I want to buy a 20kW (72V) Air cooled BLDC Motor but how do I to calculate the capacity of the required battery current? i will use this motor just to start up gas turbine engine , which means it will run for about 50-60 sec

  • \$\begingroup\$ It depends on a lot of stuff, like what you want to use it for. When I say this, I mean you need to add detailed description, edit your question. Without this information nobody can help you. \$\endgroup\$
    – WalyKu
    Feb 15, 2015 at 14:19
  • \$\begingroup\$ i will use it to start up gas turbine engine \$\endgroup\$ Feb 16, 2015 at 7:44

1 Answer 1


So you know the power of the machine, 20kW. This will more than likely be shaft power.

\$P = T\omega\$

Power equals torque x speed.

You should have such information with regards to what this machine is to be connected to. What is important for battery current capability is the Torque.

\$T = k_t i\$

Torque = torque constant x current.

The beauty of BLDC machines is this current is the DC value and it only flows in two phases, thus is the current that will be drawn from your DClink (which may be your battery)

Make sure your battery voltage is of suitable voltage to achieve your operating speed with regards to all voltage drops (switches, cable, inductor)

This peak current will flow from the battery so initially you must ensure the battery can satisfy your peak torque point.

So to amp-hours. You need to quantify your duty. Do you operate at maximum speed and torque but only for 1second, the rest of the time a low speed low torque point.

  • \$\begingroup\$ your answer was very helpful , thank you very much \$\endgroup\$ Feb 16, 2015 at 7:42
  • \$\begingroup\$ i got 1 more question does the motor voltage affect Motor current , for example : will a 48V,20KW motor draw more current than a 72V,20KW motor ? \$\endgroup\$ Feb 16, 2015 at 8:18
  • \$\begingroup\$ Depends. The electrical machine only draws what it needs to oppose the load torque. There isn't enough information to say whether a 48V machine would draw more than a 72V machine. IF they are rated at the same speed and teh peak power point is the same speed, then yes, the 48V machine will draw more current for the same speed and load \$\endgroup\$
    – user16222
    Feb 16, 2015 at 9:36
  • \$\begingroup\$ i mean it i want the same torque and speed from the BLDC motor , will the current be more if i used 48V motor instead of 72V ? i am asking this because i want to choose the optimum number of battery cells which depends on the motor current and voltage \$\endgroup\$ Feb 16, 2015 at 10:01
  • \$\begingroup\$ Without specific information, i would say more. Think of it from a power in == power out. IV = Tw. For the same speed and torque, if the V goes up, the I goes down. \$\endgroup\$
    – user16222
    Feb 16, 2015 at 10:54

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