at every moment. When current is minimized, copper loss (I²R) falls, because copper loss rises with the square of current. So even a small reduction in current creates a noticeable drop in loss.

Copper Loss, Efficiency and Torque Density

In a SynRM a significant part of total loss is stator copper loss; there is almost no loss in the rotor. MTPA targets exactly this largest loss item. By producing the same torque with less current it reduces copper loss, keeps the motor cool and pushes efficiency to IE5 levels. This makes a big difference especially in continuously running applications that demand high torque.

MTPA also helps with torque density: because the motor can produce more torque at the same current and the same temperature limit, more torque can be drawn from a smaller frame. This supports the compact, efficient nature of the SynRM. We cover efficiency measurement and the nameplate-field difference in our article on the nameplate vs field efficiency difference, and the core difference between SynRM and induction in our IE4 asynchronous vs synchronous reluctance article.

Comparing SynRM, Induction and Magnet Motors

To understand the efficiency advantage the SynRM gains with MTPA, it helps to compare it with other motor technologies. In an induction motor the current induced in the rotor creates rotor loss; this loss both lowers efficiency and heats the rotor. In a permanent-magnet synchronous motor (PMSM) there are magnets in the rotor; efficiency is very high but it brings rare-earth magnet cost and supply risk, with a demagnetization concern at high temperature. The SynRM holds neither windings nor magnets in the rotor; rotor loss is almost nil and there is no magnet cost.

This construction places the SynRM in a balanced position between induction and PMSM: noticeably more efficient than induction, more economical and supply-safe than PMSM. MTPA control takes this balance to its best point, because it minimizes stator copper loss, the SynRM's only large loss item. Thus the SynRM becomes one of the rare motor types that can reach the IE5 efficiency class without using magnets. We cover this comparison and technology selection in more detail in our IE4 asynchronous vs synchronous reluctance article.

Efficiency at Low Load and Noise Behaviour

Another important benefit of MTPA is that it preserves efficiency at partial and low load. Many industrial drives spend most of their time running below rated load. Because MTPA minimizes current at every load level, the motor stays efficient even at low load, which noticeably raises the annual average efficiency. We explain how over-sizing eats efficiency at low load in our article on efficiency at partial and low load.

A feature of the SynRM is that, by the nature of reluctance torque, it can produce some torque ripple and the noise associated with it. Modern drives reduce this ripple with advanced control algorithms and suitable switching strategies. When the right motor design and the right drive setup come together, the SynRM runs quietly and without vibration. This shows once more why, beyond MTPA, drive-motor compatibility matters so much.

Drive Setup and Commissioning: Running MTPA Correctly

For MTPA to work correctly the drive must know the motor parameters accurately. Most modern drives, during commissioning, measure the motor's d and q axis inductances and resistance with an "autotune" (automatic identification) step. If this step is not done right, the MTPA angle is calculated wrongly and the motor runs outside the optimum; efficiency drops, the motor heats up. So in SynRM commissioning, autotune and parameter verification are critical.

• Correct motor mode: the drive must be set to SynRM mode; run in induction mode the motor is inefficient, even unstable.
• Autotune: inductance and resistance parameters must be measured correctly; use a saturation table if available.
• Current limit: the drive current limit must be set to suit the motor; too low and torque is limited.
• Switching frequency and filter: evaluate a du/dt or sine filter for voltage spikes and bearing currents.

You can find the details of drive parameterization and commissioning in our article on IE5 synchronous reluctance motor drive parameterization, and drive brand compatibility in our compatibility with different drive brands article.

Beyond MTPA: Field Weakening and the High-Speed Region

MTPA control applies in the constant-torque region up to the motor's rated speed. In this region the aim is to draw the least current for each torque. But when the motor goes above rated speed, the voltage the drive can apply hits its limit; beyond this point the "field weakening" region begins. In this region the drive lowers the flux level to keep driving the motor at higher speeds, but the torque that can be drawn decreases.

In a SynRM, field weakening can be managed more flexibly than in magnet motors, because there is no fixed magnet flux and the flux is controlled entirely by current. A well-designed drive manages the transition from the MTPA region to the field-weakening region smoothly and maintains high efficiency over a wide speed range. This feature gives an important advantage in applications needing a wide speed range, such as winders, extruders and test benches. We also cover continuous-load saving in pumps, fans and compressors in our article on IE5 saving in pumps, fans and compressors.

