Selecting Induction Motor for VFD Application

A

Thread Starter

abhi1202

I want to vary flowrate in a loop from 10 lpm to 2500 lpm. For this application I have selected a 22kW and 55kW pump. the pumps are driven by VFDs of same ratings. However for selecting a Electric motor for VFD application, I am not sure of important parameters to be considered. Kindly help me wrt specially following parameter:

i) Rated frequency and tolerance/variation in frequency limit of the motor: I guess the rated frequency should be 50 Hz. However should the variation in frequency be from 50Hz to 13Hz (supposingly I use larger pump motor for flow range of 2500 lpm to 667 lpm). I have seen a specification which says variation in frequency of -5%. So whether it is correct to select the same or not.

ii) Rate voltage and tolerance limit for the motor

iii) Is Speed range a necessary parameter to be considered for selecting motor?

iv) Insulation class: Whether only mentioning Class F insulation is sufficient or some additional features shall be mentioned?

v) Temperature of the winding: Is it important to ask for temperature monitoring?

Kindly answer wrt to above points in addition to any other point which might be important wrt above topic.
 
1. Frequency Variation is specified as ±5% for Induction Motors as per IS and IEC Standards.

2. The Voltage variation is specified as ±10%.

3. Combined Voltage and Frequency Variations is specified as ±10%.

<b>Moderator's note:</b> sent email to author to find out what  is.

4. You can specify the application of the motor and mean of source (VFD) in your inquiry and ask for supplier's offer accordingly.

5. The supplier may ask for percentage of harmonics that may be present in input supply. You may get this data from VFD data sheet.

6. Insulation class F motors can work up to internal temperatures of 155deg. Internal temperature will depend upon the ambient temperature of your location and load on the motor. I suppose the temperature may remain on higher side in VFD operated motors due to presence of harmonics. From my practical experience, motor insulation exhibit long life if outer body temperature remain up to 70-75deg and inner up to 90-100deg. You can also specify the allowed temperature rise inside the motor.

7. It's up to you and your application whether you need thermistor for temperature protection of your motor. But its always good to have one. You can have dual channel also, because after period of years thermistors get defective also.
 
B
>i) Rated frequency and tolerance/variation in frequency
>limit of the motor: I guess the rated frequency should be 50
>Hz. However should the variation in frequency be from 50Hz
>to 13Hz (supposingly I use larger pump motor for flow range
>of 2500 lpm to 667 lpm). I have seen a specification which
>says variation in frequency of -5%. So whether it is correct
>to select the same or not.

motor manufacturer mentions the variation in frequency with constant voltage. However VFD use the principle of V/f constant, so the rating of 5% frequency variation is not applicable in case of using VFD. You have to ensure in VFD configuration that constant V/f must be there. Any induction motor can be used with VFD, duty cycle to be checked i.e.S1 or s2 etc.

>ii) Rate voltage and tolerance limit for the motor

choose standard voltage and tolerances like 3-ph, 415V, 50 Hz with 5% variation. in India.

>iii) Is Speed range a necessary parameter to be considered
>for selecting motor?

speed range need to be matched with your pump operating speed. that is the only criteria.

>iv) Insulation class: Whether only mentioning Class F
>insulation is sufficient or some additional features shall
>be mentioned?

standard motor insulation class may be chosen as class F. go for it.

>v) Temperature of the winding: Is it important to ask for
>temperature monitoring?

for a 22KW or 55KW motor temp monitoring is not practically used.
It is used for higher range >150Kw more or less.

>Kindly answer wrt to above points in addition to any other
>point which might be important wrt above topic.

You may ask if further any clarification needed.
 
To elaborate a bit on the previous (excellent) response:

>i) Rated frequency and tolerance/variation in frequency
>limit of the motor: I guess the rated frequency should be 50
>Hz. However should the variation in frequency be from 50Hz
>to 13Hz (supposingly I use larger pump motor for flow range
>of 2500 lpm to 667 lpm). I have seen a specification which
>says variation in frequency of -5%. So whether it is correct
>to select the same or not.

As was said, the pump performance spec is paramount. You will likely find, for example, that at 40% speed (or somewhere near that) the pump will no longer produce sufficient pressure to overcome the head pressure in the system. At that point it ceases to pump at all, so specifying a motor capable of running slower than that is pointless.

>ii) Rate voltage and tolerance limit for the motor

Answered.

>iii) Is Speed range a necessary parameter to be considered
>for selecting motor?

See i) above.

>iv) Insulation class: Whether only mentioning Class F
>insulation is sufficient or some additional features shall
>be mentioned?

Class F is fine in most cases. But more important is to specify the WINDING INSULATION as being at least 1600V, 2000V is better. This is what a typical "inverter duty" motor will use, because you must protect against potentially damaging voltage spikes created by the capacitive coupling effect of the VFD PWM pulses and the cable length.

>v) Temperature of the winding: Is it important to ask for
>temperature monitoring?

Only if the motor is used in an area where Explosion Proof enclosures are required.

>Kindly answer wrt to above points in addition to any other
>point which might be important wrt above topic.

Also specify that you want a Shaft Grounding Bushing or at the very least, Insulated Bearings on BOTH ends of the motor. Similarly to the voltage spikes that can damage the windings, a capacitive effect between the stator and rotor can result in a voltage potential developing between them. If that potential increases beyond the dielectric strength of the lubricant for the bearings, the voltage will pass to earth THROUGH the bearings and etch the bearing races as it does. It is microscopic at first, but over time is builds up damage until the bearings fail, sometimes in a matter of weeks. Again, this is the sort of feature that often comes as part of an "inverter duty" motor, but not always. You should specifically ask for it.
 
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