Selecting a Motor for 600 RPM and 3 HP: Do I Need a Controller?

[akaf24]

Hello, for a project I need to select a motor which can rotate at 600 RPM with 3 HP of power. Do AC motors typically require a controller and drive? For example, I am interested in purchasing this. Do I need to buy an expensive controller just to operate it? How complex is the set up and controlling the motor? Thanks.

 

[Drakkith]

Can you elaborate on what you'll be doing with this motor? There are many, many different types of motors, even when they have the same RPM and HP, and providing more details will make it easier to help you.

 

[metastable]

Requirements: 3 hp | 600rpm

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BLDC Motor Assumptions: 20kv motor constant, 0.02ohm resistance (2 phase measurement - lead to lead)

Controller Assumptions: BLDC >80a motor current limit

Battery Assumptions: 12S * 3.7v nominal per cell = 44.4v nominal

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3 horsepower = 2.2371 kW mechanical

(600rpm * 2 * pi) / 60 = 62.83185rad/sec

2.2371 kW mechanical = 2237.1w mechanical

2237.1w mechanical / 62.83185rad/sec = 35.60455405976427560226Nm

Torque required @ 600rpm = 35.60455405976427560226Nm

KT motor constant (torque per amp)= 60/(2 * pi * 20kv) = 0.4774648292756860073067 Nm / amp

35.60455Nm Required / 0.4774648292756860073067 KT = 74.56a motor current

Back EMF Voltage @ 600rpm = 600rpm / 20kv = 30v

Pack Voltage 44.4v * XX.XXX% duty cycle bldc = BLDC effective voltage

(XX.XXXv effective - 30v back emf) / 0.02ohm resistance = 74.56a motor current

2347.98w electrical = 74.56a * (30v bemf + (0.02ohm * 74.56a))

2347.98w electrical / 74.56a motor current = 31.49v effective

31.49v effective / 44.4v pack voltage = 70.92% duty cycle

74.56a motor current^2 * 0.02ohm = 111.183872w copper loss

2347.98w electrical / 44.4v pack voltage = 52.88a battery current

111.183872w copper loss + 2237.1w mechanical = 2347.98w electrical------------------------

Conclusions:

With a 20KV 0.02ohm BLDC motor & 44.4v battery, you can achieve 3 horsepower at 600rpm (35.60Nm @ 62.38rad/sec) with a 74.56a motor current limit controller setting, 2347.9w electrical power consumption (52.88a battery current @ 44.4v), & 111.18w copper loss assuming the stator is physically large enough that it won't magnetically saturate with less than ~74.5a stator current.

 

[Tom.G]

The motor you specified is a 3450RPM, 3-phase induction motor rated at 3Hp. It is designed to connect to the AC power line. You could buy an expensive Variable Frequency Drive (VFD) to slow it down but you will only get about ½HP out of it that way. Either choose a motor with the correct speed and HP or use mechanical means, such as a belt drive or gear box, to couple this one to your load. You will need a 5.75-to-1 speed reduction ratio.

You will also need a Motor Starter or Contactor with overloads to start and stop this thing. Pricing is in the same general range as the motor.

Sorry,
Tom