Motor Magnetising Current?

INDUCTION MOTOR THEORY

Induction Motor Parameter Measurement

Thanks for that, Ram.

While I'm well familiar with induction motors, I hadn't seen magnetising current written up on the nameplate like that before, hence my question. It is not even very common to see it specified in any of the various motor catalogues I have (GE, Teco, SEW ...) - even had the guy at Baldor scratching his head as it does not appear on any of the datasheets or catalogues that he had access to.

I'm guessing that at 2/3 of FLA the mag current is quite high on this motor because it is a low-power inverter-duty type, yes? And this may be why it features on the nameplate, because it is a bit higher than a more general-purpose motor?

I thought it was odd too so I checked NEMA MG-1, it's not a requirement for inclusion on the nameplate, and I didn't spot the definition there either. Maybe it's just Baldor being more user friendly.

It is a 1HP motor, and that current is rated at F.L. (full load); so it looks OK to me.

Regards

Easiest three steps:

Connect motor to Power supply through ammeter and power factor meter.

Note down no load current Io and Power factor CosΘ.

Calculate Io x Sin(Cos¯¹Θ). This is magnetising component of No load current,

Or else if you do not have meters to do above experiment, ask motor manufacturer (whom you already contacted) to also please advise No Load PF of the motor.

Why not IEC,BS,NEC,NEMA,VDE etc stipulate that no-load current should be printed in the motor name plate. It will serve as a guide to check after rewinding or after many years(hrs) of service whether no-load current has changed as well as to select a capacitor for motor starting or PF correction.

Practice in those countries, where the quoted Standards apply, do not find economical to rewind small motors. These are use and throw.

For large motors, which are re-serviced, generally, data to be published on Name Plate is agreed between manufacturer and purchaser. Also Shop Test Reports include such information.

For the Electrical Equipment which is of standard design and manufactured in mass (called On Belt production), it is not economical to do standard tests as per quoted standards and .......

As equipment for extensive testing of motors are not available in every work/ repairshop,name plate information like speed,FLC, no-load current,noise level,pf, capacitor for PFC,capacitor for parallel capacitor starting etc are very essential.

Your Imag is always going to be the same irrespective of the load, Ideally if there is no load , your motor current is the same as Imag.As load increase, the motor current is the vector sum of the Imag+Iload

Imag will change if deterioration of laminations,insulation between adjacent layers of laminations,rotor,number of turns in motor winding coils etc takes place. When we test a large motor after rewinding Imag & pf are different from its original value in manufacturer's literature it means something has gone wrong.

Pnaban,

When we dismantle core, the lamination bends and straighten in handling and at times some teeth get bent, may be slightly. To straighten the bends, we hit lightly with a rubber or wooden mallet.

All above is cold working of Core Plates.

Cold working results in stresses in core plate, which changes or reduces the permeability of magnetic material.

Above results in earlier saturation of core, that means increase in magnetizing current and no load losses.

I experienced once this when rehabilitating Siemens, 1850 HP, 3.3 kV motor of Boiler Feed Pump of 80 MVA Power Plant in 1977.

After experiencing above change in core properties, I had to dismantled the finished core and got it stress relieved in GKW in India. Then GKW was known for supplying CRGO and CRNGO plates to Transformer Manufacturer and core plates to motors and alternator manufacturers in India. GKW had Stress relieving conveyor. It was a regular practice at GKW (Guest Keen William) to stress relieve all plates after punching.

After stress relieving, insulating of plates again and assembly, when tested for magnetizing test, I found that original magnetizing curve is reached.

After above experience, we added stress relieving of Stator Core Laminations as regular activity of Rehabilitation process.

Background is that I was part of Rehabilitation Team of Company and we had a developed standard check list and testing procedure before during and after completing the rehabilitation of machines. We had 3 Power Plants of 80 MVA, each with 10 Nos. of 3.3kV motors from Germany, AEG and Siemens, post war production - 1955 supplies which had lived their life by 1972 when I joined as Trainee. We started Rehabilitation in 1976-77.

For 1850hp,3.3kV motor it may be economical & justified but what is the HP/Voltage limit(minimum) of motors above which stress relieving is economical?.