Help me for select a AC motor

- T
- thanh.tong
  
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posted
15 years ago

Sat, May 3, 2008 7:01 PM

Dear all, I'm doing my project in university. My project is to replace a DC drive by a AC drive using inverter. The old DC drive parameters are:

- Power 450kW

- Speed 50 to 1000r/min (so Tn = 4298Nm) (Tn: nominal torque) and the requirements are 250 % during 60 seconds acceleration and a periodical overload of 160 percent during 30 seconds. That means I need a starting torque is 250% nominal torque during 60 seconds. (Ts = 2.5*Ts = 10745Nm)(Ts: starting torque) Now I need to select an asynchronous motor to satisfy these requirements but the problems are:

- The starting torque of AC motor is lesser than DC motor so I must select a AC motor with larger power AC motor : Ts = (1.5-2.5)Tn So I must select AC motor power larger than DC motor to satisfy the starting requirement. I selected : P(AC motor) = 1.5 * P(DC motor) =675kW So in AC drive I have: motor power : P(AC) = 675kW speed 1000r/min Tn(AC) = 1.5 Tn(DC) = 6447Nm Ts(AC) approximates 2*Tn(AC) = 12894Nm They are my first draft calculations. Am I right? Can you help me? and give me some advise. I have searched on the internet an AC motor like that but I haven't found any motor. If you have any information or help me find a motor like that. Look forward to hearing you soon. Thanks Tong,

- G
- Gerald Newton
  
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posted
15 years ago

Sun, May 4, 2008 4:06 AM

How about horsepower?

- D
- daestrom
  
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posted
15 years ago

Sun, May 4, 2008 4:25 PM

You might consider an AC motor other than simple NEMA class A. The torque-speed curve of induction motors are not all the same. While a typical AC motor may have starting torque of about 150% of running (not high enough for your application), a class C AC motor has about 200%, and a class D can be something like 270%.

Well, you're assuming starting torque is 200% of nominal here. That *may* be true if you're getting a class C motor, but you don't mention anything about what NEMA class, so I wonder.

The short-term periodic overloading you mention might be achieved by utilizing the the motor's 'service factor'. This is a rating of how much overload a motor can withstand.

Trouble is, if you went to a class D motor as I suggested above, when you have this short-term overload, the speed on a class D will drop quite a bit, whereas a class A motor will not change speed by very much at all.

You don't mention how much torque you need at the lower speeds. If you need the same torque at 50 rpm that you do at 1000 rpm, you're in trouble. This depends on the type of load you're driving. A conveyor belt or hoist for example might need just about the same torque at any speed, while a pumping or fan application the torque would drop as speed drops. A DC drive, if it uses separately excited field, might not have any trouble with the same torque over a wide speed range.

daestrom