Motors suited to use with inverters

  • Thread starter Moschetti Pierluigi
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Moschetti Pierluigi

Dear members of the mailing list,
In an inverter manual I read:
"These inverters are components for open and close loop control of variable speed drives with asynchronous standard motors, reluctance motors, PM synchronous motors with asynchronous damping cage."
As far as I know from the university, it is impossible to drive a "reluctance motor" or a "PM synchronous motor" with an inverter:
there is no feedback of the rotor position and therefore it is not possible to drive the stator magnetic field correctly. Moreover the damping cage is no means to drive a motor. I like to know your opinions and experiences. Thank you

Best Regards
Pierluigi Moschetti

Matsushita Electric Works (Europe) AG
Rudolf-Diesel-Ring 2
D-83807 Holzkirchen
Tel.: (++49) 8024-648-232
Fax : (++49) 8024-648-222
EMail: [email protected]
"Synchronous reluctance" motors (as opposed to "variable reluctance" or "switched reluctance" motors) can easily run off an inverter, or off a 50/60 Hz line. Usually a synchronous reluctance motor has a "tin can" rotor with salient (protruding)poles. When it is not in synchronicity, it acts like an induction motor -- the solid tin-can structure is not quite as efficient as a squirrel cage, but it works satisfactorily. When it gets close to the synchronous frequency, the reluctance-torque effect of the protruding poles takes over and locks it in to full synchronicity with the stator electrical frequency, so it behaves as a synchronous motor.

As to the PM motors with damping cages, I am not familiar with this design, but I presume the damping cage can provide a similar asynchronous torque production mode to the squirrel cage of an induction motor that permits it to pull into synchronous operation.

Curt Wilson
Delta Tau Data Systems

Johan Bengtsson

When the motors are running asyncronously they behave as any asyncronous motor, but not that efficient, should work.

Both motors are designed however to normally run
syncronously, but that should work since:
1. An asyncronous motor have a magnetic field rotating with the stator field (same speed), off by an angle depending on load. The rotor is rotating somewhat slower (slip), that means the magnetic field of the rotor is rotating with the speed of the rotor+slip.
2. On a syncronous motor the slip is 0, the magnetic field in the rotor is locked to a specific direction compared to the rotor. The angle between the stator and rotor field does however behave the same (depends on load).
3. What an inverter do when the motor is running to slow due to load is to increase the frequency, making the magnetic fields rotate faster. In the case of an asyncronous motor the rotor is not following exacly that speed (off by the slip), in a syncronous motor it is.

The big difference is if the rotor actually follows the syncronous speed (speed of the magnetic field in stator) or if it only almost does.

It is entirely possible to use an inverter on any
type of syncronous motor, at least theoretically. The exact algorithms to calculate the speed and position of the stator field might have to be adjusted depending on type, but it should not be that much difference.

Obvously some people have done it...

/Johan Bengtsson

P&L, the Academy of Automation
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]

Mr.Peter Clothier

Dear sir, I stumbled upon your article by accident but feel I may be of assistance. I am Technical manager for an engineering company which specialises in 'Synchronous drive systems' specifically for the Glass container manufacturing industry. Our drive products for more than 30 years utilise IEC frame Reluctance synchronous motors. Years ago these were driven by an electromechanically driven 'Frequency changer'. For the last 15 years we have used inverter drive technology to control these motors with great success. The advantage with this type of motor is the fact that full synchronous operation can be achieved without the need for any kind of feedback device. Experience has taught me certain caution however. The choice of inverter for example. Inverter carrier frequencies on some models cause resonance problems. Also there are various methods of constructing 'RELSYN' motors and some are better than others. Todays models of RELSYN drive systems are muli-inverter types which are synchronously tied together (without encoder feedback) and offer features such as 'Electronic gearbox' functions as you might find with a servo driven system. Finally if you burn out one of these motors then no problem because it is a simple winding like a squirrel caged motor and can be rewound by any reputable rewinder. Your reservations about PM motors or servo motors is correct however. These motors require commutational control of the drive amplifier to make them rotate.Henceforth an encoder or resolver is normally attached to these devices. I hope that I have been of some assistance to you. Feel free to contact me if you think that I can clarify or help further. My works E-mail address is:- [email protected]