We have a 30hp 1440rpm AC motor installed on a mixer.
Motor Pulley: 6"
Machine Pulley: 20"
According to my calculations, the mixer runs at 432rpm.

Operating Procedure: The mixer is loaded with approximately 200kg of raw material feed, along with water and several mixing compounds. The motor is then started by a "Star Delta" Starter.

Due to the high load in the mixer, the motor is unable to move in the "Star" Phase. The connected ammeter's needle touches the roof, so the current consumption is higher than 80amps (the max that the ammeter can measure).

When the Starter puts the motor in "Delta", it starts with a jerk and the ammeter reads about 60amps. After the initial 2-3 minutes, the mixer settles down and the motor's current consumption hovers between 30 and 40amps.

Considering that we install an AC Drive for the above motor, I have the following questions:

  1. Would an AC Drive be able to maintain the necessary torque at extremely low motor RPMs?
    I intend to have the AC drive slowly accelerate the mixer to the desired rpm, thereby giving the mixture enough time to settle down and reduce the need for high current consumption.

  2. Would a V/F control type AC Drive suffice for the above application?
    I am specifically looking at Siemens Sinamics V20.

  3. What kind of energy savings would an AC Drive bring for the above application?

Other Details about Connected Power Supply:
V: 440V
F: 50Hz

I would greatly appreciate any and all assistance.
Please excuse me if any assumptions are wrong, I'm not well versed with this field.
Thank You.

  • \$\begingroup\$ You might want to link to the motor controller's datasheet - and the motor's too, if you have that. \$\endgroup\$
    – user16324
    Oct 11, 2016 at 11:15
  • \$\begingroup\$ Sorry, I don't have any data regarding the motor or access to its controller's datasheet. It's very very old. Can you try and answer some of the queries without access to this data? I realize it can be difficult. Thanks \$\endgroup\$
    – user126295
    Oct 11, 2016 at 11:34
  • \$\begingroup\$ No I can't. I have no idea of the current capabilities of the drive OR the starting current of the motor. \$\endgroup\$
    – user16324
    Oct 11, 2016 at 11:39
  • \$\begingroup\$ For the details of the drive: cache.industry.siemens.com/dl/files/948/103599948/att_67109/v1/… \$\endgroup\$
    – user126295
    Oct 11, 2016 at 11:46
  • \$\begingroup\$ Got this from Siemen's website: SINAMICS V20 – an overview of the technology Voltage and power range: 1AC 200 V - 240 V, ±10 % 3AC 380 V - 480 V +10 % / -15 % 0.12 kW - 30 kW (0.17 hp up to 40 hp) Control modes: V/f, V2/f, FCC, V/f multi-point \$\endgroup\$
    – user126295
    Oct 11, 2016 at 11:47

1 Answer 1


Adding a V/F control system could be an approach. However with only V/F control the regulator can not see if the motor is acctually turning. One step further is adding an encoder to the system so the regulator can see that the motor is turning. In both situations the available starting torque will be more or less 150% at low frequencies (3Hz)

A different approach could be OLV (Open loop vector control). Here the motor can realise a torque of about 200% of their rated torque at very low frequencies. Also here an encoder is possible turning the system into an CLV closed loop vector control. Here a torque control mode is possible. Thereby the motor is controlled at torque rather then motor speed.

Try to understand the principles first and then look at your situation.The Closed loop vector control would have my preference for your mixer. For this approach you can look at the SINAMICS G120.

For your specific questions.

  1. CLV can work in a standstill condition and would deliver enough torque during startup. You can compare this with a DC motor.

  2. V/F might be enough but is not sure. For the other possibilties see above.

  3. I do not think that energy savings can be achieved. The motor has to deliver its energy whatever the situation is.

  • \$\begingroup\$ Hi, Thanks for the answer. I just need a clarification. I think till now, I've been working with a flawed assumption that the relationship between torque and motor speed depends solely on the motor itself and is independent of the drive. If I'm absorbing everything correctly, I need to look for a drive that offers greater starting torque at very low frequencies? \$\endgroup\$
    – user126295
    Oct 11, 2016 at 12:40
  • \$\begingroup\$ That is correct. Since your main problem is the starting torque you need torque control at very low frequencies. I have experience with mixer motors up to 300 kW mixing Oppanol using clv. \$\endgroup\$
    – Decapod
    Oct 11, 2016 at 12:44
  • 1
    \$\begingroup\$ Dont forget the cooling. An external driven fan is probably required. I the situation I have worked with we used watercooled motors \$\endgroup\$
    – Decapod
    Oct 11, 2016 at 12:48
  • \$\begingroup\$ The motor is going to run at extremely low frequencies for about 10-15s. After that, we would need to increase it to about 900rpm to get the mixer running at our target RPM of 400-500. (Pulley ratio of 0.5 assumed). Motor Frequency|Motor RPM|Mixer RPM Say, 3Hz|90rpm|45rpm TO 31.25Hz|937.5rpm|468.75rpm The mixer is to remain at that speed for the next 30 minutes. Do you feel the need for external cooling in such a case? \$\endgroup\$
    – user126295
    Oct 11, 2016 at 13:40
  • \$\begingroup\$ Secondly, is there any cost advantage of lowering the frequency and the resultant motor RPM? I can play with the pulleys to change the motor RPM if needed. For us, energy efficiency is of prime importance. \$\endgroup\$
    – user126295
    Oct 11, 2016 at 13:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.