Where Does the SynRM Stand Out?

An MTPA-controlled SynRM makes a difference especially in continuously running applications where high efficiency is wanted. Because it contains no magnets, its cost is balanced, its supply is secure and there is no demagnetization risk at high temperature:

• Pump and fan systems: in these continuously running, variable-load applications MTPA provides low current even at partial load, increasing annual saving.
• Compressors: in compressors running continuously at high load, the reduction in copper loss turns directly into energy saving.
• Textile and winding machines: in these applications needing a wide speed range and precise torque control, the SynRM's efficiency and dynamics stand out.
• Water and wastewater treatment: in blower and pump drives, IE5 efficiency provides serious gain in the long term.

You can find the continuous-line application in a textile spinning (ring) machine in our IE5 textile ring machine case article, and selection in water and wastewater treatment in our IE5 water and wastewater treatment plant article.

Correct Drive-Motor Matching

A SynRM reaching IE5 efficiency depends on the motor and drive being chosen to suit each other. Not every drive can optimally control every SynRM; the drive must have a SynRM control algorithm (including MTPA) and the motor's parameter set. Some manufacturers offer the motor and drive together, pre-matched; this eases commissioning and guarantees performance.

With a wrong match the motor runs but does not deliver IE5 efficiency; moreover instability, vibration or overheating may appear. So at the selection stage the compatibility of motor and drive must be confirmed. We cover the drive's DC bus voltage and supply conditions in our DC bus voltage and supply article, and shaft grounding and bearing-current protection in our shaft grounding and bearing current article.

How MTPA Energy Saving Reaches the Bottom Line

Although the copper-loss reduction MTPA provides may look like a small percentage on paper, it turns into a serious cost item in a continuously running plant. Most of the money a motor spends over its life comes from energy, so every point gained in efficiency directly lowers operating cost. MTPA, especially in applications demanding high torque and running continuously, lets the motor do the same work with less current, noticeably reducing annual energy consumption.

To assess this gain correctly, the motor's annual running hours, load profile and energy unit cost must be considered together. In a plant with high running hours, the extra investment of an IE5 SynRM over a standard motor is usually covered by energy saving in a short time and then turns into net gain. Without the right drive and the right MTPA setup, however, this potential gain cannot be fully achieved; even an IE5-labelled motor will not deliver the expected efficiency in the field. So MTPA is not just a technical detail but a control strategy that directly affects operating economics.

Checklist for a Correct MTPA-Controlled SynRM Setup

• Confirm the drive's SynRM control mode and MTPA support.
• Run autotune at commissioning; measure the d-q inductance and resistance parameters correctly.
• Set the current limit and torque limit to suit the motor.
• Evaluate filter and shaft-grounding measures for voltage spikes and bearing currents.
• Confirm motor-drive compatibility at the selection stage; prefer a pre-matched set if possible.

Frequently Asked Questions

Does a SynRM run without MTPA?

The motor turns and produces torque, but it does not run at optimum efficiency. Without MTPA the drive draws more current than necessary for the same torque; this raises copper loss, heats the motor and makes IE5 efficiency unreachable. MTPA is the key to the high efficiency the SynRM promises, so a suitable drive and correct setup are essential.

Do I have to set the MTPA angle manually?

No. Modern drives calculate the MTPA angle automatically from the motor parameters and continuously update it with load. Your job is to do autotune correctly at commissioning and confirm the drive is in the correct motor mode. If the parameters are right, the drive finds the optimum angle by itself.

Can I swap a SynRM for an induction motor on the same drive?

The drive must support SynRM control mode. A drive doing only induction (V/f or vector) control cannot optimally drive a SynRM. Before swapping, the drive's SynRM compatibility and MTPA support must be checked and a suitable drive selected if needed.

Manufacturer Stock and Fast Delivery with HEM Motor

Fully achieving the high efficiency and torque density of an IE5 synchronous reluctance motor depends on setting up MTPA control correctly and matching drive and motor correctly. The HEM Motor engineering team evaluates your application's torque and speed profile, recommends a compatible drive-motor set and secures IE5 efficiency with correct parameterization at commissioning. To bring your project to life at the highest efficiency, backed by manufacturer stock and fast delivery, contact us and request a quote.